ML19345D596
| ML19345D596 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 12/31/1980 |
| From: | SOUTH CAROLINA ELECTRIC & GAS CO. |
| To: | |
| Shared Package | |
| ML19345D595 | List: |
| References | |
| NUDOCS 8012160165 | |
| Download: ML19345D596 (375) | |
Text
.- - - - - - - - - - -
II O RE30 0 I
i SL 33 _ EM E \~~A _ S'E SMO _OG C JI NVEST GA- ON :
I
, Virgil C. Summer i Nuclear Station i Init 1
' 30C(E NO. 50/395 l
l I
)
I I
' SOU~- CARO _ VA E_EC R C 8 GAS COM3ANY I
l so,,,,,\,
JE E ER- %0
i I
i I
i -
lI I
i i
i 4
i I
I f
f EVALUATION OF SITE-SPECIFIC SEIS:llCITY AT THE l VIRGIL C. SUMMER NUCLEAR STATION I
- I ll 1
!E l1 I
i I
lI i SOUTH CAROLINA ELECTRIC & GAS COMPANY
!I l DECEMB R 1980 I
i l
1
[
TABLE OF CONTENTS Section Page I
Table of Contents . .. . . . . . . . . . . . . . . . . . . i List of Appendices .. .. . . . . . . . . . . . . . . . . iii
1.0 INTRODUCTION
. . ... . . . . . . . . . . . . . . . . I 1.1 GENERAL . . .. . . . . . . . . . . . . . . . . . I 1.2 EXECUTIVE SUliMARY OF FINDINGS . . . . . . . . . . 1 2 2.0 SEISMICITY ASSOCIATED WITH THE MONTICELLO RESERVOIR . 5 2.1 GENERAL . . .. . . . . . . . . . . . . . . . . . 5 I 2.2 SEISMICITY NEAR MONTICELLO RESERVOIR BEFORE IMPOUNDMENT . . . . . . . . . . . . . . . . .. . 5 2.3 SEISMICITY FOLLOWING DiPOUNDMENT OF MONTICELLO RESERVOIR . .. . . . . . . . . . . . . . .. . . 5 1
2.4 TEMPORAL AND SPATIAL BEHAVOIR OF SEISB11 CITY WITH RESPECT TO MONTICELLO RESERVOIR . . . . . 6 I
3.0 ASSOCIATION OF INDUCED SEISMICITY WITH GEOLOGICAL, GEOPHYSICAL, AND HYDROLOGICAL ANOMALIES . . . . .. . . 8 3.1 IN SITU STRESS CONDITIONS AT MONTICELLO . . . . . 8 3.2 ASSOCIATION OF COMPOSITE FOCAL MECHANISMS WITH FRACTURE DISTRIBUTION AND ORIENTATION .
I
. . . . . 8 3.3 CORRELATION WITH GEOLOGICAL AND GEOPHYSICAL ANOliALIES . . . . . . . . . . . . . . . . . . . . 9 3.4 HYDROLOGIC DATA AT AND NEAR MONTICELLO RESERVOIR. 10 4.0 MAXIMUM MAGNITUDE OF INDUCED EARTHQUAKE AT MONTICELLO RESERVOIR . . . . . . . . . . . . . . . . . 12 I 4.1 GENERAL . . .. . . . . . . . . . .. . . .. . . 12 4.2 SITE SPECIFIC DATA THAT IMPLY A !1AGNITUDE LD11T OF Mt = 4.0 . . . . . . . . . .. . . . . . 12 4.3 EMPIRICAL DATA - SEISMIC MOMENT AND AREA 0F INTENSITY VI SHAKING . . . . . . . . . . . . . . . 13 4.4 SU101ARY DISCUSSION . . . . . . . . . . . . . . . . 13 5.0 COMPARISON OF NEAR-FIELD EARTHQUAKE GROUND MOTION TO SSE SPECIRA AND DISCUSSION OF EFFECTS . .. . .. . . 16 I
I I 1
1 i
! TABLE OF CONTENTS (Continued) i I Section Page
6.0 CONCLUSION
S . . , , . . . . . . . . . . . . . . . . . . 17 6.1 GENERAL . . . . . . . . . . . . . . . . . . . . . 17
6.2 CONCLUSION
S REGARDING THE SPATIAL AND TEMPORAL
- DISTRIBUTION OF SEISMICITY AT MONTICELLO v.RVOIR . . . . . . . . . . . . . . . . . . . . 17 i
6.3 CONCLUSION
S REGARDI.NG THE RELATIONSHIP OF fg OBSERVED SEISMICITY TO LOCAL GEOLOGY, GEOPHYSICAL ANOMALIES AND HYDROGE0 LOGIC REGIME . . . . . . . 18
- .5
6.4 CONCLUSION
S REGARDING STRESS DISTRIBUTION IN THE j MONIICELIT RESERVOIR \REA . .. . . . . . . . . . 19
6.5 CONCLUSION
S RELATIVE IO PROABILISTIC ANALYSES OF RESERVOIR INDUCED EARTHQUAKES AT MONTICELLO j RESERVOIR , . . . . . . . . . . . . . . . . . . 20
6.6 CONCLUSION
S RELATIVE TO COMPARISON OF MAXIMUM
',g INDUCED E ;RTHQUAKE TO SSE DESIGN SPECTRUM . . . . 21 i5 6.7 SDDIARY OF CONCLUSIONS . . . . . . . . . . . . . 21 iI ie 4
il
!I
}
i ll lI
!I
!l i 11 lI.
,p.m x e---,, --m..- - ,
I LIST OF APPENDICES 1
I Seismicity Associated with Impound =ent of Monticello Reservoir II Temporal and Spatial Behavoir of Induced Seismicity at Monticello Reservoir III In Situ Stress Conditions at Monticello Reservoir IV Fault Plane Solutions and Association with Pre-existing Fractures V Association of Seismicity with Geology and Geophysical i Anocalies VI Hydrologic Data Around Monticello Reservoir VII Stress Dreps Associated with Microearthquakes at Monticello Re s e rvo s.r I VIII Association of Area of 5D1 Intensity VI and Magnitude of Induced Earthquakes IX Proability Analysis of Monticello Reservoir-Induced Seismicity X Ef fects of Near-Field Earthquake Ground Motion on Structure and Equipment Design i
I I
I I
I 1
I
I EVALUATION OF SITE-SPECIFIC SEISMICITY AT THE VIRGIL C. SUMMER NUCLEAR STATION
1.0 INTRODUCTION
1.1 GENERAL The purpose and scope of this summary report is to provide, in one coherent presentation, all of the observational data and analyses used to support the position of the Applicant that tne seismic activiti associated with Monticello Reservoir is: 1) a superficial phenomenon;
- 2) limited to low magnitude earthquakes not expected to exceed ML=
4.0 because of strong heterogenetties in physical properties and stress state beneath the site; 3) not expected to generate ground motions that exceed the original seismic design as derived from the Safe Shutdown Earthquake for the Virgil C. Summer Nuclear Station.
1.2 EXECUTIVE
SUMMARY
OF FINDINGS This evaluation addresses the key issues of the nature and extent of reservoir-induced seismicity and implications with respect to future activity that might affect the Virgil C. Summer Nuclear Station. All of the available observational data which form the basis for this evaluation are included as appendices to this report.
