ML19290D444: Difference between revisions
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?, .gf g HCP @.... - * - , | ?, .gf g HCP @.... - * - , | ||
c, < :,- | c, < :,- | ||
1,g , i ~ '$.f 4 qq | 1,g , i ~ '$.f 4 qq f_ ## * 's | ||
f_ ## * 's | |||
,jk % | ,jk % | ||
'Tj,k '' ' | 'Tj,k '' ' | ||
| Line 303: | Line 300: | ||
*}p ' | *}p ' | ||
g.. | g.. | ||
4 , | 4 , | ||
. JSM. ,p l "Q ' | . JSM. ,p l "Q ' | ||
gg% | gg% | ||
4 | 4 | ||
. WC j' ;iL '': :!sR q ,,,4 9 | . WC j' ;iL '': :!sR q ,,,4 9 y; | ||
h A,; I ,- | |||
yE e .. ~ | yE e .. ~ | ||
gj , | gj , | ||
| Line 424: | Line 418: | ||
t- | t- | ||
._u a > | ._u a > | ||
,...4,g<,%} | ,...4,g<,%} | ||
s ' g<<L L | s ' g<<L L | ||
| Line 961: | Line 954: | ||
r-m.- | r-m.- | ||
0 | 0 | ||
, .. 9 .=#. 's *k # . . e I I e m he ,', | , .. 9 .=#. 's *k # . . e I I e m he ,', | ||
, 3 3 a | , 3 3 a | ||
| Line 1,023: | Line 1,015: | ||
^ | ^ | ||
o' [i f o ,lt- | o' [i f o ,lt- | ||
- # '',, i ;. ,3. , m - *; ~~, ' a , | - # '',, i ;. ,3. , m - *; ~~, ' a , | ||
c | c | ||
| Line 1,126: | Line 1,117: | ||
Y | Y | ||
{h,'Y:.j'h.'.Nl', T t.h%. **[t.,hp$.:<.fl 9,cul 2 o | {h,'Y:.j'h.'.Nl', T t.h%. **[t.,hp$.:<.fl 9,cul 2 o | ||
- (J ) ;I a 3' 9 O :{ ~h. '=' n | - (J ) ;I a 3' 9 O :{ ~h. '=' n | ||
,){. ._ , ' | ,){. ._ , ' | ||
| Line 1,139: | Line 1,129: | ||
14, bw %, e$ iSMk,., s[,,. '5 | 14, bw %, e$ iSMk,., s[,,. '5 | ||
~ ~ | ~ ~ | ||
LukyJ: y~p % a N1,y101.N y;:li A .. | LukyJ: y~p % a N1,y101.N y;:li A .. | ||
f[9 fS'.! ~ . ,,;a hw | f[9 fS'.! ~ . ,,;a hw | ||
| Line 1,151: | Line 1,139: | ||
o-[ .i'%.4.r .:: W r 3 o- >g$ | o-[ .i'%.4.r .:: W r 3 o- >g$ | ||
~ | ~ | ||
['-t b:9.n dm#i.77kN II | ['-t b:9.n dm#i.77kN II((h,.,NtryN | ||
~ | ~ | ||
s.I h.b'jy 6,--- T l | s.I h.b'jy 6,--- T l | ||
| Line 1,706: | Line 1,694: | ||
\ | \ | ||
\ | \ | ||
i | i PACIFIC t T BEACH i g nl 1 \ . | ||
PACIFIC t T BEACH i g nl 1 \ . | |||
5 mssi BAT | 5 mssi BAT | ||
/ I | / I | ||
| Line 1,738: | Line 1,724: | ||
I i | I i | ||
, I Ii th I | , I Ii th I | ||
; i c.. : | ; i c.. : | ||
( I If I O IKm i l i l l | ( I If I O IKm i l i l l | ||
| Line 1,764: | Line 1,749: | ||
E O | E O | ||
* Southernmost exposures of x/f;7 f O, Poway clost O it l y o O. | * Southernmost exposures of x/f;7 f O, Poway clost O it l y o O. | ||
2 o Eocene c o,o | 2 o Eocene c o,o | ||
' oO* W | ' oO* W | ||
| Line 1,873: | Line 1,857: | ||
5 ...... . | 5 ...... . | ||
'..,s | '..,s | ||
). ...o *l ., Faults (Kennedy, 1975) | ). ...o *l ., Faults (Kennedy, 1975) | ||
\ ' | \ ' | ||
| Line 1,885: | Line 1,868: | ||
:: j ' ' 'q.* . .,Qly | :: j ' ' 'q.* . .,Qly | ||
. . .. . . ~; \60;d: y e..C . | . . .. . . ~; \60;d: y e..C . | ||
' Olv -- Lindavista Fm. | ' Olv -- Lindavista Fm. | ||
( | ( | ||
| Line 1,929: | Line 1,911: | ||
}' ' | }' ' | ||
?, ' | ?, ' | ||
g base of Olv | g base of Olv g approx. 100m | ||
g approx. 100m | |||
: elevation | : elevation | ||
..,: .::r | ..,: .::r | ||
| Line 2,054: | Line 2,033: | ||
t pd g? - | t pd g? - | ||
ua r_y;W- | ua r_y;W- | ||
^c' rd v- | ^c' rd v-4 , .. b e al g | ||
4 , .. b e al g | |||
. g3in. , | . g3in. , | ||
p o | p o j.. m% g sS e | ||
j.. m% g sS e | |||
st | st | ||
' ; ,Nn ? w- > | ' ; ,Nn ? w- > | ||
| Line 2,118: | Line 2,093: | ||
t otS gf a | t otS gf a | ||
.ag@A h ,_ ee , | .ag@A h ,_ ee , | ||
k.w+c 5 | k.w+c 5 | ||
Q . | Q . | ||
| Line 2,164: | Line 2,138: | ||
~ | ~ | ||
ane x. | ane x. | ||
hu | hu | ||
- : ,a eit hL a m%$ | - : ,a eit hL a m%$ | ||
| Line 2,330: | Line 2,303: | ||
7 m r | 7 m r | ||
Y .'s'[$ 71,[" EcSec'[sls " " " " U E Es*I= Y " ~ * ' | Y .'s'[$ 71,[" EcSec'[sls " " " " U E Es*I= Y " ~ * ' | ||
)) y 5-y.. .. . -~.. -,.y j | |||
) | ) | ||
\g N | \g N | ||
| Line 2,401: | Line 2,374: | ||
$ km | $ km | ||
\ Io | \ Io | ||
- I t i | - I t i l 0 50 100 \ __ | ||
l 0 50 100 \ __ | |||
Figure 1. The major faults of southern California and northern Baja California. | Figure 1. The major faults of southern California and northern Baja California. | ||
84 | 84 | ||
| Line 2,612: | Line 2,583: | ||
o | o | ||
. o E) | . o E) | ||
O O' GDD o So 6 . | O O' GDD o So 6 . | ||
8 | 8 | ||
| Line 2,923: | Line 2,893: | ||
O W 0 _ | O W 0 _ | ||
$ 10-2_ - | $ 10-2_ - | ||
g _ | g _ | ||
2 4 | 2 4 | ||
| Line 2,956: | Line 2,925: | ||
h;91- p-- p' | h;91- p-- p' | ||
,s. ,. u .s. .;, , | ,s. ,. u .s. .;, , | ||
: r. , , , | : r. , , , | ||
e . e a t t | e . e a t t | ||
| Line 3,514: | Line 3,482: | ||
"H ^ h, \ | "H ^ h, \ | ||
el tpg'+,. . . | el tpg'+,. . . | ||
N | N | ||
~ | ~ | ||
| Line 3,545: | Line 3,512: | ||
~g'',ht$ v. ^ c 'm | ~g'',ht$ v. ^ c 'm | ||
.<fe c r . . t. | .<fe c r . . t. | ||
m u - | m u - | ||
t s,'b K | t s,'b K | ||
| Line 3,610: | Line 3,576: | ||
\ | \ | ||
* s.. | * s.. | ||
j | j | ||
's t | 's t | ||
| Line 3,619: | Line 3,584: | ||
+._,, | +._,, | ||
tw1N Q& 1 ^ | tw1N Q& 1 ^ | ||
((UM e=b 9~T cm ,~ | |||
c=;-Q c ==3 | c=;-Q c ==3 | ||
',jl3 E" | ',jl3 E" | ||
| Line 3,656: | Line 3,621: | ||
-. m a f: | -. m a f: | ||
,f, | ,f, | ||
*< ~* | *< ~* | ||
,ihf?/8 ^ ~ | ,ihf?/8 ^ ~ | ||
| Line 3,670: | Line 3,634: | ||
*% __f* M *M 4 g.g p- wff --.pq. . - - . . , , , . . . | *% __f* M *M 4 g.g p- wff --.pq. . - - . . , , , . . . | ||
..'3 g, . | ..'3 g, . | ||
..r' h,ae | ..r' h,ae | ||
(.4 . - x 4 g. | (.4 . - x 4 g. | ||
| Line 3,697: | Line 3,660: | ||
7 | 7 | ||
:s eIIE.AQ7 '{}'ly*;- - | :s eIIE.AQ7 '{}'ly*;- - | ||
.,,,v- , | .,,,v- , | ||
.# , % a w.v.y Yl _, [w.x 2. | .# , % a w.v.y Yl _, [w.x 2. | ||
)ty4 / . )4 .x,A _ 3,(p;; ,_4o ,y: h ]Vh lf:fEh D A | )ty4 / . )4 .x,A _ 3,(p;; ,_4o ,y: h ]Vh lf:fEh D A | ||
. ;~g | . ;~g | ||
. i : . ~ | . i : . ~ | ||
| Line 3,707: | Line 3,668: | ||
ml. x u.$a a:l-Figure 3. Strean-bed erosion in the San Diego River channel at the liighway 67 bridge over the San Diego River north of Lakeside. | ml. x u.$a a:l-Figure 3. Strean-bed erosion in the San Diego River channel at the liighway 67 bridge over the San Diego River north of Lakeside. | ||
Approximately 10 feet (3 m) of erosion has connected deep, sand borrow pits on either side of the bridge. | Approximately 10 feet (3 m) of erosion has connected deep, sand borrow pits on either side of the bridge. | ||
W '? | W '? | ||
....+-v Vi; I' h[j , | ....+-v Vi; I' h[j , | ||
| Line 3,738: | Line 3,698: | ||
,u g / | ,u g / | ||
,1#ft Mj., b | ,1#ft Mj., b | ||
..M;j 300 . | ..M;j 300 . | ||
$7 :; ; ; 2 ;; : | $7 :; ; ; 2 ;; : | ||
| Line 3,981: | Line 3,940: | ||
*yl ' ?! ' > . \_ $ l g s Y - | *yl ' ?! ' > . \_ $ l g s Y - | ||
? . ;) * | ? . ;) * | ||
*[ | *[ | ||
]; | ]; | ||
| Line 4,021: | Line 3,979: | ||
r , ,. , x i(.f~d, g | r , ,. , x i(.f~d, g | ||
e O E | e O E | ||
, f. ,4)h~g | , f. ,4)h~g h, | ||
y' v , | |||
v , | |||
L U a e | L U a e | ||
** * , , . ; d h Y. | ** * , , . ; d h Y. | ||
| Line 4,068: | Line 4,024: | ||
+e t | +e t | ||
a w x . wwa p, a | a w x . wwa p, a | ||
e,g... , aonm,4gwy4w 4,, & | e,g... , aonm,4gwy4w 4,, & | ||
> 0 | > 0 | ||
^, sa . . _,o- - | ^, sa . . _,o- - | ||
-L", | -L", | ||
| Line 4,078: | Line 4,032: | ||
x %'y %, , | x %'y %, , | ||
f ,V; MM.es r | f ,V; MM.es r | ||
f | f (( M, N _ ;R y y N fN ' f[ N '*rb'' Q ''s_ * | ||
. #ND <,_f ba | . #ND <,_f ba((M yd1 5o mm | ||
,.7%. / S I l | ,.7%. / S I l | ||
.. i i C | .. i i C | ||
| Line 4,150: | Line 4,104: | ||
, , , N f$ lip Plan e s | , , , N f$ lip Plan e s | ||
- ^ ^ - | - ^ ^ - | ||
, g -x ,- y . p .. g og_ g~.,:_:_-;. | , g -x ,- y . p .. g og_ g~.,:_:_-;. | ||
Os=0s [' | Os=0s [' | ||
| Line 4,232: | Line 4,185: | ||
figure 5. sequence of sections through a lanJslide in the Poway area. 110 t e similarity to Figure 4 I72 | figure 5. sequence of sections through a lanJslide in the Poway area. 110 t e similarity to Figure 4 I72 | ||
> ' " ' " ' ' . ' " +~7 | > ' " ' " ' ' . ' " +~7 ((jp%T:? pL QT,Q[#{,T*f7Eq). t5 ;5 E N g | ||
** .g | ** .g | ||
. . , ~ | . . , ~ | ||
| Line 4,309: | Line 4,262: | ||
". *y**"e 7 pu 7 gd,u 4 ,a 5 .- | ". *y**"e 7 pu 7 gd,u 4 ,a 5 .- | ||
44 | 44 | ||
>. r, | >. r, n ) ( ',- . | ||
n ) ( ',- . | |||
( | ( | ||
$ 4{C 3 e% | $ 4{C 3 e% | ||
| Line 4,397: | Line 4,348: | ||
'$f. r ;l[ n,, y ; it_ ,. . m p.w L~ ';;2 u;f,3, , | '$f. r ;l[ n,, y ; it_ ,. . m p.w L~ ';;2 u;f,3, , | ||
n ~ .,n . | n ~ .,n . | ||
.s k ,r o ,- . . . . . , | .s k ,r o ,- . . . . . , | ||
g- " ipf , * ' | g- " ipf , * ' | ||
| Line 4,452: | Line 4,402: | ||
~ | ~ | ||
f . 4e i | f . 4e i | ||
' "'A | ' "'A Figure 14. Viejas Mountain debris flow, right center of photo (l" = 4,000 f t.+) . | ||
Figure 14. Viejas Mountain debris flow, right center of photo (l" = 4,000 f t.+) . | |||
s | s | ||
*h ': r :; | *h ': r :; | ||
| Line 4,581: | Line 4,529: | ||
. . .:..~. ..v. .n.. | . . .:..~. ..v. .n.. | ||
.. ... : . :.:~.: | .. ... : . :.:~.: | ||
~.~. . . : . . | ~.~. . . : . . | ||
*...~ . . | *...~ . . | ||
| Line 4,589: | Line 4,536: | ||
i: * | i: * | ||
.;;:n. .. ::.::a..e. | .;;:n. .. ::.::a..e. | ||
.r s.., y, | .r s.., y, LA 3 OLLA . | ||
LA 3 OLLA . | |||
: : ~ :. .. ;a > | : : ~ :. .. ;a > | ||
. ::...: ::n:. ..?.:.:*i .~ | . ::...: ::n:. ..?.:.:*i .~ | ||
:;v.. : ::.:.:v..: ::::: .... | :;v.. : ::.:.:v..: ::::: .... | ||
.I 2 .::. . ... ,e | .I 2 .::. . ... ,e | ||
.:. :::::;.v.:::.:. | .:. :::::;.v.:::.:. | ||
M - * * ; .-ll . | M - * * ; .-ll . | ||
| Line 4,616: | Line 4,560: | ||
""7 i ...:::: | ""7 i ...:::: | ||
o s ' | o s ' | ||
- T ./ f,...:'.l.'h $,* !,b..:..:.: bI.Uk1 . . | - T ./ f,...:'.l.'h $,* !,b..:..:.: bI.Uk1 . . | ||
o .. | o .. | ||
| Line 4,665: | Line 4,608: | ||
g TO MEDIUM EXPANSIVE SOILS --: | g TO MEDIUM EXPANSIVE SOILS --: | ||
a e< , | a e< , | ||
3 AREAL DISTRIBUTION OF LOW -, | 3 AREAL DISTRIBUTION OF LOW -, | ||
- :- - ;: ~ :A . | - :- - ;: ~ :A . | ||
| Line 4,830: | Line 4,772: | ||
gggc,p A t %.: | gggc,p A t %.: | ||
f m -{W {. j | f m -{W {. j | ||
%R '*g:Agg,%jf&(Q.~_,?g | %R '*g:Agg,%jf&(Q.~_,?g | ||
'n- + A . :- | 'n- + A . :- | ||
| Line 4,883: | Line 4,824: | ||
.h, -. '.Mj: | .h, -. '.Mj: | ||
, x. m - <- ~.,t.- | , x. m - <- ~.,t.- | ||
_ ,g;4 $ 7*. . +=rn ' , ++v.-..- * [.p',g,N.yt 4. , y _ , | _ ,g;4 $ 7*. . +=rn ' , ++v.-..- * [.p',g,N.yt 4. , y _ , | ||
'G '- | 'G '- | ||
| Line 4,892: | Line 4,832: | ||
fQ-i? *s - | fQ-i? *s - | ||
3.. | 3.. | ||
9 _/> % >vene $'rene . | 9 _/> % >vene $'rene . | ||
6/ | 6/ | ||
| Line 4,901: | Line 4,840: | ||
The buttress at the foot of Froude Street, the rocky headland south of Hill Street, and the southeast-striking tunnels inland and ad-jacent to the dog-leg promon tory nor th of Froude S t reet have been little changed during the 18 year period between these photographs. | The buttress at the foot of Froude Street, the rocky headland south of Hill Street, and the southeast-striking tunnels inland and ad-jacent to the dog-leg promon tory nor th of Froude S t reet have been little changed during the 18 year period between these photographs. | ||
(Older photograph courtesy of San Diego Ti tle Insurance and Trust Lor tpany) . | (Older photograph courtesy of San Diego Ti tle Insurance and Trust Lor tpany) . | ||
g-, -r - | g-, -r - | ||
*" __ . - v w a,- | *" __ . - v w a,- | ||
| Line 4,963: | Line 4,901: | ||
3 (Q \.- 'J ~ | 3 (Q \.- 'J ~ | ||
l I | l I | ||
- ~%): ,f% gg | - ~%): ,f% gg | ||
* 33 - | * 33 - | ||
| Line 4,991: | Line 4,928: | ||
Of OCEANSIDE O 'e Ant $gAo T ENCINITA5 4 C 3 00' _ | Of OCEANSIDE O 'e Ant $gAo T ENCINITA5 4 C 3 00' _ | ||
A SEACH gCoRDERO SoRREN g SCRIPP$ INSTITUtloN of OCEANOGRAPHY L A JOLL A A E Sub SET CLlHS SAN DIEGO f | A SEACH gCoRDERO SoRREN g SCRIPP$ INSTITUtloN of OCEANOGRAPHY L A JOLL A A E Sub SET CLlHS SAN DIEGO f | ||
CoRONADO | CoRONADO e . -e 118' 3 0' 118' 0 0 117 | ||
e . -e 118' 3 0' 118' 0 0 117 | |||
* 30' Figure 1. Locatnun udp Coas tal erosion is related to bedrock structure. Faults in north San Diego County have been napped by Ziony, et al.(1974). Additional faults have been identified and mapped in the sea cliffs, and on the sea floor from aerial photographs. Faulting has brought sof t, easily-eroded formations into contact with the coastline, accounting for im-portant areas of erosion. Accelerated crosion occurs particularly where weaker bedding dips seaward. | * 30' Figure 1. Locatnun udp Coas tal erosion is related to bedrock structure. Faults in north San Diego County have been napped by Ziony, et al.(1974). Additional faults have been identified and mapped in the sea cliffs, and on the sea floor from aerial photographs. Faulting has brought sof t, easily-eroded formations into contact with the coastline, accounting for im-portant areas of erosion. Accelerated crosion occurs particularly where weaker bedding dips seaward. | ||
Research was funded by (10AA, Office of Sea r, r ,n t (fic. 04-8-tiOI-189) , | Research was funded by (10AA, Office of Sea r, r ,n t (fic. 04-8-tiOI-189) , | ||
| Line 5,006: | Line 4,941: | ||
\, a ,- | \, a ,- | ||
a tz pr. - | a tz pr. - | ||
..,.._.~- q gmE~BTE363) | ..,.._.~- q gmE~BTE363) | ||
~ | ~ | ||
| Line 5,151: | Line 5,085: | ||
* 1 Y~ | * 1 Y~ | ||
n - | n - | ||
4 | 4 w';yr | ||
w';yr | |||
':b :W v' y;,w;;c w w ,9j Figure 6A. Oblique view | ':b :W v' y;,w;;c w w ,9j Figure 6A. Oblique view | ||
' ;.' g,3 | ' ;.' g,3 | ||
| Line 5,159: | Line 5,091: | ||
, ' e %i east of a portion of | , ' e %i east of a portion of | ||
-y .,- -- . -[ g south Solana Beach i . . | -y .,- -- . -[ g south Solana Beach i . . | ||
? ,,: .,. , , . . prfor to bluff-top S. I hhS'ip'* ~...<, f E.,. .,l '.)D".U M | ? ,,: .,. , , . . prfor to bluff-top S. I hhS'ip'* ~...<, f E.,. .,l '.)D".U M (( # b ' I. {,f . | ||
development. Shore g *' $'., | development. Shore g *' $'., | ||
AW.47%A.,, Processes Lab.,5.l.0. | AW.47%A.,, Processes Lab.,5.l.0. | ||
: h. _ '. | : h. _ '. | ||
| Line 5,271: | Line 5,202: | ||
- ~'- a | - ~'- a | ||
,/ . | ,/ . | ||
'y | 'y e | ||
f | |||
.; ,, t | .; ,, t | ||
- rg .- w , .. | - rg .- w , .. | ||
| Line 5,285: | Line 5,215: | ||
. ,, ,y q c %<;~ - | . ,, ,y q c %<;~ - | ||
Vfl R$ RI~0 ; we .hj.. | Vfl R$ RI~0 ; we .hj.. | ||
x;y.ql | x;y.ql | ||
_ & :~ | _ & :~ | ||
| Line 5,294: | Line 5,223: | ||
-w .s Q. ,' , _ | -w .s Q. ,' , _ | ||
.- - 4.n- | .- - 4.n- | ||
.. m- % f5 g, | .. m- % f5 g, | ||
.y c F94 | .y c F94 | ||
| Line 5,368: | Line 5,296: | ||
~ . | ~ . | ||
i ',9 s.:n _. | i ',9 s.:n _. | ||
>y I, p r- ' ,dp c_ ;, - \ . ,..c.- r_A;? ~y s ? J / \'- 1.r s . | >y I, p r- ' ,dp c_ ;, - \ . ,..c.- r_A;? ~y s ? J / \'- 1.r s . | ||
tb, ' , 3 "/ 4r: + .. | tb, ' , 3 "/ 4r: + .. | ||
| Line 5,391: | Line 5,318: | ||
' ~ | ' ~ | ||
c | c | ||
\,,4.=';Qg7,k?&g | \,,4.=';Qg7,k?&g | ||
'* 5 ':/ | '* 5 ':/ | ||
| Line 5,423: | Line 5,349: | ||
( | ( | ||
< . ;, :2 r-"<)yf Ni dj#ho s | < . ;, :2 r-"<)yf Ni dj#ho s | ||
v e ) | v e ) | ||
, \,..t 3, 8 T y f.Y . | , \,..t 3, 8 T y f.Y . | ||
| Line 5,510: | Line 5,435: | ||
a | a | ||
{ ~ _ , , ,. . K. e A g ',y 9 . ' c Th 4 .t\, | { ~ _ , , ,. . K. e A g ',y 9 . ' c Th 4 .t\, | ||
% -*k".v G -N y .. I * ; - 'w. _k | % -*k".v G -N y .. I * ; - 'w. _k | ||
- - ~.% i 1 | - - ~.% i 1 | ||
| Line 5,521: | Line 5,445: | ||
f. | f. | ||
y 's i' % | y 's i' % | ||
* Qs., ~-r | * Qs., ~-r 3 . .$. | ||
3 . .$. | |||
"g h 0 ~ j ~ , | "g h 0 ~ j ~ , | ||
.,.* . ( | .,.* . ( | ||
| Line 5,607: | Line 5,529: | ||
,e | ,e | ||
* s,,. , ,,'A , , | * s,,. , ,,'A , , | ||
,. ' tar as..e , y (. , a v -s | ,. ' tar as..e , y (. , a v -s | ||
.n, s 2'O , | .n, s 2'O , | ||
| Line 5,615: | Line 5,536: | ||
~ ' | ~ ' | ||
~ . * ' s. , .' | ~ . * ' s. , .' | ||
g | g 1 N , | ||
1 N , | |||
-3., , . ,, | -3., , . ,, | ||
r | r | ||
| Line 5,716: | Line 5,635: | ||
-sy "'~ d [ k h -h h[ [ I- ''' ' ,- ' _Y;. j M$nyhi>mr,.. | -sy "'~ d [ k h -h h[ [ I- ''' ' ,- ' _Y;. j M$nyhi>mr,.. | ||
f: | f: | ||
s cs '\' 'h , s:s b w - Q - | s cs '\' 'h , s:s b w - Q - | ||
. -c . | . -c . | ||
Latest revision as of 09:37, 16 March 2020
| ML19290D444 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 11/30/1979 |
| From: | Abbott P, Elliott W SAN DIEGO ASSOCIATION FOR GEOLOGISTS |
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EARTHQUAKES AND OTHER PERILS SAN DIEGO REGION Edited by: Patrick L. Abbott William J. Elliott Department of Geological Sciences Engineering Geologist San Diego State University P.O. Box 541 San Diego, CA 92182 Solana Beach, CA 920/5 Prepared for Geological Society of America field trip by San Diego Association of Geologists November,1979 i
COPYRIGHT @ 1979 San Diego Association of Geologists All rights reserved. No part of this book may be reproduced without wri tten conser .he publisher. For information contact the San Diego Association of Geo-logists, c/o Patrick L. Abbott, Department of Geological Sciences, San Diego State University, San Diego, CA 92182. Library of Congress Catalog Card Number: Printed by Fidelity Printing, San Diego, CA il
PREFACE As the Earth and its processes are dynamic, so also is our knowledge of the Earth. Since the GSA Cordilleran Section meeting in San Diego in 1961, new information and new interpretations of Southern Cali fornia/ North-ern Baja Cali fornia/Of fshore geology have been added to the library. Some of these new, revised, and of ten heatedly disputed data are presented herein. There is never a last word in geology, but to the extent possible, we have tried to bring together some of the latest words - including opposing viewpoints. it is hoped that the field trip, plus the written text, will stimulate worthwhile discussion and help direct the next round of data collection and interpretation. William m. Elliott Pat ri ck L. Abbot t July, 1979 ACKNOWLEDGEMENTS Ve would first like to express our appreciation to the authors for thei r time and efforts expended in preparing these papers. Much of the quality of this volume must be att ributed to the invalu-able assistance of Lynn Henry, who participated in every phase of produc-tion. Everyone will appreciate the inimitable touch added by John Holden's original cartoons. Patricia Bell drew the landslide sketch. A signi ficant amount of camera-ready copy was typed by Kathy Jessup with assistance from Marge Neun. Publication of this book was made possible by the generous contribu-tions f rom the geotechnical fi rms listed on the following pages. iii
CONTRIBUTORS CATLIN AND COMPANY, INC. 7841 El Cajon Blvd. La Pesa, California 92041 CONVERSE WARD DAVIS DIXON 1440 S. State College Blvd., Suite 4H Anaheim, California 92806 DAMES & MOORE 1100 Glendon Avenue, Suite 1000 Los Angeles, California 90024 WILLIAM J. ELLIOTT Consultant P. O. Box 541 Solana Beach, California 92075 ENVICOM 4521 Sherman Oaks Avenue Sherran Oaks, California 91403 FUGRO, INC. 3777 Long Beach Blvd. Long Beach, Cali fornia 90807 TED FUNNEKOTTER Geophysics P. O. Box 575 Escondido, Cali fornia 92025 GE0 CON, INC. 6645 Convoy Court San Diego, California 92111 li.~ER-CITY i' S0ILS, INC. 7075 Mission Gorge Road San Diego, California 92120 1RVINE CONSULTING GROUP, INC. AND ITS SUBSIDIARIES: SAN DIEGO S0ILS ENGINEERING, INC. 4900 Mercury Street San Diego, California 92111 and IRVINE SOILS ENGINEERING, INC. 18003 Sky Park Circle Irvine, California 92714 WILLI AM S. KR00SK05 & ASSOCI ATES, INC. 4320 Vandever Avenue San Diego, Cali fornia 92120 LARIVE DRILLING C0., INC. 753 Gretchen Road Chula Vista, Cali fornia C. W. LA MONTE & ASSOCIATES, INC. 8145 Ronson Road, Suite H San Diego, California 92111 iv
CONTRIBUTORS LEls' TON AND ASSOCIATES, INC. 1797S Sky Park Circle, Suite H I rvine, Cali fornia 92714 MEDALL, ARAGON, WORSWICK & ASSOCIATES, INC. 2044 Cotner Avenue Los Angeles, California 90025 DOUGLAS E. MORAN, INC. 12791 Newport Avenue , Sui te G Tustin, Cali fornia 92680 MULTl SYSTEMS ASSOCIATES 4007 Camino del Rio South, Suite 208 San Diego, California 92108 CLAUDE B. PARKER Geotechnical Consultant 740 Metcal f Street, suite 7 Escondido, Cali fornia 92025 ROBERT PRATER ASSOCIATES 10505 Roselle Street San Diego, California 92121 GARY S. RASMUSSEN & ASSOCIATES, INC. P. O. Box 5488 San Bernardino, California 92412 SHEPARDSON ENGINEERING ASSOCIATES, INC. 1083 North Cuyamaca St reet El Cajon, California 92020 DAVID D. Sit lTH AND AS'o0CI ATES Envi ronmental Consul tants P. O. Box 1338 La Jolla, California 92038 SOUTHERN CAllFORNIA S0ll AND TESTING, INC. P. O. Box 20627 Sar Diego, California 92120 WESTEC SERVICES, INC. 3211 5th Avenue San Diego, California 92103 WOODWARD-CLYDE CONSULTANTS 3467 Kurtz Street San Diego, Cali fornia 92110 v
CONTENTS FAULTS AND LINEAMENTS IN THE BASEMENT TERRANE OF SOUTH-CENTRAL SAN DIEGO COUNTY, CALIFORNIA Paul M . Me r i f i e l d and D . L . La ma r . . . . . . . . . . . . . . . . . . . . I SEISMICITY OF THE ! EGO REGION James A. Hileman..................................... 11 IMPLICATIONS OF FAULT PATTERNS OF THE INNER CAllFORNIA CONTI-NENTAL BORDERLAND BETWEEN SAN PEDRO AND SAN DIEGO H. G. Greene , K. A. Bai ley , S. H. Clarke , J. I. Ziony, and M. P. Kennedy................. .. 21 FAULTING OFFSHORE SAN DIEGO AND NORTHERN BAJA CALIFORNIA Mark R. Legg and Michael P. Kennedy.................. 29 ACTIVE AND POTENTIALLY ACTIVE FAULTS: SAN DIEGO COUNTY AND HORTHERNMOST BAJA CALIFORNIA R. Gordon Gastil, Ronald Kies and Douglas J. Melius.. 47 ROSE CANYON FAULT: AN ALTERNATIVE INTERPRETATION Richa rd L. Th reet . . . . . .. ....... . ................ 61 GEOPHYSICAL SURVEY OF THE LA NACION FAULT ZONE, SAN DIEGO, CAllFORNIA Mon te Ma rsha l l . . . . . .... ........... .. .... ...... 73 SEISMICITY AND FAULTING IN NORTHERN BAJA CALIFORNIA James N. Brune, Ri cha rd S . Si mons , Cece l l o Rebol la r, and Alfonso Reyes.... ........ ........ ...... ... 83 INSTRUMENTAL SEISMICITY OF THE SAN DIEGO AREA Ri ch a rd S . S i mon s . . . . . . ....... ................. 101 PROBABillTY OF EARTHQUAKE GROUND ACCELERATIONS IN SAN DIEGO C. B. Crouse... . ..... ............................. 107 THEORETICAL ASPECTS OF TSUNAMIS ALONG THE SAN DIEGO C0ASTLINE W.G.VanDorn....................................... 115 vi
CONTENTS TSUNAMI HISTORY OF SAN DIEGO Duncan Carr Agnew................. .................... 117 EARTHQUAKE HISTORY OF SAN DIEGO Duncan Carr Agnew, Mark Legg and Carl Strand........... 123 THE 1862 EARTHQUAKE IN SAN DIEGO Mark Legg and Duncan Carr Agnew....................... 139 REGIONAL METEOROLOGY Donald I. Eidemiller................................... 143 FLOODS AND CHANGlHG STREAMS Howard H. Chang........................................ 151 THE 1916 FLOODS IN SAN DIEGO P a t r i c k L . Ab b o t t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 LANDSLIDES AND DEBRIS FLOWS IN SAN DIEGO COUNTY, CAllFORNIA Michael W. Hart........................................ 167 EXPANSIVE S0ILS IN SAN DIEGO, CALIFORNIA Richard P. While and Louis J. Lee............ ......... 183 SEA-CLIFF EROSION AT SUNSET CLIFFS, SAN DIEGO M i c h a e l P . Ke n n e d y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 C0ASTAL EROSION IN SAN DIEGO COUNTY, CAllFORNIA Gerald G. Kuhn and Francis P. Shepard.................. 207 ROAD LOG TO SELECTED GE0 LOGIC HAZARDS, SAN DIEGO METROPOLITAN AREA Grego ry T. Fa rrand and Wi l l i am J . E l l i ott . . . . . . . . . . . . . . 217 vil
FAULTS AND LINEAMENTS IN THE BASEMENT TERRANE OF SOUTH-CENTRAL SAN DIEGO COUNTY, CALIFORNIA by Paul M. Merifield and D. L. Lamar Lamar-Merifield, Geologists Santa Monica, CA 90401 INTRODUCTION The Peninsular Ranges of San Diego County, between the coastal plain and interior deserts, are underlain primarily by Late Mesozoic batholithic rocks and associated roof pendants of metamorphosed Paleozoic and Mesozoic rocks (Jahns, 1954). The northwest-trending San Jacinto and Elsinore fault zones (Figures 1 and 2) are the dominant structural features of the region. Images from spacecraft and high altitude aircraf t photography have provided a new perspective of the area. From a study of Gemini and Apollo photographs, Lowman (1969) noted northeast-trending lineaments, expressad by prominent valleys, that were not explained on existing geologic maps. These features, as well as the San Jacinto, Elsinore and other northwest-trending faults, are apparent on the Skylab image reproduced in Figure 3. A number of west-northwest-trending lineaments and one north-south-trend-ing lineament are also visible. As indicated in Figure 2, some of these lineaments have been previously mapped as faults, but other lineaments are not explained on existing maps. The geology of the area is shown on the Geologic Map of California (Rogers,1965; Strand,1962) and the Geo-logic Map of San Diego County (Weber,1963), which are compilations of mapping by Larson (1948), Merriam (1955,1958) and Everhart (1951). This earlier mapping was at reconnaissance scales of 1:62,500 or smaller and, for the most part, without the benefit of aerial photographs. Large areas between these early maps were covered by a cursory reconnaissance for purposes of the small-scale compilation by Weber (personal communication, 1974). More detailed investigations of selected areas are in progress (Todd,1977a,b; Hoggatt and Todd,1977). 1
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Using small-scale imagery from Skylab and Landsat spacecraft, and color infrared imagery from high-altitude RB-57 and U-2 aircraft, our attention was focused on prominent lineaments not explained on existing maps and not readily apparent on the ground or in large-scale aerial photographs. Field investigations, aided by large-scale aerial photo-graphr., were undertaken to determine the nature of the most prominent of the unexplained lineaments. Several lineaments were identified as faults by displaced contacts and/or by well-developed breccia zones and slicken-sided shear surfaces aligned with the lineament. Others were determined to be the result of erosion along foliation or joints. The origin of still others could not be established with certainty (Lamar and Merifield, 1975; Merifield and Lamar,1976). The work described in this paper was supported by NASA-Johnson Spacecraft Center Contract NAS 2-7698 and U.S. Geological Survey Contract No. 14-08-0001-13911. SAN YSIDRO CREEK FAULT The northeast-trending San Ysidro Creek fault was first recognized as a prominent 7-km (4-mile) long lineament on satellite images by Lowman (1969). Discovery of exposures of gouge up to 7 m (20 feet) wide with striated shear surfaces parallel to the lineament demonstrates the exis-tence of the fault. Data are insufficient to determine the slip direction on the San Ysidro Creek fault, but striations on shear surfaces suggest predominantly horizontal (strike-slip) movement. The ends of the San Ysidro Creek fault appear to terminate against northwest-trending faults (Figure 2). SAN DIEGO RIVER FAULT The northeast-trending San Diego River valley southwest of the Elsi-nore fault forms a 30-km (20-mile) long lineament separated 10 km (6 miles) from the southwest end of the San Ysidro Creek fault. Al though the San Diego River lineament and San Ysidro Creek fault are approximately aligned, Skylab imagery, as well as larger scale air photographs, clearly show that these features do not connect. The northeast end of the San Diego River lineament appears to abut the Elsinore fault. 3
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gEVico x;lometers u m i Figure 2 - Faults and lineaments recognized on Skylab and ERTS images and previously mapped faults, southwestern California. Abbreviations: HaCL: Hatfield Creek lineament; HCL: Henderson Canyon lineament; LPCL: Los Penasquitos Canyon lineament; LRL: Loveland Reservoir lineament; PC: Pine Creek lineament; PCL: Previtt Canyon lineament; SRL: Sutherland Reservoir lineament; SVVL: San Vicente Valley lineament; WML: Woodson Mountain lineament. 4
Some previous maps (Sauer,1929; Miller,1935; California Department of Water Resources,1967; Jennings,1973) show a fault or inferred fault along the San Diego River, while other5 do not (Everhart,1951' Merriam, 1958; Strand, 1962; Rogers, 1965; Fitzurka, 1968). None of the previous maps or reports which indicate a fault describe any field observations of fault zone exposures or offset rock contacts. In a detailed study along a segment of the San Diego River valley, Fitzurka (1968) found a right-separation of contacts between Julian Schist and plutonic rocks of from 300 to 600 m (1000 to 2000 feet). A fault along the river, in the area mapped by Fitzurka, was obscured by alluvium, anu . eported no direct evidence of faulting. Our field work substantiated the apparent separation observed by Fitzurka and also re-vealed a probable 420-m (1400-foot) right-separation of a schist body within quartz diorite along the same trend to the northeast. An en_ echelon pattern of zones of sheared and altered breccias and alignments of straight canyon segments, saddles and benches was also discovered along the river valley. The orientation of the en echelon pattern is consistent with right-slip along a shear zone. THING VALLEY FAULT One of the most prominent lineaments seen on satellite images stretches for 15 km (9 miles) in a ncrth-northeast direction through Thing Valley in southeastern San Diego County. In several places along the lineament, breccia, fault gouge and slickensided shear surfaces are exposed. A minimum of 100 m (300 feet) of right-separation is demon-strable on the fault at the southwest end of Thing Valley. The true sense of slip is indeterminant. A recent, more detailed study by Sawicki (1978) concluded that the lineament was an expression of erosion along intrusive contacts, foliation and jointing, as well as faulting. Just as in the case of the San Diego River lineament, the Thing Valley lineament is marked by a series of short fault segments rather than a continuous single trace. Although the relationship is obscured by alluvium, the north end of Thing Valley fault appears to be displaced 700-1300 m (2300-4300 feet) in a right-lateral sense by the south branch of the Elsinore fault as 5
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mapped by Merriam (1955) and Buttram (1962). However, Todd (1977a) con-cluded that rock relations probably do not permit large-scale lateral displacement on the Elsinore fault, and this northern segment may be re-lated to a thrust fault north of the Elsinore fault. The north end of the fault abuts the north branch of the Elsinore fault at Agua Caliente Hot Springs; a concentration of breccia at the fault intersection may provide a conduit for the hot water. WEST-NORTHWEST AND EAST-WEST-TRENDING FAULTS AND LINEAMENTS In addition of the northeast-trending faults described above, several west-northwest to east-west faults have been identified. The most promi-nent of these, the Barrett Lake fault (Figures 2 and 3), was mapped by Weber (1963) over a 40-km (24-mile) length in a west-northwest direction from El Cajon to Campo. Our attention was drawn to additional li,eaments of this trend by studies of Skylab, ERTS and RB-57 photos. Fault breccia and slickensided shear surfaces have been found along two of these linea-ments which are referred to as the Canyon City and Warren Canyon faults. Due to the lack of distinctive contacts between rock units, the amount and sense of displacement on these faults is indeterminant. An apparent 200-m (600-foot) left-separation of flows dipping 40 northeast in the Santiago Peak Volcanics was observed at the eastern end of the east-west-trending Otay Mountain lineament. The exposures are not adequate to prove that the separation is due to faulting and no other evidence of faulting was observed along this feature. CHARIOT CANYON FAULT A north-south-trending lineament was studied and named the Chariot Canyon fault by Allison (1974a,b), who reported 8 km (5 miles) of right-separation based on the distribution of Julian Schist and plutonic rocks. Our examination of a number of exposures in Chariot Canyon did not reveal a single fault separating Julian Schist on the west from granitic rocks on the east but a broad shear zone that appears to occupy the width of the canyon. Steeply dipping, slickensided shear surfaces were observed U to strike between north-south and N30 W. The foliation in schist and gneiss is locally undulatory but has a general strike confonnable with the shear surfaces and the trend of the canyon. 7
LINEAMENTS NOT DUE TO FAULTING Several other lineaments which show no evidence of faulting were in-vestigated; they are indicated in Figure 2. In some cases, the expcsures were sufficient to make us reasonably certain that the feature is due to erosion along foliation (Henderson Canyon lineament) or joints (Sutherland Reservoir lineament). Intrusive contacts along the Sweetwater River in Cuyamaca Rancho Park curve to the right as they cross the river valley, and no distinct break is evident. Thus, the Sweetwater River lineament (Figure 2) corresponds to the axis of a small flexure or perhaps shear (slip) fold. The Witch Creek lineament is shown as a fault by Jennings (1973);ourstudyindicatesth$tacontactinbasementrockisnotdis-placed along this feature, and no evidence of faulting was found. The origin of some of the other lineaments could not be established with certainty because of the lack of exposures and mappable contacts. TECTONIC IMPLICATIONS Of particular interest is the relationship of the northeast- and west-northwest-trending faults to the northwest-trending faults of the San Andreas set. If the northeast-trending faults form a conjugate shear system with the presently active northwest-trending right-slip faults, the northeast-trending faults should be left-slip. However, a right-slip component is probable on ,the Thing Valley and San Diego River faults. Also, the northeast and west-northwest-trending faults are restricted to pre-Tertiary rocks and nowher^ ,ut the northwest set. It is possible that the northeast and west-northwest sets were formed by an older stress system unrelated to the presently active system. East-northeast, west-southwest crustal shortening is consistent with the fault pattern and probable right-separation of the Thing Valley and San Diego Rivtr faults. No conclusive evidence of the slip direction on the west-northwest-trending faults has been found, although lef t-separation on the east-trending Otay Mountain lineament is possible. Data concerning slip directions on the faults studied are not, therefore, adequate to prove this hypothesis. However, on the basis of north-south-trending thrust faults and mylonitic zones, Sharp (1968) has also suggested an earlier period (middle Cretaceous to Eocene?) of east-west crustal shortening in 8
the eastern Peninsular Ranges. The plate tectonic model of a subduction zone parallel to the continental margin during the Masezoic (Hamilton, 1969; Hill,1971) is also consistent with earlier east-west crustal short-ening. REFERENCES Allison, M. L. ,1974a, Geophysical studies along the southern portion of the Elsinore fault: M.S. thesis (unpub.), San Diego State University, 229 p. Allison, M. L. ,1974b, Tectonic relationship of the Elsinore fauh zone and the Chariot Canyon fault, San Diego County, California: Ab. tracts with program, Geological Society of America, Cordilleran Section,
- p. 138.
