ML20151H077
| ML20151H077 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 04/12/1988 |
| From: | Barnhard T, Harding S, Urban T INTERIOR, DEPT. OF, GEOLOGICAL SURVEY |
| To: | |
| Shared Package | |
| ML20151H012 | List:
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| References | |
| NUDOCS 8808010133 | |
| Download: ML20151H077 (12) | |
Text
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k EVIDENCE OF POSSIBLE QUATERNARY FAULTING IN PUCET SOUND l FROM A MULTICHANNEL MARINE SEISMIC-REFLECTION SURVEY by Samuel T. Harding, T.C. Urban, and Theodore P. Barnhard U.S. Geolostcal Survey Introduction In April 19P7 a 461 -km mul ti chennel marine seismic-reflection survey was run in Puget Sound (fig. 1) to investigate the possi bil tl es of Quaternary rapiting in the Puret Sound region and to establish a framework for integration of numerous single-channel seismic-reflection profiles and well-log data into ongoing geologic studies of the area.
To position the track lines in PuPet Sound and the Strai ts of Juan de Fuca, a combination of Loran C ar.d CPS (elobal Positioning System) systems were used. The track 115e9 were first positioned by Loran-C readings. At points on the line where GPS positions could be made, the Loran-C line wea repositioned to these points. A backup na vigation system was also used where the radar screen was photographed and tied to shot-noint numbers to be used should the Loran-C/ CPS syst em f ail or if there had heen a need or addi tionel information.
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Narrow waterways limited the size of the boat and length of hydaophone streamer used for the survey. The system consisted of a 15-in{150m)watersun, and a 24-channel hydrophone streamer (6.2'-m groups) to record at a 1-ms samp2e rate. Figure 2 shows the relation of the boat to the watergun and streamer.
Of the 15 lines run. lines 1 thru 12 were r ecorded to 2.000 ms with a shot interval of 6.25 m. line 13 was recorded to 1,000 ms and processed at 12-fold.
Lines 18 and 15 were recorded to 1,000 ms wi th a shot interval of 3 17 m and processed to 24-fold.
The data were processed using DTSCO software of the Branch of Petroleum Geology. Because a short hydrophone streamer was l usad, processing packages for sea-bottom ruitt ple suppression which are based on the ability of the programs to separate events by the amount of normal moveout could not be used. The short streamer did not show enough difference between rea' events and multiple events to apply a dip selecting filter. To reduce these mul ti pl es , a series of cascading deconvolution programs to suepress both t he s ea-bot t om m ul ti pl es and int erbed mul ti ples (pegleg multi ples) were used for a first pass at the data. In the furture, wavelet processing techniques will be employed where warranted. The deconvolution programs enta11e d digi tising water depths and applying a sp! king deconvolution operator from the water bottaa through the section to remove the pegleg multiples generated in the layers below the watar-sediment interface. A gaping decenvolution was then applied with the gap det ermined from the depth of water. This suppressed the water-bottom 5
m ul ti pl e in most places. Other than spe ctral whi tening, standard marine processing sequence was used.
8808010133 880615 PDR ADOCK 05000344 I P PDC
Numerous a t ngle-channel marine sel sni c-refl ection nurveys k have been conducted in Puget Sound with good r osul ts. Therefore, it was hoped that a multichannel system, designed for skallow penetration, woul d improve the da t a r esolution and augment previous ningle-channel work. While multichannel data vields better r esults than single-channel dat a wher e sea-bottom mul ti pl es are a problem and where the CDP method i ncr eas es the r eflection resolution, single-channel data supplies more information for s tructures j ust belcw the water bottom.
Background information and experience obtained by the Geological Survey were utilized te conduct this marine study (Lee and others, 1987: Shedlock arid H ar di ng , 1982). Quat ern ary f aulting investigations using seismic-refl ect! on t echniques are documented i n Shedic ek and Harding (1982), Harding and others (1983), Crone and Harding ( 19 8 2' ) . Skipp and Hardi ng (1985). Harding (1985),
H a r di rig (1985b), Harding and Stewart (1986), Whitney and others (198'), Harri ng and others (19*6), Har di ng and Barnhard (19P7) and Shedlock and Harding (1982).