The most important conclusions are:
(1) Induced seismicity is limited in both space and time to superfi-cial zones beneath and in the immediate vicinity of the reservoir.
(2) Accurate hypocenter depths determined from the 10-station network (magnetic tape data) are all shallow (<3 km), and the largest magnitude that has occurred is ML = 2.8.
(3) Because of the lateral heterogeneities in rock properties and stress conditions observed to exist, only small fault areas (N1 km or less) can experience movements in any one earthquake occur-rence; this implies that there is an upper bound of ML % 4.0 on the size ',: gnitude) earthquake than can occur under the prevailing conditions at Montfcello Reservoir.
1 I
I (4) Focal mechanisms for induced earthquakes and hydrof racturing data f rom two deep (1 k:. wells indicate in situ stress conditions favoring thrust-type faulting at shallow depths.
(5) In situ stress measurements in the two deep boreholes indicate variable stress levels both vertically and laterally; absolute horizontal stress dif ferences (maximum-minimum) reported by Zoback range from a few bars to approximately 100 bars. The observed data imply the existence of a s' : css barrier at varying depths beneath the site that will limit the vertical extent of reservoir-induced seismicity.
(6) The distribution of fracture density and fracture orientations observed in the two boreholes is noauniform and is characterized by local concentrations of fractures in limited depth intervals with different dominant orientations in each; focal mechanisms of nearby induced earthquakes have nodal planes generally cor-I responding to these orientations.
I (7) Geologic maps, surf ace magnetic and aeromagnetic anomaly maps, gravity anomaly maps, radiometric maps all indicate significant lateral variations in rock properties at shallow depths beneath I the site. These variations are associated with the upper portions of plutons that have been intruded into the older metamorphic rocks of the Piedmont Province and that are nos near the surface in this area.
(8) The close spatial association of the clusters of well-located inducei Teismic events with boundaries of plutonic bodies as
, indicated by correlative geological geophysical data (item 7 above) and the association of specific focal mechanisms with dif-ferent orientations of fractures observed in the two boreholes (item 6 above) makes a strong case that the effect of the reser-voir impoundment has been to relieve local remanent stresses around the plutons that have, over geologic time, been brought near the surface in this area (i.e., into a stress environment i
I I <
I
I different from that in which the plutons were initially emplaced) .
These stresses are highly variable from one point to another over the area.
(9) The lateral and certical heterogeneity in: rock properties, stress levels and orientations, permeability, and induced seismic activity, coupled with the limited spatial extent laterally and with depth of the induced activity, all point to the conclusion that the effects of reservoir impoundment are surficial (top 2 to t 3 km) and reflect minor local adjustments where the ef feu tve stress has changed as the pore pressure from reservoir loading has changed.
(10) Limited data on the distribution of permeability indicates that there are significant lateral and vertical variations, with higher permeabilities associated with zones of higher fracture density in the boreholes. There is a general decrease of permeability with I depth towards the bottom of borehole No. 2, the only one with com-plete data available.
I (11) The initial spectral analysis of several of the induced earth-quakes indicates average stress drops of a few bars; the maximum E reported value is 17 bars.
(12) Frequency-of-occurrence vs magnitude 4 ats for induced activity at Monticello (and elsewhere) are highly atypical compared to the natural tectonic behavior in the region (and elsewhere). The transient nature of the induced activity (non-steady-state) in l both time and space, high "b" values, and limits on possible fault size mentioned above, all suggest that the usual extrapolations of f requency-of-occurrence vs magnitude plots for tectonic earth-quakes to infer the occurrence rates f ar larger events are not valid and should not be used. Instead a maximum cut-of f magnitude is likely in this setting, estimated to be ML N 4.0. The
( spatial scale of local heterogeneities in rock properties and the 1
l I z
I
I I stress regime at Monticello Reservoir ( l km or smaller) implies maximum fault dimensions consistent with this bound.
(13) Overall, this evaluation has revealed that the effects of reser-voir impoundment are very limited in extent laterally and in depth and that these surficial ef fects are highly unlikely to increase the probability of a large tectonic event (Intensity VII) at the site. ( A separate critique of the Richard B. Russell Report accompanies this site-specific evaluation and shows that the analysis leading to the design for a magnitude 5.5 induced earth-quake is unduly conservative and without justification for that site or Monticello Reservior.)
(14) Near-field ground motion spectra under various assumptions of magnitude and location were generated and evaluated vis a vis the design spectra for the plant structure. The tie between local magnitude estimates and ground motion at the site was tade using the strong motion spectra for the largest induced earthquake at Monticello Reservoir (Mt = 2.8). These results together with a I reexamination of both the regional seismicity in the Piedmont Province and new hypotheses conce.ning the Charleston earthquake (reported separately in accompanying submission) shows that the Virgil C. Summer plant design is adequate.
I I
I I
I 1
l I lI
- - _ r --
I O
r 1 2.0 SEISMICITY ASSOCIATED WITH I,iE MONTICELLO RESERVOIR 2.1 GENERAL Monticello Reservoir is located near Parr, S.C., and forms the upper reservoir of the Fairfield Pu= ped Storage Facility as well as the cooling reservoir for the Virgil C. Summer Nuclear Station. The reser-voir has a maximum surface area of 6,800 acres and a storage volume of I 400,000 acre-feet at normal maximum water surface elevation of 425.0 ft (129.6 m). The maximum daily withdrawal for electrical generating purposes is 29,000 acre-feet, lowering the pool to elevation 420.5 f t (128.2 m). Figure 1, Appendix I shows the locations of Monticello Reservoir and other pertinent features.
2.2 SEISMICITY NEAR dONTICELLO RESERVOIR BEFORE IMPOUNDMENT A permanent station of the South Carolina Seismic network, Jenkinsville (JSC), is located within 5 km of the Virgil C. Summer Nuclear Station and Monticello Reservoir and has been operational since October 1973. Local events recorded at JSC with S-P intervals of 2 see or less have been analyzed regularly and have averaged about one event per 6 days from October 1973 to October 1977. Figure 2, Appendix I, shows the distribution of events during November 1977 near Parr Reser-voir (the lower reservoir formed along the Broad River for the pumped storage f acility) when water was pumped into the lower reservoir, raising water levels by some 8.5 ft.
2 . ., SE.SMICITY FOLLOWING IMPOUNDMENT OF MONTICELLO RESERVOIR Figure 3, Appendix I, shows the filling history of Monticello Reservoir and associated seismicity with time. Filling commenced on December 3, 1977 and full pond elevation of 425.0 ft. was reached on February 8, 1978. This was followed by a rapid increase in activity until the first week in March af ter which the seismic activity sharply i declined to a level which was still much greater than the preimpound-ment background level. As shown on Figure 3, Appendix I, the increased activity occurred in two swarms with.an increase of several orders of magnitude in seismic energy release.
I lI
I I Appendix I describes the interpretation of the swarm activity following initial filling (Dec. 3-Dec. 25, 1977), up to full pond (Dec. 25-Feb. 8) and the immediate period following attainment of full pond (Feb. 8-March 7, 1978) in terms of loading effects, pore pressure diffusion effects, and pore pressure ef fects af ter full pond elevation was attained.