Buttram, G. N. ,1962, The geology of the Agua Caliente quadrangle, Cali-fornia, M.S. thesis (unpub.), Unive. sity of Southern California. California Department of Water Resources, 1967, Ground water occurrence and quality, San Diego Region: Calif. Dept. Water Res. Bull. 106-2, 233 p. Everhart, D. L. ,1951, Geology of the Cuyamaca Peak quadrangle, San Diego County, California: California Division of Mines and Geology, Bulle-tin 159, p. 51-115. Fitzurka, M. ,1968, Geology of a portion of the San Diego River valley, California: Senior thesis (unpub.), San Diego State University,19 p. Hamilton, W. ,1969, Mesozoic California and the underflow of Pacific mantle: Geological Society of America Bull . , v. 80, p. 2409-2430. Hoggatt, W. C. and V. R. Todd,1977, Geologic map of the Descanso quad-rangle, San Diego County, California: U.S. Geological Survey Open-file Report 77-406. Hill, M. L. ,1971, Newport-Inglewood zone and Mesozoic subduction, Cali-fornia: Geological Society of America Bull . , v. 82, p. 2957-2962. Jahns, R. H., 1954, Geology of the Peninsular Ranges Province, southern California and Baja California: California Division of Mines and Geology, Bulletin 170, Chap. 2, p. 29-52. Jennings, C. W. ,1973, State of California, preliminary fault and geolo-gic map, scale 1:750,000: California Division of Mines and Geology, Preliminary Report 13. Lamar, D. L. and P. M. Merifield,1975, Application of Skylab and ERTS imagery to fault tectonics and earthquake hazards of Peninsular Ranges, southwestern California: NASA Contractor Report, CR-146985. 9
Larsen, E. S. , Jr. ,1948, Batholith and associated rocks of Corona, Elsinore, and San Luis Re., quadrangles, southern California: Geo-logical Society of America Memoir 29. Lowman, P. D. ,1969, Apollo 9 multispectral photography; Geologic analysis: NASA Goddard Space Flight Center, Greenbelt, Md. , X-644-69-423. Merifield, P. M. and D. L. Lamar,1976, Fault tectonics and earthquake hazards in parts of southern California: Final Report, Skylab EREP 463 results, NASA Contractor Report, CR-144477. Merriam R.,1955, Geologic map of Cuyapaipe quadrangle, California, scale 1:62,500: unpublished map (Cuyapaipe quadrangle presently designated Mt. Laguna quadrangle). Merriam R. ,1958, Geology and mineral resources of Santa Ysabel quad-rangle, San Diego County, California: California Division of Mines and Geology, Bulletin 177, 42 p. Miller, W. J. ,1935, Geomorphology of the southern Peninsular Ranges of California: Geological Society of America Bull . , v. 46, p.1535-1562. Rogers, R. H. ,1965, Geologic map of California, Santa Ana Sheet: Cali-fornia Division of Mines and Geology. Sauer, C.,1929, Land forms in the Peninsular Ranges of California as developed about Warner's Hot Springs and Mesa Grande: University of California Publications in Geography, v. 3, p. 199-290. Sawicki, D. A. ,1978, A structural and petrographic evaluation of the Thing Valley Lineament, San Diego County, California: M.S. thesis (unpub.), San Diego State University, 95 n. Sharp, R. V. ,1968, The San Andreas fault system and contrasting pre-San Andreas structures in the Peninsular Ranges of southern California, In, Dickinson, W. R. and Grantz, A. (eds.), Proc. Conf. on Geologic Problems of the San Andreas Fault System, Stanford University Publi-cations, Geological Sciences, v. XI, p. 292-293. Strand, R. 3. ,1962, Geologic map of California, San Diego-El Centro sheet: California Division of Mines and Geology. Todd, V. R., 1977a, Geologic map of Agua Caliente Hot Springs quadrangle, San Diego County, California: U.S. Geological Survey Open-file Re-port 77-242. Todd, V. R. ,1977b, Geologic map of Cuyamaca Peak 7h' quadrangle, San Diego County, California: U.S. Geological Survey Open-file Report 77-405. Weber, F. H. ,1963, Geology and mineral resources of San Diego County, California: California Division of Mines and Geology, County Re-port 3, 309 p. 10
~
SEISMICITY OF THE SAN DIEGO REGl0N by James A. Hilenan Fug ro , Inc. Long Beach, CA 90807 INTRODUCTION San Diego occtpies a unique location with respect to the distri-bution of southern California seismicity. Al though the San Diego area has experienced only a limited number of small, locally generated earth-quakes, it is surrounded, at least on the landward side, by areas with much more active seismicity and moderate to large (M=6 to 7.1), damaging earthquakes. The historical record of seismicity in southern California is too short to fully express all of the seismotectonic processes that might be present. This paper reviews the seismicity that has been obser-ved in the region around San Diego out to distances of about 200 km, somewhat more distance to the northwest and southeast. Another paper by R. S. Simons (this volume) provides more detail on San Diego's local shocks. Instrumental observations of earthquakes began in the southern Cali-fornia area in 1926, and one of the early seismograph stations was esta-blished at La Jolla in 1927. The La Jolla station operated until 1952 and was then supplanted by a station at Barrett, about 50 km east of San Diego. Over the years, particularly recently, more stations have been added to the southern Calif ornia seismographic network until there are presently about 160 stations. Although operated for many years primarily by the Seismological Laboratory of the California institute of Technology, the network is now a joint effort of the Seismological Laboratory and the U. S. Geological Survey, with other agencies supporting some stations. As the network was augmented, the ability to determine earthquake loca-tions improved greatly. Every earthquake hypocenter determination has an accompanying uncertainty. When particular shocks are potentially sig-nificant because of their proximity to a site of investigation or their possible relationship to faults, the location uncertainties must be con-sidered car- fully. Generally these uncertainties are expressed as a loca-tion quality in seismicity catalogs. 11
Several data sources are available for seismicity in the San Diego region. Early earthquakes, known mainly by reports of the ef fects of shaking, are catalogued by Townley and Allen (1939) for the period 1769 to 1928. The Caltech Seismological Laboratory is the original source for most of the instrumentally determined locations since 1932: H i l eman et a_1_ (1973) for 1932 to 1972, Friedman, et al_ (1976) for 1973 and 1974, and preliminary listings for subsequent shocks are available from the Seismological Laboratory. The California Division of Mines and Geology has combined the Caltech data with data from other sources to prepare a file of all California shocks reported f rom 1900 to 1977 (Real, et al, 1978); an extension to include pre-1900 shocks is forthcoming soon. The National Geophysical and Solar-Terrestrial Data Center (NGSDC) maintains a catalog with worldwide coverage of earthquakes; but the recent, smal le r shocks (M less than 3) in southern Cali fornia are not included because of thei r great number, in addition, special studies of the seismicity data have been placed into the public record for environmental reports and safety analysis reports for various critical facilities such as the San Onof re Nuclear Generating Station about 80 km northwest of San Diego. With current regulatory requirements, some level of assessment of seismic hazards is being required for practically all engineered projects. Faults shown in the figures have been compiled f rom two sources. Those faults in California represent the t races identified as Quaternary by Jennings (1975) on his Fault ap of Southern California. The faults shown i n Mexi co a re f rom Gas t i l , e_t_ a_1_. (1971 ) and a re p robab l y l a rgel y Quaternary in age. SIGNIFICANT EARTHQUAKES OF THE REGION For this discussion, the San Diego region is taken to include the enti re area shown in Figure I . Consideration of the large distances within this areu is warranted because many of the earthquakes shown have been felt in San Diego, and the occurrence of maximum earthquakes postu-lated for sone of the faults would be felt strongly in San Diego (C. B. Crouse, this volume). Ccesiderable variation exists in the certainty with which epicenters can be assigned. The locations with the greatest speculation are indicated 12
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in Figure I by a circled question mark. For example, the 1812 shock south of the Los Angeles area is shown near the mission San Juan Capi-strano where damage was most severe, but the epicenter might well have been on either the Elsinore or the Newport-Inglewood faults. Shocks occurring in northwestern Mexico even today are located with less accu-racy than those in southern California because nearly all the seismo-graph stations are in California. For the region, 34 shocks with magnitudes of about 6 or greater are shown. Rupture from aa 1857 earthquake along the San Andreas fault extended Ireto the northern portion of the region, but the shock isn't shown on the figure because maximum rupture occurred farther north. Presumably, the population density has been high enough that no shock of magnitude 6 or greater has passed unrecorded in the region since 1900 (probably earlier for nost of southern California and possibly later in Baja California). In the last 80 years, 26 large shocks have affected the region, about once every three years on the average. However, the actual occurrence is not regular in time, and the " average" could easily be adjusted by a judicious choice of regional limits. The larger earthquakes are clearly not regular in thei r spatial dis-tribution (Figure 1). For example, the San Jacinto fault system (inclu-dir.g the Imperial fault) has been the rest active fault system during our limited period of observation. Using seismicity recorded since 1912, Brune (1968) estimated that the San Jacinto fault has slipped at the rate of 1.5 cm/y r. Geologic data (Sharp, 1967; Clark, et al., 1972) indicate about 0.3 cm/yr over the past 2 m.y. These different rates can be recon-ciled because the seismic strain release could be episodic. Although about 14 shocks of magnitude 6 or greater (Figure 1) can be associated with the San Jacinto fault system (there are others along the same trend farther southeast), the largest shocks observed seem tu be limited to about magnitude 7 The 1940 imperial Valley earthquake has been assigned magni-tude 7.1 in some sources (Cof fman and Von Hake , 1973), and earthquakes with magnitudes of 7.0 and 7.1 occurred on the Colorado Del ta (just southeast of Figure 1) in 1934. Thatcher, et al. (1975) have estimated the seismic slip along the San Jacinto fault and have identified two significant, 40-km gaps; one between Cajon Pass and Riverside along the northernmost portion of the 14
fault (where an 1899, Vill shock is shown) and the other between Anza and Coyote Mountain (where the 1937 earthquake is shown). These gaps were suggested as likely sites for future moderate-sized earth ,uakas, magnitude 6 to 7, along the San Jacinto fault. Another feature of the seismicity distribution that is readily apparent is the concentration of large shocks in northwestern Mexico, mostly south of about 32 N latitude. These earthquakes seem to re-present an active region rather than any particular, dominant fault. Many of these epicenters in Mexico are uncertain by 15 km or more (Hi leman , e_t_ _al_, 1973) . The remaining larger shocks in the region are more widely scat-tered and reflect lower levels of activity. Only one or two large shocks have been observed for ar.y particular fault. The lack of larger eart5quaPes within about 100 km of San Diego is an equally significant featt re of the seismicity distribution. Is this local area dif ferent from the surrounding ones , or is the seis-micity record misleading because of its limited duration? The dis-tribution of smaller shocks, Figures 2 through 4, can provide some insight, but geologic data are also needed to provide a longer time sample and better understanding. REGIONAL SEISMICITY Instrumentally determined epicenters for earthquakes with mag-nitudes of 4 or greater in the San Diego region are shown in Figure 2 using Caltech data files. Similar data are found in the worldwide catalog file, NGSDC, which contains some shocks as early as 1915 for this region and occasionally reports dif ferent magnitudes. The distribution of magnitude 4 and above seismicity since 1932 (Figure 2) is similar to that of the larger earthquakes. A lower limit of magnitude 4 was chosen to assure uniform representation of the seismicity throughout the region. For smaller shocks , the dis-tribution of seismograph stations influences whether suf ficient data are available for epicente s to be determined. The San Jacinto fault shows high activity along its socinern half, but considerably less along the northern hal f where several large shocks occurred prior to 1932. Af tershocks have not been removed from this data set; but if 15
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, quakes of magnitude 4 and greater. Data are h a,a the Seismological Laboratory of the California Institute of Technology.
16
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they are, the same distribution results along the San Jacinto fault (Thatcher, et,al, 1975). The zone of high activity in Mexico is also apparent as is the low level of seismicity around San Diego. The mag-nitude 4 and above seismicity shows a concentration along the southern one-third of the Elsinore fault where large shocks have not been obser-ved. There are scattered epicenters in the of fshore area. The San Diego area is one of historically low seismicity for shocks of magnitude 4 and greater, as demonstrated in Figures 1 and 2. When smaller shccks are considered, the area is shown to hav r a scattered distribution of microscismic activity. Figure 3 shows the earthquakes of all magnitudes in the Caltech catalog in the vicinity of San Diego for 1932 through 1975. Figure 4 is a sim*lar plot for 1976 through 1578. Some of the shocks in these figures have magnitudes down to 1.0 or less. When .3nsidering such small earthquakes, the distribution here is incomplete because not all small shocks are recorded well enough at a suf ficient number of stations to allow epicenter determi-nation. Figure 3 shows 44 years of data. During much of that tine, the .aismographic network was considerably less dense and less capable of locating small events than i t is now. Figure 4 sho.es the data for the past three years, when there has been a more systematic effort to locate the smaller shocks. The surrounding areas (Los Angeles Basin, along the Elsinore and San Jacinto faults, and in northern Mexico) are much more active than the San Diego area, and they are not shown be-cause the epicenter plot would be saturated with symbols. There are only a few shocks greater than magnitude 4 in the area, and most of these are located of fshore. The epicenter distribution seems somewhat random except for an indistinct N45oW trend offshore. Many of these smaller shocks have location uncertainties on the order of 5 to 15 km, and they are few enough that clusters and trends are not strongly de-fined. Many of these shocks are being relocated to reduce thei r un-certainties.
SUMMARY
Only small shocks have been observed in the vicinity o' San Diego twithin 100 km), and this area seems characterized by a lower level of seismicity than much of the rest of southern Cali fornia. However , many 18
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large earthquakes, with magnitades up to about 7, have occurred in the region and have been felt moderately in San Diego. Because the seis-micity record is short compared to some seismotectonic processes, geo-logic data arc also needed to supplement the data base. REFERENCES Brune, J. N.,1968, Seismic noment, seismicity and rate of slip along major fault zones: Journal of Geophysical Research, v. 73, p. 777-784. Clark, M. M. , Grantz, A., and Rubin, M. ,1972, Holocene activi ty of the Coyote Creek fault as recorded in sediments cc Lake Cahuilla: U. S. Geological Survcf Professional Paper 787, p. 112-130. Coffman, J. L., an/ Von Hake, C. A., 1973, Earthquake History of the United States. National Oceanic and Atmospheric Administration Publication 11-1, Washington, 208 p. Friedman, M. E. , Whi tcomb, J. H. , Allen, C. R. , and Hileman, J. A,1976, Seismicity of the southern California region 1 January 1972 to 31 December 1974: Seismological Laboratory of the California Institute of Technology, Pasadena, 93 p. Gastil, R. G., Phillips, R. P., and Allison, E. C., 1975, Reconnaissance geologic map of the state of Baja California: Geological Society of America Memoir 140, 170 p. Hil eman, J. A. , Al len, C. R. , and Nordqui s t, J . M. , 1973, Seismicity of the southern California region 1 January 1932 to 31 December 1972: Seismological Laboratory of the California Institute of Technology, Pasadena, 486 p. Jennings, C. W., 1975, Preliminary fault and geologic map of southern California, in, Crowell, J. C., (ed.), San Andreas fault in southern California: California Division of Mines and Geology Special Report 118, plate 1. Real, C. R. , Toppazada, T. R. , and Parke, D. L. ,1978, Sarthquake catalog of California January 1,1900 to December 31, 1974: Cali fornia Divi-sion of Mines and Geology Special Publication 52, 15 p. Richter, C. F. ,1958, Elementary Seisnology: W. H. Freeman and Co., San Francisco, 768 p. Sharp, R. V., 1967, San Jacinto fault zone in the Peninsular Ranges of southern California: Geological Society of America Bulletin, v. 78,
- p. 705-730.
Thatcher, W. , Hileman, J. A. , and Hanks , T. C. ,1975, Seismic slip dis-tribution along the San Jacinto fault zone, Southern California, and its implications: Geological Society of America Bulletin, v. 86,
- p. 1140-1146.
Townley, S. D., and Allen, M. W., 1939, Descriptive catalog of earthquakes of the Pacific coast of the United States,1769 to 1928: Seism. Society of America Bulletin, v. 29, p. 1-297 20
IMPLICATl0tlS OF FAULL PATTERNS OF THE INNER CALIFORNI A CONTitlENTAL BORDERLAND BETWEEN SAN PEDRO AND SAN DIEGO by H. G. Greene, K. A. Bailey, S. H. Clarke, and J. l. Ziony U.S. Geological Survey Menlo Park, CA 94025 and M. P. Kennedy California Division of Mines and Geology La Jolla, CA 92093 INTRODUCTl0ll Marine geophysical surveys by the U.S. Geological Survey in the Gulf of Catalina and San Diego Trough show a complex structural pattern. P rel imina ry interpretations of offshore continuous seismic reflection profiles suggest that the dominant structural pattern of the inner part of this area, between Palos Verdes Hills and the Mexican border, reflects late Cenozoic right-lateral wrenching aiong two major fault zones. The character of this tectonic activity is similar to that described by Moody and Hill (1956), liarding (1973), Eaton (1923-24,1933), and Reed and Hollister (1936) for the flewport-Inglewood fault zone onshore. A closely-spaced marine geophysical data-collection grid (4 to 8 km track-line spacing) betwe'n San Pedro and San Diego (Figure 1) has enabled us to study in detail the complex structure of this region. Our data consist of moderate-resolution, deep penetration 120 kJ sparker profiles, high-resclution, moderate penetration boomer profiles, and high-resolution, shallow penetration 3.5 kHz seismic reflection records. ilavigation was by a precision, t rans ponde r-based positioning system; maximum error in positioning is approximately 50 m. DISCUSS 10ll The predominant structural grain within the Gulf of Santa Catalina and San Diego Trough has a northwest-southeast trend (Figure 1). Two major fault zones within these areas bound a relatively undeformed structural block, here referred to as the Catalina block (Figure 1). The llewport-Inglewood-Rose Canyon fault zone forms the northeast boundary of this block, and the Palos Verdes itills-Coronado Bank f aul t zone forms the southwest boundary. Both of these fault zones are composed of discontinuous, generally right-stepping, en-echelon 21
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faults and associated tolds. No single fault within either zone appearr, to continue uninterrupted for more than 40 km. This pattern resembles other fault zones of Cali fornia, onshore and offshore, which are composed of short, en,- echelon faults in relatively narrow (1 to 10 km wide) zones. The Ucwport-Inglewood-Rose Canyon fault zone, which extends offshore at Newport Beach, appears to have influenced developnent of the eastern slope of tFe Gulf of Santa Catalina physiographic basin. The zone is defined at the surface by discontinuous, generally northwest-trending faults and folds within Tertiary and Quaternary strata; these structural features form a discrete belt that extends for at least 240 km from near the Santa Monica hountains into Baja California. To the south, near Oceanside, the faul ts of this zone step to the west and continue southward to La Jolla. Onshore and northwest of Newport Beach, the faul t zone extends northward across the western Los Angeles Basin and appears to terminate abruptly at the Santa Monica fault (Barrows, 1974; Ziony, et_ al., 1974; Jennings, 1977'. Hoody and Hill (1956) and Harding (1973) postulate a right-slip wrench tectonic model for the Newport-Inglewood f aul t zone in the Los Angeles Basin. Features suggesting this same sense of motion have been noted offshore alo the southern extension of the zone: e.g., Scripps submarine canyon appear', to be a right-laterally of fset head of the La Jolla submarine canyon. The inner, north-trending segment of La Jolla Canyon also is fau't-controlled; it probably was formed by erosion along a shear zone created by motion along the Rose Canyon fault. The Palos Verdes Ilills-Coronado Bank f aul t zone extends f rom Santa Monica to Loma Sea Valley and beyond. The segment of the faul t zone near San Pedro forms the western margin of the Catalina block and is well-defined and continu-ous. However, farther south it is discontinuous along the eastern edge of Lasuen Knol l . Here strands of the fault one step westward, directly along the western edge of Lasuen Knoll. This fault zone may be traced southward for 30 km o r mo re , to i ts intersection wi th a more north-trending f aul t. From this inter-section, the Palos Verdes Hills-Coronado Bank fault zone continues southward as two separate segments ski rting the castern edge of Coronado Bank. We suggest that the fault zone has exerted structural control on the developnent of Loma Sea Valley and the eastern slope of the Coronado Bank during the Quaternary. The length, trend, and character of these two major offshore fault zones are comparable to the W' ' ttier-Elsinore and San Jacinto faul t zones onshore. Sho rt , en-echelon, second-order faults are associated with each major fault zone and commonly splay from the primary faults at angles from 20 to 40 degrees. 23
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Second-order fold axes are similarly related to these fault zones. These struc-tural relationships follow the stress pattern for wrench faulting described by Moody and Hill (1956) and Wilcox, et al. (1973) and suggest tha. che offshore zones represent through going, right-slip faults within the under!ying basement rocks. Major structural and physiographic features within and bounding the Cata-lina block are compatible with the nodel of wrench tectonics. We suggest that La Jolla submarine canyon, for example , is a graben that has formed as the result of tension associated with dilation within the Catalina block. Coronado Bank, Point Loma, and other banks and ridges within and adjacent to the Catalina block appear to be horsts produced by compression. Horst and graben topography and buried sedimentary basins and ridges in the San Diego region (Michael Kennedy , o ra l comm. , 1978) also could be an expression of wrench tectonics. CONCLUSIONS The Gulf of Santa Catalina-San Diego Trough region of the southern Cali-fornia continental borderland contains a major structural block, here called the Catalina block, which probably was formed and is presently being influenced by wrench tectonics. The Catalina block is bounded by two major fault zones, the Newport-Inglewood-Rose Canyon fault zone and the Palos Verdes Hills-Coronado Bank faul t zone, and appears to have undergone slight deformation caused prin-cipally by right-slip along the bounding fault zones. Dif ferences in the rates of right-slip along these fault zones could result in elongation and rotation of the block. REFERENCES Barrows, A. G., 1974, A review of the geology and earthquake history of the New-port-Inglewood structural zone, southern California: Cali fornia Division of tiines and Geology Special Report 114, 115 p. C rowe l t , J . C . , 1974, origin of late Cenozoic basins in southern California, In, Tectonics and Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 22, 204 p. Eaton, J. E., 1923-24, Structure of Los Angeles basin and environs: Oil Age,
- v. 20, no. 6, p. 8-9, 52 (1923) and v. 21, no. 1, p. 16-18, 52, 54 (1924).
Eaton, J. E., 1933, Long Beach, California, earthquake of March 10, 1933: Amer-ican Association of Petroleum Geologists Bulletin, v. 17, p. 732-736. Emery, K. O., 1960, The sea off southern California: New York, John Wiley and Sons, 366 p. 26
G reene , H. G. , C la rke, S. H. , J r. , Field, M. E., Linker, F. l., and Wagner, H. C. ,1975, Preliminary report on the envi ronmental geology of selected areas of the southern California continental borderland: U.S. Geological Survey Open-File Report 75-596. lia rd i ng, T. P. , 1973, Newport-Inglewood trend, California--an example of wrench-Ing style of deformation: Arerican Association of Petroleum Geologists Bulletin, v. 57, p. 97-116. Howe l l , D . G . , S t ua r t , C. J., Platt, J. P., and Hill, D. J., 1974, Possible strike-slip faulting in the southern California borderland: Geology,
- v. 2, p. 93-100.
Jennings, C. W., 1977, Geologic nap of Cali fornia: California Geologic Data liap Series, California Division of Mines and Geology, scale 1:750,000. Junger, Arne, 1976, Tectonics af the southern Cali fornia borderland, in, Howell, D. G. (ed.), Aspects of the Geologic History of the California Continental Borderland: American Association of Petroleum Geologists Pacific Section, Miscellaneous Publication 24, p. 486-498. Junger, Arne, and Wagner, H. C.,1977, Geology of the Santa Monica and San Pedro Basins California Continental Borderland: U.S. Geological Survey Miscel-laneous Field Study 820. Moody, J. D., and Hill,it. J., 1956, Wrench-fault tectonics: Geological Society of America Bulletin, v. 67, p. 1207-1246. Moo re , D . G . , 1969, Reflection profiling studies of the California continental borderland--st ructure and Quaternary car bidi te basins: Geological Society of America Special Paper 107,142 p. Reed, R. D., and Hollister, J. S., 1936, Structural evolution of southern California: American Association of Petroleum Geologists Dulletin,
- v. 20, p. 1529-1704.
Shepard, F. P. , and Emery, K. 0. ,1941, Submarine topography of f the Cali fornia coast: Canyons and tectonic interpretation: Geological Society of America Special Paper 31, 171 p. U.S. Geological Survey, 1972, Santa Monica-Baja California zone of deformation: U.S. Geological Survey Professional Paper 800-A, p. A159 Vedde r , J . G. , Beye r , L. A. , J unge r , A. , lioc re , G. W. , Robe r ts , A. E . , Tay l o r , J. C. , and Wagne r, H. ". , 1974, Preliminary report en the geology of the continental borderland of scuthern California: U.S. Geological Survey Miscellaneous Field Studies Map 624, 34 p., 9 sheets. Wilcox, R. E., Harding, T. P., and Seely, D. R., 1973, Basic wrench tectonics: American Association of Petroleum Geologists Bulletin, v. 57, p. 74-96. Yeats, R. S., 1976, Extension versus strike-slip origin of the southera California bo rde r l an d , in, Aspects of the geologic history of the California continental bo rde r l and : American Association of Petroleum Geologists, Pacific Section, Hiscellaneous Publication 24, p. 455-485. Ziony, J. I., Wentworth, C. M., Buchanan-Banks, J. M., and Wagner, H. C., 1974, Preliminary map showing recency of faulting in coastal southern California: U.S. Geological Survey Report, 11iscellaneous Field Studies 11ap 585. 27
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FAULTING OFFSHOPE SAli DIEGO AND NORTHERN BAJA CALIFORNIA Mark R. Ecgg and Michael P. Kennedy 1 Institute of Geophysics and Planetary Physics 2 California Division of Mines and Geology Scripps Institution of Oceanography University of California La Jolla, CA 92093 INTRODUCTION The offshore area considered in this discussion (Figure 1) comprises the inner continental borderland of southern California and northern Baja California, Mexico. It is bounded on the west by the San Clemente f ault zone and on the east by the shoreline. It extends north to Santa Catalina Island and south to Punta Santo Tbmas, Baja California. Although the northern and southern boundaries are well removed from metropolitan San Diego, it is important to discuss several major faults that extend from these boundari'es to the San Diego area, since they directly affect the seismic hazard in San Diego. It is becoming more apparent, as more data are collected, that a significant portion of the carthquake hazard in the San Diego area is from offshore sources (Anderson, 1979; Legg and Ortega , 1978). It has been estimated that as much as 20s of North American-- Pacific relative plate tectonic motion in southern California occurs off-shore (Anderson, 1979). This paper describes the significant faults in the offshore region which directly affect the San Diego area. Legg (1979) suggested that faults in the inner southern California borderland are capable of accommodating most, if not all of the Quaternary offshore relative plate motion. The offshore faults are divided into four major fault zones as sug-gested by Junger (1976) and Legg (1979). These zones are represented by one or more relatively long and continuous f aults with many sub-parallel, en echelon, or conjugate faults forming a wrench fault zone as shown by Wilcox, et al (1973) . The fault zones are, from west tu 2st: (1) Santa Cruz-San Clemente-San Isidro; (2) San Pedro-San Diego Trongh-Maximinos; (3) Palos Verdes Hills-Coronado Banks-Agua Blanca; (4) Newport-Inglewood-Rose Canyon-Vallecitos-San Miguel. Three of these zones pass onshore within the area shown in Fig. 1. 29
Many faults studied have existed since the middle Miocene, when the spreading center to the west of North America collided with the continent, and a triple junction migrated southward along the Pacific coast of Baja California (Atwater, 1970). These faults have had recurrent movement to the present. Total amounts of displacement in the offshore area are un-known in general, because of the obvious difficulty in finding genuine
" piercing points". Most estimates of displacements on offshore faults have been based upon bathymetry (Shepard & Dmery, 1941; Krause, 1965; Legg , 1979) .
SANTA CRUZ-SAN CLEMENTE-SAN ISIDRO FAULT ZCNE The first, and probably longest and most continuous fault zone in the inner continental borderland consists of the San Clemente-San Isidro fault (Moore , 1969; Legg , 1979) . There are many sub-parallel and oblique con-jugate faults that also show sea-floor breaks associated with the San Clemente-San Isidro fault zone, especially in the vicinity of Fortymile, Boundary, and Navy Banks (Loc. 3,4,5, respectively, Fig. 1). The San Clemente-San Isidro f ault appears to be more than 350 km in length and quite continuous in nature. It has dramatic sea-floor scarps along much of its length. The most familiar escarpment along this fault forms the eastern side of San Clemente Island ( Loc. 1, Fig. 1) which shows a total vertical relief of up to 2300 m (Junger, 1976). Lonsdale (1979) observed a 50-75 m high scarp, with an upper slope inclined 60 degrees, and with mounds of barite believed to hav-s been deposited by hydrothermal activity, along the San Clemente fault in the vicinity of the Navy sub-marine fan (Loc. 6, Fig. 1). The San Clemente fault continues southward, through the San Clemente Rift Valley (Loc. 2, Fig.1) , (Shepard & Dmery, 1941), and along the northeastern margin of the San Clemente basin. The fault forms the western face of Fortymile Bank (Loc. 3, Fig. 1), which might suggest 25 miles (40 km) of right-lateral, strike-slip displacement between Fortymile Bank and San Clemente Isl&nd (Shepard & Emery,1941) . Also in the vicinity of Fortymile Bank are several sub-parallel faults associated with the San Clemente-San Isidro fault zone, some of which appear to be splays of the ma'n fault. In particular, one at the south- y ern end of Fortymile Bank turns more easterly and appears to trend toward Navy Bank (Loc. 4, Fig. 1) and beyond, possibly connecting with the 30
Maximinos fault zone to the south. This branch is suggested by Legg (1979) to be the previously inferred connection between the San Clemente and Agua Blanca faults (Moore, 1969) , although Legg (1979) finds a " gap" (Loc. 5, Fig. 1) in the continuity of this fault southeast of Navy Bank. The main trace of e san Clemente fault continues south along the northeast side of Sar ilemente basin where it has a small bend (125 km long) south of Navy Bank. This bend (Loc. 6, Fig.1) trends more westerly (N65 W) than the -ical strike (N45 W) of the San Clemente fault and has a 250 m hign ridge associated with it. This ridge is cannonly cut by the most prominent trace of the fault zone. Where this fault does not displace rocks of the ridge itself, it lies along its base. There are many small, sub-parallel reverse faults cutting the sea-floor along the flanks of.the ridge. The more westerly strike of this possibly com-pressional feature is suggestive of right-lateral, strike-slip along the northwest trending San Clemente fault. In addition, apparent vertical offsets of as much as 500 m (Junger, 1976) and additional compressive feature a along this fault indicate that there is also a significant dip-r.ip component. Moore (1969) and Legg (1979) trace the San Clemente fault southward, connecting it with the San Isidro fault offshore Mexico, at a point more than 30 km southwest of Punta Banda and the Agua Blanca fault. Major sea-floor scarps alternating from west-side up to east-side up are common along this part of the fault zone, and may further indicate combined strike-slip and dip-slip faulting (Loc. 7, Fig.1) . The offshore Santo Tomds fault of Krause (1965) (Loc. 8, Fig. 1) has 15 km of reported left-lateral offset and is truncated by the San Clemente-San Isidro fault as shown by the long, eastward-facing escarpment (Krause, 1961; Moore, 1969). The onshore Santo Tomis fault passes offshore at Bahia Soledad (Loc. 9, Fig. 1) and appears to have had right-lateral movement ( Allen , e t al, 1960; Suarez , personal communication), which is inconsistent with the movement reported offshore by Krause (1961, 1965). Legg (1979) concludes that the Santo Tomds fault offshore is not continuous with the Bahia Soledad (or onshore Santo Tomds) fault, but is probably closely related to the San Clemente-San Isidro fault zone. 31
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In summary, the known length of the San Clemente-San Isidro fault is more than 350 km . If this fault were to rupture along half of its mapped length, it could conceivably produce an Mg17 earthquake as inferred from *he fault length versus magnitude relationships of liousner (1969) The frequency of occurrence of such large events, if they occur at all, is probably much lower than that of the San Andreas fault. The largest earthquake recorded along the San Clemente-San Isidro fault zone since 1932 was Mg = 5.9 , occuring on December 25, 1951 at the southern tip of San Clemente Island (Loc. 10, Fig. 1). Due to the close proximity of the San Clemente-San Isidro fault zone to San Diego (and because of the tsunami potential) an event of this size or greater, with associated sea-floor displacement, could be very destructive to the coastal cities of San Diego, Tijuana, and Ensenada. Moderate earthquakes (Mt 4-5) occur along this fault zone every few years, demonstrating the seismically ac-tive nature of this major, offshore fault zone. SAN PEDRO-SAN DIEGO TROUGH-MAXIMINOS FAULT ZONE The San Pedro-San Diego Trough-Maximinos fault zone lies somewhat closer to San Diego. Principal faults of this zone are the San Pedro (Loc. 11, Fig. 1) and Santa Catalina faults (Loc. 12, Fig. 1), the San Diego Trough and Thirtymile Bank faults, and the Maximinos fault. There may not be one continuous, through-going fault in this zone, but based upon current studies of seismic profiles of these faults, they appear to be sub parallel or en echelon segments of a deeper, continuous wrench fault system. The Santa Catalina and San Pedro faults are discussed in more detail elsewhere (Vedder, e t al. ,1974; Junger & Wagner, 1977). To the south, the San Diego Trough fault forms the major component of this zone (Loc. 13, Fig. 1), extending from the central San Diego trough to a point about 20 km southwest of Punta Salsipuedes (Loc. 14, Fig. 1). At its southern end the structure is very complex, and the fault may be continuous with, or en schelon to, the Maximinos fault or branches of the Agua Blanca fault (Legg, 1979). The San Diego Trough fault breaks the sea-floor with alternating east- and west-side up scarps (as high as 10-20 m) suggesting strike-slip. The fault splays along its strike many times, but re-connects with sub parallel branches, suggestive of wrench 34
faulting as described by Wilcox, et al. (1973) . Eastward-facing scarps (Loc. 15, Fig. 1) of the San Diego Trough fault act to block the down-stream end of the Coronado submarine canyon and force the channel to the south (Shepard & Dmery, 1941; Shepard & Dill, 1966; Dme ry , e t al. , 1952 ) . The Maximinos fault extends from a splay in the Agua Blanca fault, near Valle Santo Tomds, Mexico (Loc. 16, Fig. 1), and passes through Cadada Maximinos, and offshore near Punta Los Maximinos, turning more northerly to a point where it connects with the San Diego Trough and/or Fortymile Bank fault zones. The details of faulting at both the northern end of the Maxininos fault and southern end of the San Diego Trough fault are extremely complex and not completely understood. There are two or more major, sub-parallel faults associated with the Maximinos fault that trend offshore north of Bahia Soledad (Loc. 17, Fig. 1) and along the channel of a submarine canyon that heads near Bahia Soledad (Loc. 18, Fig. 1). To the north, these sub parallel branches splay and trend towards Navy Bank (Loc. 4, Fig. 1). Based upon 3.5 kHz echo-sounder records, all of these faults either break the sea-floor, or displace the upper sedimentary layers interpreted as Quaternary in age ( Leg g , 1979). The Maximinos fault passes through a small canyon north of the Canada Maximinos im-mediately before it passes offehore (Loc. 19, Fig. 1). Here it is seen to have scarps with very youthful appearances, right laterally offset stream channels, and aligned ground-water barriers as manifested by contrasting vegetation. Scarps are uplifted to the south and west along the main branch of the Maximinos fault. In summary, the San Diego Trough-Maximinos fault zone is considered herein to be a principally right-lateral, strike-slip, northwest-trending, Quaternary fault zone. The main, through-going traces of the San Diego Trough fault lie within 40 km of metropolitan San Diego, where the eastward-facing scarps force the Coronado Canyon to turn southward along the base of the Coronado escarpment. The presence of sea-floor scarps suggests a small d5 -slip component, but certainly not as great as that observed along the San Clemente-San Isidro fault zone. The San Diego Trough-Maximinos fault zone appears to have very few earthquake epicenters located near it in the southern portion, although the inaccuracies in the epicentral 35
locations in the southernmost part of the area do not allow definite con-clusions regarding activity. Some of the smaller faults associated with this system might suggest possible connections with the Sca Clemente fault n' ar Navy Bank and Fortymile Bank, but Legg (1979) finda " gaps" within the sedimentary cover through this area. PALOS VERDES HILLS-CORONADO BANKS- AGUA BLANCA FAULT ZONE Greene, et al. (this volume) discuss the Palos Verdes Hills fault zone in more detail. The Palos Verdes Hills-Coronado Banks- Agua Blanca fault zone is typified by high vertical relief at Palos Verdes Hills (not shown in Fig. 1), Coronado Banks (Loc. 21, Fig. 1), Islas Los Coronados (Loc. 22, Fig. 1), Descanso Shelf-Ridge < Loc. 23, Fig. 1), Islas de Todos Santos (Loc. 24, Fig. 1) and Punta Banda (Loc. 25, Fig. 1). Vertical displacements of several hundred meters occur locally along these segments, even though the major component of slip is lateral. At the southern end of the offshore portica of this fault zone, the Agua Blanca fault shows clear evidence of vertical movements as shown by dramatic Quaternary sea-flool scarps (Loc. 26, Fig. 1). Right-lateral, strike-slip is suggested by stream of f sets on Punta Banda (Allen, et al., 1960; Gastil, 2t al., 1975), offsets in the Punta Banda (Loc. 27, Fig. 1), Salsipueden (Loc. 28, Fig. 1), and Coronado ( Loc. 2 9, Fig. 1) submarine canyons (Legg, 1979), and by the configurations of Punta Banda and the Coronado Banks ( Legg , 1979), Iagg (1979) suggests 11 km of post-Pliocene displacement along the Coronado Banks fault by realigning the north bank with the south bank. Allen , et al . (1960), Gastil, et al. (1975), and Suarez (personal communication) suggest that not more than 20 km of right-lateral displacement exists along the onshore segment of the Agua Blanca fault. Since the San Diego Trough-Maximinos fault zone joins the Agua Blanca fault onshore, the total dis-placement on this, plus that on the Coronado Banks fault zone is probably limited by the amount suggested for the onshore Agua Blanca fault zone. The region just offshore from Punta Salsipuedes is very complex, and direct connections between the Agua Blanca and Coronado Banks fault have not been established. Krause (1961, 1965) inferred northward continuation of the Agua Blanca fault through the Islas Los Corcnados using magnetic data. The Agua Blanca fault is also very complex along and to the north 36
of Punta Banda. There are at least two sub parallel traces of this fault that lie along opposite sides of Punta Banda, forming the Punta Banda horst (which includes the Islas de Todos Santos). The main trace of the fault on the north side of Punta Banda marks a major structural boundary between the punta Banda basement ridge high and the sediment-filled Valle Maneadero (Loc. 30, Fig. 1) which extends into Bahia Todos Santos. The faults on the south side of Punta Sanda offset the Punta Banda submarine canyon (Loc. 27, Fig. 1) as much as 4 km in a right-lateral sense, and control the shape of a northwest trending canyon to the west of the Islas de Todos Santos (Legg, 1979). The main trace on the northeast side of the Punta Banda ridge passes along the east side of the Islas de Todos Santos, forming a steep escarpment. This trace curves through a 25 - 35 angle between Punta Banda and the Islas de Tcdos Santos. East of the Islas de Todos Santos, acoustic basement highs are juxtaposed where the Valle Maneaderc ends in an east-west trending normal f ault that trends toward the Ensenada breakwater. This relationship was observed on gravi-metric and magnetic data by Serrano (1977). Parthcr north along the fault, and southwest of Punta Salsipuedes, the acoustic basement is again deeper (%500+ m) to the east, and sea-floor scarps are present along the main trace of the Aqua Blanca fault. The fault passes very near the coast at Punta Salsipuedes, and there are no data close enough to the shore to accurately delineate its northward extent. The Coronados Banks fault zone is found west of Punta Salsipuedes, along the Descanso ridge (Loc. 23, Fig. 1), within the sediment-filled channel between the ridge and the coast. To the north, the fault splays around Middle and South Coronados Islands, with one trace passing very near to the west side of Middle Island. Coronado Canyon (Loc. 29, Fig. 1) cuts transversely across the Coronados Banks fault zone and a right-lateral offset is suggested. Loma Sea Valley (Loc. 31, Fig . 1) is eroded along the Coronado Banks fault and its main trace lies along the steep, western flank of the valley where a major basement discontinuity exists (Legg. 1979). In addition to the main trace of the fault, numerous sub-parallel fault traces lie along the more gently sloping eastern side of the Loma Sea Valley. Leg g , e t. al. (1977) suggested that the Coronado Banks fault zone 37
extends northward from the Coronado Banks along a N45 W trend of earth-quake epicenters (Fig. 2). The trend of epicenters becomes more diffuse in the vicinity of Lasuen Knoll ' Loc. 20, Fig. 1), perhaps because of complexities in the fault zone (Greene, et al., this volume). South of Coronado Banks ( Loc. 21, Fig. 1) , the activity appears to cease at atout the latitude of south San Diego Bay until the Salsipuedes area where carthquake swarms have been recorded (Hileman, e t al. , 197 3) . In summary, the Palos Verdes Hills-Coronado Banks-Agua Blanca fault zone is a complex, nulti-part, Quaternery zone of deformation suggestive of wrench faulting (Wilcox, c al., 1973; Legg, 1979). Carthquake epi-centers located near these faults, as well as questionably faulted Holocene sediment , indicate that this fault zone is active. However, it is signifi-cantly less active seismically than the San Clemente or Elsinore f aults, based upon data reported by Hileman, et al. (1973). Its close pro::imity to the San Diego greater metropolitan area (12 5 km) makes it of great interest with respect to producing moderate-sized (M = 5-6) earthquakes. Although large vertical relief is present along this fault zone in the vicinity of San Diego, fault movement appears to be dominated by strike-slip, and so tsunami generation is probably less likely in this fault zone than from the San Clemente fault, (notwithstanding the possibility of seismically induced submarine slumping and associated sea wave generation along the steep slopes of the Coronado escarpment or the Loma Sea Valley) . NEUPORT-INGLEWOOD-RCSE CANYON-VALLECITOS-SAN MIGUEL FAULT ZONF The Newport-Inglewood-Rose Canyon-Vallecitos-San Miguel fault zone passes closest to metropolitan San Diego as a series of sub-parallel, c i; echelon, and conjugate faults characteristic of a wrench zone (Wilcox, et al. ,1973; Harding , 197 3) . The Newport-Inglewood zone is described else-where (Harding, 1973; Barrows, 1974; Ziony, et al., 1974), and is especi-ally well known for the 1933 Long Beach earthquake (M = 6.3). Barrrss (1974) discussed the similarities between the Newport-Inglewood and South Coast Offshore fault zones, and Euge, + ul. (1973) suggested a branch of the Rose Canyon fault passes northeastward, onshore in the Oceanside area (Loc. 32, Fig. 1). Moore (1972), Moore and Kennedy (1975), and Kennedy, et al. (1978) have mapped in detail, the offshore portions of the Rose Canyon fault zone in the innediate San Diego area. 38
118*3 0' 11 8 ' O' 117*3 0' 11 7 ' O' 116*30' 33* 30' . _ , , 33*30,
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118*3 0' 11 8 ' O' 117*30' 11 7 ' O' 116*30' Figure 2: Map showing epicentral locations of earthquakes in the offshore region of southern California and northern Baja California, Mexico. The data covers the period from 1 January, 1932 to 30 September, 1976, and are from the Caltech seismograph array in southern California. Sources for the locations are Ililcnan, a al. (1973), Friedman, vt al. (1976), Fuis, et al. (1977), and Simons (1977). Magnitudes for the earthquakes in the figure range from M =1.5 to M =5.9, and the epi-centers have been coded for accuracy of the location. Note the linear trend of more accurately located epicenters that delineate the Coro-nado Banks fault zone, and also the trend of epicenters further off-shore that marks the San Clemente-San Isidro fault zone. 39
North of Point La Jolla, Kennedy, et al. (1978) described four distinct fault patterns constituting the Rose Canyon fault zone in that area. The westernmost faults (Loc. 33) trend northwesterly, and are, in general, either overlain by 5 m of unfaulted Quaternary sediments, or lie totally within older (Late Cretaceous to Tertiary) accustic basement. Most of these fault segments are relatively short (s10 km), discontinuous, sub-parallel or cn schelon. The central zone of faulting forms the almost symmetrical, La Jolla graben, through which the La Jolla submarine canyon is cut (Loc. 34, Fig. 1). Faults in i513 sub-zone consist of short segments (41 km) with thin , discrete zones of slip, and stratigraphic separations of more than 9 m. The general trend of the La Jolla graben is IS0*-80*W, although individual fatdt segments may strike fran WNW to !!NE. Fault or fault-line scarps are observed, and disruption of the acoustically trans parent, uppermost sedimentary layer in this zone suggests Holocene activity. A nearshore sub-zone of northwest trending faults, to the cast of the La Jolla Canyon, is described by Kennedy, ct al. (1978), (Loc. 35, Fig. 1). The faulte are discontinuous, sub-parallel and en cchelon breaks similar to those observed onshore in the Rose Canyon fault zone (Kennedy, 1975; Kennedy & Pete rson , 1975; Kennedy , e t al. ,1978) , and display some surface manifestations such as sea-floor scar rs and small suLmarine canyons. Reflection profiles indicate these faults to be nearly vertical with the faulting extending into the near surf ace, Quaternary sediments. Kennedy, et al. (1978) suggest a right lateral offset of 2.5 km since early Pleistocene, by realignment of the Scripps Canyon with an abrupt change in course in the lower La Jolla Canyon (Loc. 35, Fig. 1). Along the onshore coastal plain, Kennedy (1975) delineated many northeasterly-trending fa ults that pass offshore. Vertical separations on these faults is small (%10 m), and Kennedy st al. (1978) do not observe these faults, in general, in their seismic profiles. Onshore, however, displacement of the Pleistocene Bay Point Formation (N120,000 years) is observed locally on some of these small faults (Kennedy, 1975). As the Rose Canyon fault zone is traced south, the main faults form a gentle S-shaped curve around Mt. Soledad (Loc. 39, Fig. 1) and Mission and San Diego bays. Structural relief of these highs and lows is suggested to be a consequence of alternating local compression and tension respectively, created by the predominantly strike-slip movement across the bends in the Rose Canyon fault zone (Moore & Kennedy,1975; Kennedy , et al. ,1978) . 40
Complex, multi-part, en cchclon, right-stepping faults characterize the zone throughout the San Diego area with Mission Bay, San Diego Bay and La Jolla Canyon forming structural lows in the tensional zones, and Mt. Soledad (Loc. 39, Fig. 1) and Point Loma forming the structural highs in the compressional zones. Moore and Kennedy (1975) described faulting in the San Diego Bay and offshore bight (Loc. 36, Fig. 1), and suggested that these faalts form the western side of a graben. They observed Quaternary (and in some places, Holocene?) displacements along the faults in San Diego Bay (Loc. 37, Fig. 1), which display strike-slip character as expressed by fault bounded anticl'nes, chevron-shaped dilational synclines, and other complex folds not characteristic of dip-slip drag. The castern boundary of the San Diego Bay structural low (graben) is marked by the La Naci6n fault, (Loc. 38, Fig. 1), described by Artim and Pinckney (1973) and Marshall (this volume). South of Point Loma, the Point Loma anticline, and associated f aults, are observed in the seismic profiles of Legg (1979) and Kennedy and Welday (1979). This feature is traced as far south as Rosarito Beach, Baja California, Mexico ( Loc . 40, Fig . 1) , by Legg (1979), but it trends too close to the shoreline to be seen in their profiles farther south. Con-nection or relation to the Tres Hermanos or Agua Blanca faults is possible, but not known at this time. Krause (1961, 1965) also observed this fault in his magnetic data, just south of Point Loma, and, perhaps, offshore of Punta Descanso. In swnmary, the Newport-Inglewood-Rose Canyon-Vallecitos-San Miguel fault zone is characterized by right-stepping, en echclon faults with Quaternary to Holocene offsets in many places. Gastil, et al. , (1975) and Brune and Sbmons (this volume) discussed the details of the Vallecitos and San Miguel fault zones, and Greene, ct al. (this volume) discussed the details of the Newport-Inglewood zone. Curvature in the Rose Canyon fault zone bounds prominent structural lows in Mission Bay, San Diego Bay, and La Jolla Canyon, and structural highs at Mt. Soledad and riint Loma. This vertical relief is suggested to be a result of the right-stepping, cblique-slip along the Rose Canyon fault Zone, forming local regions of tension and compression. To the north, the fault zone merges with the Newport-Inglewood fault zone; to the south, it apparently merges with the Vallecitos-San Miguel fault zone, although a connection with the Tres Hermanos or 41
Agua Blanca fault zones is also possible. Since the Rose Canyon fault zone passes directly through the San Diego metropolitan area, it may pose the greatest seismic hazard to the city. The seismicity of this zone has been very low since the establishment of the Caltech seismograph network in southern California (Simons,1977), although several small (Mn=3.5-3.7) earthquakes have been located within this zone. The occurrence ' moderate sized (M g=5-6) earthqpakes along this fault zone , within heavily porulated regions of the San Diego coastal area could cause extensive damage. CONCLUSIONS The faulting in the inner continental borderland offshore from San Diego County, and northern Baja California, Fbxico is a region extensively deformed and tectonically active. The faults in the region are predominantly strike-slip in character, and form a part of the broad shear zone associated with the San Andreas fault and the North American-Pacific tectonic plate boundary in southern California. The four fault zones discussed are the Santa Cruz-San Clemente-San Isidro, San Pedro-San Diego Trough-Faximinos, Palos Verdes Hills-Ooronado Banks-Agua Blanca, and Newpcrt-Inglewood-Fbse Canyon-Vallecitos-San Miguel. All four zones are typical of major wrench fault zcnes as described by Wilcox, et al. , (1973). These zones are typified by one or more relatively continuous main faults and numberous smaller , sub-parallel, en cchelon, and oblique faults. Transversely-oriented folds near steps or curves in the main faults were also observed. All of these fault zones show signs of Quaternary activity, and in many areas, sea-floor displacements, f a ulted Holocene sediment and/or associated seismicity. Previous seismic-risk studies in the area (McEuen & Pinckney, 1972) briefly mentioned the hazard from offshore faults, but only the San Clemente and Rose Canyon faults were known in any detail at that time. Now it is known that the San Clemente-San Isidro fault zone is currently active with moderate seismicity, and it appears capable of large , though infrequent, earthqpakes. Its distance from the San Diego metropolitan area lessens the hazard fram the more frequent, moderate-sized (M =4-6) n events, although some of these have caused slight damage in San Diego (Agnew, et al. , this volume). Closer to the populated coastal area of San Diego, the San Dicgo Trough and Coronado Banks fault zones may pose a significant earthqpake 42
hazard from even moderate-sized events. Perhaps the greatest hazard from earthquakes may be presented by the Rose Canyon fault zone which passes directly through metropolitan San Diego. Small earthquakes in this zone (M =3.5-3.7) have caused slight damage (Legg , et al,1977) , and moderate-sized events located within the metropolitan area could cause more exten-sive damage. Quantitative studies of the risk in San Diego including the newly collected offshore fault data have only recently begun, thus it cnn-not be said which, if any, of these offshore faults pose the greatest earthquake hazard to the city. It can be stated that the four fault zones described appear to be capable of generating earthquakes large enough to be damaging to metropolitan San Diego. ACKNOWLEDGEMENTS This paper represents prcliminary results of research supported by the NOAA offico of Sea Grant, project #04-8-M01-189, California State Resources Agency project # R/CZ-4 3, NSF grant #EAP76-84324, and U.S.G.S. contract #14-08-0001-17699. REFE RENCE_C_ Allen, C.R., Silver, L.T., and Stehli, F.G., 1960, The Agua Blanca fault - a major transverse structure of northern Baja California, Mexico: Geological Society America Bulletin, v . 71, p. 357-482. Anderson, J.G., 1979, Estimating the seismicity from geologic structure for seismic risk studies: Seismological S&:iety America Bulletin, V. 55, p. 753-797. Ar tim , E.R., and Pinckney, C.J., 1973, La Nacidn fault system, San Diego, California: Geological Society America Bulletin, v. 84, p. 1075-1080. Atwater, J., 1970, Implications of plate tectonics for the Cenozoic tectonic evolution of western North America: Geological Society America Bulletin, V. 81, p. 3515-3536. Barrows, A.G., 1974, A review of the geology and earthquake hirtory of the Newport-Inglewood structural zone, southern California: Californ-ia Division Mines Special Report 114, 155 pp. 43
Emery, K.O., Butcher, W.S. Jould, H.R., and Shepard, F.P., 1932, Submarine geology off San Diego, California: Journal of Geology, v. 60, p. 511-548. Euge , K.M. , Miller, C. , and Palmer , L. , 1973, Evidence for a possible onshore extension of the Rose Canyon fault in the vicinity of Ocean-side, California: Geological Society America Bulletin, Abstracts with Program, v. 5, n. 1, p. 39. Friedman, M.E. , Whitcemb, J .H . , Allen C.R. , and Hilr..uan , J.A., 1976, Seismicity of the southern California region 1 January, 1972 to 31 December 1974: California Institute Technology, Division Geological
& Planetary Sciences Contr>ibution # 2734, 404 pp.