This survey was conducted to cross th* geophysical anomalies outlined by Yount and others (1985) and also to inves tigat e the possibl e acti ve s tructures as rescribed by Gower and others (1985). Individuals from the Washington Department of Natural R es our ces , the University of Washington, and the Pacific Geosciences Center who were familiar with the regional geology were consulted to det ermine t he location whi ch would yield the best results. At this time the data have been processed t unito-mly. In aress of interest, further processing is required to enhance f eatures which seem to show interes ting struct ures.
Geophysical Anomaly Striking Eas t-Wes t Across Seattle The most persistent f eat ur e t hat could be traced from line to 11ae appears alorg the track lines crossing the geophysical lineament that crosses east-west through Seat tle. A persistent acoustically transparent zone occurs alonC the strike of this lineament. This came as no surpri se, because at both Restoration P oi nt and at Alkt P oi nt where Tertiar y rocks outcrop as the Blakely Formation, the strata dip at a near-vertical angle. This explains the lack of reflected energy f r cit deeper beds across this arts. While close spacing of the geophone groups reduced the effects of spatial aliasing during processing, the short streamer limited the aperture of the receiving system for picking up steeply dipping beds. An example of the a cous ti call y transparent chara ct eris ti cs on one of the reflection lines in Lake Washington is shown in figure 3. This section is nearly void of seismic energy from CDP 3700 south to about CDP 2800. A s*rong band of refl ecto s at 4r0 ms occurs in the northern part of Lake. This band i s flat near the eas t side of northern pontooa bridge, changes dip at at CDP 4400, and at CDP 3^50 is at ;
300 ms. Without any well control in the immediate vacinity, it l can only be assumed that this band of reflectors is coming from
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the upper part of the Tertiary. This same structure is seen on lines 1 and 2 on the east side of Mercer I sl an d . The nor t hwar d- l dipping reflectors b el ie ve d to be from the Tertiary are present, l 2
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[' but they are not as pronounced as those on line 3 on the west side of Mercer Island. This may be due to the f act that the channel on the east side of the !sland is very narrows therefore, side-channel r e fl ections as well as the other reverberations may have degraded the signal. Structures ser.n on the upner parts of the section from the water bottom to 300 ms do not appear to correlate from one Ifne to the other, indicating that these structures have no great lateral extent. Since there is no corr el a t i on between these layers al ong the s ei smi c-reflict i on lines, between-line c orr el a t i ons were not expected. Layering above the Tertiary strata is gl a ci al in nri gin and is expected to be chaotic.
If t hi s a cous tl call y transparent :one had been an isolated observation, it could have been attributed to surf ace conditions; however, this was not the case. A broad band of little or no sei smi c r efle cte d energy return f ollows the geophysical lineanent whi ch 1* definer by a steep gravity gradient and a l'ne of Tertiary rock outcroping, from the aeri al extent and persistence of this feature frem Lake Washington (lines 1 and 3) to line 5 at l Alki Point and line 4 at R es t or ation Pol nt , it was con cl u de d that the phenomena does not have a surficial ori gi n but that this lack of seismic energy is due t o the deeper structure across a broad area which crosses Seattle and Puget Sourd.
There are indications of youthful f aul ting , as seen f r om t he sei sr! c-refl ecti on lines that cr os s this feature both in Lake Washington and eff Bainbridge Island. One f aul t is indicated on the figure 3 If our cor r el at i on is r et s ona bl e . there is 50 m of veritical displacement of the Tertiary reflector on the south s!de of the fault. The ,*ediments above t hi s f aul t! nc of the Tertiary reflector also appear to be faulted, but our best guess is that this fault ts down to the south. This i ndi ca t es a complex subsurface-faultlag pattern which hints of a strike-slip origin f er th* entire :one. Indi ca t i ons of recent f aul ting wher e line 5 crosses the nouth of Blakely Hirbor will be di s c us s e d in
- more detail at a later time.