2.4 TEMPORAL AND SPATIAL BEHAVIOR OF SEISMICITY WITH FESPECT TO MONTICELLO RESERVOIR Appendix 11 presents a uetailed sumary of the important aspects of the distribution in time and space of seismic activity following impoundment of Monticello Reservoir.
Af ter the. initial spurt of seismicity following impoundment, there has been a marked decrease in seismicity. The decreased level of seis-micity was interrupted by discrete swarm episodes, the most prominent being the October 1979 swarm of 700 events, 19 of which were equal to or greater than magnitude 2.0 (see Figure 2, Appendix II and Table 1, Appendix II).
Of the 70 larger events (2.0 < ML < 2.8) between December 1977 As I
and December 1979, 54 (77 percent) occurred in three swarms.
descrioed in Section 3.0, it is inferred that the seismicity is con-trolled by local geology, lateral and vertical variability of stress, and the hydrologic regime at and near the reservoir.
l As shown on Figures 3a-3h and Figures 4a-4g of Appendix II, the 1
seismicity began in discrete locations and spread, coalescing with time l into distinct clusters. Most of the epicentral growth was completed by the end of the first year (Dec. 1978). The seismicity has been con-fined primarily to the top 2 km of the crust, with only 2 percent of the events being deeper, but less than 3 km (Tables 2 and 3, Appendix II). Moreover, there does not appear to be a systematic shallowing or deepening of seismicity with time (Figures 5 and 6, Appendix II) nor does there appear to be any systematic relationship between the depth of earthquakes and their magnitudes (Figure 7, Appendix II).
1
- I l
~. _
I Figures 8a-8c of Appendix 11 show the locations of events recorded during the period July - December 1978 (Figure Sa) with two east-west cross sections showing their distribution with depth (Figure 8b). Note that the seismicity can be grouped into five clusters (Figure 8c).
Figures 9a-9k show the locations of seismic events recorded for the period January - October 1979 and Figure 10 shows the cumulative loca-tions between July and October, 1979. Comparing Figures 8c and 10, note that the seismicity can still be defined by the same five cluster I areas.
Finally, there has been ao noticeable change in the magnitude of the largest events over the 3 year monitoring period, the largest being the August 27, 1978, ML = 2.8 event. As discussed in Section 3.0, it is inferred that several factors (e.g., variable stress barriers both I laterally and with depth and a heterogeneous stress field associated
<ith heterogeneities in physical properties) limit the occurrence of earthquakes to small events of Mg < 4.0 in the upper 2-3 km of the C ru s t .
I I
I I
I I
I I
I I
I I 3.0 ASSOCIATION OF INDUCED SEISMICITY WITH GEOLOGICAL, GEOPHYSICAL, AND HYDROLOGICAL AN01\ LIES The patterns of induced seismicity correlate closely with a variety of observations that reflect in situ rock properties in this area. Collectively, these comparisons show that there are significant variations laterally and vertically in in situ conditions and that the induced activity is closely associated with shallow plutons. Tnere is no evidence in any of thase data that the induced events are coalescing towards reservoir-sized planes of potential movement.
3.1 IN SITU STRESS CONDITIONS AT MONTICELLO RESERVOIR Appendix III describes the available data on in situ stress condi-tions at the site. Principally the data come from two deep (1 km) borehole s at the site in which hydrofracturing stress estimates were made and f rom earthquake focal mechanisms presented in Appendix IV.
Figure 2, Appendix III summarizes the borehole stress measurements for the two wells. It is clear that the conditions vary significantly in I the two wells both in magnitude of the maximum stress dif ference and the depths where they approach lithostatic stress levels. In addition, there are overcoring data from two locations as given in Table 2 and Figure 5 of Appendix III.
3.2 ASSOCIATION OF COMPOSITE FOCAL MECHANISMS WITH FRACTURE I DISTRIBUTION AND ORIENTATION
, Appendix IV summarizes composite focal mechanism solutions for various clusters of activity that have occured and all of them show thrust mechanisms. The apparent inconsistency between this result and the stress orientations from some of the borehole measurements is discussed in Appendix III. Data on foliation and joints within 10
(
miles of the site are presented in Appendix IV along with new data on the distribution and orientation of fractures in the two boreholes; the distribution is variable with depth and between the two boreholes.
l A comparison was made between the composite fault plane solutions and these fracture data. Overall, the conclusions are:
l lI l
1
- 1. There are a large number of fractures in the area.
- 2. The orientation and density of fractures varies from place to place and with depth. That is, the region in the vicinity of I the reservoir is characterized by a heterogeneous distribution of fractures.
- 3. The striking agreement between the poles of the observed frac-tures and the poles of the fault plane solutions for nearby I
events suggests that the seismicity is occurring along a net-work of pre-existing fractures which are not continuous in their spatial extent either laterally or vertically. This observation and the borehole fracture data put a constraint on the size of the fractures along which movement is occurring both for single events and clusters of events.
These data together with the variable spatial pattern in seismi-city over the area suggest that there are stress barriers laterally and vertically which limit the extent of any single fault movement. This rationale is developed in Appendix III. Further, the effect of shallow earthquakes in this stress environment (thrust faulting) is to release the strain energy in the shallow region above the stress barrier and not to influence or be influenced by conditions below. This implies a decrease in induced seismic activity with time, which has generally g
B been observed (see Appendix II).
l lE 3.3 CORRELATION WITH GEOLOGICAL AND GEOPHYSICAL ANOMALIES
[
E Appendix V presents comparisons of the seismicity patterns (Figure
- 6) with the geologic map (Figures 1 and 6), the ground-based magnetic
! anomaly map (Figure 2), the aeromagnetic anomaly map (Figure 3) the Bouguer anomaly map (Figure 4), and the aeroradioactivity anomaly map I = (Figure 5). The results of making these comparisons are summarized as follows:
s
- 1. The >baticello Reservoir is located in an area characterized i by high f requency (short wave length) anomalies and small out-W Crops.
I I
I I 2. The spatial association of the outcrops with these high fre-quency anomalies indicates that they are related, and the causative bodies are shallow and of limited extent (s 1-2 km).
- 3. The spatial association of seismicity with the flanks of high frequency anomalies strongly indicates that the seismicity is located in zones where there are strong gradients in physical conditions.
- 4. The spatial distribution and correlation of the seismicity and the geophysical anomalies imply very heterogeneous material property and stress conditions both laterally and vertically.
These results, coupled with observations of significant heterogeneities in fracture density and orientation (Appendix IV) and hydrologic conditions including permeability ( Appendix VI) strongly suggest that only small zones can experience movement in any single earthquake occurrence. This implies I
that there will be a maxim n fault dimension of about 1 km, hence a maximum magnitude for earthquakes in this environ-ment cf ML% 4.0.
3.4 HYDROLOGTC DATA AT AND NEAR MONTICELLO RESERVOIR Appendix ~ describes the available data on the hydrologic regime at the site a 4 how it changed after impoundment of the reservoir.