Fuis, G.S., Friedman, M.R., and Hileman, J.A., 1977, Preliminary catalog of earthquakes in southern California, July 1974 - September 1976: U.S. Geological Survey, Open File Report # 77-181. Gastil, R.G., Phillips, R.P., and Allison, E.C., 1975, Reconnaissance geology of the state of Baja California, Mexico: Geological Society America Met.oir 140, 170 pp. Ibrding, T.P. , 1973, Newport-Inglewood trend, California - an example of wrench style deformation: American Association Petroleum Geologists Bulletin, v. 57, p. 97-116. Hileman , J. A . , Allen , C. R . , and Nordquist, J.M., 1973, Seismicity of the southern California regio. ,1 January 1932 to 31 December 1972: Cal-ifornia Instituto Technolog.', Division Geology and Planetary Sciences Contribution # 2385, 64 pp. Housner, G.W., 1969, Engineering estimates of ground shaking and maximum earthquake magnitude, in Proceedings of Fourth World Conference on Earthquake Engineering, p. 1-13. Junger, A., 1976, Tectonics of the southern California borderland: in Howell, D.G. (ed . ) , Aspects of the Geologic History of the California Continental Borderland: American Association Petroleum Geologists, Special Publication 24, p. 486-498. Junger, A. , and Wagner, H.C., 1977, Geology of the Santa Monica and San Pedro basins, California continental borderland: U.S. Geological Survey, Misc. Field Studies MF-820. Kennedy, M.P., 1975, Geology of the San Diego retropolitan area, Californ-la Division Mines, Bulletin 200, p. 1-39. 44
Kennedy, M.P. , and i cerson, G.K., 1975, Geology of the San Diego metro-politan area, California, Section B - Eastern San Diego metropolitan area: California Division Mines, Bulletin 200, p. 40-56. Kennedy, M .P . , Tan , S .S . , Chapman , R.H. , and Chase , C.W. , 1975, Character and recency of faulting, San Diego metropolitan area, California: California Division Mines Special Report 123, 33 pp. Kennedy, M.P., Bailey, K.A., Greene, H.G., and Clarke, S.H., 1978, Recency and character of faulting offshore from metropolitan San Diego, Calif-ornia: Final Technical Report - FY 1977-1978, U.S. Geological Survey, 47 pp. Kennedy, M.P., and Welday, E.E., 1979, Recency and character of faulting offshore from urban San Diego, California: California Division of Mines Map Sheet (in press). Krause, D.C., 1961, Geology of the southern continental borderland west of Baja California, Mexico: Scripps Institution of Oceanography, Ph.D. dissertation (unpub.), 205 pp.
, 1965, Tectonics, bathymetry, and geomagnetism of the southern continental borderland west of Baja California, Mexico: Geological Society America Bulletin, v. 76, p. 617-650.
Legg , M . , Agnew, C., and Simons, R., 1977, Earthquake history and Seis-micity of coastal San Diego County, California, 1800-1976,
- n Kuhn, G.
(ed.), Coastal zone Geology and Related Sea Clif f Erosion: San Dieguito River to San Elijo Lagoon, San Diego County, California, Project Study
# 11695-0800E-KUHN, San Diego County Board of Supervisors.
Legg , M.R. , and Ortega , V .W. , 1978, New evidence for major faulting in the inner borderland off northern Baja California, Mexico: Abstract in Ess, v. 59, p. 1134. Legg, M.R., 1979, Faulting and earthquakes in the inner borderland offshore southern California and northern Baja California: M.S. Thesis (unpub.) , scripps Institution of Oceanography, 75 pp. Lonsdale, P., 1979, A deep-sea hydrothermal site on a strike-slip fault: M.S. submitted to Nature. McEuen, R.B., and Pinckney, C.J., 1972, Seismic risk in San Diego: Trans-actions San Diego Society Natural History, v. 17, p. 33-62. Moore, D.G., 1969, Reflection profiling studies of the California cont-inental borderland; structure and Quaternary turbidite basins: Geological Society America Special Paper 107, 142 pp. 45
Mocre, G.W., 1972, Of fshore extension of the Rose Canyon fault, San Diego, California: U.S. Geological Survey Paper 800-C, p. C113-C116. Moore, G.W., and Kennedy, M.P., 1975, Quaternary faults at San Diego Bay, California: U.S. Geological Survey, Journal of Research, v. 3,
- p. 589-595.
Serrano. A.G., 1977, Anomalias gravim6tricas y magneticas de la Bahia de Todos los Santos: Tesis, Universidad Autonoma de Baja California, Ciencias Marinas, 52 pp. Shepard, F.P., and Dill, R.F., 1966, Submarine Canyons and Other Sea valleys: Chicago, Rand McMally Co., 381 pp. Shepard, F.P., and Dmery, K.O., 1941, Submarine topography off the California coast: canyons and tectonic interpretations: Geological Society America Special Paper 31,171 pp. Simons, R.S., 1977, Seicmicity of San Diego, 1934-1974: Seismological Society America Bulletin, v. 67, p. 809-826. Vedder, J.G., Beyer, L.A., Junger , A. , Moore, G.W., Roberts, A.E., Taylor, J.C., and Wagner, H.C., 1974, Preliminary report on the geology of the continental borderland of southern California: U.S. Geological Survey , Misc. Field Studies MF-624. Wilcox, R.E., liarding . T.P . , and Seely, D.R., 1973, Basic wrench tectonics: American Association Petroleum Geologists Bulletin, v. 57, p.74-96. Ziony, J.I., Wentworth, C.M., Buchanan-Banks, J.M., and Wagner, H.C., 1974, Preliminary map showing recency of faulting in coastal southern California: U.S. Geological Survey, Misc. Field Jtudies MF-585. 46
ACTIVE AND POTENTIALLY ACTIVE FAULTS: SAN DIEGO COUNTY AND NORTHERNMOST BAJA CALIFORNIA by R. Gordon Gastil, Ronald Kies and Douglas J. Helius Department of Geological Sciences San Diego State University San Diego, CA 92182 INTRODUCTION This discussion includes 1) faults which create the eastern escarpment of the Peninsular Ranges, 2) faul ts of the eastern edge of the Pacific coastal basin, and 3) the transverse strike-slip fault systems from San Diego south. The plate boundary faults system to the northeast and east of San Diego (Elsinore, San Jacinto, Cucupa, Cerro Prieto faults) are not included. EASTERN ESCARPMENT FAULTS This system of down-to-the-cast, dip-slip faults are responsible for the uplif t of the castern edge of peninsular California. They presumably extend from Mount San Jacinto in the north to the southern tip of Baja California. Segments of this fault system, north of the Elsinore fault, have been disrupted and obscured by the more northwesterly trending strike-slip faults. SIERRA JUAREZ FAULT From Coyote Valley (along the Elsinore fault, San Diego County), a zone of faults extends south-southeast along the eastern escarpment of the Sierra Juarez. At the south end of Laguna Salada, this zone appears to widen eastward to include the Sierra de la Tinaja. What happens to this zone as it approaches the northern end of Valle San Felipe is not known, but it is currently being studied by students of. Richard Merriam at the University of Southern California. The old erosion surface of the Sierra Juarez stands as high as 1,830 m (6,000 feet) and locally is covered by Miocene basalt and andesite, and older conglom-erates; the surface is down-dropped by a series of step faults to elevations near sea level west of Laguna Salada. A gravity survey of Laguna Salada (Kelm, 1972) suggests that the crystalline floor of that depression is about 6,000 m below sea level. Thus the fault has on the order of eight km of vertical sepa-ration. Exposures of the fault planes are best observed along the eastbound portion of U.S. Interstate liighway 8 and along Itexican liighway 2. Almost all of these 47
does not support hypotheses that (a) the East Paci fi c Ri se passed m.Jer the cen-insula between ten and five nillion years ago, or that (b) the peninsula has been tipped up on one flank of an asymmetric rise. Antithetic faults of the desert ranges f acing the escarpment are, however, consistent with the hypothe-sis that they are part of a collapse structure related to the soreading of the gulf (Figure 1). The lack of comparable structures on the continental side of the spreading centers may indicate that the spreading is asymmetric (Figure 2). FAULTS OF THE PACIFIC BASIN hARGlii The Sweetwater-La llacion fault system of southwestern San Diego County (Figure 3), faults southwest of Tijuana, faults of the La Mision area, trend north to northwest and are predominantly down-to-the-west, normal faul ts. Gravi-ty surveys indicate that these faults lie in the zone where the sedimentary sec-tion thickens rapidly to the west (Elliott, 1970). Detailed geologic mapping across the La Nacion-Sweetwater fault zone, and gravity studies (Marshall, this volume), show that the stratigraphic section thickens to the west across many of the fault breaks, indicating down-dcopping that dates back to at least Miocene time. SWEETWATER-LA NACION FAULT ZONE A compilation by Lee Van der Hurst (1976) (Figure 3) shows that en echelon, discontinuous faults extend f rom Montezuma Road (just south of Mission Vailey), south to San Ysidro where they apparently intersect the Rose Canyon (?) fault system (Figure 3). Most of the traces show of fsets of the Lindavista Formation (early Pleistocene) and thick sections of strata mapped as Bay Point Formation (e.g. just cast of Lincoln Acres). Some older river terraces intermediate in age between the Lindavista and Bay Point Formations (Richard T. Higley, unsub-mitted MS thesis at SDSU) show no measurable offset. There are no scarps associated with these fault traces and Elliott and Hart (1977) reported radio-carbon dates as old as 13,000 years in unfaul ted alluvium. Movement on these faults may be di fferential subsidence across the hinge-line of the Pacific margin basin. A minimal thickness of post-Pliocene deposi-tion has been accompanied by minimal amounts of movement on the fault zone, it is not believed to be a continuous fault rupture capable of a major earthquake and it probably has no genetic relation to the Rose Canyon-San Miguel fault system. TRANSVERSE STRIKE-SLIP FAULTS Two very di f ferent northwes t-t rending, st ri ke-r li p f aul t systens cross the western and central portions of the area under consideration. These are the 48
Elevated Peninsula Gulf Depression Y/ M ** N. k ,' Figure 1. Antithetic fault b ocks along eastern escarpment of the Sierra Juarez. Uplif ted Peninsula Collapse caused Miocene gulf deposits by spreading
.~ ~'^ Y Pacific N North American Plate wi-- +
g \ Plate
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Mant!e Miocene basalt Figure 2. Relation between spreading of the Gulf and origin of faults along eastern edge of Pacific Plate. 49
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4> u a s, O f 9 ut i o e a um 3 C POST LiWOA VISTA vtRTICAL SEPARATION E POST SAN 08Et0 FORMATION vtRtlCAL SEPanaftom g MigtMuu ytRTICAL OFFSET ON 144 OtEGO Po#MATion e'OTM OF FAULT Lf et il PACPORTIC4AL TO vtRTIC AL SEPAR AT'9N ALL L'41401Catto saukT5 Amt Doom 70 THE WEST Figure 3 Soestwater - Le Nacion Foult System
faul ts , some wi th faul t gouge several mete rs thick, dip to the southwest , wi th the sense of slip down-to-the-southwest (an ti thet i c) . However, large topographic drops down-to-the-east probably mark the positions of unexposed faul ts dipping to the northeast. The fault system is visualized as in Figure 1, with one or several mester faults down to the east, and many secondary faults on the down-dropped block inclining to the southwest to intersect the master fault at depth. The valley which lies between the Sierra Juarez and the Sierra de la Tinaja (Gasti1, et, al.,1975) is a graben between the master fault system to the southwest and a secondary fault to the northeast. At most places along the Sierra Juarez escarpment there is li ttle, if any, evidence of recent fault motion. However, at the northern end of the Sierra de la Tinaja there are several places where fault scarps appear to cut the alluvial surface. SIERRA SAN PEDR0 HARTIR FAULT From San Matias Pass (Mexican Highway 15), it is possible to trace a dis-crete fault at, or close to, the boundary between crystalline rocks and alluvial fill for approximately 100 km to the southeast. South of that point the separa-tion on the master fault dimiaishes rapidly, the fault zone widens, and the importance of antithetic faalts increases. From the top of the escarprent (3,080 m Mount Diablo), to the base of the sedimentary fill (Slyker,1964), is an elevation difference of 5 km. It is possible that the desert ranges to the east (Sierra San Felipe and Sierra Santa Clara), are antithetic fault blocks related to the master fault, as is the Sierra de la Tinaja to the north. Brown (1978) studied the recent fault scarps near Arroyo Agua Caliente near the southern end of the Sierra escarpment. Five or more uplif ts of several meters each were found to have occurred here during the past few thousand years. An earthquake of Ri-hter magnitude 5.4 was centered there in 1975. Microseisms have not been detectee along the Sierra San Pedro Martir fault. REGIONAL TECTONICS OF THC SIERRA JUAREZ AND SIERRA SAN PEDRO HARTIR FAULTS Uplif t of the eastern edge of peninsular Cal;fornia bears an analogy to the uplif t of the eastern edge of the Sierra Nevada of Alta California. The sedi-mentary record of the California gulf depression (Gastil, et al., 1979) indi-cates that rapid uplift of the range began about ten million years ago and was active at times during the Pliocene and Pleistocene. The eastern margin of the California gul f depression has not shared this history of upli f t. The record 51
topographically distinct, but weakly active, Agua Blanca system, and the topo-graphically obscure, but very active, San Miguel system. AGUA BLANCA FAULT The Agua Blanca and associated Santo Tomas faults are distinctive for their west-northwest trend, which is more westerly than other strike-slip faults asso-ciated with the Pacific-Horth American plate margin. Abundant topographic evi-dence for right-lateral slip along the northwestern part of the Agua Blanca fault (Allen, et,al., 1960) indicates Holocene notion, but seismic records (James Grune, parsonal communication, 1978) indicate no major earthquakes along the fault since rer,rds for the area began, and microseismic activity is less than on the San Miguel system. Al though Allen, et_ al . . (1960) report of fsets of basement rock up to 20 km and of Quaternary (?) gravels up to 4 km, the strike-slip component of the fault does not extend east of Valle Trinidad. Careful mapping of San Matias pass (Gastil, et al., 1975) shows that the fault dies out completely before reaching the eastern escarpment of the peninsular uplift. SAN MIGUEL FAULT ZONE This zone of faults, collectively named the San Miguel by Reyes, et, al,. (1975), is actually at least four separate en echelon faults: f rom north to south, the Calabasas, Vallecitos, San Miguel, and Tres Hermanos. These faults (identi fied in Gastil, et al. ,1975) are currently being mapped in greater detail by students at San Diego State University and investigators at CICESI, Ensenada. Falle de Calabasas. On the 1975 map, the fault extends f rom La Hiedra about 30 km to Valle de Las Palmas. Mapping by Frazer (1972) improved the understanding in the Valle de Las Palmas area and extended the fault zone to the northwest. The northeast strand of the Calabasas fault appears to displace Quaternary alluvium immediately northwest of Mexican Highway 3 Along the southern edge of Valle de Las Palmas are essentially uneroded scarplets in bedrock that could be of recent origin. Falle de Vallecitos. A nearly continuous fault extends 65 km from the Eocene conglomerates southeast of Tijuana to Rosa de Castillo, midway across the batholith. There is no evidence of this fault of fsetting anything younger than the crystalline basement rocks. An unpublished map by Raymond Elliott in the area west of Rosa de Castillo shows lateral separation of a pluton boundary by approximately three km. At its northwestern end the fault appears to die out in, or be covered by, Eocene conglomerates. 52
Falle de San Miguel. As shown by the 1975 ma p (Ga s t i l , e t_ _a__1_. ) o f t he State of Baja California this fault consists of two segments, one extending along the eastern side of Valle San Rafael to southeast of San Salvador, and a second extending from Mezquite southeast to the road east f rom El Rodeo. The northern of these two segments is easily followed on air photographs; the latest (most clearly expressed) f aul t traces appear to separate vertical dikes of Mes-ozoic age by only 100 m or so. The southeasterly of the two segments broke over a distance of 17 km in 1956 (Shor and Roberts,1958). The northwestern portion of the 1956 breaks appear en echelon to the Valle San Rafael segment. These segments appear to form a set of right-stepping, en echelon segments, analogous to the steps of the Elsinore and San Jacinto fault zones, and to the transform faults of the Gulf of California. Falle de Tres Hermanos. Midway between the Agua Blanca and San Miguel faults is a fault approximately 45 km in length with pronounced topographic expression. Recency and sense of motion are unknown. Activity along the San Miguel Fault. Microseismic studies of northwestern Baja Cali fornia, a cooperative investigation by Mexican and American seismolo-gists (Reyes, et,al., 1975) indicate that this is an active area with broadly distributed epicenters. The strands of the San Miguel fault which broke in 1956 only displace Miocene or older conglonerate by a few meters. The zone has not been successfully traced northwest toward the Rose Canyon fault system or southeast to intersect either the Agua Blanca or the Sierra San Pedro Marti r faul ts. ROSE CANYON FAULT _ Recent detailed mapping by students at San Diego State University (Kies, 1979, and Melius, 1979) shows that although there may be pre-Pleistocene defor-mation of pre-Pliocene strata of Mount Soledad, the Rose Canyon and associated faul ts represent predominantly post-Linda Vista, right-lateral strike-slip motion (Figure 4). Although Kennedy, et al., (1975) and Kern (1973) inter-preted horizontal separation during Quaternary time on the order of one kilo-meter, based on coastal offset and the of fset of the Pliocene pinchout beneath the Lindavista Formation, it could not be proven that these features did not re s ul t f rom unrelated erosion. Kies (1979) pointed out that key piercing points are provided by the west-ernmost of the beach ridges illustrated by Peterson (1970). The north-trending beach ridge, which is truncated by the Rose Canyon fault at Ardath Road, is 53
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believed to have paralleled more easterly ridges, it probably curved southeast-ward across the area now occupied by Mount Soledad to connect with a remnant ridg.e on Clai remont Hesa (Figure 5). Along the Mount Soledad fault, south of the Easter cross, are fault slivers of beach ridge protected f rom erosion by small grabens in the fault zone. If this beach ridge deposit is part of the Ardath Road beach r?dge, then they demonstrate right-lateral separation of at least half a kilometer, and possibly a kilometer. Figure 5 also locates two small patches of beach ridge farther west on Mount Soledad, but connecting these would require a right-swinging flexure or left-lateral motion on the fault. Until a technique for discriminating between the dif ferent beach ridges is developed, the separation evidence cannot be considered conclusive. However, it tends to independently substantiate estimates made earlier by others, and is probably the best evidence yet presented. Near the mouth of Rose Canyon (point A, Figure 4), a strand of the Rose Canyon fault zone of fsets the lower portion of the Bay Point Formation (post-Lindavista Pleistocene) and is overlain by the upper portion. At point B, there is a 0.6 m vertical separation in Pleistocene strata which includes a kitchen midden in its upper portion. If the midden deposit was in place at the time of motion, it indicates movement subsequent to human occupation (generally considered less than 40,000 years; J. Philip Kern, personal communication) . At point C, Liem (1977) reported a fault separation younger than 27,000 years on the basis of radiocarbon dating. South of the San Diego River, Eocene, Pliocene, and early Pleistocene strata are steeply inclined (up to vertical) due to deformation by northwest , north , and northeast-trending faults. As in the Mount Soledad area, variations in the stratigraphic section and angular unconformities indicate deformation perhaps as early as Eocene time, and certainly pre-San Diego Formation (Plio-cene). Near the south end of Congress Street, point D, steeply-tilted San Diego and Lindavista formations have been reverse faulted over the lower part of the Bay Point Formation. The upper part of the Bay Point unconformably over-lies this fault, but is i tsel f uptilted to the east and separated one foot by a different reverse fault. Based upon the inclination of faulted beds and the character of the faul: zones, i t would appear that a major fault zone (prob-ably involving strike-slip movement) is located west of Heritage Park, just east of Congress Street, and west of the exposures on Washington Street. 55
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G Figure 5 Diagrammatic representation of relation between beach ridges (from Peterson, 1970) and strike-slip movement along Rose Canyon fault. POSSIBLE CONTillulTY BETWEEll THE ROSE CAllY0il AllD SAl1 lilGUEL FAULT SYSTEtiS A number of authors have suggested the possibili ty of a connection between the Newport-Inglewood, Rose Canyon, and San fliguel f aul t systems (e.g. Wiegand, 1970; Kennedy, et al., 1975; Reyes, et al., 1975). The hypothesis that this system is part of the Pacific-Ilorth American plate boundary (lioore and Kennedy , 1975) would seen to require such a continuity. Bedrock exposures across this hypothetical fault, however, occur southeast of Presa Rodriquez (point R, Figure 6), and no northwest-trending faul t trace is evident on air photographs. In August, 1978, an earthquake measuring 3.5 on the Richter scale had an epicenter in this exposed bedrock area (point E, Figure 5) (Brune, et al . , this volume). Subsequently, Robert Washburn (unsubmitted senior report, SDSU), investigated the area of the epicenter for ground evidence of faults, modern or ancient, and found equivocal evidence for a significant northwest-trending fault. 56
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. ~8 m.y. ondesite h
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/
/ / O o o A,,, .. i
/
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C % N s s Np OO oco o o 7% 07 % O *j fatte Of Valjec ito , Figure 6. Evidence for the "Tijuana lineament." The lineament is shown by small circles; 5 = geophysically-determined faults and thermal wells; Y = surface faults; H = Agua Cliente hot springs; R = Presa Rodriguez; E = cpicenter of August, 1978 earthquake; F = Holocene fault scarps. 57
However, several features are suggestive of a " lineament" through this area. First, there are northwest trending faults (or fault-like features) in the San Ysidro area (Kennedy, et_ al_.,1975; points S and Y, Figure 6). Second, there are warm wells in the San Ysidro area (Herbert, 1977; point S, Figure 5) and a large hot spring at Agua Caliente (point H, Figure 5). Third, the northwest trend of Tijuana Valley. Fourth, the differences in Eocene stratigraphy be-tween the north and south sides of the " lineament"; to the north are the Poway and La Jolla groups with the distinctive Poway-rhyolite clast population and to the south are the Delicias and Buenos Aires formations (Flynn, 1970) with the Las Palmas clast population (Hinch,1972). Also, to the south, basalt (circa 14 m.y.) is interbedded with marine and non-marine Miocene strata and to the north, andesite (circa 8 m.y.) overlies non-marine Miocene strata. No basalt occurs within the area of south of the lineament (Figure 5). Fifth, a set of faults (between points H and R on Figure 5) mapped by Flynn (1970) south-west of the lineament do not cross the lineament (Voorhees, 1975. Evans, 1976, Scheidemann, 1976, unsubmitted MS thesis, Higley, 1979). Finally the distribu-tion of the Pacific boundary s aults (Kennedy, et_ al. ,1975, Vanderhurst, 1976, Kennedy, ej aj ., 1975, Minch,1972) south of the lineament suggest one or more kilometers of right-lateral separation. It may be that the lineament is a recurrent structural feature dating back to the early Cenozoic with small Pleistocene and Holocene movement on a variety of individually minor but interconnected faults. REGIONAL TECTONIC SIGNIFICANCE OF THE TRANSVERSE STRIKE-SLIP FAULTS The concept that the right-lateral motion on scattered fault traces through northwestern Baja California is part of the plate boundary motion between the Pacific and North American plates, is hard-pressed to explain the apparent dis-continuity of these fault segments. Specifically, why does the Agua Blanca fault show recent offset only along its western portion? Where is the connec-tion between the Rose Canyon and Calabasas faults? What happens to the San Miguel fault to the southeast? Apparently, recent fault motion is being superposed on much older, perhaps late Mesozoic, ruptures. The late Mesozoic lineaments exposed today (e.g. the Barrett Reservoir lineament of San Diego County; Herifield and Lamar, this volume) were deep in the crust at the time they formed. Thus, many of the features seen along them are the alignment of plutonic inclusion and gneissic textures, perhaps providing evidence of tectonic flow in the crystalline rocks. Where modern stresses within the wide and complex plate boundary involve the slalic crust of the San Diego-northern Baja California area, many of these old deformation zones provide zones of relative weakness for strain release. At present, strain of the peninsula may be widely distributed rather than concen-trated on a narrow, transpeninsular fault zone. 58
REFERENCES Al len , C. R. , S il ver, L. T. , and Stehli , F, G., 1960, Agua Blanca fault - a major transverse structure of northern baja California, Mexico: Geological Society of America Bulletin, v. 71, p. 457-482. Brown, L. G., 1978, Recent fault scarps along the eastern escarpment of the Sierra San Pedro Martir, Baja Cali fornia: Mas ter's Thesis (unpub.), San Diego State University. Elliott, W. J., 1970, Gravity survey and regional geology of the San Diego embay-ment, southwest San Diego County, California, in, Allison, E. C., et al. (eds.) Pacific Slope Geology of Northern California and Adjacent Alta Cal i fo rn ia : AAPG, SEPM, and SEG Pacific Sections Guidebook, p. 10-22. Elliott, W. J., and Hart, M. W., 1977, New evidence concerning age of movement of the La Nacion faul t, southwestern San Diego County, California, in, Farrand, G. T. (ed.) Geology of Southwestern San Diego County, California and Northwestern Baja California: San Diego Association of Geologists Guidebook, p. 53-60. Evans, M., 1976, Geology of the Cerro Colorado - Cerro Jesus Maria area southeast of Tijuana: Senior Report (unpub.), San Diego State University,16 p. Flynn, C. J., 1970, Post-bathol_ithic geology of the La Gloria-Presa Rodriguez area, Baja Cali fornia, Mexico: Geological Society of America Bulletin,
- v. 81, p. 1789-1806.
Frazer, M. , 1972, Geology of Valle de las Palmas: Senior Report (unpub.), San Diego State University, 18 p. Gas t il , R. G. , Phil l i ps , R. P. , and All i son, E. C. , 1975, Reconnaissance geo-logic map of the State of Baja Cali fornia, Mexico: Geological Society of America Memoi r 140, 170 p. Gastil , R. G. , Krummenacher, D. , and Minch, J. A. ,1979, The record of Cenozoic volcanism around the Gulf of California: Geological Society of America Bulletin (in press). Herbert, B., 1977, Geochemical evaluation of subsurface temperature f rom well water in the southwest corner of San Diego County, California: Senior Report (unpub.), San Diego State University, 23 p. Kelm, D. L., 1971, A gravity and magnetic study of the Laguna Salada area, Baja California, Mexico: Master's Thesis (unpub.), San Diego State University, 103 p. Kennedy, M. P., Tan, S. S., Chapman, R. H., and Chase, G. W., 1975, Character and recency of faulting, San Diego metropolitan area, California: Cali-fornia Division of Mines and Geology Special Report 123, 33 p. Ke rn , J . P . , 1973, Late Quaternary deformation of the Nestor terrace on the east side of Point Loma, San Diego, California, In, Ross, A. and Dowlen, R. J. (eds.), Studies on the Geology and Geologic Hazards of the Greater San Diego Area, California: San Diego Association of Geologists Guidebook, p. 43-45 Kies, R. P., 1979, The Rose Canyon f aul t zone from Pt. La Jolla to Balboa Ave. , San Diego: Senior Report (unpub.), San Diego State University, 57 p. 59
Liem, T. J.,1977, Late Pleistocene taaximum age of f aul ting, southeast flission Bay area, San Diego, Cali fornia, In, Fa rrand, G. T. , (ed.), Geology of Southwestern San Diego County, California and Northwestern Baja California: San Diego Association of Geologists Guidebook, p. 61-66. Melius, D. J., 1979, A study of the Rose Canyon and associated faults: Senior Report (unpub.), San Diego State University. Hinch, J. A., 1972, The Late Mesozoic-early Tertiary f ramework of continental sedimentation, northern Peninsular Ranges, Baja California, Mexico: Ph.D. dissertation (unpub.), University of Cali fornia, Riverside, 192 p. Moore, G. W. , and Kennedy, M. P. ,1975, Quaternary faul ts at San Diego Bay, California: U.S. Geological Survey Journal of Research, v. 3, p. 589-595 Pete rson , G. L. , 1970, Quaternary deformation of the San Diego area, southwestern Ca l i fo rn i a , in, Allison, E. C., et al. (eds.), Paci fic Slope Geology of florthern Baja California and Adjacent Alta California: AAPG, SEPM, and SEG Pacific Sections Guidebook, p. 10-22. Reyes , A. , B rune , J. N. , Ba rke r , T. , Cana les , L. , Mad ri d, J . , Rebol l a r, J . , and Hunguia, L. ,1975, A micro-earthquake survey of the San Miguel fault zone, Baja California, Mexico: Geophysical Research Letters, v. 2, p. 56-59. Scheidemann, R. C. , Jr. ,1975, Correlation of the Otay and Rosari to Beach formations: Senior Report (unpub.), San Diego State University. Shor, G., and Roberts, E. E.,~1958, San Miguel, Baja California Norte, earth-quakes of February, 1956: a field report: Seismological Society of America Bulletin, v. 48, p. 101-116. Slyker, R. G.,1964, A gravity investigation of the western part of the La Jolla quadrangle, San Diego County, California: Senior Report (unpub.), Sa, 'lego State University. Van der Hurst, L. , 1976, A map of post San Diego Formation vertical separation on the La Nacion fault zone, southwest San Diego County: Senior Report (unpub.), San Diego State University, 24 p. Voorhees, B. J. ,1975, Stratigraphy and structure east of La Presa Rodriguez, northwestern Baja California, Mexico: Senior Report (unpub.), San Diego State Universi ty. Wiegand, P. ,1970, Evidence of a San Diego Bay-Tijuana Fault: Association of Engineering Geologists Bulletin, v. 7, p. 107-121. 60
ROSE CANYOU FAULT: AN ALTERNATIVE INTERPRETATION by Richard L. Threet Department of Geological Sciences San Diego State University San Diego, CA 92182 INTRODUCTION Understanding of the geology of the San Diego area has been broadened significantly through the recent work of Kennedy and Moore (e.g. Kennedy and Moore, 1971; Kennedy, 1975). However, I register strong opposition to many of their interpretations of the structural geology of the Rose Canyon fault zone (e.g. Moore, 1972; Moore and Kennedy, 1975; Kennedy, et al_., 1975; Kennedy, 1975). The present paper is intended to show some fundamental problems wi th Kennedy and Moore's structural interpretations and to offer alternative in-terpretations. This discussion of the Rose Canyon fault zone is divided into three parts: (a) extent of the fault zone and its degree of continuity with the Newport-Inglewood fault zone to the northwest and with the San Miguel faul t zone to the southeast; (b) sense and amount of slip; and (c) strain rate and evaluation of seismic risk. Figure 1, based primarily upon the mapping by Kennedy (1975), shows the location of generally acknowledged onshore geologic features that are significant tectonic elements in discussion of the Rose Canyon fault zone. EXTENT OF THE ROSE CANYON FAULT ZONE IN THE SAN DIEGO AREA AND BEYOND Moo re (1972) published an interpretation of the of fshore extension of the Rose Canyon fault northwest from La Jolla, and postulated a possible connection with the Newport-Inglewood fault zone to the north, and with the San Miguel fault to the south in Baja California. These interpretations carry implications of severe seismic risk for the San Diego area and should be examined critically. Moore and Kennedy cited Ziony (1973) as support for an extension of the Rose Canyon fault zone " southeast of San Diego Bay" to a connection with the San Miguel fault. However, Ziony's map clearly does not extend the Rose Can-yon fault zone any farther southeast than the northeast corner of San Diego Bay. Ziony's text does postulate a "through going fault system in the base-61
N.. ~ ' . ' nc.i TECTONIC SKELETAL ELEMENTS IN VICINITY OF
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ment that would compare in length and trend with the Whittier-Elsinore and San Jacinto fault zones" -- a convenient sidestep of the lack of evi-dence for such continuity in the superjacent rocks in the San Diego region. The concept of continuity between the Rose Canyon and the San Miguel fault zones is contradicted by recent mapping of both basement and superjacent rocks in northwestern Baja California (Gastil, et al., 1971). Moo re 's (1972) own work on a northwesterly, of fshore continuation of the Rose Canyon fault apparently took liberties wi th the contemporaneous geolcgic maps by Kennedy and with implications of a bathymetric chart of the La Jolla submarine canyon (Buffington, 1964). Moore shifted the on-shore coastal portion of the fault trace from the generally acknowledged northwest trend to a north-northwest t rend of fshore, completely ignoring the 6 km (3 miles) length of straight-line, fault-controlled La Jolla submarine canyon which trends northwest. Moore may have mapped merely the margin of the coastal shelf at about the 100 m (328 feet) depth con-tour, along with associated submarine slumping and landsliding, which could have been mistaken for tectonic features. By limiting geophysical traverses to a relatively narrow belt within 10-15 km (6 to 9 miles) of the coastline, Moore failed to evaluate the possibility of a northwest-erly position of the Rose Canyon faul t zone lying at,or beyond, the west end of his traverses. The faulting shown near the west end of his geo-physical section C-C' is at least as convincing as the disruptions he chose for placing the fault much closer to shore. An implication of Moore's (1972), abrupt bending (lef t-stepping) of the Rose Canyon fault trace past Mt. Soledad deserves comment here, in preparation for subsequent discussion. He states that uplift of Mt. Soledad "is believed to have resulted from compression there as a con-sequence of right-lateral strike-slip moverent along the fault." While contemporaneous thinking on "the Palmdale Bulge" may have i n f l uenced Moore's development of a "model" for Mt. Soledad, introduction of a seemingly good hypothesis as scienti fic " evidence" is specious. As an alternative, draping of late Mesozoic and Cenozoic sedimentary rocks ove r a dip-sl ip hors t block between the Rose Canyon fault and the Mt. Soledad fault to the southwest would be an equally satisfactory "model", in the absence of definitive evidence on the sense of slip on the Rose Canyon faul t zone. 63
SEf4SE AllD AMOUllT OF SLIP Of1 THE ROSE CAllY0f1 FAULT Z0f1E The reason for my opposi tion to Moore and Kennedy's (1975) conclusions on sense and amount of slip on the Rose Canyon fault is thei r lack of understanding of the fundamental difference between separation and slip (iii l l , 1959); especi-ally when they repeatedly use the ambiguous te rm "d i spl acemen t" (Kup fe r , 1960) , and introduce the completely meaningless terms " strike-slip separation" and "di p-sl i p sepa ra t ion". Confidence in their interpretations is further weakened by an obvious commitment to a "model" of right-lateral slip, on the basis of such vaguely supported statements as 1) "... features suggestive of strike-slip d i s pl acemen t , such as minor f ault-bounded anticlines , chevron-shaped dilational synclines, and other complex folds that are not in harmony with dip slip drag." (Noore and Kennedy, 1975, p. 589), and 2) "...the Rose Canyon fault zone broadens and becomes en echelon at the San Diego basin...tle believe that the basin is an area of tension at a j unction between right-stepping strands of the right-lateral f aul t zone." (itoore and Kennedy , 1975, p. 589). I have no idea what kind of mechanical "model" might support the fi rst contention. Termina-tion in a zone of distributive en echelon faulting is not unique to strike-slip faults, in fact, the existence of such en echelon terminations is practically fatal , mechan ical ly , for significant amounts of strike slip. A source of misunderstanding by floore and Kennedy (1975) may be thei r apparent equation of the present distribution of the Pliocene San Diego Forma-tion and several Eocene formations with the configuration of "the San Diego basin". Several statements (p. 592, their Figure 3), appear to say they be-lieve the line generated by the intersection of the Pliocene / Eocene unconfor-mity surface and either, 1) the present topographic surf ace or, 2) the sub-Lindavista unconformity surface is equivalent to the shoreline of deposition ot' the San Diego Formation. They state that:
"The north edge of the San Diego basin has been offset 6 km right laterally, as marked by the Eocene-Pliocene unconformity at flission Bay..."
and use this as evidence of 6 km of right-lateral slip on the Rose Canyon faul t. I can accept 6 km of right-lateral separation, as well as the neces-sary few hundred meters of down-to-the-west dip separation, based on the geo-metry of a regional unconformity that dips gently south and is broken by an essentially vertically-dipping fault. However, their associated statement 64
(1975, p. 593):
"We believe that the larger part of the displacement on the faul t zone is strike-slip, and that the generally more easily observed vertical displacerent is an effect of it.",
demonstrates a naivete that cannot go unchallenged. They appear to be saying that either, 1) lateral separation causes dip separation, or 2) that strike slip can cause dip separation, but that dip slip cannot cause strike separation. A stronger case for not using the term " displacement" could not be imagined! Reference to liason Hill's (1959) block diagram and a simple considera-tion of the geometry of the normal projection of a fault plane, as shown in any elementary structural geology textbook, confirms that ds = ls x tan a where ds is dip separation, 1s,is lateral separation, and p is the rake (or pitch) of the trace of the marker bed or surface on the fault surface. Inasmuch as the dip of the Rose Canyon fault is acknowledged to be essen-tially vertical, the vertical separation is essentially equal to the dip sepa ra t ion. In the case of the east-striking surface of the Pliocene-Eocene unconformi ty, the rake of the trace of the unconformity on the fault would be essentially equal to the dip of the unconformity, and the dip separation would be approximately 10% to 20% of the amount of lateral separation. Both lateral separation and dip separation (and any other separation one cares to specify) are caused by slip, but Hill has shown clearly that the sense and amount of slip are infinitley equivocal in such a case. Drag is simply a variation of separation and gives no more or no less equivocation on sense of slip, in view of the generally flat-lying attitudes of the Cenozoic strata in the San Diego region, as well as the northwest-striking attitudes of the late Mesozoic and Cenozoic strata that have been deformed in the vicinity of lit. Soledad, the rake of marker beds on the northwesterly-trending Rose Canyon fault must be essentially O' Thus, any observed dip separation can have been produced only by a dip-slip component of the net slip; with the amount of lateral separation as meaningless as dividing by zero in mathe-matics and the amount of lateral-slip component of the net slip ranging f rom zero to " infinity", with complete equivocation. One must be careful to avoid the specious equation of separation and slip. In fact, maddeningly, with certain combinations of rake of the marker bed and rake of the net slip, the sense of separation can be opposite to the 65
sense of respective slip-component and grossly dif ferent in magni tude (Threet, 1973). Thus, the solution of sense of slip and amount of slip on a fault requires discriminating recognition of genuine evidence for separation versus slip. Kennedy and tiocre's other criteria for " displacement" must be looked at critically. Let us sta-t wi th thei r statement that:
"The 200-m depth contour has been offset about 4 km right l a te ra l l y where the faul t zone passes out to sea near Point La Jolla."
(tiocre and Kennedy, 1975, p. 593). A literal, but nonsensical, meaning of this statement is that slip occurred after the USC&GS nautical chart was prepared. A probably intended meaning is that the configuration of the offshore bottom topography has shown no change, other than 4 km (2 miles) right slip on the Rose Canyon fault zone during the past 4,000,000 years, thereby providing the same sort of documentation of strike slip that was provided by the celebrated fence near San Francis-co in 1906. I cannot conceive of anyone else taking this " evidence" seriously. This initially more palatable statement must also be looked at cri-tically:
"The coast on opposite sides of the fault zone where it passes out to sea near Point La Jolla has rocks of similar resistance to erosion and a similar structural elevation of the Lindavista Formation. The southwest coast has been coved seaward right laterally about 1 km to form the point."