The Geophysleal Anomaly in llood Canal 4
A geophysical anomaly in Hood Canal (Yount and others, 1985) extends i nt o Dabob Bay and may *xtend into a land fault at the '
northern end of Dabob Bay (see flg. 4). T hi s is a west-facing 7b-m feature that lies abova a s t eap r efl ect ion di scont inui ty and is coincident with the tra ce of a fault reported by Gower. The Tertiary hed5 dip t o t he east and cannot be correlated beyond CDP 12.450. If the sediments (CDP 12,300) near the water botten are of T er' ! a"y age, vertical di s pl acem ent on the fault is on the order of 700 m.
Fault mapped by Wagner and others in Admiralty Inlet The Admiralty Inlet fault of Gower and others (1985) shows fault G extending northwest but not extending int o A dmi ral ty Inlet. However, Wagner and Wiley (1980, 1983) do extend this 3
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(- f aul t no+thwest through the iniat.
this proposed extension of the fault.
Lines lines '0A and 11 cross Tertiary reflectors on the south ends of these lines can be e oen. Although a f ault may exist here, its sense of motion is different than indicated Cower and others (1985). Warner and Wil ey (1980, 1983) show the cxtension of the fault to be down-to-the-west at the north end and down-to-the east at the south end of the extension to Cower's and others (1983) fault C. Other f aults are present in the Straits of Juir da Fuca: Macl eo d and others (1977) have mapped several.
f aul ts .
Our refiertion lines ha ve crossed several of these Of these, the only f ault having any surf ace or discernible subsurf ace expression is f ault C (Ma cl eod an d others ,
1977), which appears es a large warp or the sea fl oor (also shown in Wagner and Wiley, 1983) and as a possi ble reflection discontinuity on line 10. There is a slight bump on the ocean floor on line 8 where this f ault i s mapped. The other faults cannot be seen on the seismic sections.
Possible faults f ound associated wi th the Seattle geophysical anomaly i
The largest and s teepes t s ea fl oor s t ep was f ound j us t off Alk' Point i n wes t Se at tle (fig. 5). The height of the step in places
' is 115 m and can be mapped for 1 km as a very steep water-bott.on ;
scarp. This in itself does not imply f a ul ti ng. There are a (
' numbe" of possib)e explanations for such a feature in glacial l terrain. Our case for this being of fault origin is as f oll ows : (11 The band of refl ectors seen on the hanging wel' 50-i ms-wide cannot be corr el at e d with a similar band found on tAe f oot wall side of the f aul t i n the middl e of Puget Sound (not I shown on fig. 5).
' This implies that these surfaces were once coan*etar and late- displaced *y faulting. (2) The morphology of the ne ar-s urf ace strat a is si mil a r to what woulc be expected of a i i
revera e f aul t. A cartoon depict.93 this type of morphology is I shown at the corner of figure 5. The position of the tensile j
structures as well as the uplifted features have a striking similarity t o that shown in the cartoon. The strong reflector at '
about 500 ms is terminated along a proposed f aul t plane.
unfortunately this reflector cannot be seen on the hanging-wall sida of the fault. This work was reported by Harding and (
Barnhard (1987). !
On the west side of Puget Sound near Restoration Point, (
t here are t wo ses-bot t om features coincident with Bla kely Har bor ;
which, on the surface, look like a Fraben structure. The southern graben boundary may connect to a fault mapped by Walden l j
r (1967). The northern east-west f aul t is not mapped. There are
' irdications on the seismic seetion that theso may well be Taylts, but the su' surface seismic-reflection data are s pars e. This area t l
1s in the middle of the Seattle ge4 physical anomaly where very l little seismic energy is ret urneo.
that indicate the northern step ,
There ar* broken reflectort i the sea floor is f&ulted. I t
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( Possible f aults espped in Commencement Bay
- There have been been two propooed f aults mapped in Commencement Bay by other geophysical studies. One mapped by Rogers (1970) is shown by Gower and others (1985) and also Yount and others (19R5) show a possible eas t-wes t-trending f aul t i through Commencement Bay and crossing Valson Island. Sy1 wester 1 (1971) maps a similar fault but with a slightly dif f erent strike f i..
in this same area using single-channel s ei smi c-r efle e tion data. !