I Also, permeability data with depth are available for borehole no. 2 and they show that it is variable, higher in more fractured zones, and that it decreases to low values toward the bottoo of the well. Ove rall, the data indicate significant variations laterally and with depth in the permeability beneath the reservoir area. If pore pressure changes provide the trigger for the induced seismicit: , the observed initial migration of the seismic activity and subs 4aent clustering of seismicity ( Appendix 11) represents tne transient behavior as the pore pressure " front" propagates (at different rates) throughout the rock volume beneath and around the reservoir. Because of this heterogeneous I I -
I l
I permeability distribution, pore pressure changes cannot cause simultan-l eous changes in effective stress over large planar areas beneath the t
j reservoir.
I i
1 4
\I i
lI i
I I
I I
I I -
11 -
I
I I 4.0 MAXIMUM MAGNITUDE OF INDUCED EARTHQUAKE AT MONTICELLO RESERVOIR 4.1 GENERAL There are several lines of eridence that imply a limiting magni-tude for induced earthquakes at Monticello Reservoir. The evidence is in two general categories: site specific data contained in the appendices (Appendices II, III, IV, V, and VI); and an empirical body of data tnat describes the maximum observed seismicity for other I Piedmont reservoirs and the relationship of seismic moment to areal distribution of MMI VI shaking.
. 4.2 SITE SPECIFIC DATA SUGGESTIVE OF A MAGNITUDE LIMIT OF Mr = 4.0 The data sussarized in Appendices 11 through VI provide the following points of argument relative to a magnitude limit of ML=
4.0:
Comparison of the spatial and temporal patterns of the induced seismicity ( Appendix II) cemonstrate an overall decline in the rate of seismic activity, suggesting that the stored strain is being relieved and not replenished. This is also supported by I Figures 3a-3h and 4a-4g, showing the spreading of seismic activity with time to cover almost 90 percent of tne area be-neath and adjacent to the reservoir in the first year of
=onitoring, with little further expansion.
- The scale of in situ rock heterogeneities (Appendix V, Associa-I tion of Seismicity with Geology and Geophysical Anomalies, and Appendix IV, Fault Plane Solutions and Association with P-existing Fractures), in conjunction with highly variable in situ stress conditions (Appendix III) places a limit of 1 km or less on possible source dimensions for a single earthquake occur-rence; calculations using a Brune Model with an assumed 25 bar stress drop and a source dimension of about 1 km gives N 4.0 as this limiting magnitude (Appendix IX).
ML I
I
I
- Magnitude vs f requency of occurrence plots for induced seis-micity at Monticello given in Appendix IX suggest a limiting magnitude of about ML = 4.0.
4.3 EMPIRICAL DATA - SEISMIC MOMENT AND AREA 0F INTENSITY VI SHAKING Appe:ndix VIII presents a synopsis of an analysis by Hanks et al.
(1975) relating seismic moment to the areal distribution of MMI VI for 47 larger southern California earthquakes. Figure 1 of Appendix VIII compares Hanks' data to similar data for eastern North America. The resulting comparison suggests a correspondence of MMI VI to magnitude values of ML = 3.4 and 2.6, respectively, for two curves fit through the eastern North America data.
Data gathered and interpreted by Severy et al. (1975) suggest that the largest induced etrthquake in the Piedmont is MMI VI and f rom available data, the areal distribution of 3D11 VI shaking is small.
Based on the relation of seismic moment to area of MMI VI shaking, the associated earthquake magnitude is ML < 4.0.
4.4
SUMMARY
DISCUSSION The site-specific data presented in this report are extensive and definitive in characterizing the physical setting beneath and in the vicinity of the Monticello Reservoir and the seismic behavior induced by ita impoundment. Collectively, the evidence shows a highly hetero-geneous environment ::ith variabilities on a scale that limits the fault dimension associated with any single earthquake occurrence to ^'l km.
Intensity data on other reservoir-induced earthquakes in the Piedmont Province, together with empirical relations connecting the area of MMI I VI shaking, seismic moment, and local magnitude provide an independent approach to infer a maximum reservoir-induced magnitude at Monticello.
However, the applicability of experiencc_ from ot.her reservoir sites, even those within the Piedmont Province, to the Monticello site is open to serious question as discuss,ed in the accompanying evaluation of the Richard B. Russeli report. Therefore, the conclusion that ML = 4.0 is the maximum magnitude that can be induced by the Monticello I
Rese voir impoundment is based primarily on site-specific data in this report. The following table summarices in approximate order of impor-tance the evidence used to infer this limiting magnitude.
CONSTRAINTS OS THE SIZE OF THE MAXIMUM INDUCED EARTHQUAKE AT MONTICELLO RESERVOIR, ARRANGED IN APPROXIMATE ORDER OF IMPORTANCE Type of Observational Evidence Constraint
- 1. Heterogeneity in site-specific characteristics
- a. geological and geophysical <1-2 km scale of variability.
anomalies (rock type, magnetic, gravity, radio-I metric).
<1 km scale of variability,
- b. variability in focal mechanisms within and among clusters. especially with depth.
- c. stress conditions from <<1 km scale of variability boreholes and overcoring. with depth; stress barrier at shallow depth.
- d. fracture density and <<1 km scale of variability, orientation in boreholes and correlation with com-posite focal mechanisms vs depth.
g e. permeability from boreholes scale of variability <<1 km 3 and water wells. vertically, <1 km laterally.
- 2. Seismicity in space and time.
I a. growth and distribution' discrete events or small, i of induced activity. independent clusters; 98%
<2 km depth, all <3 km.
- b. frequency of occurrence vs ML Mogi Type III; ML = 2.8 I maximum to date; trend suggests cut-off ML < 4.0; overall decline of activity with time.
l 1
i lB l
l l
I -
1 l
l l
l
- 3. Spectra (limited amount of data), stress drops typically small (average 5 bars); max.
observed 17 bars.
- 4. Intensity data frora other reser- maximum t1MI VI; small area I
voirs in Piedmont Province, of 3D11 VI shaking gives inferred ML < 4.0.
I
,I l
I I
'I I
I ,
f mgy-,..ew we-en,n,,wm,w%y,_,w,wy, pw e ww.. ,% , , , - ,,.,,w,.yc----w-w-a-yem-- y--.vq-- -
t
5.0 COMPARISON OF NEAR-FIELD EARTHQUAKE GROUND MOTION TO SSE SPECTRA AND DISCUSSION OF EFFECTS 4
Based on discussions and findings within this report, it is con-cluded that ML = 4.0 is the ::cximum induced event expected to occur at Monticello Reservoir. This event is associated with a zero period acceleration value of 0.14g (less than the SSE design acceleration) which ias no adverse effect on equipment anc structures.
Fo r conservatism, the Applicant has also selected a ML = 4.5
+
evet.t (considered in the near-field sense) to compare with the SSE design spectra. This comparison is presented in detail in Appendix X.
Even though the ML = 4.5 event spectrum does exceed the Virgil C.
Sum er SSE spectrum in the high frequency region above S10liz , it is demonstrated in Appendix X that the original design nargin for tne structures and the qualification of equipment are still adequate when the higher damping values permitted in NRC Regulatory Guide 1.61 are invoked and the conservatism of the artificial time history method that was used in the original seismic analysis is removed by a more reasonable statistical basis for estimation of ground motion.