(Moore and Kennedy , 1975, p. 593). Differential erosion of the modern seacliffs at the seaward edge of the elevated Nestor Terrace (Kern, 1973, 1977) adequately explains the bight at La Jolla Cove and the I km mis-alignment of the N-S line of seactiffs on the west side of La Jolla and the U-S line of scacii ffs at Black's Beach and Torrey Pines State Park, north of La Jolla. Why else would the late quaternary Nestor Terrace be preserved as a rock--defended tread approximately I km wide on the hard Cretaceous sandstones of the La Jolla headland south of the fault, and be reduced to mere scraps of the shoreline angle just barely preserved in tiny pockets in the crumbling cliffs of soft Eocene shales and sand-stones on the downthrown (northerly) side of the Rose Canyon fault zone? Kern's work clearly emphasizes these relationships, as well as the fact that the shoreline angle and back edge of the Hestor Terrace wrap around - 66
the northerly side of the La Jolla headland, documenting the prior ex-istence of " Point La Jolla" and the absence of signi ficant strike slip on the Rose Canyon fault during the past 100,000 years. As for Moore and Kennedy's claim of "similar structural elevation of the Lindavista Formation", I can only let the reader try to reconcile thei r (1975,
- p. 592) acknowledged 130-meter dif ference in elevation of the old wave abrasion platform at the base of the Lindavista Formation on opposite sides of the Rose Canyon fault at Mt. Soledad. I cannot accept this and preceding definitions of "similar".
Regarding " offset" of the late Pleistocene Day Point Formation on the Nestor Terrace, Moore and Kennedy (1975, p. 593) and Kennedy (1975,
- p. 35) accept Kern's findings on the Rose Canyon fault at La Jolla, but Kennedy's accompanying geologic map inconsistently indicates that the Rose Canyon faul t is concealed unconformably beneath the Bay Point For-mation. Kern's work appears to be definitive for slip on the Rose Can-yon fault during the past 100,000 years, inasmuch as he had based his conclusions on piercing points of a geologic line that provides genuine evidence on slip -- the vertex of the shoreline angle, a dihedral angle, at the landward edge of the Nestor Terrace abrasion platform.
Kern (1977) showed a minimum of 54 m of vertical separation of the shoreline angle of the Nestor Terrace, although he conceded that his vertical control north of the Rose Canyon fault must be projected about 2 km (1 mile) southerly to the fault. Nevertheless, the elevation dif-ference of the very gently inclined shoreline angle vertex is probably not very sensitive to such projection, and whatever figure one accepts (on the order of 50 to 60 m) must be regarded as a direct measure of the amount and sense of dip-slip component of the net slip during the past 100,000 years. In regard to the matter of lateral separation of the shoreline angle (which must also equal the lateral-slip component of the net slip in this special case), Kern (1977) acknowledges that his projection to the fault is subject to error, because the shoreline of 100,000 yea rs ago also had a bight in it that was about as great as the present bighc. This is indicated by the fact that the accurately mapped shoreline angle of the Nestor Terrace wraps around the north side of Point La Jolla, almost to the Rose Canyon fault. Kern's projection of the " shoreline 67
angle" for 2 km southerly from the Scripps Pier (north of the Rose Canyon fault), admittedly had to depend on a somewhat vague geonorp-hic expression of the "back edge" of the terrace cover, rather than of the shoreline angle itself, approximately along the 25 m (80 f t.) topographic contour. Kern's choice of position shows a southwesterly-trending curve in such a way that the map position of the " shoreline angle" could be brought to the faul t (from the north) at just about the same place that the directly mapped shoreline angle on the up-thrown block (south of the fault) projects northeasterly and easterly to the fault. Kern interpreted a 150-meter r ight-lateral separation (although his Figure 8 inconsistently scales 230-240 m of right-late-ral separation) of the " shoreline angle", which would mean 150 n. (or 230-240 m) of right-slip component of the net slip. The acknowledged latitude in interpolating the position of the shoreline angle across the Rose Canyon fault could allow an interpretation of no strike slip since Nestor Terrace time. In other words,- the amount of dip slip on the Rose Canyon fault during the past 100,000 years is rather accu-rately and unequivocally known to be several tens of meters, while the strike slip is much less definitively known and .may range from zero to a few hundred meters. STRAIN RATE AND EVALUATION OF SEISMIC RISK The geometric consequences are that dip separation must equal dip-slip component of net slip with perhaps no strike-slip cor.1po-nent, i n this local case of predominantly flat-lying beds, or of northwest-striking beds raking essentially 0 on a northwest-striking faul t. Thus, the differential amounts of dip separation of different rock units (800 m of post-Eocene, 130 m of post-Lindavista, and 50 m of post-Bay Point) f rom Moore and Kennedy (1975) and Kennedy (1975)
~I yields a long-term rate of dip-slip strain on the order of 10 mm/yr.
Moore and Kennedy used values for " strike-slip displacement" to assess seismic risk in the greater San Diego area, particularly with the further assumption of a "model" that the Rose Canyon fault zone is "related" to (continuous wi th?) the Newport-inglewood and San Miguel , faults. From this cnodel" Moore and Kennedy (1975) used: 68
(a) a rate of strain of 1 mm/yr, which may be an order of magnitude too high, (b) Brune's (1968) "model" of seismic moment, (c) Bonilla and Buchanan's (1970) work on length of historic fault rupture length versus earthquake magnitude, and (d) the records of a few historic earthquakes renote from San Diego and of questionable relevance for the discon-tinuous and en echelon pattern of the minor league faults of the San D Ego area. Moore and Kennedy's (1975) conclusion was that ".. . strain for a larger earthquake may be accumulating." because of a hypothetical, gross defi-ciency of seismic events according to thei r "model". They estimate that a strong earthquake with a Richter magnitude of approximately 6.5 could be expected to have a recurrence interval of as little as 300 years in the San Diego area. Of course, if one considers: (a) the essentially aseismic history of the San Diego area, (b) one order of magnitude lower rates of strain that can be documented rather than merely postulated, (c) a large difference in meaning between length of fault rupture and length of fault trace (especially in view of the tendency to extend and connect faul ts more or less indefinitely, despite local evidence of lack of continuity) and, (d) the doubtful connection of the Newport-Inglewood-Rose Canyon-San diguel faults and their supposed control of behavior by a " San Andreas system" (Moo re , 1972) , then one might conclude that the seismic risk in San Diego is much lower than in many other areas of southern California and that a reasonable re-currence interval for strong local earthquakes soars up into the thousands of years. Published accounts indicate that both the documented geologic, long-term rate of s't rain, and the historic seismicity in the vicinity of the major league faults in southern California, are from two to three orders of magnitude as great as those which can be established firmly for the San Diego area. CONCLUSIONS It is my opinion that Moore and Kennedy have done a disservice to San Diegans and to consulting geologists who must provide objective eval-uations of the character of the local faul ts and of the extent of seismic for the San Diego area. Instead of playing academic games with geologic 69
"models" based on extrapolation of equivocal observations to the scale of global tectonics, we should be continuing to investigate and reason through mul tiple-working hypotheses , in a manner that is worthy of sci-entific credibility. Mark Twain's famous lesson from meander cut-offs on the Mississippi River and his sarcastically " scientific" conclusion is appropriate to remember of ten -- that model building may of fer such a " wholesale return of conjecture for such a trifling investment of fact." REFERENCES Bonilla, M. G., and Buchanan, J. M., 1970, Interim report on world-wide historic surface faulting: U. S. Geological Survey Open-File Report, 32 p. Brune, J. N. ,1968, Selsmic moment, seismicity, and rate of slip along major fault zones: Journal of Geophysical Research, v. 73,
- p. 777-784.
Buffington, E. C.,1964, Structural control and precision bathynetr: of La Jolla submarine canyon, California: Marine Geology, v. 1,
- p. 44-58.
Gastil, R. G., Phillips, R. P., and Allison, E. C., 1971, Reconnais-sance geologic map of the state of Baja Californla: Geological Society of America Memoi r 140. Hill, M. L. ,1959, Dual classification of faults: American Association of Petroleum Geologists Bulletin, v. 43, p. 217-221. Kennedy, M. P., L975, Geology of the San Diego metropolitan area, Cal i fornia: California Division of Mines and Geology Bulletin 200 (Part A), 39 p. Kennedy, M. P. , and Moore, G. W. , 1971, Stratigraphic relations of Upper Cretaceous and Eocene formations, San Diego coastal area, California: American Association of Petroleum Geologists Bulle-tin, v. 55, p. 709-722. Kennedy, M. P., Tan, S. S., Chapman, R. H., and Chase, G. W., 1975, Character and recency of faulting, San Diego metropolitan area, Cal i fo rnia: California Division of Mines and Geology Special Report 123, 33 p. Kern, J. P. ,1973, Late Quaternary oeformation of the Nestor Terrace on the east side of Point Loma, San Diego, California, in, Ross, A., and Dowlen, R. J., (eds.), Studies on the geology and geologic hazards of the greater San Diego area, Cali fornia: San Diego Association of Geologists Guidebook, p.43-45 Kern, J. P.,1977, Origin and history of upper Pleistocene marine terraces , San Diego, Cali fornia: Geological Society of America Bulletin, v. 88, p. 1553-1566. 70
Kupfer, D. H. ,1960, Problems of faul t nomenclature: American Association of Petroleum Geologists, v. 44, p. 501-505. Moore, G. W., 1972, Of fshore extension of the Rose Canyon faul t , San Diego, California: U. S. Geological Survey Professional Paper 800-C , p. 113-116. Moore, G. W., and Kennedy, M. P., 1975, Quaternary faults at San Diego Bay, California: U. S. Geological Survey Journal of Research, v. 3, p. 589-595 T h ree t , R. L., 1973, Down-structure method of viewing geologic maps to ob-tain sense of fault separation: Geological Society of America Bulletin,
- v. 84, p. 4001-4004.
Ziony, J. i. ,1973, Recency of faulting in the greater San Diego area, Cali-fornia, In, Ross, A., and Dowlen, R. J., (eds.), in, Studies on the geology and geologic hazards of the greater San Diego area, California: San Diego Association of Geologists Guidebook, p. 68-75. 71
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GEOPHYSICAL SURVEY OF THE LA NACION FAULT ZONE, SAN DIEG0, CALIFORillA by Monte Marshall Department of Geological Sciences San Diego State University San Diego, CA 92182 INTRODUCTION To better define the location, number of scarps and total throw, detailed detailed gravity traverses were made across the La llacion fault zone. Results of these surveys and preliminary interpretations are presented herein. The gravity data used were collected as senior projects by San Diego State University s t uden ts (Bair, 1976; Lotharce r, 1978; Stoll, 1979). LA NACION FAULT ZONE GRAVITY SURVEY Six, approximately E-W gravi ty traverses were made across the La Nacion fault zone as mapped by Kennedy, et al. (1975). The lines are spaced 2-7 km (1-4 miles) apart fron Otay Valley on the south to Montezuma Road, 22 km (13 miles) to the north (Figure 1). Readings were taken at intervals of 150-300 m (500-1000 feet) over a 2-3 km (1 to 2 mile) length on either side of the fault zone. The instrument used for the southern two lines was the San Diego State Uni vers i ty Uorden gravineter (E-132), and, for the northern four lines , the Universi ty of Cal i fornia, Rive rs ide, La Coste-Romberg gravimete r. Data were corrected for elevation and latitude. Terrain corrections were found to be necessary only on the Montezuma Road traverse. RESULT 5 General. Gravi ty increases in a series of steps f rom west to east (Fig-ure 2). This increase ranges f rom about 18 agals along the southern traverses to only 1 mgal on the northern lines (Table 1). Similar gravity increases along these traverses were observed by Elliott (1970) in a regional gra,ity sur-vey of the San Diego area. His combined gravity and well-loj data showed the geology under southern San Diego to be that of a sedimentary basin, centered about 8 km (5 miles) southwest of the southern end of San Diego Bay. Under-lying about 1830 m (6,000 feet) of Upper Cretaceous and younger sedimentary de-posits at the center of the basin is the basement, probably Santiago Peak 73
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MILES - -- Figu e 1. Location nap of geophysical t rave rs es . tio, liontezuraa Road; EC, El Cajon Boulevard; Un, Unive rs i ty Avenue; in, Imperial Avenue; Bo, Bonita Road-Sweetwater Valley; Ot, Otay Road. Fault traces generalized f rom Kennedy, et al. (1975). L!1, main La llacion fault trace; SW, Sweetwater fault. X, gravity-indicated positions of major scarps in basement. Table 1. La ilacion Fault Zone Gravity Data Line o T 5 X,p 2 D, g 0( E E, E g E E E Mo 1.4 200 1 225 250 350 150 -50 150 0.3 -500 0 500 EC b.8 100 1 275 330 400 300 0 100 0.8 - :JO -300 700 Un 3 500 300 600 3 375 IOO -300 200 2 -1300 -500 800 im 9 1400 3 -- -- -- -- -- -- 6 -2700 -900 1800 So 18 2800 5 -- -- -- -- -- -- 14 -4500 -500 4000 Ot 16 2500 3 -- -- -- -- -- --
-4000 14 -530 3500 Table 1. 5, number of gravity-indicated scarps along the traverse; O and D , depth to the scarp base and top, respectively; E and E , elevation of scarp base and top, re s pe c t i ve l y . gg - aE are data taken along the prescat traverses f rom Elliott's (1970) regional gravity study. gg. Elliott's gravity change;gE andE E , Ili tt's estimated elevation of the west and east end of the traverse, resr ctively; ag , the elevation difference between the traverse ends.
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A tLN (800') [ ~ i f,P - _ B t Sw(900') - ff? ffPP _ I i f I i 1 I t I 1 1 0 2 4 6 8 10 12 14 16 18 20 22 24 26 HORIZONTAL DISTANCE (X 1000') Figure 2. Gravity anomalies, Ag, over rle La Nacion fault zone. Traverse locations and symbols as in Figure 1. LN and SW are traces of the La Nacion and Sweetwater faults , respectively, as mapped by Kennedy, et al . (1975).
- Denotes appreciable post-Pliocene throw where mapped trace crosses the traverse. Number in parenthesis is the geophysically-estimated throw.
f denotes the geophysically-indicated position of a fault (see text).
metavolcanic rocks, which shoals northward and eastward and outcrops several miles east of the present study area. Steps in the detailed gravi ty profiles , however, suggest that this eastward shoaling of the basement is largely accom-plished by steps or scarps. That these scarps are fault related is further suggested by correspondence between inflection points on the gravity profiles and t races of the La Nacion faul t zone, an en echelon series of high angle, down-on-the-west, normal faults (Artim and Pinckney,1973; Kennedy et al . , 1975). Gravity changes observed in this study agree fairly well with those found in the regional study by Elliott (1970, _ Table 1) . The gravi ty-computed total relief on the southern four lines is also somewhat similar in the two studies. However, the relief across the northern two lines as well as the depth to basement in the northern three lines is considerably greater in Elliott's study. The present estimation of throw is based on an assumed density contrast, op, o f 0.5 gm/cc. Since sediments overlying basement in the northern part are conglomeratic, the actual basement-sediment density contrast probably is lower than 0.5 gm/cc. Increasing the conglomerate densi ty from 2.3 to 2.5 gm/cc, for example, would reduce op to 0.3, ainost doubling the throw estimates and increas-Ing the depth to basement values as well. On the other hand, the elevation changes and depth to the basement obsersed in Elliott's study may be unreal-istically large due to the lack of well control in this area. Steps in the Profiles. Assuming that the steps in the gravity profiles are cue to scarps in the basement where denser metavolcanic rocks have been faulted against less dense sedirentary rocks, gravity data can be used to help define basement fault location, throw, and depth. Fault location is the map position of the point of inflection of S-shaped steps in gravity profiles, i.e., the point on the step where the value of gravity is midway between the lower value at the base of the step and the higher value at the top of the step. Relief on the scarps, or throw (T), is given by T (in feet) = Ag (change in gravity due to the individual scarp)/0.013ap, whe re Lp is the density contrast across the sca rp. Densities of 2.8 gm/cc for metavolcanic basement and 2.3 gm/cc for sedimentary rocks were assumed for this study, giving a op of 0.5 gm/ cc. Assuming a single, vertical scarp, the depth to the midpoint on the scarp, 2, is indicated by the slope on the S-shaped gravity steps, as measured by the horizontal distance (X;jg) between the inflection point and the point on the S-shaped step where the value of gravi ty has changed by ag/4. For 76
scarps where T<<Z, Z = Xy g ; when Tt Z, i.e., the upthrown basement side is close to the ground surface, Zt l.2X g. The Three llorthern Profiles. The geophysically-indicated position of west-facing basement scarps beneath the northern three lines coincides very closely wi th the posi tion of the trace of the La Nacion fault as mapped by Kennedy,
- e_t, t a l . (1975). The increasing number of other steps and curves in the gravity profiles to the south of Montezuma Road suggests that the basement topography becomes riore i rregular, possibly due to other east- and west-facing fault scarps.
The gravity-indicated throw on the single La Nacion fault is 30 m (100 feet) beneath El Cajon Boulevard and increases slightly to 60 m (200 feet) beneath Montezuma Road, the northern limit of the mapped trace (Table 1). South of El Cajon Boulevard, the throw increases rapidly to 150 m (500 feet) beneath Universi ty Avenue. The slope, or gradient, of the gravity profiles suggests that the depth to the midpoint on the scarp increases southward f rom about 75 to 120 m (250 to 400 feet) over this 3.2 km (two mile) distance. The imperial, Avenue Profile. South of University Avenue, the g echelon, multiple strands of the La Nacion fault zone are mirrored in the multiple steps of the gravity profiles (Figure 1). The gravity-indicated positions of the two large basement scarps on the imperial Avenue line closely match the traces of the main La Nacion fault and a break to the west, the Sweetwater fault. Two fault traces east of the main La Nacion faul t apparently are not associated with any detectable basement scarps. The two western traces crossing the Imperial Avenue line are the only traces associated with significant basement relief, a total of 460 m (1400 feet), as well as appreciable throw on the San Diego For-mation at the surface (Gas ti l , et_ a_1_. , th is vol ume) . Depth to basement along the Imperial Avenue traverse, as well as that along the two traverses to the south, cannot be directly computed using the formula given above. Its use requires ei ther a single scarp or scarps so widely spaced that the steps on the gravity profile are totally independent of each other. Despite the fact that the position of the inflection point is probably affected by the close spacing of the two faults, their geophysically-indi.ated position is within several hundred feet of the fault traces suggesting the faults are high angle. The 8 km (5 mile) long traverse across Chula Vista, along Bonita Road and up Sweetwater Valley (Bo) suggests that basement there is of fset by at least five scarps having a tctal relief of almost 900 m (3000 feet). The total dis-placement in the geologically-mapped fault zone, about 400 m (1300 f eet), is qui te similar to the 430 m (1400 feet) observed in the zone to the north, as well 77
as to the displacement of 520 m (1700 feet) along line Ot to the south (Figure 2). The agreement between the geophysically-indicated positior, of basement scarps and surface traces is not nearly as good as along the traverses to the north--which is not surprising given the complexity of the surface faulting. However, major displacements of the basement seem to shif t to the eastern branches; the eastern breaks show the most displacement of the San Diego Formation. Traverse Bo has many basement scarps, partially because it is at least twice as long as the other traverses. When lines im and Ot are extended, they too will probably detect other basement scarps. Since the basement trough shoals to the north, the north-erly lines, when extended, should show fewer scarps with less relief. Line Bo suggests the presence of two faults (A and B), as yet unmapped, in Chula Vista (Figure 2). Assuming the faults are nearly vertical, the eastern fault (A) would lie in an area centered approximately midway between F and G Streets and First and Second Avenues. The western fault (B) would Ile in an area centered on Broadway,150 m (500 feet) north of H Street. Since traverse Bo does not extend west far enough to define the entire S-shaped gravity curve, the location of proposed fault B is more uncertain than that of fault A. Interestingly, possible fault B lies almost directly on line with a series of faults mapped and inferred from photographic evidence by Kennedy, et al. (1975), several miles to the north in National City. The Otay Valley Profile. Principal basement scarps on the Otay Valley traverse appear to coincide with the main La Nacion faul trace on the east (Kennedy, el al., 1975) and with their extension (with the aid of photographic evidence) of the Sweetwater fault to the west. The two fault traces lie about 300 m (1000 feet) to the east of the geophysically-suggested position of the basement scarp. This discrepancy is probably due to a combination of the westerly dip of the faults, an error in the geophysical position caused by the close spacing of the two faults, or the basement scarps actually being zones of multiple, stair-step faults extending 300-600 m (1000-2000 feet) west of the two surface traces. Although the Otay Valley traverse did not extend far enough west to define its position accurately, the gravity profile suggests a basement scarp, which, if associated with a vertical fault (C), would intersect Main Street roughly in the vicinity of Hermosa Avenue. This scarp is situated almost due south of the eastern of the two Chula Vista faults (fault A) proposed above. It also appears to be located about 300 m (1000 feet) west of the extension of a short faalt trace, mapped by Kennedy, e_t_a_1_. (1975), south of Otay Valley along Beyer Way. 78
CollCLUS l0ll5 The limited gravity data presented here suggest that the style of fault-ing in the basement rocks along the La llacion fault zone parallels that in the overlying sedinentary rocks (Figure 3). Fault spacing decreases and the amount of throw generally increases from north to south. Basement scarps are apparently en echelon and, with exception of a few possible scarps at the eastern end of the northern traverses, basenent is down to the ucst. Relief, or throw, on the mapped part of the fault zone increases from 60 m (200 feet) at the north end to about 520 m (1700 feet) along Otay Valley. Fault planes appear to be almost vertical, although the exact position of basem.1t scarps along the southern lines is obscured by closely spaced, multiple faulting, inflections in gravity profiles suggest the existence of at least two, as yet unnapped, faults or fault zones in the western Otay Valley-Chula Vista area. The nul t iple , down-to-the-wes t , basenent scarps suggested by the gravity data agree with the proposal (Kennedy,et al., 1975) that the San Diego basin is a mul ti-stepped graben. That the basin is still subsiding is suggested by the parallelism of the structural contours on the Pleistocene Lindavista terrace (Moore and Kennedy, 1975) with the basement contours (E11iott, 1970). Additlon-ally, hoore and Kennedy (1975) point out a submerged Sangamon barrier beach, south of Coronado, in the center of the basin. Faults forming this nested graben not only cut quaternary sediments, but apparently cause nuch of the local seismicity (:loore and Kennedy, 1975). Aside f rom the f aul ts bordering the Coronado escarpnent, 15 km (9 miles) west of San Diego, the currently most seismically active region in the San Diego area is the southern end of San Diego Bay (Simons, 1977). D i p-s l i p r.uvemen t on the graben faults may be genetically associated with possible strike-slip notion on the Rose Canyon faul t, which trends southeasterly from La Jolla approxinately through the center of the graben (Kennedy, et al., 1975, licore and Kennedy, 1975). How much of the current seismicity is due to strike-slip and how much to dip-slip motion is unknown because scismograph dis-tribution is not yet adequate to compute focal mechanisms. Whether wrench tectonics caused the graben to form, or whether possible strike slip simply inheri ted sone of the older dip-slip fault planes, is unknown. If the basin is possibly as old as Cretaceous (Elliott, 1970), it may simply be the eastern-most extensional feature of the southern California Borderland. 7:s
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j/ Figure 3 Schematic, block diagram of the basement scarps along the La Nacion fault zone. View looking eastward. Scarp reIIef and en echelon style generalized f rom gravi ty data.
REFERENCES Artin, E. R., and Pinckney, C. J., 1973, La Nacion fault system, San Diego, Ca l i forn ia: Geological Society America Bulletin, v. 84, p. 1075-1080. Uai r, J. F., 1976, A gravi ty survey of the La Hacion fault in southeastern San Diego, California: Senior Report (unpub.), San Diego State University. Elliott, W. J., 1970, Gravity survey and regional geology of the San Diego embay-ment, southwest San Diego County, California, In, Allison, E. C., et al. (eds.), Paci fic Slope Geology of Northern Baja California and Adjacent Alta California: AAPG, SEPli, and SEG Pacific Sections Guidebook, p. 10-22. Kennedy, it. P., Tan S. S., Chapman, R. H., and Chase, G. W., 1975, Character and recency of faul ting, San Diego tietropoli tan area, D li fornia: California Division tiines Special Report 123, 33 p. Lothamer, R. T., 1973, A gravi ty survey over a portion of the La Nacion faul t in San Diego, California: Senior Report (unpub.), San Diego State University. fioo re , G. W., and Kennedy, ft. P., 1975, Quaternary faults at San Diego Bay, California: Journal of Research, U.S. Geological Survey, v. 3, p. 589-595. Sirons, R. 5., 1977, Seismicity of San Diego, 1934-1974: Seisnological Society Ame rica Bul le t in, v. 67, p. 309-826. S tol l , R. T. , 1979, A gravity survey over the La Nacion faul t , San Diego, California: Senior Report (unpub.), San Diego State University. 81
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SEISMICITY AND FAULTING IN NORTHERN BAJA CALIFORNIA by James N. Brune and Richard S. Simons Institute of Geophysics and Planetary Physics University of California, San Diego La Jolla, CA 92093 and Cecello Rebollar and Alfonso Reyes Centro de Investigaci6n Cientifica y de Educacidn Superior Ensenada, Baja California TECTONIC SETTING The seismicity of northern Baja California is related to the numerous faults of the San Andreas system, which represent the boundary between the Pacific and North American Plates (moving right-laterally at a rate of about 6cm/yr.). The main plate boundary at present appears to be represen-ted by the San Andreas and Brawley faults near the Salton Sea, the Imperial fault at the International Border, and the Cerro' Pricto fault south of Cerro Prieto, with connecting zones of spreading (Figure 1). Although an idealized transform fault-spreading center system appears to be a fair representation of the tectonic f ramework of the Salton trough and northern Gulf of California, it does not explain the abundant seismic activity west of the trough, nor the numerous active faults which continue northwest from transform faults within the trough and which, in the ideal-ized pattern, should not be present (e.g. - the Agua Blanca, San Miguel , Laguna Salada, Elsinore and the San Jacinto fault zones). All of these are active right-lateral faults which diverge westward from the axis of the northern Gulf and Salton trough; there is no analogous system to the east. Lomnitz, et al,(1970) gave a tectonic explanation for this, i .e. , that the present-day spreading rates on individual ridge segments decrease progressively northward, resulting in a continuation of north-westerly trending, right-lateral faults on the west side of the Gulf. Why the spreading rate should decrease northward is unknown, but it was suggested that this might be the result of the interference to through going fault movements caused by the Transverse Ranges of southern California. 83
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The tectonic significance of the branch faulting west of the main fault boundary is important in understanding earthquake hazards in the southern California northern Baja California area, because a signifi-cant amount of the right-lateral plate motion between the Pacific and North American plates may be shunted offshore from southern California and connect up again with the main olate boundary, either in the Trans-verse Ranges or farther north along the Hosgri-San Simeon fault system. MAJOR FAULTS The major f aul ts are shown in Figure 1. The following paragraphs describe each of them briefly.
- 1) The Imperial fault strikes southeast, 15 km east of Calexico, and extends at least 30 km into Baja California, as observed from dis-placements that occurred during the magnitude 6.7 Imperial Valley earthquake of 1940.
- 2) The Cerro Prieto faul t, formerly called the San Jacinto fault, extends from Cerro Prieto, an old volcanic remnant, southeast for 150 km to a point near El Golfo. Earthquakes larger than magnitude 6.0, believed to have been associated with this fault, occurred in 1915, 1934, 1963, and 1966. The magnitude 7.1 earthquake in 1934 was the apparent cause of sur-face faulting near the head of the Gulf of California which was observed in subsequent aerial photographs (Biehler, et al . , 1964).
- 3) The Laguna Salada and Sierra Juarez faults. The Laguna Salada fault extends southeast from the Elsinore fault zone, cutting across the Sierra de los Cucapas Range. This region also is occupied by a series of smaller sub parallel faults, including the Cucapa and Pescaderos. The estimated epicenter of the 1934 magnitude 6.5 earthquake is about 10 km southwest of the Laguna Salada fault. The Sierra Juarez fault is a zone of high seismic activity on the western edge of Laguna Salada, in 1975-1976 it was the most active zone in the region, with many events between magnitudes 4 and 5
- 4) The San Miguel fault zone trends northwest as a series of en echelon faults, from the southern end of the Sierra Juarez fault to the vicinity of Tijuana (Gastil, et al. , 1975). Four large earthquakes, mag-nitude 6.1 through 6.8, occurred along this faul t zone in 1956 near the town of San Miguel, surface faulting associated with this earthquake was observed to extend more than 20 km (12 miles) (Figure 2). Relocation of 85
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. Reyes et al,1975 7 ' h, + Johnson et al,1976 + { 3 9 i , , , 5,0 ,, km ji Son Felipe y rigure 2. Some recent large earthquakes and microcarthquake activity along the San Miguel fault, Baja California norte, Mexico. 86
the magnitude 6.0 and 6.3 earthquakes of 1954 Indicates that they also were located along the San Miguel fault zone near the northern end of the surface faulting observed by Shor and Roberts (1958). Tectonic, geologic and seismic evidence suggest that the San Miguel fault is part of a through going fault zone capable of generating at least mod-erate-sized earthquakes (M > 61s) . This zone includes the Newport-Inglewood fault zone to the northwest, site of the 1933 Long Beach earthquake (M = 6.3), the Rose Canyon fault zone (Figure 1), and off-shore fault zones in the vicinity of San Diego, in addition to the San Miguel fault zone. However, the exact relationships between these fault zones have not been established.
- 5) The Agua Blanca fault cuts west-northwesterly across the Baja California Peninsula from near Valle Trinidad to the Pacific Ocean, 15 km south of Ensenada (Allen, el al.,1960). It continues northward offshore, sub-parallel to the coastline, possibly connecting with either the San Diego trough or the Coronado Banks fault zones (Legg and Ortega, 1978). The 6.0 and 6.3 magnitude earthquakes described above, previously located on this fault, are now thought to have occurred on the San Miguel fault, hence, no large earthquakes are known to have occurred along the Agua Blanca fault.
- 6) The San Pedro Martir fault zone extends southeast from the projected intersection of the San Miguel and Agua Blanca faults (Figure 1). It lies at the base of the spectacular fault-scerp face of San Pedro Martir Mountain, which rises to an elevation of nearly 3,050 m (10,000 feet) above San Felipe Valley. Impressive recent fault scarps exist in this zone and have recently been shown to be older than 300 years (R. Gordoi Gastil, personal communication).
GENERAL SEISMICITY Northern Baja California is an area of extremely high seismi-ity. At least 13 earthquakes of magnitude greater than 6 have occurrei since 1900 (Table 1). The large 1892 earthquake originated in this area, but has not yet been identified with a particular fault. The Caltech earth-quake catalogue (since 1932) shows considerably higher seismicity for this area than for the area north of the border, even though the percen-tage of earthquakes located is smaller because of the lack of seismograph 87
TABLE I o Earthquakes of Magnitude 6.0 and Greater in Northern Baja and Northwestern Sonora, Mexico between 1912 and 1971. DATE LAT. LONG. MAG. REGION FAULT AUTHORITY 11-21-15 32.0 115.0 7.1 Colorado Delta Cerro Prieto Eq. Hist. # 12-30-34 32.25 115.5 6.5 Colorado Delta Laguna Salada Leeds + 12-31-34 32.1 114.9 7.1 Colorado Delta Cerro Prieto Leeds 2-24-35 32.0 116.0 6.0 Sierra Tinaja San Miguel Leeds 5-19-40 32.7 115.5 6.7 Imperial Valley, imperial Caltech C) Calif. 12-7-40 31.7 115.1 6.0 Colorado Delta Cerro Prieto Caltech 10-24-54 31.7 115.9 6.0 Santo Tomas San Miguel Leeds 11-12-54 31.7 115.9 6.3 Santo Tomas San Miguel LeeUs 2-09-56 31.7 115.9 6.8 San Miguel San Miguel Caltech 2-09-56 31.7 115.9 6.1 San Miguel San Miguel Caltech 2-14-56 31.5 115.5 6.3 San Miguel San Miguel Caltech 2-15-56 31.5 115.5 6.4 San Miguel San Miguel Caltech 8-07-65 31.8 114.5 6.3 Colorado Delta Cerro Prieto Caltech
- Surf.ce faulting extended about 30 km into Baja California.
# Earthquake History of the United States Publication 41-1 U. S. Department of Commerce NOAA/EDS + Leeds, n. L., 1979, Relocation of M 5.0 northern Baja California earthquakes using 5-P times: Masters Thesis, University of California, San Diego.
C) H i l eman , J. A., Allen, C. R., and Nordquist, J. M., 1973, Seismicity of the southern California region, 1 January 1932 to 31 December 1972: Seismological Laboratory, California institute of Technology, Pasadena, California. 88
stations south of the border. Older epicenter locations in the Caltech catalogue show a wide scatter because of a lack of local station control. Lomintz, et d.,(1970), using data from a seismograph station at Rio Hardy (Figure 2), have shown that most of the epicenters fall along active faul ts. More recent Caltech locations show less scatter because of improved station coverage. The paper by Hileman (this volume), contains a discussion of the gene-ral seismicity of this region. In the following sections only specially-located earthquakes and earthquake swarms will be described. EARTHQUAKES AND EARTHQUAKE SWARMS SAN MIGUEL FAULT ZONE Micro-earthquake activity in the San Miguel fault region is very high. Reyes, et al., (1975) operated high gain portable seismographs at 22 sites within this region. Results were reported in terms of normalized rates of micro-earthquakes per day. Very high rates, exceeding 100 events / day, were observed near the southeast end of the San Miguel fault (Figure 2). Sixteen of the 22 sites recorded rates exceeding 20 events / day. The overall micro-seismicity was higher than along most of the San Andreas fault in southern California. At two sites, arrays were operated to obtain accurate locations. Hypocenter locations near the southeast end of the San Miguel fault were near the fault trace and at depths of 8 to 14 km. Relatively high micro-earthquake rates were observe at a site 70 km from Tijuana, suggesting that the north-western San Miguel fault zone is active and might pose a significant seismic hazard to the cities of Tijuana and San Diego. Johnson, e_t_al., (1976) studied the micro-seismicity of the San Miguel and Agua Blanca fault zones. Five micro-earthquake instruments were operated for two months in 1974, in a small mobile array deployed at various sites. An 80 km (50 mile) long section of the San Miguel fault zone was found to be seismically active, producing the vast majority of recorded earthquakes (Figure 2). Very low activity was recorded on the Agua Blanca faul t. Hypo-centers on the San Miguel fault ranged in depth from 0 to 20 km although two thirds were shallower than 10 km. A composite focal mechanism study showed a mixture of right-lateral and dip slip, east side up, similar to a solution obtained for the 1956 San Mir,uel earthquakes, and consistent with observed 89
surface deformation. In an unpublished 1979 study, R. S. Simons, A. Nava, and J. N. Brune relocated eight earthquakes in this region based on readings from two sta-tions in Baja California (Rio Hardy and San Felipe) plus stations of the Caltech network to the north in southern California. The earthquakes occurred between September, 1969 and January, 1970. Six of the events were clustered about the southern end of the San Miguel (and Sierra Juarez) fault zone; the other two originated in an area near the middle of the San Miguel fault (Figure 2). In a study relocating northern Baja California earthquakes with magni-tudes greater than 5.0, using S-P times, A. L. Leeds (1979) found 5 events in the San Miguel fault zone (Figure 2). 1974 PING SOLO EARTHQUAKE The Pino Solo earthquake (M = 5.0) occurred on July 8,1974 between the San Miguel and Sierra Juarez fault zones (Figure 3). The epicenter was located by using stations in the Caltech array and in northern Baja Califor-nia to obtain better epicenter control than would result from just using the Caltech array. The location of this event is important for calibrating epi-center locations in this region because its location was confirmed by using a small local array to pinpoint aftershock locations. The aftershock region was located between the San Miguel and Sierra Juarez fault zones (hatched area, Figure 3) at depths between 4 and 18 km. The aftershock zone extends in a wes t-nor thwes t--eas t-sou theas t d i rec t ion (A. Nava, in prep., 1979). There are no known through going, major faul ts in the epicentral region, although some small faults are shown on the map by Gastil, et al., (1975). The earthquake probably resulted from the complex stress system between the San Miguel and Sierra Juarez fault zones. THE AUGUST 19, 1978 CANON DE LA PRESA EARTHQUAKE On August 19, 1978, an earthquake of approximate magnitude 3.5 occurred in an area midway between the southernmost known extension of the Rose Canyon fault in San Diego and the northernmost trace of the San Miguel fault zone in Baja California (Simons, 1979) (Figure 4). The location was obtained using stations in the San Diego area, the Ensenada station, and selected stations of the Caltech network. The locale of this event is distinguished by a sharp 90
4,, N 1
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= CPM EQ of 19 Aug 1978 (-)[$) FIRST MOTIONS : -t- = Compression
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topographic feature trending northwest-southeast (Cab de la Presa) All available first motion data are consister t with right-lateral strike slip movement (Figure 4). Robert Washburn of San Diego State University (personal communication) has reported evidence of recent landslides in Cahon de la Presa and for strike-slip faulting in the C a rTon . The geological and the seismological observations both sug-gest that Cahon de la Presa may delineate an active fault between (and possibly connecting) the Rose Canyon and San Miguel fault sys-tems. LOCATION OF THE 1954 NORTHERN BAJA CALIFORNI A EARTHQUAKES A series of earthquakes occurred in northern Baja California in late 1954 (October 17, 5.7; october 24, 6.0, 5.4; November 12, 6.3, 5.0; November 14, 5.4; and numerous smaller events). The Caltech Bulletin placed the epicenters for these events a few km f rom the Agua Blanca fault (the epicenters were taken from USGS locations). These locations have been taken as evidence for activity of the Agua Blanca fault. However, the earthquakes of 1956 were clearly not on the Agua Blanca fault, but on the San Miguel fault farther north; for example, ground breakage was reported by Shor and Roberts (1958). Evidence for the locations of the 1954 earthquakes has been re-examined by Leeds and Brune (1979). Richter (1958) stated that the accuracies of the USGS locations for the 1954 events is about
- 1/4 degree, (i.e.,
- 28 km). Within these limits of error, all the events (with the exception of the October 17 event) could have occur-red on the San Miguel fault, rather than on the Agua Blanca fault.
Comparison of S-P times for the numerous 1954 aftershocks with S-P times for the 1974 Pino Solo carthquake aftershocks (for which there is an accurate location based on local aftershock recordings), indi-cates ' chat the 1954 events were only 10-20 km south of Pino Solo, i.e. , on the San Miguel f aul t and not on the Agua Blanca faul t (act-ually farther north than the 1956 events). International Seismologi-cal Centre locations for these 1954 events are also near the same location on the San Miguel fault. This new location of the 1954 events is cor istent with micro-earthquake studies of Reyes, et al., (1975) and Johnson, et al., (1976); they found no micro-earthquake activity 93
on the Agua Blanca fault but high activity on the San Miguel fault, REGION OF THE CERR0 PRIETO GEOTHERMAL FIELD Albores, et al., (1979) reported results of seismicity studies in the region of the Cerro Prieto geothermal Jield (Figure 5). These studies were conducted with local, short-period seismic arrays during 1974-75 and 1977-78. During the latter period, horizontal seismometers were used for better control on S-wave arrival times. Locations were obtained for about 200 events and composite fault plane solutions were obtained for five groups of events. The seismic activity is characterized primarily by swarms of relatively small events, although one swarm had events with mag-nitudes as high as 5.0. Epicenter locations indicate a broad distribution connecting the Cerro Prieto and Imperial faults (Figure 5). Within this distribution there are indications of trends both parallel, and oblique to, the Cerro Prieto-Ir.perial transform faul t system. Composite fault plane solutions indicate right-lateral strike-slip faulting along the northwest-southeast trending faults (A and E, Figure 5) and dip-slip faulting along at least some of the obilque faults (B, C and D, Figure 5). Most of the events near region E (Figure 5) occurred during the Victoria, Baja California earthquake swarm of March 9 to March 20, 1978. The largest event of the swarm had a magnitude of 5.0. CERR0 PRIETO FAULT REGION The Cerro Prieto fault, southeast of the Cerro Prieto volcano and steam field, is the main locus of slip between the Pacific and North American plates, and consequently, seismic activity is very high. Three moderately large his-toric earthquakes have occurred along this fault (1934, M = 7.1; 1940, M = 6.0; 1966, M = 6.3). The most recent of these events has been studied by Ebel, el a_1_. , (1978). Smaller earthquakes and earthquake swarms along the Cerro Prieto fault have been located in special studies. Lomintz, et al., (1970) found a con-centration of events along the fault during 1969 (Figure 6). The 1976 Mesa de Andrade earthquake occurred on this fault and the aftershocks were loca-ted by Centro de investigacion Cientifica y Educacion Superior de Ensenada (CICESE) in Ensenada (Figure 7). 94
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SIERRA JUAREZ FAULT ZONE Local array studies of the Sierra Juarez fault zone have not been carried out. Caltech epicenter catalogues show that this is one of the zones of highest earthquake activity in the region. Numerous magnitude 4 to 5 earthquakes were reported in 1575-1976. The activity has decre-ased considerably since the first part of 1978. Epicentral control on these events will be greatly improved once readings from Mexican stations are included with the epicenter location calculations. Leeds (1979) has located 3 earthquakes with magnitudes greater than 5 in the Sierra Ju rez fault zone. Two of them are in the northern portion of the zone; the other is at the southern end near its confluence with the San Miguel fault zone (Figure 2). BAHIA DE SAN RAMON On September 13, 1975, an earthquake of magnitude 5.2 occurred in the vicinity of Bahia de San Ram 5n, approximately midway between Punta Colnett and Cabo San Quintin (Figure 8). It was followed by an excep-tionally high level of seismic activity having the characteristics more of a swarm than a typical aftershock sequence. Using portable seismo-graphs deployed 2 days af ter the initial event, seismologists from the CICESE detected as many as 164 events in a period of 48 hours. The epicenters were clustered in a fairly limited area centered just off-shore (Figure 8). The depths of the events lay principally between 14 and 20 kilometers. The swarm continued at least through January, 1977, with a gradually decreasing rate of seismicity, although there was a return to relatively high levels of activity during April 5-19, 1976. The vicinity of Bahia de San Ramon contains no mapped faults of any consequence, and has sustained no previous known seismic activity (with the possible exception of a poorly located event in 1942). The principal geologic characteristics of the crea consist of a sequence of inactive volcanoes both onshore and offshore, with remnant flows of Quaternary basalt (Rebollar, 1977). 97
,o g -
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iAf tershocks of the December 7,1976 9 earthquake) N A km Mesa de Andrade i I I l O 50 100 Figure 7. Aftershocks of the 17 January, 1977 carthquake on the Imperial fault and the earthquake on the Cerro Prieto fault (located by CICESE in Ensenada) . 98
f Punta - 3 Colnett - o
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REFERENCES Albores, A., 1979, Seismicity studies in the region of the Cerro Prieto geothermal f!d d: Proceedings of the First Symposium on the Cerro Prieto Geothermal Field, Lawrence-Berkeley Laboratories Report No. 7098, July, 1979 Allen, C. R., Silver, L. T., and Stehli, F. G., 1960, Agua Blanca Faul t - A Major Transverse StrJcture of Northern Baja California: Bulletin of the Geological Society of America, v. 71, p. 457-482. Bichler, S., Kovach, R. L., and Allen, C. R., 1964, Geophysical framework of nor;hern end of Gulf of California structural province, in, Marine Geo!ogy of the Culf of California: American Association of Petroleum G'. ologists Memoir, No. 3, Van Andel and Shor, (eds.). Gastil, R. G., Phillips, R. P., and Allison, E. C., 1975, Reconnaissance geologic map of the State of Baja California, Mexico: Geological Society of America Memoir 140. Ebel, J. E. , Burdick, L. J. , and Stewart, G. S. ,1978, The source mechanism of the August 7, 1966 El Golfo carthquake: Bulletin of the Seismological Society of America, v. 68, p. 1281-1292. Johnson, T. L. , Madrid, J. , and Koczynski, T. , 1976, A study of microseismi-city in northern Baja California, Mexico: Bulletin of the Seismological Society of America, v. 66, p. 1921-1930. Lomni tz, C. , Mooser, F. , Al len, C. R. , Brune, J. N. , and Tha tcher, W. , 1970, Seismicidad y tectonica del Golfo de California; resultados preliminares: Geofisica In terni ciona l 10, (2), 37 (Also in English). Leeds, A., and Brune, J. N., 1979, The locations of the 1954 northern Baja carthquakss: Transactions of the Fourth Annual CIBCASIO (Centro de Investiga.: ion de Baja California and Scripps Institution of Oceanography) Meeting, N>vember 3, 1978. Leeds, A. L., 1979, Relocation of M 2 5.0 northern Baja California earthquakes using S-P times: Masters Thesis, Univeristy of California, San Diego. Legg, M. R., and Ortega, V. W., 1978, New evidence for major faulting in the inner borderland off nearthern Baja California, Mexico: Abstract in EOS,
- v. 59, no. 12, p. 1134.
Rebollar, C. J., 1977, Estudio de depallado del enjambre de San Quintin, B. C., Mexico, occurrido durante 1975: Centro investigacion Cinctifica y Educacion Superior de Ensenada Internal Report, February, 1977 Reyes, A., Brune, J. N., Barker, T., Canales, L., Madrid, J., Rebollar, J., and Munguia, L., 1975, A microcarthquake survey of the San Miguel fault zone, Baja California, Mexico: Geophysical Research Letters, v. 2,
- p. 56-59 Richter, C. F., 1958, Elementary Seismology: W. H. F reemand Co. , Inc.