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' Lina 5 does isot show either one of these f aults but this is not !
definitive because line 5 may not have started far enough to the j south to int ersect either of these fault 9 Line 4 which goes as &
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f ar south as the Tacoma Narrows Bridge in West Pass to the Wes t l I
of Vision Island does not show either one of these faults. I 1 C or'cl us i ons {
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Tha results of this invest!gation are preliminary because the information gathered from this study is only now at a point
{ where we are able to target areas where f urther processing should be done. Indications of post-Pleistocene f aulting have been }
j j
fcund on these lines, but further processing will be required te better define the nature and extent of the f aulting. {'
Acknowledgments i '
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We would itke to thank a numbea of people wi thout whone help this study could not have been accomplishec. Thanks go to Dave j Hichols (Branch of Atlantic Marine Geology) for putting t ogether >
the acquist tlon system and seeing to it that everything arrived !
i at the ri g* t place at the right t i m e', and to John Erickson i
(Branch of Pacific Marine Geology) who put together the compressor tystem for the watergun and stayed with the survey '
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until it was working properly. Richard E. Sylwester (Williamson i
)
and Assoel ates) and Mark Holmes (Branch of Pacir *4srine Geology) i h el pe d wi t h the local logis tics and their knowltoge of previous I I
seismi c work in Pucet Sound was i nval ua bl e. 'J e w oul d l i k e t o ,
l thank Mark Holmes and George White (Uni vers'.e y of Was hi n gt on) for i i
1 allowing es the use of their fac!11ttes ar a staging area. Tom Bice (Branch of Geologic Risk Ass essment ) put together the GPS f nsy1gatter s ys t em and Niegl e Font ai n ('4agn a vox) helpee us put the ;
GPS system together on a crash basis. Also, thanks to Ben j
j Huntley, Captain of the R/V Redoubt, who put in long hours u= der j I
trying conditions to ensure the success of the survey and to Kristin Dew who provid'd land-support for the survey. ',
l i References Cited i
Crone, A., J., and Hardi ng. S. T.. 19P4 The relatiorship of Late i
Quat ernary f a ul t scarps to subjacent f a ul ts. Eastern Great Basin, Utab
! Geology, v. 12 p . 292-295.
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5
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- l t
i Crore, A. J., and Harding, S. T., 1964, Near-surface faulting .l associated with Holocene f ault scarps, Wasatch f ault zone, i l Utah--a prelimi nary report , in, Hays, W. W., and Cori, P. L., l l eds., Proceedings of workshop XXVI on "Evaluations the !
regional and urban earthquake hazards in Utah": U.S. ;
j GeoloFical Survey Open-File Report 84-763, p. 241-268. I Gower, H. D., Yount, J. C., and Crosson, R. S., 1985, Seismotectonic map of the Puget Sound Region Washington:
t U.S. Geological Survey Miscellaneous Investigntions Map I- {
i 1613.
i l
Harding. S. T., 1985. Preliminary results of a high resolut t en i reflection survey across the Meers f ault, Comanche County, !
d Oklahoma (abs.): Earthquake Notes, v. 55, no. 1, p. 2. !
i j Harding, S. T. (in press), Preliminary results of hith-resolution l
j seismic-refl ection surveys conducted aero*s the Beatty scarp, !
Beatty, Nevada, and the Crater Flat fault scarp, Nevada: U.S.
j Geological Survey Bulletin.
t 4 F *.r c i n g , S. T., and Stewett, '. M., 1996 Hi gh-res ol ut i on s ei smi c-j reflection surveys near Charleston, South Carolina (abs.]:
- Earthquake Notes, v. 57, no. 1, p. 17.