I I
I
6.0 CONCLUSION
S 6.1 GENERAL Ihe conclusions regarding the maximum sagnitude event (ML = 4.0) which can likely be induced by Monticello Reservoir can be grouped into and supported by several categories of observational data. These data groups are:
- 1. the spatial and temporal distribution of seismicity at, around and beneath Monticello Reservoir;
- 2. the association of observed seismicity with local geologic phenomena (surficial); geophysical anomalies (subsurface); and the hydrogeologic regime at and around Monticello Reservoir; and
- 3. The distribution of stress indicators within and around the reservoir area which are data from in situ stress measurements
[hydrof racing (deep) and strain relief by overcoring (shal-low)}, composite focal mechanism solutions, and stress drops from spectral analysis of a .me recuc d events.
Because of the transient (non-steady state) nature of induced seismicity, the standard approach used to extrapolate frequency of occurrence vs vagnitude data to estimate recurrence intervals for tectonic earthquakes is not applicable. Alternative approaches were used which are more appropriate.
6.2 CONCLUSION
S REGARDING THE SPATIAL AND TEMPORAL DISTRIBUTION OF SEISMICITY AT MONTICELLO RESERVOIR The conclusions that can be drawn from the observational data are:
The seismic activity is limited to superficial zones beneath and immediately adjacent to the rescevoir. Accurate hypocentral depths from the 10-station network are all less than 3 km.
- The largest event observed is ML = 2.8
- The seismic activity is episodic, and has declined in overall activity rate since the initial spurt of activity during I February 1978.
- There does not appear to be any systematic relationship between magnitude and depth or saallowing or deepenire of activity with time.
- Activity has coalesced into clusters, having occupied almost all areas (90 percent) beneath the reservoir in the first year of monitoring (1978).
I 6.3 CONCLUSi3NS REGARDING THE RELATIONSHIP OF OBSERVED SEISMICITY TO LOCAL GEOLOGY, GEOPHYSICAL ANOMALIES AND HYDROGE0 LOGIC REGIME This body of data is the most supportive of limited effects from reservoir impoundment as extensive information is available f rom site-specific studies, both surficial and subsurface, carried out over the last 8 to 10 years. The conclusions one can draw from this body of data are:
- lIonticello Reservoir is located in an area of high frequency (short wave length) anomalies and limited area outcrops. The spatial association of the granodiorite, metamorphic, and mig-matitic outcrops with the geophysical anomalies and clusters of seismic events suggest a causal relationship. These perturbing bodies are shallow and of limited spatial extent (si-2 km).
- The spatial association of seismicity with the flanks (or areas of high gradient) of the high frequency anomalies strongly sug-gests that the events are occurring near the peripheries of the perturbing bodies.
- The spatial distribution and association of seismic activity l with the shallow geophysical anomalies suggests a very hetero-l geneous distribution of rock properties (laterally and with depth) across the site reacting very locally to a variable I
stress field.
l l
r L
- Composite focal mechanism solutions for clusters of shallow induced events demonstrate a mechanism favoring high angle I reverse f aulting, and they show a good correlation with orienta-tions of mapped f racture patterns at the surface and with the distribution of fracture densities and orientations observed in the USGS hydrofracing holes No. I and No. 2.
- Data on the distribution of permeability with depth in the Virgi) C. Summer site area shows that the hydrological system is complex. The most convincing data on variability of permeabili-ties with depth are observations made in the deep (%1 km) hydro-f racing holes which showed permeabilities of 10-2 mDarcies at 600m depth in hole No. 2 and 1 mDarcy at 726m depth in hole No. 1. When viewed in conjunction with the in situ stress measurements in the same holes, it is apparent that variations in deviatoric stress with depth also might create " hydrologic I barriers," or a maximum zone of influence of the reservoir or total hydrologic regime, with depth.
6.4 CONCLUSION
S REGARDING STRESS DISTRIBUTION IN THE MONTICELLO RESERVOIR AREA These data are from three principal sources: 1) in situ stress measurements from hydrofracing (deep holes No. I and No. 2) and over-coring (shallow); 2) composite focal mechanism solutions of recorded seismic events; and 3) stress drop data calculated f rom spectral analy-ses of recorded events. The conclusions which can be drawn from these data are:
- Composite focal mechanisms and hydroiracir.3 data indicate that release of the stored strain energy is achieved by high angle reverse faulting.
- The variable nature of the surface where the deviatoric stress is very low or zero indicates that distortional strain in the rock is low or near zero at the same points, forming a " stress barrier" at depth.
- The variability in the orientation of the P, T, and 8 axes of the focal mechanisms associated with the clusters of seismic activity indicate that the trends of stress trajectories are not uniform throughout the reservoir area; this creates stress bar-riers along strike, effectively limiting the size of a potential seismic source.
- Stress drops, as estimated from spectral analysis of selected earthquakes, generally average about 5 bars or less, with the one exception being the August 27, 1978, ML = 2.8 earthquake which had an estimated average stress drop of 17 bars.
- When these data are viewed in conjunction with data from Appendix IV, they support the premise that the heterogeneity (both laterally and with depth) of in situ properties permit only small areas (<1 km2 ) to experience dislocation in any single seismic event; this implies a maximum ML 1 4.0.
6.5 CONCLUSION
S RELATIVE TO PROBABILISTIC ANALYSES OF RESERVOIR INDUCED EARTHQUAKES AT MONTICELLO RESERVOIR The observed frequency of occurrence vs ML data we re extra-polated in several dif ferent ways to estimate the recurrence interval for magnitudes above ML = 2.8, the largest event that has occurred at the Monticello site.
Only the case where usual methods for tectonic regions were applied gives a short recurrence interval (14 years) for an ML = 4.0 I avent. Because of the transient nature of the induced seismicity in both space and time and heterogeneities that limit the maximum source I size, this approach is not valid. Instead, the cases that limit the maximum magnitude that can occur or at least significantly lower the probability of occurrence of larger events are more representative of the actual behavior in this setting, as deduced from all the evidence presented in this report.
To estimate the ground motion spectra and maximum acceleration for l any assumed ML event, the observed strong motion spectrum observed I
ll
for the August 27, 1978 ML = 2.8 event was used for calibration.
This relates (ties) the local magnitude scale to near-field ground motion spectral '.evels at Monticello Reservoir and eliminates the troublesome probi ta of using diverse magnitude relations obtained for other areas ad applying them to this setting.
o.6 CONCLUSiv :S RELATIVE TO COMPARISON OF MAXIMUM INDUCED EARTHQUAKE TO SSE Dr IGN SPECTRUM The maximum induced earthquake of ML = 4.0 at Monticello Reser-voir produces a zero period acceleration value of 0.14g (which is less than the SSE design acceleration); therefore, there are no effects on equipment and structures. Even an assumed around ML = 4.5 near-field earthquake does not exceed the SSE design margin.
6.7 SC:01ARY OF CONCLUSIONS The summary conclusions of this entire analysis are:
- The seismicity induced by Monticello Reservior is shallow, (<3 km), closely associated with the peripheries of shallow plutonic rock bodies of limited size (%1-2 km), and the result of a highly variable, heterogeneous stress field acting upon limited potential seismic source areas. The overall rate of tne seis-micity is declining, suggesting that the stored elastic strain being relieved through the occurrence of shallow seismicity is not being replenished.
- Because of the latercl and vertical heterogeneities in distor-tional strain in the bedrock associated wich heterogeneous physical properties, there are only small potential seismic source areas of G ka' available for any single seismic event.