Shor, G., and Roberts, E., 1958, San Miguel, Baja California norte, earth-quakes of February, 1956: A Field Report, Bulletin of the Seismological Society of America, v. 48, p. 101-116. Simons, R. S., 1979, The August 19, 1978 Canon de la Presa Earthquake: Trans-actions of the Fourth Annual CIBCASIO Meeting, November 3, 1978. 100
lilSTRUMENTAL SEISMICITY OF THE SAfl DIEGO AREA 1934-1978 by Richard S. Simons Institute of Geophysics and Planetary Physics University of California, San Diego La Jolla, CA 92093 in a previous paper (Sirons,1977), twelve quarry explosions within the City of San Diego were used to determine a crustal velocity model for the surrounding region. Parameters for this model are: hi = 1.5 km ai = 3.50 km/sec 83 = 1.90 km/see h2 = 26.5 km a2 - 6.35 km/sec B2= 3.65 km/sec h3== a3 = 8.00 km/sec 83 = 4.60 km/sec A computer program embodying this model was used to recalculate epicenters of all events previously located in the San Diego area by work at the Sel-smological Laboratory at Caltech f rom 1934 through 1974. Over 70 percent of these epicenters had been established by pre-computer, graphical tech-niques, and the velocity models, used for the computer-determined epicenters were not appropriate for this region. The 1977 relocations benefited from readings from several new stations in the vicinity (beginning in 1964), not available to scientists at Caltech. Also, a great deal of care was taken to identify and eliminate quarry blasts, which were numerous. The purpose of this paper is to summarize and update the previous paper (Simons, 1977) by applying the same model to locate earthquakes from 1975 through 1978. Since 1975, epicenter control within the San Diego area has been improved by the addition of new stations to the Caltech-USGS net-work. Those most critical to San Diego are Vista (VST), Camp Pendleton (CPT) and Julian (JUL). These stations are shown in Figure 1, along with others normally utilized for locating events in the San Diego area. Data continues to be available f rom the three non-Caltech stations in the area (SOB, SilD, and BBP). Epicenters for the period 1934-1978 are shown in Figure 2. Although 37 earthquakes are charted for the original 41-year period, a total of 15 are shown representing the past four years. This apparent upwelling in seismicity is, of course, the result of lowering the network's detec-tion threshold by adding more and closer stations. Whereas the entire 1934-1974 period turned up no more than six events of magnitude 2.5 or 101
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less, the number of similar events repor ced for 1975-1978 is eight. The num-ber of earthquakes of magnitude 3.0 or greater during 1975-1978 (one), is actually lower than the average rate for similar events during 1934-1974 (approximately 0.6 per annum). Coordinates, origin times and nagni tudes of all the relocated earth-quakes are provided in Table 1. Magnitudes given are those oriclnally determined at Caltech. The largest events (magnitude 3.7) occurred in 1358 and 1964. The depths of all events have been constrained to 4 km, because a well-located subset of the 1934-1974 events suggested this depth as a good median choice. As shown in Figure 2, the general pat ern of seismicity around San Diego since 1934 has consisted of a broad scattering of events either off-shore or less than 10 km inland, plus a handful of events more than 25 km inland. Within the limits of location accuracy, most of the epicenters shown can be associated with either submarine topographic features para-lleling the Coronado Escarpment or tha broad area of f racturing which con-stitutes the Rose Canyon fault zone. The concentration of activity around the south end of San Diego ray is conceivably associated with a series of underwater faults discovered there by Moore and Kennedy (1975) by means of acoustic profiling. The faults are reported to have definitely displaced Holocene sediments, Little is known about the earthquakes in the castern part of the study area except that, on the whole, they are not well located. It is perhaps noteworthy that the El Capi tan Reservoi r was completed in 1934; the two events located near there in 1937 may be related to subsequent filling of the reservoir. REFEREllCES Moo re , G. W. , and Kennedy , M. P. , 1975, Quaternary faults in San Diego Bay, California: Journal of Research, U. S. Geological Survey, v. 3,
- p. 589-595.
Simons, R. S., 1977, Seismicity of San Diego, 1934-1974: Sei smclogi cal Society of America Bulletin, v. 67, p. 809-826. 104
TABLE I Earthquake In the San Diego Area (32.5 - 33.0 0N, 116,75 - 117 5 W), 1934-1978 DATE TIME (UTC), LOCATION MAG. N P+ HR Miu 1.LC LAT.Oi) LONG.(W) 5 JUN 1936 05 38 $4.1 32.798 116.764 2.5 4 6 B 11 OCT 1937 04 55 07.5 32.885 116.805 3.0 3 6 B 25 OCT 1937 10 28 27.4 32.932 116.797 3.0 3 6 B 27 SEP 1938 1718072 32.639 117.401 2.5 4 8 8 11 JUL 1940 13 34 45.2 32.608 117.027 2.7 3 5 C 14 SEP 1941 13 50 52.6 32.732 117.455 3.0 5 8 B 1 JAN 1946 23 56 32.2 32.687 117 335 33 5 10 A 20 JUN 1949 18 35 30.1 32.808 117.272 3.0 4 8 A 21 sVN 1949 1939359 32.772 117.290 2.9 3 6 C 25 JUN 1949 02 13 18.8 32.796 117.175 3.1 4 7 C 4 AUG 1949 0' 24 30.5 32.605 117.102 2.5 3 6 8 5 OCT 1949 02 52 10.1 32.981 117.307 2.3 3 6 B 13 MAY 1950 12 11 36.0 32.716 117.399 2.9 5 9 B 20 APR 1952 18 01 43.4 32.648 117.340 3.2 4 6 A 20 APR 1952 21 12 27.3 32.610 117 347 3.2 4 6 A 11 JUN 1952 08 01 58.2 32.852 117.378 3.0 4 5 C 12 JUN 1952 12 45 43.1 32.627 117 262 3.4 3 5 B 12 JUN 1952 12 54 39.4 32.626 117.216 2.6 4 6 A 4 NOV 1952 19 07 31.3 32.899 116.919 3.2 3 5 C 21 DEC 1954 07 11 12.5 32.636 217.076 3.5 5 9 8 7 JUL 1956 12 41 53.2 32 905 116.868 3.5 6 12 A 14 JAN 3958 14 05 41.6 32.644 117.102 3.7 6 12 A 13 NOV 1958 09 09 01.8 32.733 117.456 3.4 4 7 A 23 NOV 1958 21 04 00.3 32.745 117.449 3.4 5 9 A 30 MAY 1961 22 01 38.1 32.612 116.779 3.0 4 7 C 4 AUG 1962 06 37 26.9 32.593 117.324 3.2 4 8 B 14 DEC 1963 02 48 14.3 32.748 117.123 3.1 4 8 B 21 JUN 1964 15 32 52.1 32.652 117.126 3.7 6 11 A 23 JUN 1964 04 54 37.5 32.659 117.143 3.6 5 10 A 15 JUL 1964 03 11 05.6 32.645 117.144 3.5 5 9 A 3 MAY 1968 07 21 54.4 32.631 117.153 3.5 6 11 A 9 APR 1971 23 00 29.0 32.598 117.043 3.3 5 10 A 18 DEC 1972 01 59 37.0 32.617 117.288 2.5 4 8 A 15 APR 1973 03 18 06.4 32.624 117.099 3.2 5 10 A 12 JUN 1974 23 48 19.8 32.684 117.088 2.7 8 15 A 14 DEC 1974 04 01 23.0 32.745 117.389 3.1 5 10 B 22 DEC 1974 15 16 30.0 32.672 117.084 2.3 4 8 B 27 JAN 1975 02 07 21.5 32.519 117.227 3.1 6 10 B 18 MAY 1975 04 42 51.6 32.610 117.269 2.5 8 13 A 1 NOV 1975 12 28 45.2 32.550 117.317 2.1 5 9 B 19 Fra 1976 08 34 15.6 32.728 117.409 2.3 7 13 A 11 MAR 1976 18 12 16.3 32.749 117 393 2.6 7 13 A 13 JUN 1976 01 19 42.5 32.617 117.283 2.7 10 17 8 10 JUL 1976 17 20 57.6 32.788 117.438 2.9 8 i2 B 27 JUL 1976 16 25 55.3 32.849 117.492 2.4 7 13 A 3 DEC 1976 10 59 37.5 32.752 117.476 1.7 5 10 A 29 JUL 1977 18 25 45.5 32 704 117 397 27 6 10 A 22 SEP 1977 04 27 26.2 32.616 117.257 2.7 8 16 8 20 NOV 1977 02 44 16.7 32.633 117.277 2.5 8 13 A 29 JUN 1978 06 55 57.8 32.715 117.353 2.4 9 15 A 17 JUL 1978 17 25 59.4 32.656 117 088 0.0 5 9 B 26 DLC 1978 02 33 33 0 32.589 117.313 2.7 9 17 A
- No of stations used in solution; + Uo. of phases used in solution;
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PROBABILITY OF EARTHQUAKE GROUi>D ACCELERATIONS 114 SAN DIEGO by C. B. Crouse Fugro, Inc. Long Beach, CA IllTRODUCT 10i'l Probabilities that various earthquake accelerations on firm ground will be exceeded in San Diego in a 50 year period and the average return periods of these accelerations have been computed. The analytical model used for the calculations was similar to standard models currently used for scismic risk analyses of this type, inputs to the analysis consisted of: (1) the loca-tion and geometry of scismic source areas in the ion surrounding San Diego, (2) recurrence curves giving the rate of occurrence of earthquakes of a given magnitude wi thin each seismic source area, and (3) an attenuation relation to predict peak accelerations on firm ground in San Diego f rom future earthquakes in the region. AtlALYSIS Analytical Model The basic assumption of the analytical model for this, and many other seismic risk studies, is that the occurrence of earthquakes within a particu-lar seismic source area are random and can be approxirated as a Poisson process. Previous studies of seismicity in southern California (Gardner and Knopoff, 1974) have shown this assumption to be reasonable. It is recognized that the Poisson assumption may not be appropriate for certain seismic sources such as the San Andreas fault. However, for this application, the San Andreas fault is far enough from San Diego that its impact on the proba-bilities would be small, regardless of the assumption made on the occurrence of earthquakes on this fault. Tne mathematical model for this study was adapted from publications by Cornell (1968), Der Kiureghian and Ang (1975) and Benjamin and Cornell (1970). Under the assumption of a Poisson process, it can be shown that this model is represented by the following equation: n m. p(a>A) = 1 - exp (-t E , E (v;; p;j)) (1) J"I ,i=1 107
where p(a>A) is the probability that a given ground acceleration will be exceeded in San Diego in t years (t = 50 years was assuned in this study) . The symbol p; is the probability that one earthquake in some small magnitude range (i) occurring in some seismic source (j) produces a peak ground accel-eration in San Diego, (a) exceeding a given acceleration level (A). The basis for computing p. . is to assume that carthquakes can occur anywhere IJ wi thin the seismic source area. The value of p., IJ is a function of the atten-uation of the peak ground acceleration wi th magnitude and distance, and the geometry and location of the seismic source area, with respect to San Diego. Details of the nethod to compute p. . can be found in Cornell (1968) or Der 1) Kiureghian and Ang (1975). The synbol, v.., represents the rean recurrence IJ rate of an earthquake of a given magni tude (M;) in a particular seismic source (j) and is computed f rom the recurrence curves. The average return period (T) of the event (a>A) is: n m. T = 1/ ( c Z v.. p..) (2) 8J 'J j=1 i=1 Inputs to Analysis The seismic source areas considered in the probabilistic analysis are shown in Figure 1. A seismic source is defined as an area where the seis-micity characteristics and potential for earthquakes are reasonably uni form. The selection of the seismic sources for this analysis was based on the dis-t ribution of seismici ty, recognized geologic provinces or major fault zones, and general understanding of the tectonics of southern California. A reason-able simpli fication of the geonetry was to consider the sources as rectangles. The San Andreas, San Jacinto and Elsinore fault systens were approximated as long, narrow rectangles. The Rose Canyon fault, which passes through San Diego, was not included in the analysis because of the lack of historic seismicity associated with this feature. Preliminary calculations indicatcJ that other possible seismic source areas not shown in Figure 1, such as a source area to the west of the Peninsular Ranges / Continental Borderlands and northe rn Mexico, had a negligible e f fect on the probabilities. Interval recurrence curves for each seismic source considered in the analys is a re shot n in Figt e 2. These curves indicate the average nunber of earthquakes per year per uni t area for nagni tude intervals of one-half unit. For example, the recurrence curve (Figure 2) for the San Jacinto fault indi-108
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SEISMIC SOURCES A. PENINSULAR RANCES CONilNENTAL BORDERLANDS
- 9. NCATHEAN MEllC0 C. LOS ANGELES BASIN
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- f. SAN ANLREAS FAULT FIGURE 1 SEISMIC SOURCES IN THE REGION SURROUNDING SAN OlEGO 109
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-5 cates about 10 earthquakes per year per square km for the magnitude inter-val 6.0 to 6.5. The values of v. . used in equations (1) and (2) were t) obtained f rom the recurrence curves.
An attenuation equation by Donovan (1973) was adopted for the study to approximate the peak accelerations in San Diego on firm ground from earth-quakes in the surrounding region. This equation was: a = 1080 (exp (0.5M)) (R + 25) -1.32 (3) where a is the peak ground acceleration in en/sec/sec, M is earthquake magni-tude and R is the hypocentral distance in km. This equation was obtained by a regression analysis of peak ground accelerations recorded during previous earthquakes. Since a significant portion of the regression data came from southern California, reasonable estimates of the accelerations in San Diego can be obtained from equation (3). Equation (3) and the associated uncer-tainty in the estimated accelerations were used to compute p. . in equations IJ (1) and (2). RESULTS AND DISCUSSION Results of the analysis, shown in Figure 3, indicate that the probability of significant ground accelerations in San Diego is relatively low compared to othe r regions in southern California. The main reason for this is that the more active seismic sources such as northern Mexico, the San Jacinto fault, and the Los Angeles basin, are too far from San Diego to appreciably affect the probabilities. The Peninsular Ranges / Continental Borderlands, the seismic source area containing San Diego, had by far the greatest con-tribution to the probabilities even though the recurrence of earthquakes in this region (Figure 2) is the lowest of all the seismic sources considered in the analysis. Whether or not the low historic seismici ty is representa-tive of the long-term seismic activity for this region is unclear, due to the relatively short time period over which seismicity has been recorded. Geologic maps (e.g. Jennings , 1975) show a few Quaternary faults within San Diego, such as the Rose Canyon and La Nacion faults. Although the potential earthquase activi ty in the inmediate future of these or other capable faults in the San Diego vicinity is not presently known, engineers should consider the possibility of moderate to large earthquakes produced by these faults in the aseismic design of important or critical facilities. 111
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FIGURE 3. AVERAGE RETURN PERIDOS AND PROBABILITIES OF EXCEEDING GROUND ACCELERATIONS IN 50 YEARS 112
REFERENCES Benjamin, J. R. , and Cornell , C. A. ,1970, Probabi l i t 4 Statistics, and Decision for Civil Engineers: Mc-G raw-Hill , 684 p. Co rnel l , C. A. , 1968, Engineering seismic risk analysis: Seismological Society of America Bulletin, v. 58, p. 1583-1605. Der Ki ureghian, A. and Ang, A. , H-S, 1975, A line-source model for seismic risk analysis: Technical report, University of Illinois, 134 p. Donovan, H. C., 1973, A statistical evaluation of strong motion data including the February 9,1971 San Fernando earthquake: Proc. SWCEE, Rome, Italy, v. 1, p . 1252-1261. F ri edman , M. E. , Wh i tcomb , J . H. , Al l en , C. R., and Hileman, J. A., 1976, Seismicity of the southern California region, 1 January 1972 to 31 December 1974: Seismological Laboratory, California Institute of Technology, 93 p. Ga rdner , J. K. and Knopof f , L. , 1974, is the sequence of earthquakes in southern California, with aftershocks removed, Poi ssonian?: Seismolog-ital Society of America Bulletin, v. 64, p. 1363-1367 Jen...ngs, C. W., 1975, Preliminary faul t and geologic map of southern California: California Division of Mines and Geology, Geologic Data tiap 1. 113
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THEORETICAL ASPECTS OF TSUNAMIS ALONG THE SAN DIEGO C0ASTLINE by W. G. Van Do rn Ocean Research Division Scripps Institution of Oceanography La Jolla, CA 92093 If one were asked to pick the safest place in the Pacific Margin f rom the standpoint of geologic hazards from tsunamis, San Diego County would be a top contender. Of the five greatest tsunamis occurring in this century, not one of them produced effects readily apparent even to a skilled observer walking along the scacoast (Table 1). The greatest single excursion of sea level (1.0 m) was recorded at La Jolla for the tsunami of itay 22, 1960, which was produced by the largest earthquake ever recorded (Richter t-tagnitude 8.5) . The strong al ternating currents engendered within San Diego Bay by this same tsunami ripped loose a few hundred feet of rotted wharfage, and caused the Coronado Ferry to discontinue service for several hours for the fi rst time in 80 years of operation. Table 1. tiaximum Recorded Local Tide Gauge Excursions
.h(cm) for Five liajor Tsunamis (USC&GS Reports)
Tsunami Date 4-1-46 11-2-52 7-1-57 5-22-60 3-28-64 Source Location Aleutians I;anchatka Aleutians Chile Alaska San Diego 37 70 46 137 110 (Broadway Pier) Oh La Jolla 43 24 61 100 76 (510 Pier) Because the above tsunanis have originated from all representative direc-tions where tectonically active tsunamigenic sources are thought to (xist a round the Paci fic, it is considered unlikely that new scrprises may alter pre-sent confidence in San Diego's immunity from tsunami damage. The physical reasons underlying this confidence are, however, more associative than theo-retical. (1) It has long been recognized that major tsunamis originate from ver-tical dislocations of large crustal blocks (105 km2 ) by a meter or so, and that such dislocations are primarily confined to the Pacific trench system and have 115
recurrence intervals of several hundreds of years. (2) Recent numerical calculations of the resulting patterns of water waves suggest that most large tsunamis are very similar in deep water, and that their local intensity depends mainly on source orientation, distance, and angle of approach to a remote point. (3) riature has providentially arranged matters so that waves from cur-rently active foci approach southern California from nearly glancing inci-dence; further protection is af forded by wave transformation and/or reflec-tion at the margin o' our broad coastal shel f. Thus, calculations of regional ef fects serve only to confi rm historical precedent as regards remotel y generated tsunami s . As regards local generation, the prognosis is similarly optimistic, but for different reasons. (1) Relative motion of the Pacific crustal plate, vis-a-vis the conti-nental plate of f Cali fornia, is principally horizontal . Hence, local earth-quakes along the San Andreas fault system are prone to strike-slip notions. This ci rcumstance is the strongest argument against the occurrence of a major tsunamigenic earthquake in this sector of the Pacific. (2) Relatively minor local tsunamis have been produced by small earth-quakes on the continental shelf north of Point Fermin, and there is at least a verbal description of a three-foot wave in San Diego Bay associated with the earthquake of May 27, 1862, the only positive evidence for signi ficant th rus ti ng of any magr.i tude is the possible connection of an offshore escarp-ment with the Agua Blanca fault (Legg and Kennedy, this volume). (3) Given the dimensions of the above fault zone, one could assume a maximum credible areal dislocation, and use present numerical nethods to esti-mate the magnitude of shoreline effects along the San Diego coastline. So fa r, this has not been done. Thus, one can conjecture that (1) it would be highly unlikely that a destructive tsunami could be produced by any reasonable dislocation (1 m) along this fault zone, and (2) that the most likely result would be minor flooding of the low-lying areas in Imperial Beach and Mission Beach, i f it happened to coincide wi th a high tide. 116
TSUNAMI HISTORY OF SAN DIEGO by Duncan Carr Agnew Institute of Geophysics and Planetary Physics Scripps Institution of Oceanography University of California La Jolla, CA 92093 The purpose of this paper is to list the available records of tsunamis at San Diego. For remote tsunamis the historical record is quite extensive; for local tsunamis data are essentially nonexistent. A remote tsunami is one observed more than a few wavelengths from its point of generation. Large runup from remote tsunamis usually occurs when the offshore topography concentrates the tsunami energy. This does not seem to have happened at San Diego. Tide gauge records for San Diego bay extend from 1854 to 1872 and from 1906 to the present. In the 92 years of record, at least 19 tsunamis have been recorded. Most have been only a few tenths of a meter in height; for comparison, the diurnal range of tide at San Diego is 1.7 meters. The largest one was caused by the Chilean earthquake of May 1960. In San Diego it had a maximum range (peak-to-trough) of 1.5 meters, and produced strong currents which caused some damage to piers and which temporarily halted ferry service to Coronado. In its recorded history (since the late 1700's) San Diego has experienced only one tsunami caused by a local earthquake. It was asso-ciated with the earthquake of May 27, 1862, which caused the most intense shaking known for San Diego (Legg and Agnew, this volume). Because the tide gauge was being repaired at the time, there is no quan-titative record of the event, but an eyewitness account by the tidal observer, Andrew Cassidy, has been preserved #. At the time of the 'In a memorandum dated May 27, 1862, pasted on page 711 of the " Emigrant Notes" ccepiled by Benjamin Hayes (manuscript CE 62, Bancroft Library, Berxeley) . 117
earthquake Cassidy was on the beach at La Playa, about 2 kilometers north of Ballast Point, on the east side of Point Loma. He wrote that, "The water in the Bay did not appear to be much agitated notwithstanding the sea run up on the beach between 3 and 4 feet, and immediately returned to its usual level." The value of 3 to 4 feet probably refers to the horizontal distance along the beach; the wave height would have been much less. Cassidy also noted falls of earth in the banks between La Playa and Point Loma. If cne of these were large enough it might have caused a single wave in San Diego Bay similar to, but much smaller than, that caused by a rockslide in Lituya Bay, Alaska, in 1958 (diller, 1960). In light of this possibility it would be premature to use this observation to conclude that tsunamigenic earthquakes can occur near San Diego. Such a conclusion, and any estimates of risk from local tsunamis, would ha ve to come from a combined study of earthquake rec urrence rates and types of faulting in the offshore area, together with model studies of tsunami generation, propa gation , and runup. All the historical evidence shows is that damaging local tsunatis have not occurred at San Diego in the last two centuries. The following liat , which is based on that of Joy (1968), is lim-ited to those tsunamis for which there is some evidence of a record at San Diego. For each date, the list gives the location ana magnitude of the causative earthquake, followed by the size of the tsunami at tide gauges near San Diego. Where possible, earthquake magnitudes have been given according to the M scale introduced by Kanamori (1977). Three terms have been used to specify tsunami size. " Range" is the maximum value from peak to trough, also called " maximum rise or fall". " Height" (taken from Ilda n n , 1967) is the maximum positive departure from normal sea level. " Amplitude" is used when the definition of size in the original source is unclear. Though it is tempting to estimate a period for a tsunami record, it may not be too meaningful. Detailed analysis of the 1960 tsunami as recorded at La Jolla showed a broad spectrum (Miller d d , 1962). Tsunami records from San Diego harbor give the general impression that the predominate periods are in the range of one-half to one hour. Unless otherwise specified, the sources of information for this list are as follows. Earthquake locations before 1900 are from lida n 118
al (1967); from 1900 to 1954 locations are from Gutenberg and Richter (1954), and M3 magnitudes are from Geller and Kanamori (1977) and Geller d al (1978); after 1954 locations and M 3magnitudes are from epicenter lists published by the U. S. Coast and Geodetic Survey and its successor agencies. M magnitudes are either from Kanamori (1977) or have been computed from moment estimates. Tsunami information is from Iida d al (1967). Other references are given in the individual listings. TSUNAMIS AT SAN DIEGO 1854-1975 1854 July 24. No source is known, but the tidal observer at San Diego noted that on this date, " Water rose & fell nearly a foot in 10 minutes - currents set up also, harbor calm." (Andrew Cassidy,
" Miscellaneous Notes on the Tide Gauge & Tidal Obser/ations at San Diego - Cal: 1853-1854". Cassidy Papers, Serra Museum Library, San Diego).
1854 December 23 Japan, 34 N, 138 E. Range 0.1 m at San Diego (Bache 1855). 1856 August 23 Japan, 42 N, 141 E. Recorded on the coast of Califor-nia (Joy, 1968). 1862 May 27. Earthquake at San Diego caused a small tsunami in San Diego Bay. See text for details. 1868 April 2. Hawaii, 19.3 N, 155.3 W (Wood, 1914). Height 0.1 m at San Diego. 1868 August 13 Chile, 18.5 S, 71 W. Amplitude 0. 8 m at San Diego (Hilgard, 1869). 1872 August 23 Davidson (1872) said that on this date a tsunami was recorded at San Diego, San Francisco, and Astoria. He used relative arrival times to infer a source in the northwest Pacific. 1906 January 31. Off the coast of Ecuador, 1 N, 81.5 W. My = 8.8. Recorded at San Diego. 1917 May 2. Kermadec Islands, 29 S, 177 W. M = 7.9. Recorded on the 8 west coast of the U. S. (Heck, 1947). 119
1917 June 25. Tonga, 15.5 S, 173 W. M = 8.4. Recorded on the west 8 coast of the U. S. (Heck, 1947). 1919 April 30. Tonga, 19 S, 172.5 W. t8= 8.2. Recorded in California (Heck, 1947). C 1922 November 10. Central Chile, 28.5 S, 70 W. M = 8.5. Height 0.2 m at San Diego. 1923 February 4. East coast of Kamchatka, 54 N, 161 E. N = 8. 3 Height 0.2 m at San Diego. 1923 April 14. East coast of Kamchatka, 56.5 N, 162.5 E. M = 7.2 (Gutenberg and Richter, 1954). Height 0.1 m at San Diego. 1927 November 4. Off Point Arguello, California, 34.5 N, 121 W. M = 7.3 (Hanks d d,1975). Range 0.006 m at La Jolla (Byerly, 19303. 1933 March 2. East of Honshu, 39.2 N, 144.5 E. M = 8.4. Height less than 0.1 m at La Jolla. 1944 December 7 Near Honshu, 33.7 N, 136 E. M" = 8.1. Height 0.1 m at San Diego. 1946 April 1. Southern Alaska, 52.75 N, 163.5 W. M = 8.4 (Kanamori, 1972). Range 0.43 m at La Jolla, 0.37 m at San fiego (Green, 1946; Symons and Zetler, 1960). 1952 March 4. Hokkaido, Japan, 42.5 N, 143 E. M = 8.1. Range 0.02 m at La Jolla (Munk, 1953). 1952 November 5. Off east coast of Kamchatka, 52.7 N, 159.5 E. M = 9.0. Range 0.24 m at La Jolla, 0.7 m at San Diego (Zerbe, 1953)" 1957 March 9. Rat Islands, 51. 3 N, 175.8 W. M = 9.1. Range 0.6 m at La Jolla, 0.45 m at San Diego (Salsman, 1955). 1960 May 22. Coast of central Chile, 39.5 S, 74.5 W. M = 9. 5. Range 1 m at La Jolla, 1. 5 m at San Diego (Symons and "Ze tler , 1960; Miller d d , 1962). Some damage to piers and moorings in San Diego Bay. 1964 March 27. Southern Alaska, 61 N, 147.8 W. M = 9.2. Range 0.7 m at La Jolla, 1.1 m at San Diego (Spaeth and BeUkman, 1964). 1968 May 15. East of Honshu, 29.9 N, 129.4 E. M = 8.2. Amplitude 0.1 m at La Jolla (Joy, 1968). 1975 November 29 Hawaii, 19.3 N, 155 W. M = 7.1. Amplitude 0.3 m at La Jolla, 0.12 m at San Diego, 0.37 m at8 Imperial Beach (Spaeth, 1976). Acknowledstements . I should like to thank B. D. Zetler for help and comments. 120
REFERENCES Bache, A. D., 1855, Notice of earthquake waves on the western coast of the United States, on the 23d and 25th December, 1854: U. S. Coast < Survey Annual Report, 1855, p. 342-346. Byerly, P., 1930, The California earthquake of November 4, 1927: Seismo-logical Society of America Bulletin, v. 20, p. 53-66. David son , G., 1872, Remarks on recent earthquake waves: California Academy of Science Proceedings, ser. 1, v. 4, p. 268. Geller, R. J., and Kanamori, H., 1977, Magnitudes of great shallow earthquakes from 1904 to 1952: Seismological Society of America Bulletin, v. 67, p. 587-598. Geller, R. J., Kanamori, H., and Abe, K., 1978, Addenda and corrections to " Magnitudes of great shallow earthquakes from 1904 to 1952": Seismological Society of America Bulletin, v. 68, p. 1763-1764. Green, C. K., 1946, Seismic sea wave of April 1, 1946, as recorded on tide gages: American Geophysical Union Transactions, v. 27, p. 490-500. Gutenberg, B., and Richter, C. F., 1954, Seismicity of the Earth and associated phenomena, Princeton, Princeton University Press, 310 pp. Hanks , T. C. , Hileman , J. A. , and Thatcher , W. , 1975, Seismic moments of the larger earthquakes of the southern California region: Geologi-cal Society of America Bulletin, v. 86, p. 1131-1139. Heck, N. H., 1947, List of seismic sea waves: Seismological Society of America Bulletin, v. 37, p. 269-286. Hilgard, J. E., 1869, The earthquake wave of August 14, 1868: U. S. Coast Survey Annual Report, 1869, p. 233 Iida, K., Cox, D. C., and Pararas-Carayannis, G., 1967, Preliminary catalog of tsunamis occuring in the Pacific Ocean: Hawaii Institute of Geophysics Data Report 5 (HIG 67-10). Joy, J. W., 1968, Tsunamis and their occurrence along the San Diego County coast: Report to the Unified San Diego County Civil Defense and Disaster Organization. Kanamori, H., 1972, The mechanism of tsunami earthquakes: Physics of the Earth and Planetary Interiors, v. 6, p. 346-359. Kanamori, H., 1977, The energy release in great earthquakes: Journal of Geophysical Research, v. 82, p. 2981-2987. 121
Miller, D. J., 1960, Giant wave in Lituya Bay: Seist .og'. cal Society of America Bulletin, v. 50, p. 253-266. Miller, G. R., Munk, W. H., and Snodgrass, F. E., 1962, Long-period waves over California's continental borderland. Part II. Tsunamis: Journal of Marine Research, v. 20, p. 31 -41. Munk, W. H., 1953, Small tsunami waves reaching California from the Japanese earthquake of March 4, 1952: Seismological Society of America Bulletin, v. 43, p. 219-222. Salaman, G., 1959, The tsunami of March 9, 1957, as recorded at tide stations: U. S. Coast and Geodetic Survey Technical Bulletin 6. Spaeth, M., 1976, Tsunamis: United States Earthquakes 1975, p. 115-116. Spaeth, M., and Berkman, S. C., 1964, The tsunami of March 28, 1964, as recorded at tide stations: U. S. Coast and Geodetic Survey Techni-cal Report 6. Symons, J., and Zetler, B. D., 1960, The tsunami of May 22, 1960, as recorded at tide stations: U. S. Coast and Geodetic Survey Prelim-inary Report. Zerbe, W. E., 1953, The tsunami of November 4, 1952, as recorded at tide stations: U. S. Coast and Geodetic Survey Special Publication 300. 122
EARTHQUAKE HISTORY OF SAN DIEGO by Duncan Carr Agnew and Mark Legg Institute of Geophysics and Planetary Physics Scripps Institution of Oceanography University of California La Jolla, CA 92093 and Carl Strand Department of Geological Sciences San Diego State University San Diego, CA 92182 The San Diego area is sometimes taken to be relatively safe seismi-cally, at least compared with other major population centers in Califor-nia. Instrumental records since 1931 show the San Diego area to be relatively free from earthquakes, but this is a very short time on which to base a seismicity estimate. Our purpose here is to give a more com-plete picture of the local seismicity, by presenting a partial listing of earthquakes felt in the San Diego area in the last 180 years. Though we have made no explicit division, different classes of earthquake are included for different periods. Before 1900, we have included every earthquake reported as felt within the present boundaries of San Diego County, Most of these were small and unimportant, and have been included only for completeness. Although 1900 is a somewhat arbi-trary cutoff, after this date the existing lists are much more complete. Between 1900 and 1931 we have included only those earthquakes for which shaking of intensity VI or above (on the Modified Mercalli scale) was reported within the county. More extensive lists for this period are those of Toppozada, si al (1978) and Townley and Allen (1939); the latter has been ably indexed by Clark (1944). After 1931 instrumental locations are available in Hilemmi, mi al (1973) and additions to it issued by the California Institute of Technology. For the immediate San 123
Diego area Simons (1977 and this volume) should be consulted instead. Intensity data from 1928 to the present are available in the annual pub-lication United States Earthauakes, published by the U. S. Department of Commerce . There seems little point in republishing this information, and we have therefore limited our list to earthquakes that caused shak-ing of intensity VI in the metropolitan San Diego area. Because of the imperfect nature of the historical record this cata-log is also incomplete before 1900, but for earthquakes of damaging intensity at San Diego it is probably complete from 1850 on. The record from the Spanish and Mexican periods is very scanty, though from about 1780 to 1830 some mission records are available. In 1850 a U. S. Army post was established at San Diego. Daily weather records kept there sometimes report earthquakes. In 1851 a weekly newspaper, the San Diego Herald, began publication, which continued until 1859 (Dawson, 1950). Though the files of this newspaper are not complete, they still provide a valuable record. The San Diego Union began publication in 1869 and is still in existence. There is a good index to both of these newspapers in the San Diego Public Library. The lack of a local newspaper in the 1860's is to some extent remedied by the notes compiled by Judge Benja-min Hayes in about 1870, and now in the Bancroft Library. For the years from 1870 to 1900 our most important sources have been the San Diego Union and the catalog of Townley and Allen (1939), which for this period is mostly based on that of Holden (1898). This in turn drew on a variety of sources; an important one was the reports of weather station observers at such places as Campo. Few of the earthquakes listed here caused any damage. Usually the only description of the.m is as being " light", " heavy", " severe", or some similar adjective. Because the use of these terms does not seem to correlate very well with intensity, we have not given an intensity unless other information was available. A mention of a place means that the earthquake was fcit at that place. If no location is given, the earthquake was reported from San Diego only. The times given are taken from the reports, and so are either local or Pacific Standard (and in any case not very accurate) . The times for earthquakes for which epi-central coordinates are given are the origin times, again Pacific Stan-dard. These coordinates and times, and the local magnitudes, are from 124
Richter (1958) and Hileman, d AL (1973). The times are to the nearest minute, and coordinates to the nearest tenth of a degree. During the time covered by this catalog, San Diego has suffered from several damaging earthquakes, but none have been very destructive. The earthquakes that have caused the strongest shaking before 1900 were those in 1800, April 1852, September 1856, May 1862, February 1892, and October 1894. The first two are inadequately documented. Of all the earthquakes in this list, the one in 1862 seems to have produced the strongest shaking, of about intensity VI-VII in San Diego (Legg and Agnew, this volume). The pre-1900 part of this list also includes at least three large earthquakes that clearly were located in the Salton Trough: !ovember 1852, November 1875, and July 1891. In the last 50 years the strongest shaking in San Diego has come from earthquakes along the San Jacinto fault zonc or in northern Baja California. 125
CATALOG DE SAR DIEGO EARTHQUAKES Abbreviations used for references cited often in the catalog are given here. All newspapers are cited according to month, day, and year, followed by page and column number. CMS - MS recollections of Herbert L. Crouch, Serra Museum Library, San Diego. CR - Climatological Records of the Weather Bureau, Record Group 27, U. S. National Archives. (Microfilm number T907. If no name is given, the records are those for San Diego or New San Diego). GD - Extracts from the diaries of Dr. Hiram W. Gould, Serra Museum Library, San Diego. H - Holden (1898). HEN - Benjamin Hayes, " Emigrant Notes", Bancroft Library CE 62, Berkeley. (The pagination used in the references is that added after the MS was written). McD - Extracts from the diaries of Dr. George McKinstry, Serra Museum Library, San Diego. PD - Extracts from the diaries of Theron Parsons, Serra Museum Library, San Diego. R78 - Rockwood (1878). SD - Extracts from the diaries of Mrs. Theodore Steinmeyer, Serra Museum Library, San Diego. SDH - San Diego Herald. SDU - San Diego Union. SFAC - San Francisco Alta California. TA - Townley and Allen (1939). USEQ - United slat.gg Earthauakes ( Annual publication of the Coast and Geodetic Survey). 126
1800 November 22. 1: 30 P.M. At San Juan Capistrano, threw down walls of mission church (then under construction). At San Diego, several buildings considerably cracked (VI?). - Bancroft (1888), v. 1, p. 654, 659; Engelhardt (1920), p. 154. 1803 May 25. San Diego, mission church slightly damaged. - Bancroft (1888), v. 2, p. 106. 1812 December 8. Mission church at San Juan Capistrano destroyed. No evidence of damage at San Diego, or even that the earthquake was felt. - Bancroft (1888), v. 2, pp. 200-201. 1843 June 23. Coast of California - H. 1849 August 16. At night , at Palm Spring oasis, 11 km SE of Vallecito.
- Bloom (1945).
1849 September 16. Santa Ysabel, felt by few. - Whipple (1850) . 1849 September 22. South of Carrizo Creek, felt by few. - Whipple (1849), Couts (1932). 1850 August 4. San Diego and the Gila River.- H. 1850 August 15. San Diego and the Gila River. - H. 1852 April 12. Midnight. "Very severe", duration 30 seconds, an adobe house "d es t ro yed" . - SDH, 4/17/52, 2:1. (Intensity possibly VII, but it is odd that no other damage is mentioned. ) 1852 November 29. About noon. A large earthquake, probably in the region of the Colorado River delta. Intensity about V ac San Diego. - Balderman, et. .a1 (1978); Agnew (1978). 1852 November 30. 8 A.M. Aftershock of the above, felt in San Diego. - CR. 1853 June 26. 8: 30 A.M. - CR. 1856 September 20. 11:25 P.M. At San Diego windows rattled, small obj ects upset (V). At Santa Ysabel and San Felipe trees shaken severely, some cracking of adobe walls (VI-VII). Felt at El Cajon, Vallecito. Not felt at Yuma. - CR; SDH 9/27/56, 2: 3; HEN, pp. 889-900. 1857 January 9 8:30 A.M. The great Fort Tejon earthquake, felt in San Diego with intensity V. - Agnew and Sieh (1978). 1859 January 26. 11 P.M. - McD 1/27/59. 1859 March 21. 6 A.M. Buildings creaked (IV-V?) . Several more felt that day. - McD 1/21/59; SDH 3/26/59, 2:4. 1859 March 25. Severe, with ground cracking near Ballena. - McD 3/25/59. (Possibly the same as the next shock). 127
1859 March 26. 2 P.M. , sensibly felt. - SDH 4/2/59, 2:4. Possibly also at Mesa Grande. - McD 3/26/59. 1859 March 30. 10 A. M. , slight. - McD 3/30/59. 1859 August 2. 10 A.M. - McD 8/2/59 1860 January 14. 7:23 P.M., " violent", lasted about 10 sec. - CR; Meteorological Journal of Andrew Cassidy (Cassidy Papers, Serra Museum Library). 1860 August 2. 5 A.M., Mesa Grande. - McD 8/2/60. 1860 August 15. 1: 30 & 4:30. - McD 8/15/60. (Mesa Grande?). 1862 April 15. Carrizo Creek stage station, all frightened. - Los Angeles Semi-Weekly Southern News 4/23/62. 1862 May 27. About noon. In San Diego, obj ec ts upset, buildings cracked, cracks formed in wet ground (VI-VII). At Temecula and Aguanga plates rattled and objects upset (V). Generally felt throughout San Diego County, from the ocean to the desert. Felt at Anaheim and Los Angeles. Not felt at Yuma. - Legg and Agnew (this vol ume) . 1862 May 28. Several, " slight". - CR. 1862 May 29 10 A.M. " Violent" at San Diego. Felt San Ysidro and Anaheim. - CR ; REN , p. 691. 1862 May 30. 3 A.M. - HEN, p. 691. 3 P.M. - CR 1862 May 31. 1-2 P.M. Temecula and 0:a Diego. - CR ; HEN , p. 691. 1862 June 1. 11 A.M. - CR. 1862 June 2. In the night. - CR. 1862 June 3 4 A.M. - CR . 1862 June 4 & S. " Light shocks at San Diego". - HEN, p. 691. 1862 June 6. Midnight . - HEN , p. 691. 1862 June 7 5 P.M. - CR ; HEN , p. 691. 1862 June 8. 5 A.M. - CR. 1862 June 13 10: 30 A.M. " Violent" at San Diego. Another about 2 P.M. - CR; HEN p. 697. 1862 June 14. 2 P.M., 10 P.M. - CR ; HEN , p. 697. 1862 June 15. 9:50 P.M. - CR . 128
1862 June 19. Noon. - CR. 1862 June 27. 10 P.M. " Violent". - CR. 1862 July 11. 9: 27 P.M. - CR. 1862 August 18. 2 A.M. - CR. 1862 October 21. 6 A.M. Many frightened and ran outdoors, no " serious damage" (V7) - Clipping from SFAC, pasted on p. 562 of HEN. 1863 January 25. 2:20 P.M. No damage, many frightened (IV?) - SFAC, 2/11/62, 1: 8. 1863 about June 25. 1:11. Lasted 30-40 seconds, buildings creaked, no damage (IV). - SFAC 7/7/63, 1:5. 1865 April 15. 12: 39 A.M. Many awakened, liquids splashed. - CR; SFAC 4/26/65, 1: 6. 1867 February 1. 6:45 A.M. - SFAC 2/18/67,1: 6. 1869 October 21. At New River stage station (near Mexicali), many alarmed . - San Diego Week 1v Bulletin 11/6/69, 2: 2. 1869 October 30. " Severe" in the desert , 100 km east of San Diego. - SDU 11/4/69, 2:1. 1871 October 27, 10 P.M., Temecula and San Juan Rancho. - McD; SDU 11/14/71, 2:2. 1872 March 26. 2: 46 A.M. The Owens Valley earthquake, marginally felt in San Diego (intensity II-III). - SDU 4/2/72, 3:3 1873 March 23 3 A.M. Cholla Valley (now southeast San Diego). - SDU 3/25/73, 3: 1. 1873 October 12. 1:15 A.M. -H. 1875 November 15. 2: 30 P.M. Earthquake probably centered in the Imperial Valley or Colorado River delta. Destroyed adobe buildings at Indian Wells and Gardner's Wells, near Mexicali (VIII), at Campo upset furniture and threw dishes off the shelves (VI), at San Diego frightened some and scopped clocks whose pendulums swung EW (IV). Also felt at Yuma and Maricopa Wells, Arizona. Six more shocks were felt at Campo in the next 2 days. - SDU 11/16/75, 3:2; 11/19/75, 3: 2 & 3: 1. 1877 January 13 Noon, 70 km southeast of San Diego. - R78. 1877 August 17. 7:30 P.M., Campo and Agua Caliente. - CMS, R78. 1877 September 4. Agua Caliente. - CMS. 129
1877 September 29 2: 30 P.M. Campo and Agua Caliente. - CMS , R78. 1877 October 23 Agua Caliente? - CMS. 1877 November 30. San Luis Rey? - CMS. 1878 July 2. About 6 P.M. Two shocks felt in the mountains east of San Diego, in some places strong enough to break crockery and upset things (V). - SDU 7/9/78,1: 3 1878 December 17 4 P.M. Campo and Yuma. - H. ; Fort Yuma CR. 1880 August 29 1:10 P.M. - SDU 8/31/80. 1880 December 19 3: 40 P.M. Felt by some in San Diego, and noticed along the coast to Los Angeles. - SDU 12/23/80, 4:4; 12/24/80, 4: 3 1880 December 21. 11 P.M. San Diego and Campo. - SDU 12/23/80, 4:4; 12/25/80, 4: 3 1880 December 22. 3: 22 A.M. Campo, all awakened (V7). - SDU 12/25/80, 4:3 1881 January 7. 6: 15 A.M. Campo, slight . - H. 1881 February 15. 6: 20 A.M. " Light". - GD. 1881 June 30. 8 A.M. Campo, sharp. - H. 1881 October 2. 9 A.M. Campo, sharp. - H. 1882 February 1. 3 o' clock. Warner's ranch. - SDU 2/11/82, 3: 1. 1882 March 11. 4 P.M. Poway, slight at San Diego. - H.; SDU 3/12/82, 3: 1. 1882 September 30. 11 A.M. Campo and Warner's ranch. - H.; SDU 10/11/82, 3: 1. 1882 October 8. 2 A.M. "Very heavy" at Warner's ranch. In San Diego, windows and crockery rattled (IV); another felt at 5:30 A.M. Felt at Spring Valley, National City. - SDU 10/11/82, 3: 1, 3: 2; GD; diary of F. A. Kimball, National City Public Library. 1883 February 6. 4:30 P.M., slight. - H. 1883 March 11. 11 A.M. Campo and Tijuana. - SDU 3/13/83, 3: 1. 1883 April 4. 1: 30 A.M. Warner's ranch. - SDU 4/7/83, 3: 1. 1883 November 11. 6:15 P.M. Poway, slight. - H. 1883 December 16. 3 P.M. Poway, slight . - H. 130
1884 July 24. 4:20 P.M. Pala. - SDU 7/27/84, 3: 1. 1884 August 26. 11 P.M. " Severe" at Jac umba , still stronger south of the border. - SDU 9/3/84, 3:2. 1885 April 1. 3 A.M. Agua Caliente. - SDU 4/3/85, 3: 2. 1885 August 19. 1 P.M. , not felt by all. - SDU 8/20/85, 3: 1. 1885 September 5. 1 P.M. - CR. 1885 September 13 4: 30 A.M. Many awakened (V) . Felt in Los Angeles and San Bernardino. - SDU 9/15/85, 3: 1; GD; H. 1885 December 1. 1:40 A.M., light . - SDU 12/2/85, 3: 1. 1886 September 29 11 : 05 A.M. , felt by few. - SDU 9/30/86, 3:2. 1886 October 8. 3:30 A.M., 4 A.M. Many awakened (V). - SDU 10/9/86, 3: 1; GD. 1886 December 4. 2:40 A.M. Rattled windows and doors (IV). - SDU 12/4/86, 3: 1; GD. 1887 April 29? 3:30 A.M. Campo, rattled dishes (IV). 4 A.M. National City. - SDU 5/4/87, 4:2; PD. (The date of the Campo shock is unc-ertain; we have assumed that it coincided with that reported from National City). 1887 June 28. 11 P.M. , some frightened. - SDU 6/30/87, 5: 1. 1887 August 24. 6 A.M., light. - SDU 8/25/87, 3:3 1888 March 7 7:54 A.M. Earthquake centered near Pasadena, intensity II in San Diego. - H. 1888 August 19 Morning, National City. - PD. 1888 October 4. 11 P.M. -H. 1889 February 6. 9 A.M., National City. - PD. (Possibly an error for the shock of 9: 20 P.M., felt strongly in San Bernardino. ) 1889 June 25. Midnight. - H. 1890 February S. 10:15 P.M. In San Diego and National City, many awak-ened, crockery rattled, no damage (V). - SDU 2/6/90, 4:1 & 5:1; PD. 1890 February 9. 4:06 A. M. Earthquake probably in the San Jacinto mountains, felt in San Diego, Lawson Valley, and National City. - SDU 2/18/90, 4: 3; PD; SD; H. 1890 February 21. Lawson Valley. - SD. 131
1890 April 3 Evening. Several light shocks at Tijuana. - SDU 4/4/90, 6: 2. 1890 June 10. 3 P.M. Buildings creaked, felt by many, lamps swayed. Also Coronado, National City. - SDU 6/11/90, 6:2 & 8:4; PD. 1890 September 22. 8:05 P.M. Chandeliers swung, rattled glass (IV7). Also El Cajon, National City. - SDU 9/23/90, 8:2; PD. 1890 late November. Two shocks felt "last week" at Campo. - SDU 12/8/90, 5:2. 1891 January 5. 8:25 P.M. Some awakened in National City. Also San Diego, Linda Vista. - PD; SDU 1/8/91, 5:2. 1891 January 6. 4:55 P.M. - SDU 1/8/91, 5:2. 1891 January 13 3 A.M. , few awakened. - SDU 1/14/91, 5:1. 1891 March 17. 7:45. At Campo , strong, no damage. Slight at National City. - SDU 3/23/91, 8: 1; PD. 1891 July 30. 6: 10 A.M. Earthquake probably centered in the Colorado River delta. In San Diego, clocks stopped, china rattled, furniture moved, many awakened (V). - SDU 7/31/91, 5:3 Also National City. - PD. In Ensenada, many awakened, no damage. - Ensenada Lower Cali-fornian 7/31/91, 1:2. In Yuma, all frightened and ran outdoors, clocka stopped, windows and crockery rattled, houses creaked, some adobe cracked (VI). - Yuma Arizona Sentinel 8/1/91, 3: 4. At Colonia Lerdo, men thrown down, buildings collapsed, ground cracked (VIII). - Yuma Arizona Sentinel 8/8/91, 3:3; San Francisco Examiner 8/13/91, 3:5. (This earthquake seetis to have inspired a great many sensationalized newspaper accounts, and it is difficult to make out just what did happen.) 1892 February 20. 9: 15 A.M. " Sharp" at Julian. - Elsinore Press 2/27/92, 1:3 1892 February 23 11:20 P.M. One of the largest earthquakes to have produced strong shaking in the San Diego area. It was felt from San Quintin (Baja California) to Visalia and Santa Barbara (11-III), and as far east as Needles and Yuma. In San Diego, many were awakened and ran outdoors, clocks stopped, crockery was upset, and much plaster was cracked though little fell (VI). In Paradise Val-ley, two buildings on stilts co_ lapsed; at Jamul, a stone kiln was cracked ; at Julian, some light objects were upset; at Campo, some adobe walls were cracked but goods were not thrown off shelves; at the Carrizo stage station adobe buildings were damaged. There are reports of landslides or falling rocks from Campo, Dulzura, Inko-pah, and other places in the mountains east of San Diego. In Escondido, some objects were overturned and goods thrown off shelves (V). The shock was " severe" but apparently caused no dam-age in Ensenada. The intensity seems to have been IV-V throughout the Los Angeles area; reports from Anaheim, Santa Ana, Los Angeles, Pasadena, San Bernardino, Ontario, Redlands, and Riverside all say that many were ankened and ran outside, that some clocks were
stopped, but that there was little or no damage. Many accounts from San Diego and piaces nearby mention a very large number of af tershoc ks , over 100 in the first day; ehocks were still being felt in the mountains east of San Diego as late as May. We have listed only the more important of these. [This earthquake and its aftershocks are the subj ect of a forthcoming Master's thesis by Carl Strand, which will contain full references.) 1892 February 24. 5 A.M. At Campo caused an adobe wall to collapse and goods to fall off shelves. Also felt at Rancho Bernardo, and possi-bly Julian, though the time there is given as 6:30. - SDU 2/25/92, 5: 2; 2/29/92, 5: 2; San Diego Ega 2/25/92, 5:3 1892 February 24. 9:30 P.M. Julian, Rancho Bernardo, Ontario, Santa Ana, Los Angeles. 1892 February 25. 2 A.M. San Diego, Julian, Beaumont, Ontario, Santa Ana , Los Angeles. 1892 March 1. 3: 20 P.M. Ensenada , San Diego, Campo, San Bernardino. - Ensenada Lcwer Californian 3/4/92, 1:1; San Diego Sun 3/1/92, 5: 2; 3/2/92, 5:2; Monthlv Bull, fal. Weather Service vol. 1, no. 7, p. 136. 1892 March 22. 9:10 A.M. " Light" at San Diego, National City. - SDU 3/23/92, 5: 1; PD. 1892 March 23 7:50 A.M. National City - PD. 1892 April S. 4:45 A.M. Campo, strong. - SDU 4/7/92, 5:2. 1892 April 19. Morning. Ensenada. - Ensenada Lower Californian 4/22/92, 1:1. 1892 April 25. San Diego. - El Cajon Vallev News 4/30/92, 2:2. 1892 May 28. 3: 20 A.M. Felt in Spring Valley, strongest in San Bernar-dino. - El Cajon Vallev News 4/30/92, 2:2; H. 1892 June 14 5:25 A.M. Felt in San Diego, strongest in San Bernardino and Riverside. -H. 1893 April 4. 11:40 A.M. Newhall earthquake, intensity II in San Diego.