I !
i ; larding, S. T., and Barnhard, T. P., 1987 Holocene faulting as indleated f rom marine seismic-refl ection survey in Puget Sound i Washington: EOS (Transactions of the American Geephysical Union), v. 6 8, no. Ah, p. 1240.
4 4
l King, Geoffrey, and Bailey, Geoffrey, 1985. The paleovnvironment of some arc *eological sites in Greece--The influence of accumulater uplift in seismically active regions: Proceedings of the i Prehistori c Soci et y, v. 51, r. 273-382.
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Ma cl eod , N. S., Tuffine, P. L., Sna vely , P. D., Jr., and Curie, R. G., !
1977. Geologic interpretatter of magnet'e and gravi ty t. nomali es in the Straits of Juan de Fuca U.S.-Canada: Canadian Journal of Earth Sci ences, v. 14 no. 2, p. 223-238.
Rogers, W. P., 1970, A geological and geophysical study of the central Puget Sound 1owland: University of Washington, Ph.D.,
l Seattle, ;
Washington, 123 p. l l
! S he dl ock , X. M., and Harding, S. T., 19'2, Mirsissippi seismic
)
surveys coophysicai Researen tetters, v. 9, p. 1275-:278. j t
l 6 L_ ,___ - _ _ _ _ . - . __ __ ._ __._ _ __-_,_ _ .. a
i J I
k Shedlock, K. M., Harding, S. T., and Brocher. T. M., 1997. The 4
Par kfi el d shallow-r efle ction ex periment--Struct ur e of t he ;
dilattoral jog southeast of Gold Hill (abs.): EOS [ Transactions of the American Geophysical Union), v. 68, p. 1345. L
- Skipp, Betty, and Hardinr, S. T., 1985, Preliminary r eport on geology
) of Borah Peak area Idaho, incluoing interpretation of seismi c and [
- gravity data, i rt Stein, R. S., and Burknam, R. C., eds.,
i Proceedings of wor < shop XXVIII--The Borah Peak earthquake U.S.
G eol ogi c a l Survey Open-Fil e R eport 88'-290, p. 837-671 1 l l Swadley, W. C., Yount, T. C., and Harding, S. T. (in press). ;
j Reint erpret ation of the Beatty scarp U.S. Geolog! cal Survey i i Bulletin.
i Sylwester, R.
E., 1971 The determination of active fault zones in Puget Sound. WashinFton, by means of continuous seismic i j
, pref!11ng: University of Washington, M.S. thesis, Seattle, ,
Washington, 87 p. I Wagner, H. C., and Wiley, M. C., 19Po, Praliminary map of of f shere geology in Prote ction Island-Point Parbeidge area, northern Puget
)
j Soune, Washington: U.S. Geological Survey Open-File Report 83- !t i 548.
1981, nffshore Quaternary geology of the north Puget Sound-4
' easter Strai ts of Juan de Fuca region Washington, t rt Yount, J.
C., and Crosson, R. S., ;
eds., Proceedings of Workshop XIV--
Earthquake ha:ards of the Puget Sound region, Washington, U S. '
Coologion1 Survey Open-File Report 8?-19, p. 178-267.
} Waldon, H. H., 1967, Duwamish Ovad quadrangle, King and Kitsap 1
Count i es, Was hinf ten: U.S. Geological Survey Geologic Quaerantle ,
Map GC-706.
Whi tney , S. W.. Shreba, R. R., Semand, F. W., and Harding, S. T.,
1986, Recurrent Quaternary movement on the Windy Wash f ault. N ye Geologie=1 Soel ety of America Abstracts County, Nevada (abs.):
i with Programs, p. 787. l l
Yount, J. C., Dembroff, C. R., and Barats. G. M., 1985. Map ahowing j
4 depth to tedrock in the Seattle 30' by 60' quadrangle, l Washington: U.S. Geologieml Survay Fiscellaneous field Studies
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