- The evaluation has revealed that the effects of the reservoir impoundment are very limited in extent (laterally as well as vertically) and that these surficial effects are highly onlikely
_ _ _ _ - . . ~ . __ . . _ _ _ - . .. _. . _ - _. . . . __
i
~
t I to increase the probability of a larger tectonic event (Inten-1 sity VII) at the site.
iE iW j
- Ground motion generated by the maximum induced event of ML=
f 4.0 at Monticello Reservoir would have no effect on equipment and structures. For conservatism, near-field ground motion spectra for a ML - 4.5 event have also been constructed and I compared to existing SSE design spectra. These results demon-
. strate that existing design margins for structures and qualifi-cation of equipment are adequate for an Mt = 4.5 near-field
}
j event.
!I i
I I
I I
I I
I I
I I
I
,i
(
APPENDIX I l
' SEISMICITY ASSOCIATED WITH IMPOUNDMENT OF MONTICELLO RESERVOIR I
I I
I I
I I :
I I
I I
1I I
l I
I
I I SEISMICITY ASSOCIATED WITH dip 0JNDSIENT OF MONTICELLO RESERVOIR FAIRFIELD PUMP';D STORAGE DESCRIPTION Perhaps :he best documented case of microearthquake activity induced by the impoundment of a reservoir is taking place presently near Monticella Reservoir. The pumped storage hydroelectric facility I consists of tto impoundments, Monticello Reservoir and Parr Reservoir.
The two reser roirs are separated by the Frees Creek Dams and by the Fairfield powe.r house which contains eight reversible pump-turbine units having a maximum capability of 83,000 hp each. Parr Reservoir constitutes the lower pool of tne Fairfield Pumped Storage Facility, and is located on the Broad River approxiamtely 1 mile (1.6 km) to the west of the Virgil C. Summer Nuclear Station. This reservoir is formed by Parr Das which is located about 2.5 miles (4.0 km) southwest of the nuclear site (Figure 1).
Monticello Reservoir has a surf ace area of about 6,800 acres and a storage volume of about 400,000 acre-feet at a normal maximum water surface ele.vation of 425.0 f aet (129.6m). The maximum daily withdrawal for generating purposes is 29,000 acre feet, lowering the pool to elevation 420.5 feet (128.2m).
EARLY SEISMICITY NEAR MONTICELLO RESERVOIR The site o'.' the Virgil C. Summer Nuclear Station and % Monticel-lo acd Pa rr Reservoirs are within 5 km f rom JSC (Jenkinsville) per-manent station of the South Carolina Seismographic network (Figure 1).
JSC, located about 42 km northwest of Columbia has been in operation since October 1973. Local events recorded at JSC have been analyzed regularly and those with S-P time < 2 sec, average about one event every 6 daye (for the period October 1973 - October 1977). There were I a few spurts of activity recorded at JSC (S-P < 2sec) in October and November 1977. These followed (by a few days) a rise in the water level in Parr Reservoir. The increase in water level in Parr Reservoir resulted from floods in late September ard in late October. Water level of Parr Reservoir was raised f rom 257 feet (ms1) on November 15, I I-1 I
I 1977 to 265.6 feet on November 19, 1977. Each of the increases in the water level was followed a few small events (Mt N 0) (Figure I 2). The seismicity afts December 3 is related to the impoundment of the Monticello Reservoir, and is discussed in the next section.
SEISMICITY ASSOC' i. WITH IMPOUNDMENT OF MONTICELLO RESERVOIR Af ter the construction of the Frees Creek Dams (Figure 1), the Monticello Reservoir was filled by using water f rom Frees Creek and by pumping the water from Parr Reservoir using the pump-turbine units.
I Figure 3 shows th filling history of the reservoir. Water was pumped from a height of ?l9 feet (97.3m) on December 3,1977 to a full pond elevation of 423 feet (129.6m) on February 8, 1978 (bottom row in Figure 3). Seismic activity (recorded at JSC) began about 3 weeks after initial impound =ent. The cumulative number of events increased steadily to about February 8, 1978, af ter wMch there was a very rapid increase until about March 7, 1978. There was a sharp decline in activity after the first .eek of March to a level which was still much greater than the preimpoundment background. These three periods of I activity are easily apparent from a graph of the cumulative number of earthquakes.
I The increased activity appears to have occurred in two swarms -
the first in the period January 10-25, 1978 and the second between I January 25 and March 10, 1978. These are seen in a plot of the daily number of events (middle r e Figure 3). There was an increase in the seismic energy release by two orders of magnitude associated with the increased activity (top row). Events with ML > 2.0 are also shown in Figure 3, with the largest swarm of ML > 2.0 events occurring between February 10 and March 7,1978.
The graph showing the cumulative number of events is perhaps the most interesting. There are three distinct breaks in the slope of the curve. The first occurs about 3 weds af ter the beginning of filling of the reservoir. The second occurs when the lake is filled and the third about 3 weeks later. The activity is shallow and the 3-week I I-2 I
I I delay probably reflects the time for pore pressure diffusion to reach the hypocentral locations. The focal mechanisms for these events are predominantly N- and NW-trending thrust mechanisms, which fits the observation of the change in slope of the cumulative seismicity at the time of filling. We know that the loading of water in a thrust en-vironment inhibits failure, while pore pressure diffusion drives the Mohr circle towards failure. In the period December 3 to December 25 1977, the predominant effect is '. hat of loading, with minor ef fects, due to pore pressure. However, between December 25, 1977, and February 8, 1978, both effects are present and both are increasing. The increase in the pore pressure effect (delayed by 3 weeks) is greater than the stabilizing effect due to loading. Consequently we see an increase in the activity. Af ter the lake is filled, there ;s no further change in the loading effect; however, the pore pressure ef fect continues to increase for about 3 weeks, and we see an acceleration in the seismicity, which then levels off. The seismicity has now been monitored continuously for almost 3 years and we observe an overall decrease in it.
The temporal and spatial behavior of this induced microearthquake activity is discussed in Appendix II.
l I
lI 1
.I I
I II
'.- 3 I
.~...
.1G* '.,
. ;, ,. . , p. c ,
- .... . t) . ...- s
.(\-.. t a -
r, ,
n ! :l l i. ,
C ,p. -
I~
Q * ' * ' ' ' ' 7 .
'., (
q %*, .....g.- 1,-
- . , s -
e k'r - '
i
_i y.<
- . . *s ,... .
i 1
. - r,1
_: h .
- 1.*y;d s
.S*
._::g:f:." _ . 1,
- \
{
' ',, -J' . f: < j. . .
= t, .
( g, '
.Q , , %;, .,, J t* , l;i '_.f \ l I y
v>f . \ ' .- R,q-. ; f.- u \ - e
-.' i ,.'
3 / !
i, - 3
..:., q , i).y,
.s
, j ,,
- t.
-._..). b s. -
ma}
,.i.a. . , , , , ,\e i .. ,t6-(
.1 - s a -
. .i. ;
T. ('.. c. .i.
-^..
F-a . . 3 * .f:) . ..
y < .
c3s 'n m. 's.v .v. .
\. '
M. ::::. ,
t.
).
.i n .