- H.
1893 May 18. 4: 35 P.M. Earthquake centered near Ventura, felt as far south as San Diego. - TA. 1893 August 9. 11:02 A.M., 4:07 P.M. -H. 1894 July 29. 9: 12 A.M. Earthquake centered near Mojave, felt in Escon-dido. - TA; SDU 7/30/94, 5: 4. 1894 October 23 3: 03 P.M. In San Diego, people frightened and ran out-side, buildings creaked, some cracks and fallen plaster (VI). At Otay and Buckman Springs, rocks fell off hillsides (VI+?). - SDU 133
10/24/94, 5:4; San Diego Syn 10/23/94, 5:4. In National City, all frightened and ran out, no damage reported. - National City Record 10/25/94, 3: 1. In San Juan capietrano, clocks stopped, windows broken, crockery overturned (VI). No damage at Oceanside. In Santa Ana, people frightened and ran outside. - SDU 10/24/94, 1:5. Felt strongly at Escondido and Valley Center. - Escondido Times 10/25/94, 3:2; 11/1/94, 3: 4. Also felt at Coronado, Campo, Los Angeles, P.iverside, San Bernardino, Needles, Ensenada. - SDU 10/24/94, 5:4; 10/28/94, 5:2; Riverside Daily Press 10/23/94, 3:2; Needles Eyg 10/23/94, 2:1; Ensenada Lower Californian 11/2/94, 2; TA. Most aftershocks felt in the mountains east of San Diego. - SDU 10/30/94, 5:4. The exact location of the epicenter of this earth-quake is unclear; it seems to have been in the Peninsular Ranges, probably near the Mexican border. 1894 October 26. 5 A.M. " Strong" in Lakeside and Alpine, felt in Ense-nada. - SDU 10/28/94, 5:2; Ensenada Lower Californian 11/2/94, 1. 1894 October 27. 11 P.M. In San Diego, windows rattled and lamps swung (IV). Felt in Los Angeles, and strongly to the east of San Diego. - SDU 10/28/94, 5:4; 10/30/94, 5:4. 1894 November 17 5 P.M. Campo. - H. 1894 November 19 10 A.M. Felt on upper stories in San Diego (II). Also Julian, Rancho Bernardo. - SDU 11/20/94, 5:4; H.; Escondido Times, 11/22/94, 3: 2. 1895 September 18. 8:25 P.M. Campo, sharp. - SDU 9/23/95, 5: 1. 1896 April 12. Jamul, light . - SDU 5/1/96, 2:2. 1896 July 3 9:27 P.M. - H. 1896 September 30. Descanso. - H. 1897 February 16. Descanso. - TA. 1897 February 25. Descanso. - TA. 1897 May 15. About 4 A.M. - TA. 1897 May 22. 6:55 A.M. "Very distinct". - SDU 5/23/97, 5:1. 1897 September 6. Descanso. - TA. 1897 October 27. 9:07 A.M. " Slight" in San Diego. Also Morena Dam, Campo. - SDU 10/28/97, 5: 1. 1897 November 12. Descanso. - TA. 1897 November 22. Descanso, Escondido, Fallbrook. - TA. 1898 March 3 2: 30 A.M. Descanso, light . - TA. 134
1898 April 21. Descanso. - TA. 1898 June 23 1:44 P.M. Descanso. - TA. 1898 June 24. 2:45 P.M. Descanso. - TA. 1899 April 14. Cuyamaca. - TA. 1899 June 1. Morena Dam. - TA. 1899 July 21. 4: 45 P.M. , no damage. - SDU 7/22/99, 6: 2. 1899 July 22. 12: 32 P.M. Cajon Pass earthquake, "quite strong" in San Diego, but no damage (IV). - TA; SDU 7/23/99, 7:4. 1899 July 27. Campo, heavy. - SDU 8/3/99, 7:1. 1899 August 21. - SDU 8/22/99, 6: 2. 1899 October 28. Morena Dam. - TA. 1899 December 25. 4:25 A.M. San Jacinto earthquake. In San Diego, many awakened and ran outdoors, clocks stopped, some glass broken (V). - SDU 12/26/99. 1915 November 20. 4:15 P.M. Near Cerro Prieto, Baja California (32 N, 115 W, Mg = 7.1) Intensity V-VI in the San Diego area. - TA. 1918 April 21. 2: 32 P.M. Near San Jacinto (33.75 N, 117 W, Mt = 6.8). In San Diego, many frightened , some cloc ks stopped, a few small objects upset (V). Intensities V-VI in San Diego County, largest in the northeast. - SDU 4/22/18, 2:5-6; Townley (1918). 1919 December 31. 6:35 P.M. At Warner Springs, adobe walls cracked (VI). Felt strongly (intensity about IV) in San Diego, El Cajon, Julian , Elsinore , Hemet , Corona. - TA. 1920 October 5. 9:48 A.M. At Warner Springs, trees shaken, all frightened and ran outdoors (VI). Also Aguanga, Hemet, San Diego.
- TA.
1927 August 14. 6:48 A.M. Epicenter near Barrett reservoir. Intensity V+ at Alpine and Jacumba. Felt San Diego, Oceanside. - Carnegie Institution of Washington Yearbook, 21, 419; TA. 1929 December 2. 3: 24 A.M. Epicenter near Ensenada. Intensity V+ in San Diego. Also felt Jamul, Escondido. - USEQ. 1934 December 31. 10: 45 A.M. Colorado Delta (32 N, 114.8 W, Mg = 7.1). Intensity V-VI throughout San Diego County. In San Diego, some cracks in buildings, fallen plaster, broken windows. Slight damage also reported from Alpine, Coronado, Lemon Grove (V). Intensity IV reported along the coast from Solana Beach north, also Ramona and Escondido. - USEQ. 135
1942 October 21. 8: 22 A.M. Borrego Valley s.3 N, 116 W. N = 6.5) . Maximum acceleration .026 g at San Diege. Intensity VII i'n Carrizo Gorge. Intensity VI (cracked plaster and broken glass) in Campo, Lakeside, Miramar, San Diego, Santa Ysabel, Warner Springs. Inten-sity V reported from Aguanga, Anza, Escondido, Mesa Grande, Mount Laguna, Oceanside. Intensity IV reporteu generally along the ccTst. - USEQ. 1949 November 4. 12: 42 P.M. Northern Baja California (32.2 N, 116.5 W, g = 5.7). Maximum acceleration .017 g at San Diego. Intensity V-VI in San Diego, La Jolla , Campo (some plaster cracking, trees and bushes shaken, dishes rattled). - USEQ. 1951 December 25. 4:47 P.M. Near San Clemente Island (32.8 N, 116.3 W. Mg = 5.9). Maximum acceleration .014 g at San Diego. In Del Mar and San Diego, goods fell off shelves, some plaster cracked, trees and bushes shaken (VI). Intensity V reported from Barrett Dt n, Mount Laguna, Pala. Intensity IV at Alpine , Campo, El Cajon, Escondido, Jamul, Julian, Leucadia, Oceanside, Santa Ysabel. - USEQ. 1954 March 19. 1: 54 A.M. Santa Rosa Mountains (33.3 N, 116.2 W, = 6.2). Maximum acceleration .016 g at San Diego. Intensity (VI (some broken glass and cracked plaster) in Boulevard, Jamul, La Jolla, La Mesa, Warner Springs. Intensity V in Aguanga, Campo, Chula Vista, Del Mar, Descanso, El Ca jon , Escondido, Julian, Leu-cadia, Oceanside , Pala , Ramona , San Diego, San Ysidro. - USEQ. 1956 February 9. 6:32 A.M. El Alamo, Baja California (31.7 N, 115.9 W, Mt = 6.8). Maximum acceleration .013 g at San Diego. Intensity VI Ecracked plaster reported) in Campo, Chula Vista, El Cajon, Imperial Beach, La Jolla , La Mesa , National City, San Diego, San-tee. (In many of these places the damage, though present, was described as " slight"). Intansity V reported from Cardiff, Carlsbad, Del Mar, Escondido, Lake Hodges , Leucadia , Pala, Poway, San Marcos, San Ysidro. - USEQ. 1964 December 22. 12:54 P.M. Northwest of Ensenada (31.8 N, 117.1 W, M = 5.6). Maximum acceleration .034 g at San Diego. Intensity VI ab Imperial Beach, La Mesa, National City, San Diego. Intensity V at Campo, Chula Vista , El Cajon, La ? :la, San Ysidro. - USEQ. 1968 April 8. 6:29 P.M. Borrego Valley (33.2 N, 116.1 W, M 6.4). Maximum acceleration .029 g at San Diego. Intensity VHL =in the Borrego Mountain - Ocotillo Wells area. Intensity VI at Alpine, Borrego Springs, Campo, Chula Vista, Del Mar, El Cajon, Encinitas, Escondido, Julian, Lakeside , Leucadia , Ramona, San Diege, San Yri-dro. Intensity V in La Jolia. - USEQ. 136
REFERENCES Agnew, Duncan Carr, 1978, The 1852 Fort Yuma earthquake - two additional ac counts: Seismological Society of America Bulletin, v. 68, p. 1761-1762. Agnew, Duncan Carr, and Sieh, Kerry, 1978, A documentary study of the felt effects of the great California earthquake of 1857: Seismolog-ical Society of America Bulletin, v. 68, p. 1717-1729 Balderman , M. A., Johnson, C. A., Miller, D. G., and Schmidt, D. L., 1978, The 1852 Fort Yuma earthquake: Seismological Society of Amer-ica Bulletin , v. 68, p. 699-709. Bancroft, Hubert Howe, 1888, The History of California. San Francisco, The History Company. Bloom, L. B., 1945, From Lewisburg to California in 1849 - notes from the diary of William H. Chamberlain: New Mexico Historical Review,
- v. 20, p. 249.
Clark, Jane T. , 1944, Index to descriptive catalog of earthquakes of the Pacific coast of the United States, 1769-1928: Seismological So-ciety of America Bulletin, v. 34, p. 35-62. Couts, Cave, 1932, From San Diego to the Colorado in 1849: the journal and maps of Cave J. Couts. William McPherson, ed. Los Angeles, Arthur M. Ellis. Dawson, Muir, 1950, Southern California newspapers 1851-1876: Historical Society of Southern California Quarterly, v. 32, p. 5-40, p. 139-174. Engel hard t , Zephyrin, 1920, San Diego Mission. San Francisco. Hileman , J. A. , Allen , C. R. , and Nordquist , J. M., 1973, Seismicity of the southern California region, 1 January 1932 to 31 December 1972. Pasadena, California Institute of Technology. Holden, Edward S., 1898, A catalogue of earthquakes on the Pacific coast: Smithsonian Miscellaneous Collections 1087. Richter, C. F., 1958, Elementary Seismology: San Francisco, W. H. Free-man . Roc kwood , C. G. , 1878, Recent American earthquakes, American Journal of Science, v. 115, p. 22-25. 137
Simons, Richard S., 1977, seismicity of San Diego, 1934-1974: Seismolog-ical Society of America Bulletin, v. 67, p. 809-826. Toppozada , T. R. , Parke , D. L. , and Higgins , C. T., 1978, seismicity of California 1900-1931: California Division of Mines and Geology Spe-cial Report 135. Townley, S. D., and Allen , M. W. , 1939, Descriptive catalog of earth-quakes of the Pacific coast of the United States, 1769-1928: Seismological Society of America Bulletin, v. 29, p. 1-297. Whipple, A. W., 1850, Extreet of a journal of an expedition from S .n Diego, California to the Rio Colorado from September 11 to December 11, 1849 31st Congress, 2nd Session, Senate Executive Document 19 138
THE 1862 EARTHQUAKE IN SAN UIEGO by Mark Legg and Duncan Carr Agnew Institute of Geophysics and Planetary Pt ysics Scripps Institution of Oceanography University of California La Jolla, CA 92093 fhe earthquake of May 27, 1862, is of special importance in making seismic risk estimates for San Diego. Most earthquakes that have caused damage in San Diego have been located in the Imperial Valley or northern Baja California. Although its location is not completely determinable, the 1862 earthquake seems to have been closer to the San Diego metropol-itan area. This earthquake serves as a good example of the fragmentary nature of the historical record of California earthquakes: despite its size, it has received almost no mention in earthquake catalogs. The purpose of this paper is to give a trief description of the earthquake, based on contemporary documents. A list of these is given at the end of the paper; the numbering of that list will be used in the citations. The main shock occurred at about noon on May 27, 1862. Two accounts (1,2) say that there were two shocks separated by some minutes, the second being the stronger, in San Diego (the present Old Town) this shock stopped clocks and upset bottles and tumblers (1,2,4,6) so that "many sets of crock-cry were demol i shed" (2) . The bell at the Army depot was set ringing (2). Understandably enough, all the people ran outside in fright (1,2,4),and many slept outside because they feared further earthquakes (1,7). There apparently were no injuries, and no buildings were destroyed. However, many of the accounts (1,2,4) mention damage to buildings, primarily cracking; i t should be remembered that in 1862 imst buildings in San Diego were either adobe or poor masonry. 'he newspaper repc.*ts say that "various adobe houses" (4) were " cracked throuah and through". Some specific examples were the Pico adobe, which sustained several cracks, one passing through the wall; the Bandini adobe; and the 2-story Fitch adobe, which was "much sprung on its side wall" (1). Of masonry buildings, the Whaley house cracked in several places, and the lighthouse tower suf fered several cracks, one going through the wall (1,4). However, the light was not thrown out of adjustment, nor was any glass broken (1). Some f rame buildings were racked so that windows and doors were 139
loosened in their frames (1), and windows and door hinges were broken. Several accounts (1,2,4) mention cracks in low ground near the San Diego River, which washed over its banks (1,4). At La Playa (on Point Loma) , cracks formed on the beach, water came out of the sand on the tidal flats, and a piling that had just been driven into the mud was shaken loose. Some bluff banks on the east side of Point Loma collapsed (1, p. 711, 4). This shock was felt in Los Angeles, where i t wa s termed " light" (5) and in Anaheim (1). At Temecula and Aguanga it rattled plates on the shelves; at Aguanga it also caused a pile of sacks to fall over (1). At Mesa Grande it seemed to last about 10 seconds, and caused the building roof to creak (1). I t was also fel t at Lake Henshaw, El Cajon, Carlsbad, Rincon del Diablo, Vallecito, San Luis Rey, San Dieguito, San Felipe, and the Cuyamacas (1,4), it was not fel t at Fort Yuma (4). There are two lists of aftershocks (1,6). These show that earthquakes were felt every day at San Diego up to June 8,12 days af ter the first shock, and relatively frequently for the rest of June (see Agnew e _t_ a_1. , this volume). An aftershock at 10 a.m. on May 29 was described as " violent" at San Diego (6), and was felt at San Ysidro and Anahein (1). One in f.he af ternoon of May 31 was felt at San Diego (6) and Temecula (1). A relati-ly large aftershock occurred at 10:30 a.m. on June 13; it was strong at So., Diego (1,6) and also felt at Penasquitos (1), but was not generally felt in Los Angeles or San Bernardino (1). Based on the descriptions given be, we estimate that this earthquake caused shaking in San Diego of about intensity VI to Vil on the Modified Mercalli scale. The upsetting of smi.ll objects and the extent of building damage (cracking but no serious damage) both suggest intensity VI, although the damage seems to have been greater than that associated with intensity VI shaking from more recent earthquakes, such as the 1968 Borrego Mountain event. Ground cracking is usually associated with intensity Vill, but this is certainly too high, Judging by the ef fects on buildings. It is possible that this reflects higher intensity on marshy ground. There is not enough information to estimate intensities elsewhere, except that they were lower, possibly IV-V, in the Temecula-Aguanga area, and lower still in Los Angeles. The distribution of intensities suggests that San Diego was closer to the epicenter than any other place for which we have reports. This seems 140
to be confi rmed by the af tershock records , though there is an obvious bias because San Diego is the orly place for which there is a contin-uous record. That the carthquake was not felt at Fort Yuma would seem to rule out a source in the Imperial Valley. On the whole, a location south or west of San Diego seems most likely; in the absence of more information we can say little else. SOURCES (1) Benjamin Hayes, " Emigrant tiotes", MS CE-62, Bancrof t L ibrary , Berkeley. (In the Bancrcf t Library pagination, pp. 690-697 are transcripts of a diary for May-June 1862; pp. 709-710 is an account of the 1862 earthquake; p. 711 is a memorandum f rom Andrew Cassidy describing the effects of the earthquake at La Playa). (2) San Francisco Daily Alta Cali fornia, June 17, 1862, p. 2. (3) San Francisco Daily Alta California, Ju a 18, 1862, p. 1, col. 3 (4) Los Angeles Star, June 21, 1862, p. 2, col. 2. (5) Los Angeles Semi-Weekly Southern flews, fiay 28, 1862, p. 2, col. 2. (6) Weather Records, New San Diego. (in the Climatological Records of the Weather Bureau, Record Group 27, U. S. flational Archives) . (7) Letter of Augustus Ensworth to Cave Couts, May 28, 1867, e.t Diego. (Couts Collection, Hunt;ngton Library, San Marino). 141
e% v W\\/ 1 'kamba 142
REGIONAL METEOROLOGY by Donald I. Eidemiller Department of Geography San Diego Stata University San Diego, CA 92182 Geologic hazards such as flooding, landslides, silting, and erosion a re i n mos t instances caused by meteorological conditions associated with the seasonal weather regimes occurring within an area. In San Diego County, it is the distribution of winter precipi tation associated wi th mid-lat i tude cyclonic circulations, and occasional late summer tropical cyclones that a re principally responsible for these hazards. San Diego County lies on the sou thern edge of d ry summer , sub t ropical North Ame ri ca. From wes t to east, a n umbe r o f c l i na t i c zones ranging from steppe to humid to arid are found within a relatively short linear distance (Figure 1). a.,. , .-------- , . . . . ... ... ,.
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143
The coastal terraces have a mild climate and one of the lowest seasonal temperature ranges within the continental United States (averages of 13 C in January and 220 C in August). Semiarid (steppe) conditions prevail with annual precipitation varying from 9 to 10 inches (229-254 mm) , mos t of i t falling in winter. Moving east, interior valleys and foothills are more typi-cally Mediterranean, as they have a wider seasonal temperature range and annual rainfall measuring f rom 11 to 24 inches (279-610 mm) . The annual pic-ture is one of low humidity and relative mildness. On the highest summits of the Peninsular Ranges precipitation is more abundant. Crests of the Laguna and Palomar Mountains average nearly 40 inches (1016 mm) a year. Winters are relatively cold and total snowfall averages 2 to 3 feet (60-100 cm). Summe rs are warm. On the eastward side of the mountains aridity becomes the key word in expressing the climate. Hot summers and mild winters are characteristic of this mid-latitude desert (Figure 1). Seasonal Summaries Summer. During the summer months mid-latitude cyclones track too far north to influence the San Diego County area. The eastern Pacific subtropical high pressure cell becomes nearly stationary approximately 1000 to 1500 nautical miles (1800-2700 km) northwest of the southern California coast. The anti-cyclonic (clockwise) flow of air around this high results in persistent northwest winds over the San Diego County area lying west of the Peninsular Ranges. This flow is enhanced by the presence of a heat-induced surface " thermal trough" of low pressure lying over the desert regions of interior California and Arizona. Low clouds and fog become prevalent over coastal southern California on almost a daily basis as a result of this onshore flow of marine air. Due to the above-mentioned meteorological conditions, the areas of San Diego County lying west of the mountains are normally f ree of storm activity during the season. During summer, the desert areas east of the mountains are normally covered by continental tropical air at the surface and capped by very dry superior ai r aloft. This situation produces an extrenely arid climate, but when this combination is occasionally disturbed by an easterly flow of moist tropical air, thunderstorm conditions are created. That is, extremely high temperatures and high humidities occur in the mountains and deserts, and high level thunderstorms appear over the eastern slopes of the Peninsular Ranges where heating and orographic lif ting are at a maximum. These storms can result in flood producing rainfall over limited areas. 144
An easterly flow of moist tropical ai r occurs under the following meteorological conditions. (1) A low pressure center over Baja California or northwestern Mexico will produce an upper level casterly flow into the San Diego area. Moisture will be advected f rom the Gulf of California. If the upper level flow has sufficient east to west strength, thunderstorms created under these conditions have been known to move westward toward the coast af ter their development over the desert and mountains. Normally, thunderstorms will not occur along the coast because of the stability of the atmosphere and the absence of sufficient coastal hcating needed to produce large convective cells. (2) Tropical storms usually tend to dissipate long before reaching the latitude of San Diego County. However, impulses of moist tropical air f rom these storms are advected into higher latitudes at upper levels resulting in an easterly flow, and in the appearance of middle and high clouds with possible showers and thunderstorms occurring. This type of circu-lation can cause flooding on a limited scale. The annual probability of a tropical storm reaching the San Diego area is between 5% and 10% (Eidemiller, 1978). The most notable example of one of these storms occurred on September 10, 1976 when tropical storm Kathleen moved across northern Baja California, Mexico, and the Imperial Valley of southeastern California. This was the strongest tropical cyclone to reach southern California so far this century. Despite Kathleen's high winds and low pressures, it will probably be remembered best for the severe flash flooding that resultea. Rains began over the desert on September 9, then spread northwest to the coast by evening. Heavy runof f and rapidly rising streams prompted flash flood watches in many areas of the southwestern United States. Nearly all mountain areas from Los Angeles to San Diego County had over 5 inches (127 mm) of rain with the highest peaks (Laguna Mountains) . eceiving greater than 10 inches (254 mm). The rapid flow of extremely moist trop. cal ai r up the eastern and southern slopes of the mountainous terrain created a, ideal situation for very heavy rains, which resul ted in heavy flooding in the normally dry desert areas of San Diego County. Precipitation was not high west of the mountains. Two tropical storms weaker than Kathleen entered this area in 1977 and 1978, but neither created any significant problems. 145
No tropical cyclone of hurricane intensity has occurred within the San Diego area during the past 109 years. The closest approach was by hurricane Katrina (September 1967) which came ashore at the northern end of the Gulf of California, but was shortly thereaf ter downgraded to a trop-ical storm. With probabilities of less than 1%, hurricanes are not consi-dered a serious threat to San Diego County, but should one ever occur, heavy flooding and related damage would be expected. Fall. Fall can be described as a transitional period between the summer stratus regice with occasional impulses of tropical air, and the winter frontal season. During the fall stratus become less prevalent , and the occurrence of large-scale of fshore flow becomes more f requent, generating some of San Diego's most severe heat periods (Santa Ana winds). Storr s of suf fi-cient intensity to produce flooding and related damage are extreme 'y rare during the fall season. Winter. Tracts of upper level troughs and frontal systems begin to assert their influence f arther south with greater frequency and intensity during winter. Three di f ferent cyclonic (counterclockwise) patterns can influence the San Diego County area during this season.
- 1. The first type is characterized by an active surface cold f ront which passes rapidly through southern California. A relatively strong upper level cold trough accompanies the surface ci rculation, but the t rough remains wel l to t.c north and east of San Diego (Figure 2). Precipitation produced by this type of cyclonic ci rculation can vary f rom light to mode rate. No particular hazards are usually associated with this type of frontal activity.
- 2. The second type occurs when cyclonic ci rculations are created at latitudes south of 35 N in the castern North Pacific. This may result in a flow of warm, moist, unstable air to the coast of California, causing very heavy, topographically influenced rainfall. Fronts are sometimes embedded in this southwestern flow, which then permits the San Diego County area to receive the full effect of the warm and cold f ront passages, as well as the effect of orographic lif ting (Figure 3). Outstanding examples of this type of cyclonic circulation which caused flooding and other problems occurred between December 18-27, 1921; February 10-18, 1927; February 14-March 5,1941 ;
Harch 10-15, 1941; and January 18-27, 1969 146
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- 3. A prolonged period o^ heavy rainfall may occur in this area whenever the thi rd type of cyclonic circulation f requents the eastern North Pacific.
When a new short wave impulse of cold air enters an upper level trough in the Wes te rl ies , it has a twofold ef fect. First, it causes the trough to deepen, which results in a strong southerly flow of moist air alof t along its south-eastern margin. Second, it retards the eastward movement of surface lows and accompanying f ronts (Figure 4). This quasi-stationary circulation extends the duration of precipitation f rom a period of hours to several days. Examples of these weather conditions occurred on the following dates: January 14-19 and 24-29, 1916I ; Janua ry 16-18,1952; March 7-17,1952; December 31, 1976 through January 8, 1977; January 6-8 and 17-20, and February 6-14, 1978; March 1-6 and 10-12, 1978; January 15-19, 1979; January 31 - February 3 and 21-24,1979 l r. f '
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ll '. ' \ $' ~ ) -/ Figure 4a. Quasi-stationary surface Figure 4b. Deep upper-level trough low off the central California and closed low induces a south-coast extends duration of pre- erly flow of moist air at all cipitation in southern Califor- levels over southern California nia f rom hours to days. (Surface (500 millibar Chart - 00Z, Chart - 00Z, 5 March, 1978). 5 March, 1978). I Charles M. Hatfield, the " Rainmaker" is credited by many for the large amount of precipitation which fell during this period. However, it was merely coincidental that he was in the San Diego area at a time when the quasi-stationary type of circulation was present. This weather pattern produce'd record-breaking amounts of rain throughout most of the western United States in 1916. 148
Spring. Spring is similar to fall in that it is also a transition period. Less precipitation is expected as the tracts of upper level troughs and surface cyclonic storms begin retreating farther north. This allows the subtropical anticyclone to reintensify, establishing once again the stratus regirre of late spring and summer. Sultt%RY Flooding, landslides, erosion and relaced problems are most apt to occur in the San Diego County area under the following meteorological con-ditions: the infrequent invasion of tropical circulations during summer, and the occasional winter occurrences of (1) open wave, mid-latitude cyc-lones developing further south than normal, and (2) the combination of quasi-stationary surface and upper level circulations. REFERENCES Eidemiller, D. I., 1978, The f requency of tropical cyclones in the South-wes tern Uni ted S tates and florthwes tern itexico: Tempe, Arizona Labo-ratory of Clirutology, Arizona State University Clirutological Publi-cations , Scienti fic Papers ilo, l. U. S. Departrent of Commerce, Ai r Weather Service, ESSA, and NOAA, 1939, et seq: Daily Series Synoptic Weather 11aps, Part I, Northern llemi-sphere Sea Level and 500 ftillibar Charts 1899-1939, U. S. Depart-ment of Commerce, Vashington D. C. 149
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FLOODS AND CHANGING STREAMS by Howard H. Chang Department of Civil Engineering San Diego State University San Diego, CA 92182 INTRODUCTION Streams in San Diego County, like those in nest of the semi-arid south-west, are epheme ral in nature. But unlike many other ephemeral streams, they are usually more di.3turbed by such human activities as damming, sand mining, and bridge and highway construction accompanying rapid urbanization in the river envi ronment. Such changes distort the natural equilibrium of streams and in the process of restoring equilibrien, streams undergo changes. Streams will adjust to new conditions by changing their cross-sectional shape, slope, roughness, bed-material size, or meandering pattern. Within the existing constraints, any one or a combination of these characteristics may adjust as streams seek to maintain the balance between their ability to t ra ns po r t , and the load provided, in this paper, severci case histories of changing streams will be described, a computer model for estimating stream erosion and sedimentation will be introduced, and finally, some social impacts of " changing streams" will be outlined. FACTORS RESPONSIBLE FOR STREAli CllANGES AND CASE HISTORIES for a natural stream, the slope, cross-sectional shape and plan con-figuration, are delicately adjusted to provide just the veloci ty required for the transportation of the load supplied from the drainage basin. The concept of graded stream as introduced by liackin (Mackin, 1948) is a condi-tion of equilibrium in streams serving as agents of transportation. With the constant changes in water discharge and other parameters, the final equilibrium is never fully attained in natural streams, although each channel is continuously readjusting i ts condition toward equilibrium. Environmental changes may occur naturally, as in the case of climatic vari-ation or changes in vegetative cover due to forest fires. What is unique in San Diego County is changing streams caused by human activities. This paper will focus on this aspect of changing streams. ISI
One major factor contributing to stream aggradation and degradation in sand mining in the stream bed. River sand deposits are formed by natural processes of hydraulic sorting wherein the clays and fine silts are washed away as suspended load during floods leaving sand in the stream bed. River sand is the most abundant mineral resource in the county. To meet the demand for rapid urban growth, sand is being mined from the stream bed at an increasing rate, leaving scattered borrow pits in nearly all major streams. During a flood, the borrow pit usually acts as a sediment detention basin in which most of the bed load (sand) settles. As the flood water leaves the pit with its bedload depleted, it picks up new sand from the stream bed to satisfy its transport capacity and thus causes erosion down-stream f rom the pi t. Examples of stream-bed erosion due to sand mining are shown in Figures 1 and 2. A borrow pit can also induce stream bed erosion upstream from the pit during low flow; this is because a borrow pit lowers the base level of an inflowing stream when the water level in the pit is low, in reaching this lower base level, the inflowing stream cuts into the stream bed to form a gully which extends gradually upstream in the process of erosion. This type of gully formation is shown in Figure I where a gully forned in the winter of 1976-77 upstream f rom a large borrow pit. The sand carried into the borrow pit was deposi ted as a del ta (Figure 2). There are nunerous other case histories of stream-bed erosion caused by stream-bed mining. Examples f rom the San Diego River are shown in Figures 3 and 4. The photograph in Figure 3 was taken in the spring of 1978, soon af ter a major flood of the magnitude of a 10 year flood. Deep scour lowered the stream bed under the Highway 67 bridge north of Lakeside and thus con-nected sizeable borrow pits which were present on both the upstream and down-stream sides. The erosion reached a depth of around 10 feet (3m) and en-dangered the safety of the bridge which caused its being closed to traffic until repai rs were made. The photograph in Figure 4 was taken in February, 1979, at the Magnolia Avenue bridge crossing of the San Diego River in Santee. This brioge, completed in 1977, had its pile caps buried deep in the sand bed. Erosion has since exposed the pile caps as well as the piles. Another important factor contributing to stream bed erosion in the San Diego area is the encroachrent on stream channel width by bridge embank-152
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nents. As shown in Figure 5, the Via de Santa Fe bridge south of Rancho Santa Fe, on the San Diegui to River was built with long road embankments in order to reduce the bridge span. These embankments have seriously en-croached upon the natural width of stream flow, resul ting in accelerated flow velocity and accompanying erosion. ticasurements made in the spring of 1978 showed a maximum erosion depth exceeding 15 feet (4.5m). In this particular case, crosion was not caused by encroachnent alone as borrow pits also exist in the river bottom. C011PUTER SlitULATION OF STREAft CilANGES DURlflG A FLOOD A computer program has been developed to simulate stream changes during the passage of a flood (Chang and Hill, 1976). The flow diagram showing the najor steps of computation is shown in Figure 6. In using this program, a river reach is defined by a series of cross-sections. The progressive vari-ations of the strean bed and water-surface profile is first computed. Then, sediment discharges at all sections are computed using a sediment formula, such as the Engelund-ilansen fo rmu l a , the modified Einstein formula, etc. The difference in sediment movement rate at adjacent sections indicates stream change: greater inflow of sedinent causes deposition while greater outflow causes crosion. The amount of change for each time increment is computed and applied at each cross-section to obtain the corrected stream-bed profile. This iteration continues until the desi red tire period is covered. The potential stream changes in the San Dieguito River near the Via de Santa Fe bridge have been sinulated using this computer program during the passage of the 100 year flood. Figures 7a and 7b show respectively, channel topography before the storn, and during the peak flood predicted to undergo erosion as deep as 25 feet (8u). Eros ion of this magni tude would resul t in totei exposu e of the piles with a high probability of the bridge being washed out. During this extrere process of stream change, borrow pits in the stream slowly disappear as shown in Figures 7a and 7b. Figurc 8 shows the variations of sediment discharge at dif ferent time intervals as part of the computer output. During the early intervals, the sediment discharge shows great variations alc,9 the channel because of the nonuniform channel profile due to the encroachment and borrow pits. But as time increases, the sediment discharge becomes more uniform as the channel profile undergoes adj ustments. After the tenth hour, the sediment discharge 155
7 croachment y" ' iN~ ~.nto the San Diegulto River at the Via de V.,f Santa Fe bridge cross-tv ing, south of Rancho Santa Fe. J
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becores quite uniform, indicating that the equilibrium channel profile has been nore or less reached. SOCIAL lMPACTS OF STREAM CHANGES Envi rencental cons i de rat ions requi re eval uat ion of st reams in their natural or existing conditions. This usually reans precisely delineating the limits of a 100 year flood. Flood hazards are reduced by restricting developrent wi thin the floodway. National flood plain mapping, under a Fede ra l insurance Agency program has been going on for the last 10 years; one of i ts goals is to develop precise maps showing the limits of 100 year floods along major stream courses. Presently, flood levels are determined using computer programs which assume rigid channel boundaries during floods, therefore the effects of erosion and sedimentation in streams are not con-sidered. While rigid boundary assumptions are quite valid for neny streams, they are not accurate enough for " changing streams." For example, as a result of stream bed erosion that occurred in the winter of 1978, the 100 year flood level in the San Diego River channel near the Magnolia Avenue bridge was lowered by about 4 feet (1.3 m) in compu-tations using the newly eroded (post 1978) s tream profi le. Under the old (pre 1978) computed flood level, large overbank areas were subject to inunda-tion, making development extremely expensive. Using the rigid boundary assumption, the flood level near the San Dieguito River inuth in Del Mar is computed to be high enough to cause ex-tensive flooding of several hundred homes. These homes have never been flooded in historical times, even before the completion of rajor dans up-stream. This is because during each previous major storm , , sand bar blocking the river routh was invariably renoved by the flood water, resul-ting in a larger opening and lower flood level. The Federal Insurance Ad-mi ni s t ra t ion has become awa re of "changi ng s t reams" and i ts impact on flood plain mapping. Hopefully, studies will be started soon to adopt a novable bed program for use in selected stream courses. REFtRENCES Chang, H. H., and Hill, J. C., 1976, Computer codeling of crodible flood channels and deltas: Journal of the Hydraulics Division, Proc. American Society of Civil Engineers, v. 102, HY10, p. 1461-1477. Mackin, J. H., 1948, Concept of the graded stream: Geological Society of Ameri ca Bul le t in , v. 59, p. 463-S12. 158
THE 1916 FLOODS IN SAN DIEGO by Patrick L. Abbott Department of Geological Sciences San Diego State University San Diego, CA 92182 INTRODUCTION Noteworthy floods occur in the San Diego area on an unpredictable time scale. Until the last three Scars, no floods capable of generating widespread concern occurred for about four decades. However, in southern California, wetter than average years tend to follow dry years in irregular cycles averaging about 12 and 15 years duration, respect i vely (Ganus ,1977) . Recent wetter than normal years, with accompanying flooding, have height-ened local i n te re s t in floods. Since the biggest events usually attract the nost attention, the intent of this article is to review the parameters of San Diego's severes t historic floods , those of January , 1916. When evaluating the local flood records , an interesting relationship appears -- that is, there is a lack of correlation between . major floods and annual rainfall (rainfall years run from July 1 and June 30). For example , at the downtown San Diego rain gauge, 12.55 inches (319 mm) of precipitation was recorded for the 1915-1916 rainfall year yet had a peak flood on the San Diego River that was about 450 percent larger than the largest flood during the 1921-22 rainfall year, during which 18.65 inches (473 mm) of precipi-tation was recorded at the same station. The largest floods in San Diego history have occurred when heavy rains have fallen en ground al ready satu-rated by a recent storm. On the other hand, if the precipitation events were spaced far enough apart. then the typical high evaporation losses in the area dried the soil. Empty pores in the dried soil were then able to accept large quanti ties of water f rom subsequent storms, wi th a resul tant reduction in peak flood height. METEOROLOGICAL CONDITIONS lH JANUARY, 1916 U. S. Weather Bureau reports (1916) for San Diego describe two very wet weeks occurring f rom January 14 to 20, and f rom January 24 to 30,1916. During the first week, a low pressure air mass crossed the coastline in central California on the 17th, slowed down over southern California and 159
yielded heavy rains, then noved northeast on the 19th. Because light rains had fallen during the previous few days, the heavy rains of the 17th and 18th caused severe flooding. Meteorological conditions that produce extended periods of heavy rainfall are described by Eidemiller (this volume) as cyclonic ci rcu-lation in the eastern florth Pacific made quasi-stationary by an upper level im-pulse of cold air. The second major occurrence of this conditio1 in January, 1916, caused very heavy rains on the 26th and 27th. This downpour settled on already saturated ground and nearly-full reservoi rs which resul ted in record setting peak discharges. RAllirALL Af1D RUl10FF During the 17 days, January 14-30, 1916, the downtown San Diego rain gauge recorded 7.08 inches (180 mm) of precipitation compared to the average a_nnual precipitation (1912-1975) at that station of 9.79 inches (249 mm). In general, p reci pi ta tion in San Diego County increases eastward, from the coast to the mountains , and thus , away f rom the downtown area. The unusually heavy rainfall from these two closely-spaced storms is shown for selected stations (Table 1). Other rainfall data, helpful in understanding the 1916 floods are those of mean precipitation over some of San Diego County's major drainage basins during the January 14-30 interval (Table 2). TABLE 2. HEAF 1 DRAltlAGE BASlfl PRECIPITATl0ll Drainage basin lican precipi tat ion Rainfall ange area in basin East West San Diego River 434 Sq. miles 19.9 inches 7.08 inches 32.34 in. (112,400 hectares) (505 m) ( lo mm) (821 m) downtown Cuyamaca Reservoir Sweetwater River 181 sq. miles 21.8 inches 8.55 inc.hes 28.68 in. (above Sweetwater (46,880 hectares) (554 mm) (217 m) (728 cin) Dam) Sweetwater Descanso Dam Otay River 99 sq. miles 19.2 inches 9.39 inches 25.45 in. (above lower Otay (25,640 hectares) (488 mm) (239 m) (646 cin) Dam) Lower Otay Dulzura Dan Significant flood crests occurred on the 17th and 18th due to runoff from the first major storm system (week of January 14-20) . The second storm sys tem 160
TABLE I. DAll.Y PRECIPITATION R[CORC$ JANUARY, 1916 tievation g H 16 17 18 19 20 21 22 y 2 4,, 2J g ,2,], g 2), g San Diego River basin 87 feet near costh - inches .53 .09 95 1.55 .31 .80 -- -- -- -- tr .21 .22 2.19 .06 .17 --
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.03 .23 1.63 1.53 8.54 1.30 1.12 --
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- inches -- 2.12 1.02 6.36 4.93 1.33 67 -- -- - .02 38 73 3.11 '.07 .09 .80
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(week of January 24-30) dumped less total water than the fi rst, but had the highest single-day rainfall (January 27). The rains of the second wet week fell on saturated ground and the resultant runoff set the historic records for San Diego area ri ve rs (Figures 1 & 2). Summaries of runoff and discharge data are shown in Table 3. TABLE 3 RUNOFF AND DISCHARGE DATA Peak discharge on Jan. 27 Total runoff (cubic feet /second)i (ac re-feet )? San Diego River 75,000 213,000 (J a n . 1-31) Sweetwater River 45,500 110,000 (Jan. 16-31) (at Sweetwater Dam) Otay River 23,500 -- -- (at Lower Otay Dam) il cubic-foot /second(cfs) = 7.5 gallons = 28.4 liters 1 acre-foot = 326,000 gallons = 1.234 kiloliters On J anua ry 2 7, 1916, the peak discharge of 23.500 cfs was overtopping the 40,000 acre-feet capacity Lower Utay Reservoir. A 'arning ,15 sounded through-out the Otay River valley to evacuate to higher ground -- nost people heeded the warning. Sapping at the base of the dan was roving boulders , and at 5:05 p.m. the stress on the dan was so great that the s teel core tore from the tra at its center and the dam opened outward like a pai r of gates. It took 2.5 hours for the rese rvoi r to evacuate (Figure 3). The wall of water was variously described as from 6 to 20 feet high (2 to 6ra), it moved the 10 miles (16 kn) to the southern end of San Diego bay in 46 minutes. HATFIELD THE RAINMAKER A fascinating personal s tory un folJed along eith the floods -- the Saga of Charles Hatfield. His 1916 ra i nnaking ef fo rts acre the basis for the 1957 film, "The Rainnaker". In 1915, San uiego was s u f f e r i n c, c ro, a aater shortage as the main reservoir kept dwindling for lack of replenis ment. Hatfield appeared be-fore the City Council and declared that he could fill the rese rvoi r to overflowing wi thin a year if guaranteed $10,000 upon his success. Since there was no 'ee due if he failed, the City Council apparently reasoned that it was a 'no-loss" situa-tion; they okayed the deal, provided Hatfield coul < ce he caused the rains 162
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should they materialize. Shortiy af ter his tower and vats were constructed and in operation. the January 14-20 and 24-30 storms tracked through San Diego unleashing thei r record runof fs. Twenty-two persons drowned in San Diego County, mostly in the Otay River valley after the Lower Otay Dam failed. Following the floods, the San Diego City Counci1 was faced w!th Iawsuits total 1ing $3,500,000. When Hatfield's lawyer pursued his client's fee the Council agreed to pay i f Hatfield would acknowledge causing the rain and, hence, the floods and resultant damage. On balancing the credits and debits of this transaction, Hatfield decided to forego his $10,000 guarantee. Despite the loss of his fee, and the unfortunate floods, Hatfield considered the 1916 rains to be the never-again obtainable zenith of his rainmaking career. He then switched to another line of business and apparently " produced" no other rains throughout the remaining 42 years of his life. FLOOD COMPARISONS AND EXPECTATIONS lt is interesting to compare the peak flow of the 1916 flood with the flood crests experienced during January, 1978 and January, 1979 These most recent runoff events have caused appreciable economic losses and shortened tempers, yet the volumes of water conveyed to the ocean pale when compared to the 1916 runoff (Table 4). TABLE 4. PEAK DISCHARGE OF SAN DIEGO RIVER (in lower part of the valley) January 27, 1916 75,000 cubic-feet /second January 15, 1978 15,000 cubic-feet /second January 31, 1979 17,000 cubic-feet /second When contemplating the photograph of the 1979 flood in the lower San Diego River valle 't is sobering to multiply that volume of water by five to esti-mate the ef.-.c of a 1916-size flood (Figure 4) . Can it happen again? Or have conditions in the San Diego River basin changed such that it could not happen again? It is comrnonly believed that the threat of flooding along the lower San Diego River has been substantially reduced since completion of the San Vicente and El Capitan reservoirs in the late 1930's. This is certainly true for any storm that occurs when water levels in the reservoirs are low. Howeve r , given a replay of the 1916 rain sequence, the fi rst storm would likely fill, or 164