W =\ - 1 -
j i
- . :5 ' \
', )
, c~
c'r = * .
. , .,- l ..z
.p.* -. .'/) 0* j. . .
i
.M s -=- . .
- ). s.. f.
. - . Q( . . r., cec .. :y'. ,. / .' , i
- 6
)
..: z: :. .. ..u. ' .
- .. .; w,, -
1 -
\
T. 2 - i
.g y %...f. . . , . ....:. .. t ;.4 ', ,
tJ
,t .
- 1 .
- - x. , 3 ;..
o ts
~ g. v .) s ,
\ .
,). ,.. *[ , :k -
,I
- ]
..Q.'.e,. *
' 'rjD 2 [. ' "h ':MONTIC
- - E L L Oij{ .
4' (I)J '.n*.k".*o .f/.**
\ N.h .- .s -- .:M P R E S., R VOiH s
~( . . , .. , , ,
0
).w,s **
.p. . ,,. l
.. a
,raEEs S
ff n.g m \',.
.l.
. s ) * ,.'.!*[k. )" g CREEN
- , ..
- ; .'. .' :t9 :* ' :, ,, :. d.- \.
onu y& '.
s .) >
.- :h < x. . /, -
t
- l
.\ .!
d , . . .. _ t U 0.:.y /
l, ,f. \ N '. , ,. .A 4 ,- , ,
.a .p' .- 3
) S 'u i
M'c4c f.
. . '. / i,*
\
s / N '* .;
- I
"',,,- . ; f. . \ \ l:
. ,,- - ,,. \. ........
e ,e A. %1c,**...
. PARR \ . .
4
- o. .q-nEsEnvoia ---- raEEs .,.....u....... ...
.s u .e..ru . ...
t
[J u1 3. j. ., .
- 4 4 '.
- ' . at x CREEK SITE 1, "u'.'.*.*.*.,'.*.*'s.te.co...........n
.. u..
' p v.. ts. , Y ' Y '*
- g cd::- s :. cc av.reva e.L es
' -~~ ^E L* . . ~. Tr. :' ~^' * * ' f,(h.: l~ $. .i 't ,
1 QQQ~Q{
. r- .a cg MCA,E. : :. ;: . . * , 2 , e d f ** e i s -
/ ....
.r N..t.. 4 :.* - ) .... ; ..<..........................ou.
enese. ..er..cere..r .*..o.....e
(}' \ 1 - %
s' Na
- f ",1(
- .;.y'c.
e ,
6 South Coren.no Electric a Gas Ca PQM. .g ,apo A (f- ** g. 'i'- .'l.. g.
<M.o')' .,
4
- ~ ~ ' 7
_g ' ' '1, < e. ; Vergil C. $ veneer pe. clear $tetton
=l %. )s.~> . . . . . , ,g . ' . /- \ . N'. ' .
ys/p') { $ * *g*.e '. ' - , , . , , ., , , . . .
PARR DAM M,sp of Site Vicinity Shs, wing
,[}.. ** * (g *
',i ',, )
- E
-- . . . . a
./> # .L, ..f... . f. .b.k .,
.a. c. . .c . .
4 -
Figure 1
) . l m ~
m 'oo uO dz t
0 6 a
1 T
e '
~
n R
m .E B
M
.E
_ C M -
n E
.D n
n m -
g W -
R
.E W B M
2 e
. M .E V 7 7
r u
g e O .O N 9 1
i F
L n E
e V E
L
- R ;
E J
- e T A
W .
- n
- R R
-m A
P R
E
. B
.O
-. W n.O T
C
.m ni
.e - -
_ 3 9 5
. 5 5 5
. 2 2 2 y j_ fl< _
.e e
.m j !; 1, . !ti i, : l!i
QP] SEISMIC ACTIVITY ASSOCIATED
- %~
WITH IMPOUNDMENT OF c :~7 MONTICELLO RESERVOIR
!O 'O ~
}$ ]l1 I
I: [g 14 -
. (( j %{'
f pf l QI f us sn 1e q a
-h "E 12 y ]l ll ll It il I i II l 117 ll il ll ll 11 ilq 10 20 30 10 20 304 10 20 30 to 20 20 to 20 30 to 100 -- # ,6
~
>- ~ 3000 b 50 -
g-J bE _ RJ le 8 " la _..n n l!
!Im,,,f'N El I} d l4 %
c'%
!! !! A l'.
Mru'ut_ p
!! n !! !!-
na g _r NOO w Of tr f5 t i5m y
~
G b f ULL POPJD 1000 h
# '~
,. 3 4
W (,_,#./~ L AstE LEvit Mg > 20 - 500 Nu HG 5f AHf D 350 -
'_, ,, / -
y . _ _ _ _ _ . _ _ __l. _ - - - -
" )g Il !! 11 11 ll ll ll ll ll ll !! il Il ll 11 !!
10 20 30 10 20 30l 10 20 30 10 20 28 10 20 30 10 l NOvtM62 ' OtClutt a JA r 4U A R Y FEBRUARY M A R CH APflL 1977 1978 Figure 3 b
_.mmm u ,,.... . a.i__ ._ _ __._ -.__ _- _-. - _ _ _2 _ .= _-a e ___m a_._m ..s e..- a 4 m- ...a
., 2 _ _m ___. _m .A a _ _ _.Js.-. _ s APPENDIX II TEMPORAL AND SPATIAL BEHAVIOR OF INDUCED SEISMICITY AT 210NTICELLO RESERVOIR E
I I I lI l l I I l I , I l l l
..___z___.-_.._.-_,__,,.