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nearly fill the re se rvoi rs , leaving little or no flood-storage capacity for a second major storn arriving shortly after the first. Another signi ficant change in flocd runoff characteristics is the ef fect of urbanization on peak discharge. San Diego development has en-tailed the usual laying of pavement for streets and erecting roofs for buildings. Thus, much of the ground in the urban areas is covered and unable to accept the fallen rain into its pores. Studies of runoff following urban-ization in other areas have shown that for a given rainfall pattern, peak discharge nay be more than three times as great (Leopold, 1968; Young, 1975). In the absence of quantitative studies describing the changed conditions in the San Diego area, one can only speculate on the Jegree of flooding given a rerun of the 1916 rainfall sequence. RE FE REllCES Eidemiller, D. l., 1979, Regional meteorology, in, Abbott, P. L., and Elliott, U. J., (eds.), Earthquakes and Other Perils, San Diego Region: San Diego Association of Geologists Guidebook. Ganus, W. J., 1977, is southern Cali fornia ready for a wet period? , in , Abbott, P. L. , and Vi ctoria, J. K. , (eds.) , Geologic Ilezards in San Diego: San Diego Society of flatural ilistory, 96 p. Leopold, L. B., 1968, liydrology for urban land planning: United States Geologi-ca: Survey Circular 554. Lockwood , fle rbe rt , 1972, The Rainmaker, in, Fallout from the Skeleton's Closet: Uailey and Associates, San Diego, v. 1, p. 71-75 licGl ashan , H. D., and Ebert, F. C., 1918, Southern Cali fornia Floods of January, , 1916: United States Geological Survey Water-Supply Paper 426, 80 p. U. S. Ucather Bureau, 1916,lionthly Weather Review, v. 44, no. 1. Young, I; . P., 1975, Geology, the Paradox of Earth and fian: lloughton-ftifflin Co., 526 p. 166
LANDSLIDES AND DEBRIS FLOWS IN SAN DIEGO COUNTY, CALIFORNIA by Michael W. Hart Geocon, Inc. San Diego, CA 92111 INTRODUCTION in the past decade, geologists in San Diego County have made sig-ni ficant progress in the identification and analysis of ancient land-slides and mudflows. Previously, landslides in San Diego were relatively unknown. Primarily responsible for the recent advances in understanding "the secrets of mass-wastage" has been the rapid population growth during the past 10 years which forced developnent in marginally stable canyons and hillsides bordering and incised into the broad, flat, coastal terraces. This rapid urban expansion onto hillsides followed similar growth patterns in the Los Angeles and Orange County areas to the north. Because most of the ancient landslides lacked the classic features described in textbooks , the impending problems went essentially undetected for some developnents. The presence of large-scale landslides was suspected for several years by some geologists working in the region, but it was not until grading operations dissected some large landslides that the proof was over-whelming. Now, only 11 years after the first documented, large, ancient landslides, San Diego geologists routinely recognize the significance and extent of this hazard to developnent. The western part of San Diego County is characterized by a series of broad and extensive Pleistocene marine terraces, capped with marine con-glomerate and sandstone, deposited during regression of the Pleistocene sea. The terraces, are underlain by gently westerly dipping sedimentary rocks of mostly Eocene, Miocene, and Pliocene ages. Sone coastal areas underlain by Cretaceous sandstones and shales occur f rom Point Lone, at the west end of San Diego Bay, to La Jolla (Figure 1). Most of the larger landslides occur in clay-rich Eocene and Miocene rocks. Only on Mt. Sole-dad, a large anticlinal feature located east of La Jolla, does the piesence of steeply-dipping, bedding surfaces contribute to the landsliding process. 167
The sedimentary rocks described in detail by Kennedy and Moore (1971), occupy a relatively narrow coastal strip extending inland to a position approximately parallel to Interstate 15 in the north, and the Santee / San Ysidro areas in the south (Figure 1). The remainder of San Diego County, with the notable exception of a portion of the Anza-Borrego Desert, is underlain by Cretaceous granitic rocks and prebatholithic metavolcanic and metasedimentary rocks. Many large, ancient landslides and mudflows also occur in this terrane, but they have not been as thoroughly studied, nor are they believed to be as common as in the sedimentary rock terrane in the western part of the County. LAf1DSL I DES IN WESTERN SAN DIEGO COUNTY Ancient landslides that occur in Eocene and Miocene rocks of San Diego County, most notably the Santiago, Friars, and Otay formations, are typically of the bedding plane glide (block glide), or composite slump and bedding plane glide variety. These landslides are usually up to three or four thousand feet in width and of equal, er siightly lesser length (Figure 2). Most of the landslides investigated have a well-defined basal slip-surface that is typically planar with a dip between horizontal to about 10 degrees in the direction of sliding. Several well-documented cases of landslides that moved up the basal slip surface are known in the a Carlos area of San Diego. One large blockslide, or composite la. lide, that was studied in detail during grading for a subdivision, was observed to have occurred on a reverse dip of approximately two degrees for most of its 120 m (400 feet) length (Figure 3). Most of the large ancient landslides in San Diego County, which the author has investigated over the last 10 years, have been inter-preted as composite slump / block glide movements. That is, the head of the slide rotated backward on an axis parallel to the slope, but the remainder of the slide followed stratification planes and tended to remain intact as a more or less single unit. Landslides of the block-glide variety do occur, but they are difficult to distinguish from the composite type of slides because the head of the slide sometimes resembles the graben-like features fo rmed by slumping. The typical block glide slide resembles Hough's (1957) idealized translatory slide (Figure 4). A detailed investigation of a landslide in the Poway 168
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area revealed a section similar to the idealized translatory slide except that the di rection of movement of Wedge A in Figure 4 is di f ferent f rom the movement that occurred in the equivalent wedge in the landslide (Figure 5). It is through this mechanism of backsliding that the graben at the head of a block glide landslide is usually formed. AGE OF SLIDING lt is becoming apparent as more landslide studies are completed, that the ages of large, ancient landslides in the San Diego area vary greatly. There are well-documented cases (Hart, 1972 a,b) of landslides with crosional in-version of relief in the eastern part of San Diego City and in the Poway area. There are also very young and very large landslides such as those occurring near the Mexican border (Figures 6, 7, 8). Only a few landslides have been dated by the C method, and most of these have been dated by obtaining carbon from buried topsoils. Although it was thought that this procedure gave a fairly accurate age of sliding, recent work by specialists in quaternary geomorphology indicates that topsoils exposed at the groun I surface may be much older than previously believed. Certain soils on tit west slope of the Sierra Nevada Mountains of California, for instance, have reported ages of up to 100,000 years or older. It is obvious tNn, that ages determined by dating topsoils are subject to some rather ser-ious errors. There have been probably less than 10 landslides dated in the San Diego area. Ages obtained range f rom approximately 6,000 to 24,000 years (P i nck r.ey , et al 1979, in press'; Hart, 1977). These dates obtained f rom soils are in good agreement with dates calculated by Stout (1969) for landslides in Orange County, Cali fornia that were determined f rom carbonized wood f ragments buried near the toe of landslides. One of the most interesting features about landslides is their changing morpholcgy with age, and that there are many landslides of advanced years that have been so thoroughly obscured by erosion that they defy recognition by standard reconnaissance methods. Landslides of this type probably exceed the time range of C' dating as indicated by Stout (1969). Landslides with crosional inversion of relief have been reported by Hart (1972b) in both Poway and San Diego City. Landslides such as these represent serious problems to the engineering geologist because they are dif ficul t to detect, and thus 170
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they present one uore reason why geologists should periodically inspect grading projects and prepare "as graded" geologic maps. LANDSLIDE CAUSES Eckel (1958) summarized the causes or factors that must be present to produce a landslide. In his classic paper on landsliding, he stated that since all slides involve failure of earth material under stress, the ini-tiation of the landslide process can be assessed according to those factors that contribute to high shear stress and those that contribute to low shear strength. Factors contributing to high shear stress include the removal of lateral support through erosion or construction, the addition of surcharge weights such as rainwater or the accumulation of slopewash, and earthat;kes. The primary factors contributing to low shear strength are the presence of weak clay materials, high pore-water pressure, and viscous drag caused by seepage. Bedding plane Faults. In an earlier discussion on landslides in San Diego County (Hart, 1973), a comment concerning the necessity for deter-mining the how and why of landsliding was made as follows: "One must try to answer the basic question of why did this slide occur on this hill and not on the one over there, or why not 200, 300, or 1,000 feet to the right or left?" Pinckney, et, t a_l, (1979, in press) have shed new light on the subject of the how and why of landsliding in their discussion of the influ-ence of bedding plane faults on landslides. They report evidence of the widespread occurrence of remolded clay seams or bedding plane faults in the Eocene sed'mentary rocks of the western part of the County. Strength tests performed on typical materials encountered in the shear zones revealed re-sidual angles of internal friction (0) ranging f rom 6 to 12 degrees and residual cohesion values (c) of less than 200 psf. These same materials, i f not " pre-sheared" or remolded by movement, would yield much higher shear strengths termed " peak strengths" (typical values for 0 of approximately 20 degrees and "c" values in excess of 1,000 psf). If the lower, or residual strengths, art: assumed whe 1alyzing the susceptibility of a natural slope to landsliding, it then becomes easier to understand why such massive landslides occur. For instance, a bedding-nlane glide landslide in the San Carlos area occurred on a basal slip-surface with a reverse dip of two degrees (Figure 3). If a pre-existing shear surface (bedding plane fault) was present prior to sliding, then the occurrence of 174
a landslide proceeding up-dip is not as difficult to comp rehend. Even so, there appears to be a factor missing when such a landslide is analyzed. The assumption that a pre-existing shear surface with low residual shear strength was present is, in itself, usually not sufficient to allow a landslide to proceed up-dip for long horizontal distances. Apparently, the missing factor in the analysis is the occurrence of high pore-water pressures or the buoyancy ef fect that such high pore pressures have on materials above the slip-surf ace. When such ef fects are calculated, it results in a nearly 50 percent decrease in resistance to shearing forces. This tends to support Stout's (1969) belief that the cause of sliding in the San Juan Capistrano area was excessive precipitation in late Wisconsin ti me (l ate Pleistocene) . Stout cites evidence such as fossil tree ring studies, which indicates rainfall during the Wisconsin interval of more than double the present rean. Evidence gained to date indicates that there were probably two pri-mary causes of ancient landslides in the San Diego area. The first was the widespread occurrence of bedding plane faults in the Eocene and Miocene sedimentary rocks, and second, the presence of a high water table (high pore pressure) and associated seepage pressures beneath canyon slopes. Ove r-i r r i ga t i on. There is another signi ficant aspect related to the role of high pore water pressures in the landslide mass which many geolo-gists (and perhaps even soil engineers) have been ignoring in their analysis of a particular site's susceptibility to landsliding. Although San Diego has a near-desert climate with a mean annual precipitation of approximately 250 mm (10 inches), Sorben and Sherrod (1977) showed that the over-irrigation of landscaping in residential subdivisions is the equivalent of 1250 to 1500 mm (50 to 60 inches) of rainfall per year. This is contributing to a general buildup of the water table under some developed areas. This means that the present equivalent precipitation excoeds the Late Pleistocene pre-cipitation when nest of the large landclides occurred. Are conditions again becoming favorable for the fornatic- / large landslides along bedding plane faul ts or the reactivation of pres y stable ancient slides? I t seems that the answer may be yes, as der 2ed in several examples which follow. Examples, in the late 1950' a large 124 lot, residential subdivision was graded in the Skylark Drive a of Oceanside, California. In 1978, af ter a winter with precipitatic.. about double that of the normal , approxi-mately 10 lots, including eight hones and a portion of Skylark Drive, began 175
to move af ter almost 20 years of stabili ty. Knowledge of this imminent dis-aster was discussed among local geologists and an informal visi t was made to the site. Af ter viewing the damage and studying aerial photographs taken in 1953, prior to the grading operation (Figure 9), it was obvious that the subdivision had been constructed on an ancient lands lide which had probably becone reacti-vated by heavy rainfall and a possible rise in the water table. Today, almost two years af ter novenent began, several homes have been vacated and are in the process of slowly breaking up. A scarp has developed at the head of the slide (Figures 10 & II) that is approximately 3 to 4 m (10 to 12 feet) in height. Horizontal movenmnts lower in the slide mass appear to be on the order of 1 m (three or four feet). The slide is still progressing slowly, and with each week addi tional damage is created. Although a subsurface investigation has not been completed, it is believed that a slow buildup of the pore water pres-sures within the slide inass was primarily responsible for its reactivation. Another example of an ancient landslide probably reactivated by excessive pore pressure exists in a residential subdivision located near the northern terminus of Carlton Hills Boulevard in the Santee area. In a situation similar to the Skylark Drive subdivision, the Carlton Hills Boulevard subdivision was constructed over 10 years ago (approximately 1969) on an ancient landslide. A large portion of that landslide reactivated during the wet winter of 1978. Two separate investigations of this landslide were undertaken, one financed by the County of San Diego, and a later one by the af fected homeowners. Results of these investigations indicated that the piezometric surface existed at , or very near, the ground surface near the head of the reactivated portion of the slide and a much lower piezonetric surface existed near the center and toe. Slope stability analyses indicated that the landslide would have had a factor of safety against failure of nearly two without the loss of shear resistance caused by t.he ef fect of excessive pore pressures. At present, almost two years af ter reactivation began, several of the approxinately 20 homes situated on the slide have been abandoned because of severe damage, in addition, water, sewer and gas mains are in constant need of repair and the danger of continued move-ncnt re ma i n s . Honeowne rs involved in the Carlton Hills Boulevard slide have forned an association and hired a consultant to assist them in determining ways to halt the s l i de 's movemen t. Consideration has been given to both dewatering and unloading the head of the slide by grading, however, each method represents severe economic hardship for the homeowners. 176
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Another s low-moving landslide occurred in a densely developed area during the winter of 1978. The evidence strongly suggests that this modern slide developed solely as a result of excessive pore water pressures above a remolded clay layer or bedding plane fault. Aerial photographs taken prior to develop-ment indicate that the presence of an ancient landslide in this locality is unlikelv. The geologic consultant for the homeowners indicated that the slide, located near the intersection of Main Street and Westwind Drive in El Cajon, has severely damaged at least three of the 17 year-old residences and has caused lesser damage to approximately five others. According to the consulting soil engineer, groundwater was encounter ed in their exploratovy borings near the head of the slide at a depth of I m (four feet) below grade. When punps were installed and the water table was lowered to approxir.ately 5 m (15 feet), the slide stopped moving. This landslide is the most sobering for several reasons. First, according to Richard L. Threct, the geologic consultant, the failure plane occurred on a highly plastic clay scam (possibly bentonite) within the Eocene Mission Valley Fo rma t i on , a unit not previously known to contain such clays or to be prone to landsliding. Second, the failure surface was planar and occurred on a reverse dip of several degrees. Third, if it is true that no ancient landslide existed previously, then this may represent the first case of a modern landslide devel-oping as a result of excessive pore pressures above a plastic clay scam. Land-slides such as this one may becorce increasingly common in the San Diego area as development centinues and groundwater levels rise. LAllDSLIDES AllD DEBRIS FLOWS 111 BASEMEllT ROCKS-EASTERil Sail DIEGO COUllTY SETTillG Basement rocks occur in the eastern two-thi rds of San Diego County and form the rugged foothills and peaks of the Peninsular Ranges. Cretaceous granit-ic rocks, cons isting primarily of coarsely crystalline granodiorites, quartz diori tes and gabbros make up the bulk of the basement rocks , howeve r , p reba tho-l i th i c re t a vo l can i c an d me t as ed i men t a ry rock s a re common . The netasedimentary rocks consist of well-foliated mica schists and lesser quartzi tes that are found as generally lenticular roof pendants on the batholi thic rocks. The metavol-canic rocks, naking up the Santiago Peak Volcanic unit, are composed chiefly of lightly metamorphosed andesi tes, breccias, and volcanically-derived sedimentary ro cks . Many large, ancient landslides and mudflows have been recently dis-covered in these rocks. 178
LANDSLIDES Probably the first large landslide discovered in the prebatholithic rocks was by two students at San Diego State University (Miller and Maulis, 1962). This landslide (Figure 12) is located on the cast flank of Granite Mountain in the Anza-Borrego Desert. The Granite Mountain slice is nearly 0.8 km (0.5 mile) in width at the toe, and approximately 1.4 km (4,500 feet) in length f rom crown to toe. It occurs in folded and faul ted Triassic schist cut by numerous pegmatite dik's. Foliation in the schist dips approxinutely 50 to 60 degrees northeast and is believed to have been a major cause of sliding. The total amoun t o f l a te ra l novemen t , as evidenced by displacement of dikes and stream channels, is 0.25 to 0.04 km (800 to 1,200 feet). Several miles to the south of the Granite Mountain slide and just north of County Highway S-2, is the Vallecitos landslide (Ha rt , 1964). This landslide, which may be part of an even larger, more subtle appearing slide, has an almost ci rcular shape (Figure 13). Although the Vallecitos slide has the characteris-tics of a classic slump at the head, there appears to be flow-banding near the lobate toe which indicates that portions of the slide may have moved as a vis-cous flow. The Granite Mountain slide also shows some evidence of flow. The fact that the Vallecitos landslide occurred in relatively unweathered quartz dior e is puzzling, except that it lies near the trace, and possibly di rectly ov( the Clsinore Fault. While ancient landslides are not common in granitic rocks, they are a subject of much interest since the causes are generally diffi-cult to determine. Not all hard-rock landslides are found in the desert areas of San Diego County. Many unnamed and little-known landslides are being discovered as engineering geologists investigate potential subdivision sites. In areas such as Jamul, Escondido, and Alpine, landslides in coarsely-crystalline granitic rocks have been discovered in outcrops where well-developed slip-surfaces are exposed. An excellent example of a slip-surface developed in granitic rock occurs east of Alpine on the north side of Interstate 8, approximately 60 m (200 feet) eas t of the Wi llows Road of f-ramp. Here a roadcut has exposed the base of a large landslide and i ts primary zone of shearing, represented by a il foo t thick zone of clay gouge and slightly plastic rock flour. DEBRIS FLOWS Several large, ancient debris flows occur on the east flank of Viejas Mountain. The largest of these features (Fi gu re 14), is approximately 0.5 km 179
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(1500 feet) in width and 2 km (6000 feet) in length. Little work has been done on these interesting landforms. Another interesting flow-like feature was dis-covered approximately 3 miles northeast of the debris flows on Viejas Mountain on the old Viejas Grade Road (Figure 15). Its shape resembles giant pincers about to close over a resistant outcrop of granitic rock. Ilote in the photo-graph in Figure 15, hcw the flow appears to have gone around this outcrop and that there is evidence that many of the smaller gullies in the vicinity post-date the flow. Li ttle is known of the age of these debris flows. It is prob-able that they occurred during the same period as the large landslides of the coastal sedimentary belt previously described. They likely formed in a simi-lar manner to recent debris flows, that is, triggered by heavy rains on deeply weathered soil mantle and possibly also as a result of ancient forest or brush fires. C0llCLUS 10:ls Understanding of local landslides and related mass wastage phenomena has greatly increased during the past 10 years; beginning with the discovery of tne problem in the late 1960's, to progress in understanding the causative mechan-Isms in the late 1970's. Most geologists now seem felly cognizant of the hazard that landslides represent to hillside development and the great signifi-cance that they have played in local erosional processes. Some of the more interesting work is still before us such as determining which areas might be subject to new sliding or reactivation of old landslides. N I' h. wahkbh. f
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REFEREHCES Eckel, E. B., 1958, Landslides and engineering practice: Highway Research Doard Special Report 29, Public. 544, Washington, D.C., 232 p. Ha rt , M. W. , 1364, The Elsinore fault between Banner Grade and Vallecitos Val-ley, San Diego County, California: Senior Thesis (unpub.), San Diego State University. Hart, M. V. ,1972a, Landslides of west-central San Diego County, Cali fornia: Master's Thesis (unpub.), San Diego State Unive rs i ty, 78 p. Ha rt , M. V. , 1972b, Erosional remnants of landslides, west-central San Diego Coun ty, Cali fornia: Bulletin of Association of Engineering Geologists Bulletin, v. 9, p. 377-393. Hart, M. V., 1973, Landslide provinces in San Diego County, California, in Ross, A., and Dowlen, R. J. (eds.), Studies on the Geology and Geologic Hazards of the Greater San Diego Area, California: San Diego Association of Geologists Guidebook, p. 47-52. Ha rt , M. W. , 1977, Landsliding, an alternative to faulting in San Ysidro, Ca l i fo rn i a , in Farrand, G. T. (ed.), Geology of Southwestern San Diego County, Cali fornia and Worthwestern Baja California: San Diego Associa-tion of Geologists Guidebook, p. 37-42. Hough, B. K., 1957, Basic Soils Engineering: Ronald Press Company, 513 p. Kennedy, M. P. and Moore, G. W., 1971, Stratigraphic relations of Upper Cre-taceous and Eocene formations, San Diego coas tal a rea, Ca l i fo rn ia : Ame r i - can Association Petroleum Geologists Bullet in, v. 55, p. 709-722. Miller, H. J., and Maulis, V. R., 1962, Rockslide in Earthquake Valley quad-rangle, San Diego County, California: Senior Thesis (unpub.), San Diego State University, 23 p. Pinckney, C. J., Streif, D., and Artin, E. R., 1979, The influence of bedding-plane faults in sedimen ta ry forma tions on landslide occurrence, western San Diego County, Cali fornia: Association Engineering Geologists Bulletin (in press). So rben , D. R. , and She rrod , K. L., 1977, Groundwater occurrence in the urban env i ronmen t , San Diego, California: In Farrand, G. T. (ed.), Geology of Southwestern San Diego County, California and Northwestern Baja California: San Diego Association of Geologists Guidebook, p. 67-74. Stout, M. L., 1969, Radiocarbon dating of landslides in southern California and engineering geology implications: Geological Society America Special Paper 123, p. 167-179. 182
EXPANSIVE SOIIS IN SAN DIIXD, CALIFORNIA by Richard P. hhile and Inuis J. Ice Vbodward-Clyde Consultants San Diego, CA 92110 I?7fRODUCTION
'Ibe population increase in the t.hited States during the recent years has, to a large extent, been concentrated in the suburbs of the larger cities. 'Ihus, there has been a significant effort to prepare master plan-niry of these areas to best utilize the land at the lowest cost (Gizienski 1965). In this regard, the geologist and engineer must provide input to the planning process for proinsed developnents on the effects of subsurfac conditions, such as expansive (swelling) clay soils (Holtz, G.W. and Hurt, S.S. ,1978). Expansive soils are relatively widespread in the San Diego area, and can have a significant effect on plans and developnent costs.
'Ihe problem of expansive coils was generally identified as being a significant problem in San Diego in the early 1960's, and has becane more
'Ihe major area of important as San Diego's population has increased.
It is, therefore, important that concern has been in residential housing. during initial land develognent planniry, the identification and location of expansive soils be carefully considered.
'1he distribution of expansive soils in San Diego can be generally Ebrmations related to the areal distribution of the geologic formations.
which contain the most highly expansive raaterials are the Delmar, Friars, Sweetwater, Otay, and the Quaternary coastal terraces. 183
The classification of expansive soils is generally based on conven-tional laboratory tests, and correlated with experience. This infonnation can then be used, along with environmental factors, to evaluate potential heave and possible damage to structures. Several design and construction methods have been used successfully in the San Diego area to reduce the risk of severe structural damage fran expansive soils. (bologists and engineers cannot guarantee solutions for every swlling soil problem. Ibwver, damage can be mitigated by having a general knowledge of successful design, construction, and maintenance methods (Woodward, Clyde, Sherard and Associates, 1968). DISTRI"UTIOJ AND CIASSIFICATIQJ OF EXPANSIVE SOILS IN SAN DIEGO The distribution of expansive soils is generally a result of the geologic history, paleo climatic conditions, and the types of mineral alteration. In order to provide an indication of the location of swelling soils in San Diego, the geologic formations in San Diego which contain materials with swlling potential have been identified, and their distribu-tion obtained fran geologic maps (Weber,1963; Kennedy,1975; Kuper and Castil, 1977.) This information is discussed in the following paragraphs, and is summarized in Figure 1 and Table 1. The geologic fo. . nations in the San Diego area may be subdivided into four groups: 1) Jurassic and Cretaceous hard-rock units; 2) Cretaceous nederately indurated sedimentary fonnations; 3) poorly to moderately indurated 7brtiary and Quaternary sedimentary rocks; and 4) largely uncon-solidated Quaternary sedimentary units. The geologic column shown in Table 1 presents the various formations found in the San Diego area; a descrip-tion of each soil unit is shown, as w11 as a classification according to expansive soil characteristics. Ebr the purpose of this paper, the potent-ially expansive soil groups indicated in Table 1 are categorized as follows: la. Igneous and metamorphic rock with localized expansive surface soils, 184
t I lb. Sedimentary rocks with localized expansive surface soils,
- 2. Iow to mediun expansive soils,
- 3. Low to mediun expansive soils with localized distribution of highly expansive soils, and
- 4. Mediun to highly expansive soils.
Onitted fran categories 2, 3 and 4 is the soil mantle which is gene-4
< rally relatively thin (less than 5 feet), arxl occurs over most of the San Diego area. These surficial materials exhibit a wide range of expansive characteristics, fran low to ve: / high, and should be considered in any developnent as a potential problem.
The general extent of the five expansive soil categories in San Diego is shown on the attached map (Figure 1) to illustrate their approximate distribution. As the boundaries shown are highly generalized, the map is
, intended only to provide a relative distribution of the types of expansive soils in the San Diego area.
Ilard-rock units (Category la) with a low expansion potential are found generally east of a rough northwest-southeast diagonal line extending across the center of the study area. A few outlying knobs are present adjacent and just west of this imaginary line. These rocks are crystalline granitic, fine-crystalline meta-volcanic, and meta-sedimentary materials. hhile exhibiting a thin localized clayey overburden, they typically do rot present significant expansive clay problens. Sedimentary fonnations of low expansion characteristics (Category lb) and with localized surface expansive soils are found throughout the geologic column (Table 1). All except one of the Cretaceous sedimentary units, present in the Ibint Lana and Ia Jolla coastal areas, fall into this group. In the 1brtiary formations, granular rocks canInsed of nonexpansive sand-stone and conglomerate extend over a relatively large area south of Ibway, between Ibway Valley and the El Cajon Valley. In addition, nonexpansive Ibrtiary fonnations seem to be found in irregularly-shaped areas north of Mission Valley, east of Icse Canyon, and west of the hard-rock boundary. An exception to this is the nonexpansive San Diego Formation, which extends 185
TAIU1 1 CIDIICIC CollPN ixemS1Ve Soll GolC41c FotTatton G neral liscgt try, ila.wi b ation AlluY1&l bulls-Inlatui Valleys IntLttandel Mrds afd Clays i AlluVlal 10115-GAastJ1 Areas aral Strds, 511ts, afd &avels ib Cuastal (J1yons My Ibir.t l'otrat tor. Mros ard Q avels lo tow Castal arrace Ir sits of Clays and Salts 4 Sot,th San Drejo farmldvista Fottation Mruls, Csreantal Q avels, and 2 Clays with Gavels San Litso terration f are Sands IL Ctay-imrlto teach Fot1Astlun Clays, I* ntonttic Clays, ard 4 S11ts with Mtvis fw etwater totLation Clays, Mrdy Clays, arms Clayey 4 Mrds PJetMJ CurylCForate-RMay QUUp Q aVels with MndS lb Missim Valley lonation-Rusy Mrus with Iran Clay Icnws 3 Goup btajian (bryttterate-Rusy Roup Ravels with Cuttles ard Mral Ib ITlars Fctmtion-La Jolla uoup Clays with Sands 4 Scrins Fotration-IaJolla troup Mrds lo Ardath Stale-la Jolla Goup S11ts and Mrds witt. wan 3 Clays %rrey Sandstore-Ia Jolla &oup huujs it. tel thir to:1stion-ta Jolla &oup Clays with Sarus 4 Ptaunt toledad Ebrnation- Sards and Wavels la la Jolla G oup Cabrillo Fotwation fardstone- Mros and Cravels ib ibsarto & oup Caorillo toriration Congicrwrate- Mrals and Q avels 1b itasarto Roup loint II na Fot1 nation-}csarlo Sarajs with Stlts 2 Qoup Iusards tornation-imrlo 20+ Sands with Q avel it, California BatholittrHard Iccks &anttic locks la 186
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'5W t:_:; 4 AREAL DISTRIBUTION OF 33 HICH t:XPANSIVE SOILS 5 '_AKES AND RESERVOIRS Figure 1. General Distribution of Expenme Soih in San Diego 187
south fran Mission Valley and const.itutes a major portion of the rocks underlying the higher elevations of the San Diego mesa. Alluvial soils in the lower valley areas and in the narrow canyons are primarily cantosed of nonexpansive sands and gravels. Other areas of low expansive materials are found around the northern end of San Diego Bay, Mission Bay, and the low coastal areas of Ia Jolla, Coronado, and the Silver Strand. Three fonnations contain localized distributions of low and medium expansive materials (Category 2). These are: 1) the Ibint Lana Fonnation, which contains some shale beds; 2) the widespread Lindavista Fonnation; and
- 3) the recent alluvial soils of the inland valleys, such as El Cajon, Ioway, Pamona, and San Pasqual. The Lindavista Fonnation, capping the mesas, is primarily canposed of sands and cemented gravels, and is inter-bedded, particularly near the eastern boundary, with clay and clayey gravel.
Ebnnations having areal distributions of low to medium expansive soils (Category 3) are the Ardath Shale and Mission Valley Formation. The Ardath Shale is found in Ibse Canyon, the steep slopes of Mission Valley, and the steep sides of other inland valleys, such as Ibnasquitos, Cannel, and Carroll canyons. The Mission Valley Fonnation is found on the steep northern and southern slopes of Mission Valley, in the San Carlos and higher areas northwest of Mission (brge, and, to a lesser degree, in areas south of Mission Valley. In the northern portion of the study area, fonnations of high poten-tial swll (Category 4) incitrie the Delmar Fonnation, found in valleys to the east of Del Mar, Encinitas, and in the Pancho Santa Fe area. The Friars For' nation, a unit with bad expansive soil problens, is found in the valley areas further to the east of Del Mar; in the Ionasquitos and Rancho Bernardo areas; the western slopes of El Cajon Valley; Fletcher liills; valley areas of western Kearny Mesa; eastern tributaries to incolote Canyon and Murphy Canyon; and in tributary valleys to the Mission (brge area. The Sweetwater Fonnation and the Otay Fonnation now considered to be part of the Ibsarita Beach Fcunation (Kt,per and Gastil,1977), are found south of 188
Mission Valley, and in the higher elevations east of Chula Vista and IJational City. Low coastal terrace deposits of highly expansive clays are found underlying tJational City and Chula Vista, and lower oaastal terraces soeth of San Diego. IDEtJTIFICATION OF EXPAtJSIVE SOILS The potential for swell is dependent primarily on tne properties of the soil, such as gradation, plasticity, type of minerals, mineral struc-ture, density, and moisture content. 'Ihe actual amount of swell or shrink-age beneath structures or pavements is dependent not only on the properties of the soil, but also on the environmental conditions producing iroisture change, and the depth and thickness of the clayey layers. IArrerous procedures for identifying the soils that possess expansion characteristics have been developed, and have been carbined with experience to provide a basis for evaluating whether or not damage to a particular type of structure may occur. Several tests have been used by soils en-gineering consultants in southern California (Table 2). Classification systems have been developed fron these tests which generally categorize expansive soils into four broad classifications. Table 2 gives an approx-imate canparison of the currently used methods (Portland Cenent Association, 1971). EVALUATIOtJ OF POTEITTIAL HEAVE The " effective depth" or normal depth of water penetration in the San Diego area depends primarily upon the amount and duration of the annual rainfall, the type of vegetation, and the amount and frequency of irriga-tion. San Diego is in a relatively warm, arid climate where surface evaporation exceeds annual rainfall. 1hus, a moisture deficiency exists in the ground which may extend to depths of 10 to 15 feet. Because the water table is generally well below the construction grades, and because the natural water content of the clays is relatively low, the soil moisture profile in San Diego is governed primarily by natural seasonal fluctuations, and by fluctuations due to intermittent irrigation. In much of the San 189
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DOIL 'I'La'i id PALX1hNit 1;/4L13 li/di1L1'IY II.LLX 5-1 ; 10-25 20-45+ 35+ atx1 CIlsY Cufrufi (less than .uoc .a . ) 5-15t 10-25e 20-30s 30-454 11/d;t.L G.LLL ';'Is'. Seall 60 lo. durel.a r .ie U-45 4-9s 9- 1 ? t 12s+ ae;11 144 lu. durcharge 0-35 3-bt 6-lut 10t+ 144211 65U lo. aurel.orge 0-1s 1-31 J-5s 5s+ k . I .10 .'. i .1, LXIV d .b 1LU II.LL: aa. LilLL CGL17;T 'i!.b'; 0-20 20-60 60-100 1004 (Urdlbunce 2895) H A - Ed.'l .L'i Li: 0-2 4-4 4-6 b+ LX1'id.b lu. Ll/cali lun ItL UA. cLL Ll/6L i10: \,Ll;Y lilui 190
Diego area, soil water variations due to ese phenanena probably do not extend beneath depths of approximately 4 to 6 feet below tne ground surface under nonnal conditions. Thus, for most design considerations, expansive clay layers below a depth of 6 feet (measured fran finish grade) do not have a significant influence on foundation or concrete slab-on-grade design. 7he maximun total heave which may be expected under a structure can be computed fran the area under the percent swell versus depth curve (Gizienski and Ice, 1965). It is the authors' experience that in San Diego a reason-ably conservative estimate of total heave under nonnal conditions is obtained by using the maximun percent swell for air-dried soils under a 1 psi load, and an " effective depth" of approximately 4 feet. Experience has indicated that differential movement will generally not be more than 50% of the total or maximun heave; however, this can be influenced by localized moisture chanjes. 7ypical estimates of total and maximum differential heave under nonaal conditions for the various soil classification in the San Diego area are: Expansion Classification 7btal lieave Differential lieave low 0 to 3/4" 1/2" Fbderate 3/4 to 1-1/2" 3/4" liigh 1-1/2 to 2-1/2" l-1/4" Wry liigh over 2-1/2" over 1-1/4" Care should be taken to properly evaluate those conditions which may not be considered typical, and which may require detailed analysis. Such conditions include: a) Water table within 10 feet of F t grade, b) Drainage conditions which could lead to prding and/or the accanu-lation of excessive water and/or significant penetration of water below a depth of 4 feet, c) Clay strata with relatively high water contents which could
~s hrink, causing settlement, as well as heave, 191
d) A relatively thick (greater than 4 feet) layer of highly expansive clay located at or near finish grade, e) Areas which may be expsed to conditions of excessive water, such as near a swinning pool.
'IOLERABLE HEAVE FOR RESIDE 2TI'IAL STRUC'IURES tb generalization can be made regarding a mode of distru.s to struc-tures due to expansive soils in San Diego. Were these soils are present near finish grade the type of damage most conmonly observed to houses constructed on conventional slabs-on-grade consists of slab heave and breakage with the greatest amount of heave occurring in the center of lanje roans. Interior partitions and walls may inhibit slab heave, but do not prevent it, and cracks in interior wall finish are canmon. In general, damage to exterior walls is less severe.
Except where a structure is connected to a supported utility or adjacent structure, differential heave within the structure, rather than total heave, generally causes damage. 'Ihe tolerable differential novement of structures depends primarily upon the type of construction. Approximate values of differential heave for various structures that can occur without serious damage to frame or finish are listed below: a) One or two-story houses with plane brick bearing walls and light structural frame can generally tolerate approximately 1/2" differ-ential heave in a 20 feet distance. b) Wood-frame houses can generally tolerate differential movements of up to 3/4" if properly braced in the ceilings and walls. c) If the structures have sensitive interior or exterior finish such as plaster, ornamental stone, or tile facing, the differential novement should generally be limited to 1/4" to 1/2" maximum in a 20 feet distance. d) If the structure is provided with a relatively rigid raf t or mat foundation whien will tend to redistribute the loads, total heaves of up to 2 inches or more may be tolerated. 192
QUALITY STANDARDS FOR EVALUATI?K; DAMAGE The tolerable heave of a structure is generally reflected in the acceptable extent of observable damage. The Ihne Owners Warranty Program (llCW) has developed sane quality standards that are expressed in irrms of performance standards. 'Ihe format is designed for easy conprehension by both layman and builder (lione Owners Warranty Program, liCW,1974). Items which pertain to possible damage due to heaving are presented in Table 3. TRF1dNI27f OF EXPANSIVE SOIIS Ibtential heaving of structures may be minimized by 1) excavating the expansive material and replacing it with properly conpacted, nonexpansive granular fill, 2) presaturation of in-place soils, or 3) using belled piers founded below the depth of swelling (and supporting the floors between these piers). In the latter case, provisions for an opening or ocrapress-ible filler beneath the floors is necessary. Ilsnage due to swelling under light structures may also be minimized by the use of 1) slab or raft foundations, 2) dry wall construction, 3) steel or reinforced concrete framing, or 4) making provisions for jacking. 193
TABII 3 O ALITY STAbT/SIE FIR EVAILAT1!JG C#EE 'IU FGI!X77FIAL STI1K7IUPJS R>ssible Deficiency PerforTrance Standard I:r4toFer drainage *1te necessary grales and c,f the site. swales stould to established to insure paper dralnage any frczn tre taxise. to standtry eter stamid rtrain in the yard 24 tuurs af ter a rain, except sales which nay drain as Icn; as 48 *4rs af ter a rain, or starp snp diset.arge. Besment or foundation ibn-structural cracks are rot wall cracks. unusual in <x;ncrete foundation walls. Such cracks greater than 1/8 inch in width are considertd excessive. Cracktry of bassent Minor cracks in anctete floor. tasment flcors are txrron. nacks exceedtry 3/16 inch width or 1/8 inch in vertical displacuent are considertd excessive. Cracking of attacted Cracks in garage slabr. in garaje slab. excess of 1/4 inch in width or 1/4 inch in vertical displaceent are cunsidered excessive. Crack 1rx;, settling, Stoops or ste;s should rut or teavtry of stcx4s settle or trave in excess of or steps. 1 inch in relation to the house structure. Ib cracks except hairline cracks (less than 1/16 inch) are accept-able in concrete stcope. Oacks in attached patios. Qacks in excess of 1/4 inch width or in vertical dis-placment are considered excessive. Cracks in concrete Qacks dich significantly sle-cn-grade flours. Impir the agearance or
}erformnce of tre finish flooriry raterial shall tot te acceptable.
Cracks in nasonry alls mall cracks are cncon in or veneer. ::ortar Jcints of masonry construction. Oracks greater than 1/8 inch in width are considered excessive. 194
REFERDEES Gizienski, S.F., 1965, Soil engineering and geology applied to land devel-opnent: Proceedings - 13th Annual Meeting of the California Council of Civil Ergineers c.nd Land Surveyors. Gizienski, S.F. and Ice, L.J. ,1965, QJmparison of laboratory swell tests to small scale field tests: Engineering Effects of Moisture Changes in Soils, Concluding Proceedings, International Pesearch and Digineering Conference on Expansive Soils, p. 108-119. Oxifrey, K. A. , Jr. ,1978, Expansive and shrinking soils-building design problems being attacked: Civil Engineering Magazine, p. 87-91. Ibltz, W.G. and liurt, S.S. ,1978, Ibne construction on shrinking and swlling soils: American Society of Civil 179 neers, i National Science Ebundation Grant tb. 05577-21212. IICW,1974, Approved standards / quality standards: Ibne owners Warranty Program, 1101-25. Kennedy, M.P. and Peterson, G.L. ,1975, Geology of the San Diego metro-p31itan area, Calitornia: California Division of Mines and Geology Bull. 200, 56 p. Kuper, ll.T. and Castil, R.G. ,1977, Reconnaissance of marine sedimentary rocks of southwestern San Diego County, In, Farrand, G.T. (ed.), Geology of southwstern San Diego County, California and northwestern Baja, California: San Diego Association Geologists Guidebooks, p. 9-16 and maps. Fortland Cement Association,1971, Beccmmended practice for construction of residential concrete floors on expansive soil: v. II - Second Printing. Weber, F.II. , Jr.,1963, Geology and mineral resources of San Diego County, California: California Division of flines and Geology County Report 3, 309 p. Woodward, Clyde, Sherard and Associates,1968, Ienedial methods applied to houses damaged by high volume change soils: Federal ibusing Mnin-istration, FifA Contract A-779. 195
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SEA-CLIFF EROSION AT SUNSET CLIFFS, SAN DIEG0* by Michael P. Kennedy California Division of tiines and Geology La Jolla, CA 92093 Sea-cli f f crosion at Sunset Cli f fs is the resul t of ocean-wave action along prominent joints that are oriented obliquely to the clif f face. This in turn has led to the developnent of surge channels and sea caves in the cliffs. Sea-cliff retreat has averaged about 3 feet (1 m) in the past 75 years, and is associated with channel and cave developnent. However, where collapse of a cave roof occurs, local sea-cliff retreat can be as much as several feet in a moment. This dramatic and sudden local ef fect may give a false impression of rapid, regional cli f f retreat. Sunset Clif fs is a scenic, residential area in the City of San Diego, forming the northernmost part of the Pacific sea clif fs on the Point Loma Penin-sula. During the past 20 years, portions of the clif f have been rapidly eroded. This cliff retreat is primarily the result of roof collapse of individual sea caves. Sea caves are formed by wave action. The er.ergy of the waves is concen-trated and funneled along prominent joint sets in the cliff rock, gradually widening these joints to form surge channels and sea caves. Wave erosion pro-gresses headwa rd, laterally, and upward in the fornbtion of sea caves, leading to uniform hollowing and eventual roof collapse. Some sea caves extend land-ward as much as 60 feet (19 m) f rom the sea-cli f f face. Since the early 1950's, roof collapse has been accelerated slightly as a resul t of the loss of Leach sand at the base of the cliffs. In the past, the sand buffered much of the now continuously exposed sea clif fs from direct wave attack during several seasons of the year. A number of single-family dwellings, public utilities, and streets lie dangerously close to several mature sea caves. As part of a detailec geologic study of the City of San Diego, the Calif-ornia Division of liines and Geology in cooperation with the City of San Diego, investigated the nature of marine erosion at Sunset Cliffs. Detailed geologie cups (scale 1:2400) were prepared to show the geologic features controlling the crosion of the sea cliffs (Kennedy, 1966). The present report, based upon that study (and later work by the U. S. Army Corps of Engineers), suggests a possible i Reprinted f rom California Geology, v. 26, February 1973, p. 27-31. 197
geologic basis for sea-clif f retreat at Sunset Clif fs. Numerous photographs were used to document the average annual rate of this retreat. GE0 LOGIC SETTING Sunset Cliffs is underlain by marine Upper Cretaceous and Pleistocene sedi-mentary rocks. These rocks form a portion of the western limb of the Pacific Beach syncline (Kennedy and Moore, 1971), the southern extension of which is submerged beneath Mission Bay. The Upper Cretaceous rocks, part of the Point Loma Formation (Kennedy and Moore, 1971), strike U 5* to 15 W and dip 3* to 10 E in the area studied (Figure 1). These rocks crop out continuously along the west-facing sea cliffs of Point Loma and are predominantly olive gray, thin-bedded, well-indurated, ma ri ne sands tone and s i l ts tone. They contain calcareous nannoplankton of Late Cretaceous (Campanian to Maestrichtian) age (Bukry and Kennedy, 1969). The flat-lying Pleistocene deposits of the Sunset Cliffs area belong to the Bay Point Formation of Hertlein and Grant (1939,1944). These rocks, where preserved, rest unconformably on the Point Lona Fo rma t ion (Figure 3). Here the Bay Point Fo rna t i on is a well-indurated sandstone which is light brown, medium to coarse grained, poorly sorted, poorly bedded, and locally well cemented. Fossil-bearing rocks at the case of the Pleistocene section (exposed at many lo-cations on the west side of Point Loma Peninsula), correlate with fossiliferous strata 13 miles (21 km) to the north. These fossili ferous strata are reported to be late Pleistocene (Sangamon) in age (Kern, 1971). The contact between the Pleistocene deposi ts and underlying Upper Crc-taceous rocks is approximately 30 feet (9 m) above sea level at the southernmost part of Sunset Cliffs, sloping to near sea level at the northernmost exposures in Ocean Beach. This suggests a slight tectonic warping toward the Pacific beach syncline during late Pleistocene time. The lowest roc :s exposed in the cli f f s belong to the Point Loma Formation. Two prominent vertical joint sets cut the Point Loma Formation at intersecting angles that strike U 30 to 40 E und il 40* to 50 W. Large rectangular blocks have been formed by the intersec-tion of these joint sets with erosion by wave action taking place readily along these joints. EROS 10NAL PROCESSES The processes of sea-cave formation in Upper C retaceous rocks at La Jolla were described by Moore (1954). The rocks at Sunset Cli f fs and La Jolla are 138
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lithologically the same and lie at similar stratigraphic positions within the Point Lora Formation. The rocks at La Jolla form the northeast limb of the Pacific Beach syncline, with those at Sunset Cliffs forming the southwest limb. The beginning stage of sea-cave developrunt is the formation of surge channels. These channels are numerous at Sunset Cliffs (Figure 2). The chan-nels, which follow the intersecting joint planes, gradually evolve into caves by progressive basal undercutting. The cave excavation is aided by the concen-tration and agitation of abrasive sand particles along these joints. Headward erosion begins along individual joint planes accompanied by gradual lateral and vertical widening. Erosion continues along a joint plane until an intersection wi th a cross-cutting joint plane is reached. Wave erosion then begins excava-tion along the intersecting plane as well as continuing headward, resulting in a broadened inner part of the cave. The caves marked Cl and C3 (Figure 2) have formed at such intersecting joint planes, as shown by their shape. nternal cave erosion progresses until either all, or part of the roof collapses. Connonly, only part of the roof collapses so that a blowhole and natural bridge are formed such as those marked Tl and T2 (Figure 2) . Upon later erosional removal of the natural bridge, or in the case of complete initial roof failure, enbayments develop such as those shown to the right and lef t of T2 (Figure 2). The photographs in Figures 4, 5, and 6 show that sea-cave development at Sunset Cliffs has taken many years. For example, there are only slight crosional changes for the 66 year interval between the photographs in Figure 5; this is evidence that tine far in excess of 66 years is needed to carve even small surge channels here. Periodic roof-collapse accompanied developnent of the sea cave shown in Figure 6 prior to the covering of its mouth by the City of San Diego in about 1960. The foreground rock that supported the wooden bridge in the 1920 photo-graph collapsed in the late 1940s. This alerted the City to possible future hazards related to similar failures. The grass-covered slope in the foreground of the 1971 photograph is underlain by about 10 feet (3 m) of artificial fill which was bulldozed over the rockfall area of the early 1950s. The 1971 photo-graph also shows that the stone bridge-support at the contact between fill and bedrock on the far side of the channel has not yet been completely eroded away. 200
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The sr.3all headland between C1 and C2 is exposed to wave attack contin-uously throughout the tidal range whereas T2, T3, C3, and C4 are actively eroded only above the positive l- to 2-foot tidal range. Dotted lines outline sea caves and tunnels; SC, surge channel; Cl, C2, C3 and C4, sea caves; Tl, T2, and T3, tunnels;W, strike of vertical joint; g , strike and dip of bedding. Figure 3. The schematic cross sect:en shows the : elation-ship between Qbp and Kp in th= face of the cliff. At the north end of the map, Qbp is almost at mean sea level; at th e south end it is about 30 feet above mean sea level. Qbp
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RATES OF EROS 10U The rates of marine erosion at Sunset Clif fs were determined largely by studying old photographs and comparing them with newer ones. The rates have been small on a regional scale but large locally (Shepard and Grant, 1947; Shepard and Wanless, 1977). The rapid local rate of sea-cli f f retreat is controIIed primarily by dif-ferential erosion of the sof ter rock. Shepard and Wanless (1970), in a state-ment regarding Sunset Clif fs, said, "Mos t of the stacks and natural bridges adjacent to the shale cliffs have shown conspicuous changes in recent years. However, at the north end of Sunset Clif fs, we have e , example where sandstone clif fs had only very minor changes since photographed in 1887." Within the area of the nap, the average annual rate of crosional retreat of sea cliffs was appraised for a 75 year period. This was accomplished by averaging rates of those areas that have had essentially no retreat, those that locally have had large arounts of atreat for this period of time. Approximately 75 percent of the sea-clif f area studied during this investi-gation has undergone no appreciable erosion during the past 75 years. Photo-graphs of the area between Froude and liill S t ree ts taken in 1950 and in 1968 (Figure 4) show very li ttle erosion for that 18 year period. Photographs taken in 1905 and 1971 of the area adjacent to Crawfish Cove at the foot of Santa Cruz Avenue (Figure 5) show that this area has undergone very minor crosional altera-tion in 66 years, liore than 20 percent of the clif f area has undergone erosion at a rate that is small, but neasurable, for tnat period of time. According to Shepard and Grant (1947), a natural arch at Sunset Cliffs collapsed in the late 1930s with only a small remnant of the buttress standing in 1947 They also reported that a sea stack 10 feet (3 m) high and 3 feet (I n) square, at the routh of a promi-nent joint-controlled surge channel at Ocean Beach, was completely eroded away between 1887 and 1946. The sea-caves and tunnels at the foot of Monaco Street (Figure 2) have been eroded locally as nuch as 5 inches (13 cm) between 1965-1972. Less than 5 percent of the sea-cliff area has undergone very rapid retreat (as much as 10 feet (3 m) in the past 75 years), and in each place where this much retreat has been documented, it is the resul t of sea-cave collapse. The partially collapsed sea cave that lies at the foot of Osprey Street (Figure 6) and extends more than 20 feet (6 m) landward f rom the sea-clif f f ace beneath Sunset Cli f fs Coulevard, retreated nearly 5 feet (1.5 n) during one year in the 202
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Q .. 8., X. , i?+.9h , &f 'kel")W &- i m, Figure 5 i'h o t o q r.w h 3 t 4 en at sunset C1ifts in 1305 (left) and 1971 (right), V i e, sou tiocas t of tra./ fish Cove ,i t the fout of Santa Cruz Avenue. This cave has developed along several proainent, northeast-striking joint planes. ;otice that the rocks at the routh of the cove, ,hich r e ; ia . n un tie r rave attack at all tidal ranges, ,till exist in the 1971 p h o t (;g r a p n _, . Very > light erosional ef fect > have occurred during the 06 year interval b e t <, .e e n these photographs. (Ol de r photograph cou r-tesy of San Diequ Title losurance and Trust Corpany; newer photograph by W. P. Reetz, Stripps Institution of Oceanography, La Jolla), 3 l 203
late 1940s. The wooden bridge and a portion of Sunset Clif fs Boulevard shown in the early photograph (Figure 6) were destroyed during that year. The average rate of sea-cli f f erosion along Sunset Cli f fs has probably accelerated slightly during the past 20 years, due to a very gradual, slight loss of beach-sand deposits that once protected the clif fs from wave action. Peacock (1965) concluded that beach deposits have dissipated at Sunset Clif fs as a result of the Mission Bay jetty development in 1951. His report sug-gested that the jetty has deflected seaward much of the southward-moving ,ong-shore current which transports sediment vital to the permanence of a sand baach along the shores of Point t.oma. Recommendations were made to build concrete cliff-revetments, seal the sea-cave openings, and construct an artificial bea:h so that further cliff failures could be minimized. Revetment construction ani sea-cave closures were completed in late 1971 following Corps of Engineers; Design Memorandum for Sunset Cli ffs--Segment B, April 1970. Artificial beaches were not emplaced because of opposition by local property owners. In their desire to keep Sunset Clif fs esthetically pleasing, they fought against the development of what they were afraid would become a busy public recreation area and won thei r case as the resul t of public hearings held by the City. The combined average rate of sea-clif f retreat at Sunset Cliffs is esti-mated to have been almost one half inch (1.25 cm) per year, or slightly more than 3 feet (1 m) for the 75 year period immediately preceding the development of the revetments and the closure of the sea caves. C0i1CLUS10NS Erosion and retreat of the sea clif fs at Sunset Clif fs is controlled lar,ely by joint planes in the cliff rock along which ocean waves and abrasive sand are funneled. Surge channels, tunnels, and sea caves have been formed by the scouring of sedimentary rock adjacent to the joints. By comparing old photographs belonging to the San Diego Title Insurance and Trust Company with later photographs and observations, the average amount of sea-cliff retreat during the past 75 years is estimated to be more than 3 feet (1 m). This average rate is high because of the rapid rates associated with local sea-cave collapse. The actual rates of sea-cli f f retreat, which are regionally negligible and locally high, are in general agreement with estimates suggested by most earlier workers who have considered sea-cliff re-treat rates associated with similar kinds of bedrock elsewhere along the coast of southern California (Emery,1941; Moore, 1954; Shepard and Grant, 1947; 204
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Figure 6. Photographs taken ay Sunset Cliffs in 1920 (lef t) and 1971 (right). View north of a collapsed sea cave at the foot of Osprey Street. The gently dipping, thin-bedded strata belong to the Upper Cretaceous Point Loma Formation. A small sliver of Pleistocene Bay Point For-nation crops out on the extreme left hand side of the 1971 photo-graph, directly above the Point Loma Formation. Artificial fill covers most of the Pleistocene rocks and comprises the grass-covered slope in the foreground as well as slopes of the parking area i r, the background flotice that the stone bridge-support has not yet beer. completely des-troyed. (Older photograph courtesy of San Diego Ti'.le Insurance and Trust Company; newer photograph by W. R. Reetz, Sc.ipps Institution of Oceanography, La Jol1a). Shepard and Wanless, 1970). Whether the sea-clif f stabilization program begun by the Corps of Engineers (as nodi fied by local citizens) will prove to be effective in stopping sea-clif f erosion at Sunset Cliffs, has yet to be determined. REFEREllCES Bukry, D., and Kennedy, M. P., 1969, C retaceous and Eocene coccol i ths at San Diego, California: Cali fornia Division of Mines and Geology Special Report 100,
- p. 33-43 Emery, K. O., 1941, Rate of surface retreat of sea clif fs based on dated inscrip-tions: Science, v. 93, p. 617-618.
He r t l e i n , L. G. , and G ran t , U. S., IV, 1939, Seology and oli possibilities of southwes tern San Diego County: California Journal of Mines and Geology, v. 35,
- p. 57-65.
lie r t l e i n , L. G. , and G ran t , U. S., IV, 1944, The geology and paleontology of the carine Pliocene of San Diego, California, part 1, Geology: San Diego Society of fla tural llis tory Memoi r, v. 2, p. 1-72. Kennedy, M. P., 1966, Marine crosion at Sunset Cliffs, San Diego, California: Cali-fornia Division of Mines and Geology, open-file report and map (scale 1:2400).
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Kennedy, fi. P. , and lioore, G. W. , 1971, Stratigraphic relations of Upper Cretaceous and Eocene formations, San Diego coastal area, California: American Associa-tion of Petroleum Geologists Bulletin, v. 55, p. 709-722. Kern, J. P., 1971, Paleoenvironmental analysis of a late Pleistocene estuary in southern lalifronia: Journal of Paleontology, v. 45, p. 310-823 lico re , D. G . , 1954, Origin and development of sea caves: The American Caver Bulle-tin, v. 16, p. 71-76. Peacock, E. G., 1965, Special study of City of San Diego Sunset Clif fs: U. S. Army Corps of Engineers Ucach Erosion Cent rol Report on Coast of Southern Cali for-nia, Appe, dix Vil, Contract W-04-193-Eng.-5196. Shepard, F. P., and Grant,'U. S., IV, 1947, uave erosion along the southern Cali-fornia coast: Geological Society of America Bulletin, v. 58, p. 919-926. Shepa rd , F. P. , and Wan less ,11. R. , 1970, Our changing coastlines: ticG raw-H i l l Book Comp.any. U. S. Army Corps of Engineers, 1970, Design memorandum for Sunset Cliffs--Segment B: Bluff stabilization with revetments, dikes and the sealing of caves, at Ocean Beach area, City of San Diego, San Diego County, California. t
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C0ASTAL CROSION lH SAN DIEGO COUNTY, CAllFORNI A by Gerald G. Kuhn and Francis P. Shepard Geological Research Division Scripps Ins ti tution of Oceanography University of California at San Diego La Jolla, CA 92093 INTRODUCTION During the past two decades, extensive land developuent has occurred along the sea cliffs, particularly north of La Jolla. Until enactment of the California Zone Conservation initiative in 1972, local agencies per-mi tted cons truction within a few feet of the cli f fs. Developers justified this practice with the arguacnt that the cliffs were not retreating at an appreciable rate. The reported retreat rates were usually based on exper-ience during the past 25 to 30 years; this was a period of unusually slow crosion, characterized by low rainfall and few storms capable of producing heavy surf. Earlier coastal erosion studies showed a far less optimistic picture of sea cliff stability. For example, Vaughn (1932) reported that poorly indurated bluffs (Quaternary terrace deposi ts) near Scripps Insti-tution of Oceanography at La Jolla retreated 3 to 6 m (10 to 20 feet) between 1923 and 1930. Shepard and Grant (1947) found the same area eroding at a sate of about one foot / year during stora periods just prior to 1947 Cli f fs of Cocene and Cretaceous sedinentary rock have retreated epi-sodically due to large rock falls (Shepard and Waniess, 1971). In many places, however, these cliffs show no indications of appreciable retreat since photographic records began, approximately 50 years ago. High rain-fall which occurred during 1977-78 and 1978-79 has brought on renewed in-terest in sea-clif f erosion rates, especially between Del Mar and Ocean-side (Figure 1), where there has been a large amount of construction in recent years. Alluvial cliffs near Scripps, which appeared stable since 1947, have once again begun to show higher rates of erosion. In 1973, extensive studies of coastal erosion began between Del Mar and Oceanside (Kuhn, 1977). Repeated measurements, with photographic coverage , was made at critical points. History of erosion was investi-gated by studying old saaps, newspapers, weather bureau reports, ships logs, records of land ownership, and through discussions with long-time residents. 207
34 00' VENICE L LoNG DEACH
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Of OCEANSIDE O 'e Ant $gAo T ENCINITA5 4 C 3 00' _ A SEACH gCoRDERO SoRREN g SCRIPP$ INSTITUtloN of OCEANOGRAPHY L A JOLL A A E Sub SET CLlHS SAN DIEGO f CoRONADO e . -e 118' 3 0' 118' 0 0 117
- 30' Figure 1. Locatnun udp Coas tal erosion is related to bedrock structure. Faults in north San Diego County have been napped by Ziony, et al.(1974). Additional faults have been identified and mapped in the sea cliffs, and on the sea floor from aerial photographs. Faulting has brought sof t, easily-eroded formations into contact with the coastline, accounting for im-portant areas of erosion. Accelerated crosion occurs particularly where weaker bedding dips seaward.
Research was funded by (10AA, Office of Sea r, r ,n t (fic. 04-8-tiOI-189) , the California State Resources Agency (R/CZ-43), and the San Diego County Integrated Planning Organization (llo . Il596-0800E). Cooperation and sug-gestions by Jeffrey D. Frautschy are appreciated. SEA-CLIFF EROSI0ff-HISTORICAL ASPECTS Evidence of crosion during the past century should not be overlooked in evaluating potential hazards of building near bluff tops. Accordingly, land, road, rail road, and topographic surveys, dating back to 1876, have been located. Some of these maps and tax assessor records suggest that 208
entire city blocks have disappeared in Encinitas since the town plat was filed in 1883 (Figure 2). Beginning in 1884, seaward property values decreased, while landward values increased. Some seaward par-cels were finally removed from the tax rolls. This apparent loss of real estate coincides with an ll year wet cycle which began in 1884 and ended in 1894. ft/ 1...,_ ,, ... q:., \.
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For example, Sorrento Valley, (Soledad Valley) south of Del Mar, was particularly hard-hit during February and March, 1884. The San Diego Union reported on March 2, 1884, that telegraph lines were down, rail-road tracks through the area were inpassable, and that two trains were:
" hemmed in on one side by landslides in the big Soledad cut, and on the other by the unsafe condition of the track, which is mostly underwater through the whole of Soledad flats."
Presently, the Sorrento Valley flood plain contains the same rail-road tr;cks, major freeway rights-of-way, and an extensive industrial deve lopmen t. In the southern portion of Del Mar, trains have been derailed, and even fallen over the sea cliffs. For example, a freight t rain went over the cliff on New Year's Eve, 1940, after the t racks were undermined by a series of heavy rains and large waves during unusually high tides, in addition to direct danage during storms, delayed damage also occurs. For example, the Self-Realization Fellowship Tenple was built in 1938, at leas t 30 feet back f rom the top of the sea cliffs. Follow-ing the 1941 storms, the bluf f, weakened by rain-saturated soil and crosion by wave action, collapsed, and the temple building was destroyed. Also, sections of the old coast Highway (101), immediately south of the Self-Realization Temple, have been moved landward, and the bluff face re-graded and dewatered because of cliff retreat. ARTIFICIAL C0ASTAL PROTECTION Artificial coastal protection, such as groins, jetties, bulkheads, sea walls, riprap, and rock debris are present along many sections of San Diego's retrea ting shorel ine. The crosive side ef fects of these artifi-cial devices are not fully understood, especially for periods of large storms and high tides, it has been observed that where such protective measures extend seaward beyond adjacent unprotected property, accelerated erosion occurs on immediately adjacent unprotected areas. This phenomenon was observed this past winter at La Jolla Shores, Ca:isbad, and Oceanside. During these January through April,1978 storms, beach-sand levels dropped te a near record low. Storm waves (accompanied by high tides) undercut the toe of cliff; unprotected areas (located adjacent to artificial pro-tections) were eroded headward in a short time. 210
SEA-CLIFF EROSI0li Ill URBAHlZED AREAS DURillG THE MAY 1973 - DECEMBER 1977 DRY PERIOD in areas where undocumented casual work suggested little or no coastal erosion, careful work, particularly north of Del Mar, has shown that many sea cli f f sections collapsed ing the 1973-1977 dry period. Failures have been typically of the " landslide" and " block-f all" variety. For the res t pa rt , cliff collapse has been instantaneous, with separation occurring along fractures and bedding planes. At least 25 block-falls have been reported along the Encinitas cliffs. Blocks vary in dimensions f rom 0.3 to 3.5 m (I to 12 feet) long, 1.5 to 33.5 m (5 to 110 feet) wide, and up to 10 m (32 feet) high. A particularly large block-fall occurred on flovember 30, 1977, at the foot of F Street in Encinitas. The cliff face, undercut by wave erosion, was probably also weakened by ground wate r in fractures. The collapse crushed the beach-access stairway, imme<liately following the collapse, water flowed f rom the cliff face at the Quaternary-Eocene un-conformity, and within the Eocene format ion below. Forty eight hours after the collapse, water ceased to flow, and then started again a few days later. RECENT BENCH AND CLIFF EkOS ION - J ANUARY TilROUGil APRIL , 1978 During the early 1973 s torms , poorly-cerented, clas t ic sedimentary rock that fell at the foot of F St reet , Enc i ni tas , on Novembe r 30, 1978, was broken up and dispersed during a single storm period by a combina-tion of wave action during high tides, heavy rainfall, and beach cobble abrasion. Older block-fall debris, in the sane area was similarly dis-persed between January and April . Beach cobble abrasion may be locally severe following rapid beach-sand depletion. Sea-caves, shea r zones , and poorly-cemen ted sedimenta ry strata were observed to be eroded by beach-cobble abrasion during periods of heavy surf and high tides. Low Quaternary clif fs, south of Scripps Institution of Oceanography, which appeared to have stabilized during the dry period f rom 1947 to 1977, were subjected to considerable erosion during the January and February 1978 storms. Houses built at thr cli f f edge, mos tly wi thout benefi t of sea walls or other protection, we re vulnerable to wave attack and cliff 211
erosion. Protective beach-sand was removed by a combination of high waves and tides, and erosion f rom rain gullying. Residents, threat-ened with land and structural losses, took desperate measures to pro-tect their belongings; old car bodies were placed at the toe of the slope, ri,, rap was cemented in place, and temporary sea-walls were in-stalled (Figure 3).
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h 1 k j _ Figure 3. h d & i_ R x-Storm waves battering homes along alluvial clif fs south of Scripps institution of Oceanography, looking south. A section of the concrete seawall in the foreground collapsed later. M. Clark photo, S.I.0., February 7, 1978. As these measures were only partially successful, some houses had to be reinforced to prevent collapse. Unprotected lots were the most severely damaged. At the Self-Realization Temple in Encinitas, a section of cliff separated along parallel f ractures and collapsed on April 26, 1978 (Figures 4A and 48). The collapse measured 3.6 to 4.9 m (12 to 16 feet) long, 34 m (112 feet) wide, and a maximum of 19 m (40 feet) high. Ground 2i2 h hD rw:- hM h= ). u cL Guuddbu i,
water flowed f rom the newly-exposed cli ff face. Rapid clif f degradation was observed wherever storm-drain pipes, fences, stairways, and lifeguard towers were present along former natural drainages. Water, collected in man-made structures, causes accelerated erosion wherever it is allowed to run directly over the cliffs. Beach-access stai nvays, located in or near these drainages, conrnonly collapse both at the top (Figure 5A), and the base of the cliffs (Figure SB). Storm drain pipe collapse often initiates severe gullying of the bluf f face. BLUFF-TOP GRADING Slope failures often occur following bluff-top grading. Also, drainage ways are of ten created along which surface runof f can pro-duce accelerated crosion of the bluff-top face. Where Quaternary terrace deposits and natural soils (including protective vegetation and case-hardened surface materials) are undisturbed, and drainage di rected away f rom the cli f fs, erosion appears to have been slow (Figure 6A). In 1971-72, along south Solana Beach, the bluff-top was excavated down 4.6 to 5.5 m (15 to 18 feet), and surface water was allowed to run over the bluff face. Upper cliff slopes have eroded, and in some cases collapsed, during periods of heavy rainfall (Figure 68). Part of the base of the cli ff has eroded headward 2.4 to 3.0 m (8 to 10 feet) between 1972-1978 (Figure 6C). Cliff collapse also occurred following extensive bluff-top grading in front of the Marine Biology Building at Scripps. Hannan (1975) reported that these cli f fs, carved into Quaternary sedimentary deposits, retreated 16.5 m (54 feet) between 1912 and 1975 In 1975, these cliffs retreated 1.5 to 2.4 m (5 to 8 feet) in places. EFFECTS OF GROUND WATER Irrigation of landscaped areas aloag the coastline has had at least three important effects: 1) it has caused the water table to rise, even during dry periods (adding to the weight of the soil, which favors landslide development), 2) it has increased porewater pressure in potential slide areas, and 3) in some cases, it has pro-duced solution cavities, favoring the formation of sea-caves some 213
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distance back from the cliff faces. CONCLUSIONS The concept of an average, long-term, regional, coastal sea-cliff retreat rate is likely valid over thousands of years. Short-term pro-vincial clif f retreat however, is episodic, and appears to be related to meteorological conditions, composition, induration and structure of clif f-forming formations, and to a c.ombination of natural and man-made erosive agents. REFERENCES Hannan, D. L., 1975, Sea clif f stability, west of the marine biology building, Scripps Institution of Oceanography, La Jolla, California: Benton Engi-neering, Project #75-9-18FG, 5 p., 2 maps. Kennedy, M. P., 1973, Sea cliff erosion at Sunset Cliffs, San siego, Califor-nia: California Geology, v. 26, p. 27-31. Kuhn, G. G. ,1977, Coastal zone geol 3gy and related sea clif f erosion, San Dieguito River to San Elijo Lagoon, San Diego County, California: Inte-grated Planning Organization Contract #11596-0800E, County of San Diego, California. Kuhn, G. G., and Shepard, F. P., 1979, Acceler ated beach-cliff erosion related to unusual storms in southern California: California Geology, v. 32,
- p. 58-59 Lee, L., Pinckney, C. , and Bemis, C. ,1976, Sea cli f f base erosion, San Diego, California: American Society of Civil Engineers, National Water Resources and Ocean Engineering Convention, April 5-8, 1976, Reprint 2708.
Pinckney, C. J., and Lee, L., 1973, Sea cliff recession study, southern one-half of San Diego County, in, Elliott, W. J. (ed.), Engineering Geologic Problem Areas, Southwestern San Diego County, California: San Diego Association of Geologists Guidebook for San Diego Chapter of International Conference of Building Officials and the San Diego County Planning Directors Association field trip. Shepard , F. P. , and G rant , U. S. , IV, 1947, Wave erosion along the southern California coast: Geological Society of America Bulletin, v. 58, p. 919-926. Shepard, F. P., and Wanless, H. R., 1971, Our changing coastlines: Mc' - -Hill Co., N.Y., 579 p. Vaughn, T. W., 1932, Rate of sea cliff recession on the property of tri Scripps institution of Oceanography at La Jolla, California: Science, v. 75, no. 1939, p. 250. Ziony, J. l., Wentworth, C. M., Buchanan-Banks, J. M., and Wagner, H. C., 1974, Preliminary map showing recency of faulting in coastal southern California: U. S. Geological Survey Map MF-585, scale 1:250,000. 216
ROAD LUG TO 5ELECTdo GEOLOGIC HAZARDS, SAll DIEGO I4ETF.0POLITAN AREA Gregory T. Farrand and Wililan J. ElIlott Cumulative liileage 0.0 Start trip f rom the Town and Country llotel at the San Diego River crossing with Fashion Valley road and proceed north. As a resul t of heavy rainfall in the winters of 1977 and 1978, the San Diego River flooded portions of liission Valley (see Chang, this volume). In the immediate area, the river rose to the base of the wooden pedestrian walkway located several hundred feet to the east and flooded portions of the Town and Country llotel property, Fashion Valley Road, and the adjacent golf course (see Abbott, this volume). 0.3 Turn left (wes t) on Fria rs Road. Turn right on llapa Street (1.7). Turn right (north) on liorena Uoulevard (2.0), then northwest on West liorena Boulevard at fork in road (2.1). Wes t tiorena Bot.levard runs subparallel and adjacent to the northwesterly-trending Rose Canyon fault zore (see articles by Gastil, et al.,Greene,et,al.,t sinons , and Th reet , this voluue). In this area, the reddish-brown sandstones ani cobbly sandstones are Late Pleistocene deposits cf the Bay Point Formation. 5.2 Turn west on Garnet Avenue. Turn right on Mission Bay Drive (5.7), left (west) on Bluffside Avenue (5.7), and right on Pacifica Drive (6.0). 6.1 Park next to the drive-in theater and walk to the cut slope located STOP 1 behind the theater. The uppermost cobble conglomerate bed has been mapped as the Pleistocene Day Point Formation. The yellowish sand-stone and conglomerate strata immediately below the Bay Point For-mation are deposits of the Pliocene San Diego Formation which are broken by the Country Club fault, a strand in the Rose Canyon fault Zone. 6.1 Continue north on Pacifica Drive. Turn lef t on Loring Street (6.4), then right (north) on Soledad liountain Road (6.9). 7.9 Turn right on Desert View Drive. 8.1 Eight homes on the west side of Desert View Drive were destroyed by landsliding in December, 1961 (Figure 1). The slide occurred during construction as a result of oversteepened cuts in faulted and jointed siltstones and claystones of the Eocene Ardath Shale. These homesites were subsequently regraded, including construction of buttress fills, and new homes have been built. Desert View Drive becomes Palomino Circle. d7 Turn right on Soledad flountain Road. Turn right on La Jolla Scenic Orive (9.6) and right on Soledad Park Road (10.0). 217
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10.2 Park by Easter Cross on Mount Soledad (elevation 811 feet) for STOP 2 an excellent view of metropolitan San Diego. Mount Soledad is, in part, underlain by a thin cap of red-brown cobble conglomer-ate and sandstone of the Pleistocene Lindavista Formation. Eocene Mount Soledad conglomerate occurs east of the cross and conglomerates of the Cretaceous Cabrillo Formation occur to the west. The northwesterly-trending Rose Canyon fault and Mount Soledad fault lie to the east and west of the cross, respect-ively. The large north-south trending canyon to the east is Rose Canyon, and the broad, flat surface beyond is Linda Vista Mesa. Note that the red sandstones and cobble conglomerate of the Lindavista Formation that cap Linda Vista Mesa, about 350 feet. 10.2 Return (west) to Soledad Park Road. Turn right (northwest) on Via Capri (10.4), lef t on Hidden Valley Road (11.3) and lef t on Ardath Road (11.7) . Go straight across Torrey Pines Road (11.9) and proceed north on La Jolla Shores Drive. Turn left on El Paseo Grande (12.8). Scripps Institution of Oceanography is on the right (north). Continue west to parking lot at Scripps (12.9) . 12.9 STOP 3 Walk about 100 yards south along the beach to observe seawalls and structural damage which occurred during recent winter storms (see Kuhn and Shepard, this volume). 12.9 Return to parking lot and continue south on El Paseo Grande. Bear right on Camino del Oro (13.2), then turn lef t on Avenida de la Playa (13.5). 13.8 Turn right on La Jolla Shores Drive. Turn right (west) on Torrey Pines Road (14.0) . 14.3 Til ted beds of the Cretaceous Point Loma Formation are exposed in cut slopes on the left. The Rose Canyon and Mount Soledad faults also occur in this area (not visible from road). 14.8 Turn right on Prospect Place. 15.0 Dear right and down ramp on Cave Street. Access to the Sunny Jim Cave (15.1) on the right (enter through La Jolla Cave and Shell Shop) was created by tunneling through Point Loma Forma-tion sandstones and shales down to an existing sea cave. Photo-graphs taken in 1906 and recently of the cave opening indicate that only minor erosion of the sea cave has occurred. Continue west cn Cave Street. 15.1 Cave Street becomes Coast Boulevard. Conti nue south on Coast Boulevard and/or Coast Boulevard South for a view of the shore-line. The coastal terrace is underlain by Late Pleistocene Bay Point Formation and Cretaceous Point Lore Formation. 219
15.9 Continue along shore on Coast Boulevard. 16.3 Coast Boulevard becomes Ravina Street. Turn right (south) on La Jolla Boulevard (16.4) and then right (south) on itission Boulevard (19.2). South on Pacific Beach Drive, tiission Boule-vard is built on a sand spit. Mission Bay, on the left, was, until development began in the 1950's, a slough with tidal flats called False Bay (Figure 2). 21.5 Turn left on West ttission Bay Drive. Turn right on Sunset Cliffs Boulevard (22.6). The San Diego River Channel (23.1) was con-structed by the U. S. Army Corps of Engineers in conjunction with dredging and development of tiission Bay. 23.4 Bear right on Sunset Cliffs Boulevard. Turn right (west) on Del Mar Avenue (24.8) and proceed to the dead end (25.0) . Park for Stop 4. 25.0 Prior to April,1979, an abandoned two-story apartment building STOP 4 existed on the now vacant lot on the south side of Del fiar Avenue. When the westerly one-third of the building foundation was undermined by sea-clif f crosion, the building was condemned and demolished. The underlying materials consist of weakly-cemented sandstones of the Bay Point Formation which are in turn underlain by indurated sandstones and shales of the Point Loma Formation. 25.2 Return to Sunset Cliffs boulevard and turn right (south). 25.7 Stop near the sea cliff at second parking lot on right. Differ-STOP 5 ential wave erosion along northwesterly- and northeasterly-trending joints within the Point Loma Fermation can be observed here (Kennedy, this volume). 25.7 Turn around and go north on Sur. set Cli f fs Boulevard. Turn right (east) on Point Loma Avenue (25.9), lef t on Catalina Boulevard (26.7), right on Chatsworth Boulevard (26.8). Bear right on Lytton Street (28.9), turn left (north) on Rosecrans Street (29.0). Keep right on Rosecrans at the Interstate Highway 5 South sign (29.7). Rosecrans becomes Taylor Street (30.0). Turn right (southeast) on San Diego Avenue (30.l) which then becomes Congress Street. This is the Old Town area of San Diego. 30.7 Go straight across (southeas t) San Diego Avenue on Congress Street. Stop at Congress Street cul-de-sac (30.9). 30.9 in the cut slope behind the apartment building is an exposure of STOP 6 steeply tilted and faulted beds of the San Diego Formation and Pleistocene Lindavista Formation. The Old Town Fault, a strand of the Rose Canyon faul t zone, has been napped about 1000 feet to the north of this stop by Kennedy, et al., (1975). Backtrack along Congress Street. Turn left (soutIi)Tn Old Town Avenue (31.0) and south on Interstate liighway 5 (31.2) . 220
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35.9 Take Coronado Island of f ramp (State Highway 75), pass through toll gate (38.1) and continue northwest on 4th Street. 38.1 A strip of land along the southeast side of Coronado Island is underlain by fill materials derived f rom channel dredging oper-ations in San Diego Bay. 38.3 4th Street becomes 3rd Street. Turn left on B Avenue (38.5), then lef t on 6th Street (38.8). 38.9 A gently sloping, topographic lineament with relief of several meters can be traced south-southwest f rom 4th St reet to the west end of Glorietta Bay. This lineament may be interpreted as a possible fault-line scarp since it is approximately aligned with an offshore fault (Coronado f aul t) identi fied by Kennedy , et al . , (1977) by seismic reflection profiling. I t coul d al so s idpTyTe an erosional feature. 39.0 Turn right on Pomona Avenue and continue south. Turn left (south) on Orange Avenue (39.7). Orange Avenue becomes Silver Strand Bou'.evard (39.8). The Silver Strand is a tombolo that connects Coronado Island to the mainland at Imperial Beach. An estimated sand loss of 1,400,000 cu. yds./yr, f rom Silver Strand Beach (Nords trom and inman, 1973) is periodically replenished with sand from channel dredging. 47.2 Silver Strand Boulevard becomes Palm Avenue. Continue east on Palm Avenue through imperial Beach. 50.1 Turn right (southeast) on Beyer Boulevard. Turn right (south) on East Beyer Boulevard (52.8), then lef t (southeast) on San Ysidro Boulevard (53.9) which is also referred to as Border Village Road. Bear lef t (northeast) up the paved alley (54.1) and go right (southeast) alongside railroad tracks past steel rail road depot building (54.2) . Just before reaching the USA / Mexico international Border fence, turn left (east) across rail road tracks onto a di rt road (54.3) . Follow this dirt road up the hill parallel to the Border fence. 54.4 Stop adjacent to fenced, abandoned transformer station on the STOP 7 left. The cut slope located at the back of the station exposes a steeply inclined shear zone which has been cited by Kennedy, et al., (1975, Figure 43) as evidence for extending the San Ysidro fault to the USA / Mexico border. An al ternative interpre-tation of this exposure (Hart, 1977) is that the shear zone is instead due to landsliding. Turn around and return to San Ysidro Boulevard. 54.7 Turn right on San Ysidro Boulevard. Turn right on East Beyer Boulevard (54.8). East Beyer Boulevard becomes Otay Mesa Road (55.9). Continue north-northeast. 222
56.5 Light grey sandstones of the Miocene Otay Member (Kuper and Gastil, 1977, and Scheidemann, 1977) of the Rosari to Beach Formation (Minch, 1967) are exposed in cut slopes on right. 57.0 Contact between yellowish cobble conglomerate and fine sandstones of the Pliocene San Diego Formation and the Miocene Otay Member. 57.3 Coarse, angular cobble to boulder conglomerate is exposed in cut on left. The majority of the top of Otay Mesa is capped by these locally-derived sediments which are probably Pleistocene terrace deposits. 57.7 Turn right (south) on Dillon Trail, a dirt road located about 100 feet before the stop sign at intersection of Otay Mesa Road and road leading to Interstate Highway 805. Row of eucalyptus trees are on the lef t side of road (58.2). Turn right (west) at fork in road (58.6), heading tos 3rd wire fence and shed on right side of road (58.7). Turn left (south) at the City of San Diego Sur-vey nonument sign (58.9) and bear lef t (southeast) at fuck in road (59.0). Follow road southeast along rim of Otay Mesa. 59.1 The hummocky terrain south of Otay Mesa is the San Ysidro land-STOP 8 slide (see Hart, this volume). Recent landslide features such as blocked drainage and well-developed topographic benches and head scarps can be observed (Figure 3). 59.1 Continue southeast along mesa rim. 59.3 San Ysidro slide /Tijuana River flood plain overlook. Ponds in a STOP 9 graben of the landslide can be observed during periods of high p reci p i ta t i on. The lineaments described by Kennedy, et al. , (1975) as faults and interpreted by Hart (1977) as laiidsTide scarps can be seen here. 59.3 Proceed southeast along mesa rim. 59.6 in bottom of swale, turn left (northwest) on di rt road and proceed back toward eucalyptus trees and Dillon trail . 60.9 Turn right (north) on Otay Mesa Road f rom Dillon trail . Turn left (west) at stop sign (61.0), proceed toward Interstate Highway 805 Turn right (north) on interstate Highway 805 (61.7). Take Calif-ornia Highway 94 east (73.0). 76.2 Eocene Stadium conglomerate is exposed in cut slope on the right. 78.2 Jurassic Santiago Peak Volcanics are exposed on the lef t. 79.4 Take California Highway 125 north. Turn right on Severin Drive / Fuerte Drive of f ramp (81.5), lef t on Severin Drive (81.9) and left (northwest) on Amaya Drive (82.4). 82.7 Turn right on Fletcher Parkway. At this intersecti n the Eocene Stadium Conglomerate is exposed. 223
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arcuate head scarps, hummocky topography and ponds, 1%3 edi t ion , San Ysidro 15 minute quadrangle, surveyed 1%1. t.' wism,'%. . p : s Mu 0,w d ul @;l - dH ejrdia u 3,
83.7 Turn left on Navajo Road. Then right on Fanita Drive (84.0) . 84.5 Red-brown c'hble conglomerate and sandstone of the Eocene Pomerado Co glomerate and the underlying light grey to tan sandstone aid mudstone of the Eocene Mission Valley Formation are exposed on right at top of hill. 84.8 Eocene Stadium Conglomerate is exposed on the right. The cobbles of the Stadium and Pomerado Conglomerates consist predominantly of distinctive, slightly metamorphosed rhyolite and rhyodacite "Poway" clasts. The conglomerate being mined for aggregate on the north wall of Mission Valley (northeast of Town & Country Hotel) is primarily Stadium Conglomerate. 85.0 Light grey to brown sandstones of the Eocene Friars Formation are exposed on the right. 85.8 Turn right (east) on Fanita Rancho Road. Bear lef t on Todos Santos Drive (85.8). 86.0 On the right side of Todos Santos Drive is the site of the STOP 10 Fanita Corona Subdivision landslide described by Hannon (1970) in which eight houses were destroyed and several others dis-tressed by landsliding in 1965-66. The active portion of this slide occurs within a larger ancient landslide mapped by Hart (1972). The larger ancient landslide extends up the hill toward the water tank. Sedimentary rocks in this region consist of shallow-dipping, pale green to light grey claystones, mud-stones and sandstones of the Eocene Friars Formation. Many of the ancient landslides and reactivated landslides in the San Diego region occur in the Friars Formation. 86.1 Turn right on Fanita Rancho Road. Turn right on Fanita Drive (86.3), right on Mission Gorge Road (87.2) and lef t (north) on Carl ton Hills Boulevard (87.4). Proceed to north end of Carlton Hills Boulevard to Stop 11. 89.0 Following heavy rains in the winter of 1977-78, an ancient STOP 11 landslide was reactivated that affected about 25 homes, some of which were damaged and subsequently abandoned. In an effort to stabilize the slide, remedial grading was done to remove some of the driving forces from the head area (see Hart, this volume). Surface drainage structures were constructed to reduce water infiltration. Disrupted streets, sidewalks, and utilities were also repai red and most of the homes are still occupied. 89.1 The two homes on the west side of Carlton Hills Boulevard at Swanton Drive are located at the westerly boundary of the active portion of the slide. These homes underwent severe distortion. The head scarp of the reactivated portion of this landslide can be observed about 200 feet north of Carlton Hills Boulevard cul-de-sac. Retrace route along Carlton Hills Boulevard. 225
90.3 San Diego River. 90.7 Turn right on Mission Gorge Road. Turn right on Father Junipero Serra Trail (92.6) and follow through Mission Gorge along the San Diego River. 93.3 Old Mission Dam on the right was constructed of terra cota brick between 1803-1816 by the Franciscan fathers to provide water to the Mission San Diego de Alcala in Mission Valley. 95.3 Turn left (east) on Mission Gorge Road. View soil slips in cut slopes on right. The geologic materials consist of Eocene Friars Formation claystones and sandstones. Turn right on Golfcrest Drive (95.8) and right (west) on Monteverde Drive (96. 3) . Park near 6855 Monteverde (96.6). 96.6 Rowena landslide. In early 1978, an ancient landslice was re-STOP 12 activated, damaging six homes at the top of a cut slope along the west side of Rowena Avenue (uphill to the east). Six other homes, situated at the toe of slope (along the east side of Monteverde Drive), were also threatened by novement at the toe of the slide. Plastic sheeting was placed over the slope to reduce water infiltration during the 1978-79 rainy season. Move-ment continues and the slide is now encraoching within a few feet of several homes at the toe. Proposed remedial measures include removing some of the homes at the top, regrading the slope to a 3 1/2 (horizontal): 1 (vertical) slope and construction of a shear key and buttress fill. 96.6 Turn around and return to Golferest Drive. 97.0 Turn right on Golfcrest Drive. Turn right on Navajo Road (97.6), left on College Avenue (99.6), right (west) on Interstate Highway 8, exit at Hotel Circle (107.0) and go eest. Turn left on Fashion Valley Road (107.3) . 107.3 End of trip at Town & Country Hotel. REFERENCES Hannan, D. L., 1970, Engineering geology of a landslide area at Otay Mesa, San Diego, and a landslide in Fanita Corona Subdivision, Santee, California: Senior Report ('unpub.), San Diego State University. Hart, M. W. ,1972, Landslides of west-central San Diego County, Cali fornia: Masters Thesis (unpub.), San Diego State University. Hart, M. W., 1977, Landsliding, an alternative to faulting in San Ysidro, California, in, Farrand, G. T., (ed.), Geology of Southwestern San Diego County, California and Northwestern Baja Cali fornia: San D: ego Association Geologists Field Trip Guidebook, p. 37-42. 226
Kennedy, M. P., Tan, S. S., Chapman, R. H., and Chase, G. W., 1975, Character and recency of faulting, San Diego metropolitan area, California: California Division of Mines and Geology Special Report 123 Kennedy , M. P. , and Velday , E. E. , 1977, Character and recency of fault-Ing of fshore f rom urban San Diego,13 Studies on Surface Faulting and Liquefaction as Potential Earthquake Hazards in Urban San Diego, California: Cali fornia Division of Mines and Geology, p. A A-15 Kuper, H. T. and Gast il , G. , 1977, Reconnaissance of marine sedimentary rocks of southwestern San Diego County, h, Farrand, G. T., (ed.), Geology of southwestern San Diego County, California and north-western Baja California: San Diego Association Geologists Fieldtrip Guidebook, p. 9-15 Minch, J. A., 1967, Stratigraphy and structure of the Tijuana-Rosarita Beach area, northwestern Baja California, Mexico: Geological Society America Bulletin, v. 78, p. 1155-1178. Nordstrom, C. E. and inman, D. L., 1973, Beach and cliff erosion in San Diego County, California, h, Ross, A. and Dowlen, R. J., (eds.), Studies on the geology and geologic hazards of the greater San Diego area, California: San Diego Association Geologists Fieldtrip Guidebook,
- p. 125-131.
Ross, A. , and Dowlen, R. J., (eds.),1973, Studies on the geology and geologic hazards of the greater San Diego area, California: San Diego Assoc-lation Geologists Fieldtrip Guidebook, 152 p. Scheideman, Jr. , R. C.,1977, Correlation of the Otay and Rosarito Beach Formations, in, Farrand, G. T., (ed), Geology of southwestern San Diego County, California and northwestern Baja California: San Diego Association Geologists Fieldtrip Guidebook, p. 17-27 227
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