E TEMPORAL ASD SPATIAL BEHAVOIR OF INDUCEP SELSMICITY AT MONTICELLO RESERVOIR INTRODUCTION This appendix describes the temporal and spatial c.istribution of induced seismicity at the Monticello Reservoir commenci ng with observsf. tions shortly before impoundment. In trying to understand the n "tre of the temporal and spatial distribution of seismicity it is important to recognize that changes in the instrumentation in tne study period allowed significantly improved hypocenter determinations starting in September 1978. STATION DEPLOYMENT About 3 months before impoundment of the Monticello Reservoir, South Carolina Electric & Gas Company (S.C.E.6G.) installed a four-station microseismic network. These stations 1, 2, 3, and 4 are located in a triangular array, with station 1 in the center serving as a recording site (Figure 1). The data are written on Helicorders. The instrumentation was installed by Teledyne-Geotech Corporation. I cataloging the number of events, those recorded at JSC were used. For The local magnitude, ML , of the events was based on their duration D in seconds according to the relation: ML = -1.83 + 2.04 log D. JSC was also used, as it provided continuous data since 1973. For the location of seismic events, meaningful data from the S.C.E.6G. network were obtained from about a week before the reservoir I was impounded. Between February and September 1978 these instruments were suppiamented with 2 to 6 MEQ 800 portable seismographs. Towards the end of May 19.'8, the U.S.G.S, installed a six-station permanent I network (stations 5 - 10, Figure 1). Data are telemetered to station 5 and recorded on magnetic tape. Continuous, meaningful data with proper timing were obtained from about the last week of August 1978, when the S C.E.6G staticas were incorporated with the U.S.G.S. network. This forms the Monticello seismographic network. At present the analog l I II-1 I
tapes are played back (with several months delay) at the U.S.G.S. office in dolden and returned to the University of South Carolina for analysis. Summarizing, for counting the daily number of events, and cal-r culating their magnitude, data recorded at station JSC were used. Fo r routinely obtaining the epicentral locations of events, data recorded oa the S.C.E.&G network were used. Better hypocentral location data were obtained from the !onticello network for day each in July and August 1978 and then continuously from September i, 1978. Due to a delay in obtaining tape playbacks, data analyzed from the Monticello network covers the period July 1978 - October 1979. TEMPORAL BEHAVIOR OF SEISMICITY The temporal behavoir of seismicity between December 1977 and October 1980 is shown in Figure 2 anu tabulated in Table 1. In this period the seismicity is characterized by four episodes of swaralike activity. The seismicity following the initial impoundment occurred as an intense swarm, with up to 100 events / day. This phase persisted until early March 1978. During the 5-week period January 21 - February 26, 1978, there were 23 events with 2.0 f ML f 2.8. In the period from March - July 1978 the intense activity decreased; there were only three larger events (Mt > 2.0). The second swarmlike episode occurred in the period from the last week of August to early November 1978. During I this period only 13 larger events (2.0 < ML < 2.8) occurred, however 2 of the largest events !!L = 2.8 ocurred during this period and were associated with peak horizontal ground accelerations up to 0.25 g at the ground surface. The seismicity decreased and stayed at a low level until the third swarm occurred in October 1979. This flurry of actici-ty included 19 relatively large events (2.0 f ML f 2.8). The fourth suarm occurred in July-August 1980, and included two relatively large events (2.0 f ML f 2.5). In each of these episodes of activity, we note several characteristics of a swarm, among them is the atsence of a II-2 1E
I significantly larger (or main) shock. There are several events of comparable magnitude. Another interesting observation is that the I largest event in each of the first three swarms is !It N 2.8, and for the last one it is :!L N 2.5. In the plot of cumu c tive number of events as a function of time (Figure 2) we note the perturbation in the curve due to the swarms. We also note that in the " normal activity periods" or periods when there are no swarms (! arch - August 1978, December 1978 - September 1979, November 1979 - June 1980) there is a general decrease in the level of seismicity. This is noticeable from a change in the slope of cumula-tive seismicity as well as in the limited number of larger events (2.0 i lit 3 2.8). SPATIAL BEHAVIOR OF SELSF11 CITY As noted earlier, continuous hypocentral data are available from the S.C.E.&G. network (Deccabu 1979 to September 1980), whereas the data for the 1*.ger >1onticello network covers the period July 1978 to October 1979. To compare the hypocentral locations of events recorded on the !!onticello networrt (tape recording) with those on the S.C.E.5G. network (helicorder recording), about 100 events recorded on both were selected. The epicentral 1ccations obtained from the two recordings agreed well (latitudes within 10.3 minutes (N500m) for 78 percent I events, longitudes within 10.3 minutes (N450m) for 72 percent events). t l However, only 53 percent of the depths agreed within 10.6 km, whereas for 47 percent of the events, the depth differences range between _+0.6 and +2.6 km. These observations indicate that although the S.C.E.6 - network gives adequate epicentral locations, the depth estimates ata unretiable. Consequently in a study of epicentral variation with time we have used data from the S.C.E.EG. network. However, to get precise hypocentral locations (especially the depth), we have used only quality A and B data from the Stonticello network. The spatial variation in seismicity is snown in a series of figures (Figures 3a through 3h). Events were located by using HYP071, l I Il-3 I
I I and only those events located with an RMS error <0.1 sec and ERll < 1.0 km were considered. The periods chosen are weekly (during swarms in February 13.1 and October 1979), biweekly (January 1978 and Septem-ber 1979), and monthlyz An envelope surrounding the epicentral loca-cions for a pa rticular period is shown by a dashed line; an envelope showing the rimulative area covered up to that time is shown by a solid line; and the new area covered in any period is shaded. Thus, the shaded area for any period indicates migration of seismicity. Similar plots are made for the larger events (Figures 4a through The initial seismicity (Decemter 1977 - January 1978) began in unconnected locations (probably at locations were the rocks were highly jointed and located within easy access of the reservoir water). It is not clear if the events located to the east of tne reservior are natural or are quarry blasts. In February 1978, a period of peak activity, the epicentral locations were located on the outer periphery of the reservoir, and they spread outward. Interestingly, the larger events (ML > 2.0) occurred to the east and southeast corner of the reservoir (for most part) in January 1978. In February 1978 most of the larger events occurred in tight clusters to the west of the reser-voir. In March April 1978 there was further spreading of the seismi-city, but most of it occurred in filling the holes in between the pre-viously defined periphery of epicentral . ms . The same pattern of small epicentral growth continued until t. c' .978 (Figures 3c through 3e) by which time many of the holes nal gt a and coalesed into three " puddles." In this period the larger events occurred to the SW I and NW of the center of the reservoir. In 1979 there was very little further epicentral growth (Figures 3e through 3h). This minimal growth occurred in February, August, and the first waek of October 1979. The October 1979 swarm was also associated with the larger events located to the west and southwest of the reservoir. After the first week of October 1979, there has been no further epicentral growth. I 11-4 I
I From Figures 3a through 3h we note that the epicentral growth rate was fastest in the period December 1977 - >! arch 1978. (During that period the epicenters covered approximately 70 percent of all the epi-central area occupied through December 1979). During the next nine months (April - December 1978), a further increase of about 20 percent of the total area occurred. In 1979, the growth rate was much less (as was the seismicity). SiONTICELLO NETWORK DATA AS A REPRESENTATIVE SA>1PLE After the impoundment of dienticello Reservoir in 1977, induced seismicity was observed at different locations in the reservoir vicinity. The seismicity spread in subsequent months (see above); however, most of this spreading occurred in approximately 6 months following impoundmer.c. Of the total epicentral area covered in 2 years af ter filling, over 90 percent of it was covered in the first year, and i va.Lous clusters had been defined. In the period from July 1978 to October 1979 we located 390 events recorded on the >bnticello network with quality A and B (and R>1S < 0.1 sec) . These events cover seismi-I city in almost all areas around the reservoir except to the east and southeast of the reservoir, where seismicity occurred only in January 1978. Ehus, the data recorded on the analog tapes (Sionticello network) pro' ride a representative sample of the seismicity. These data have , been used for the following:
- a. a study of the depth range of the seismicity.
l b, obtaining fault plane solutions (Appendix IV).
- c. association of seismicity with geologic and geophysical data (Appendix V).
l l I DEPTH OF EVENTS In analyzing the depth of seismicity and possible related temporal variations, we have used only those hypocentral solutions of >baticello network that had A and B quality. I II-5 l8
Of the 390 events, 80 events or 72 percent were 1 km deep or
- shallower, 383 events or 38 percent were 2 km deep or shallower and only 7 events or 2 percent were deeper than 2 km (Table 2).
Io test for any correlative increase in depth with time, histo-I, grams are plotted in Figure 5 showing the number of events as a function of depth, the data being plotted monthly. We do not see any systematic change in depth. For all periods since September 1978, using continuous data, most (in percent) of the seismicity (60-84 percent) is 1 km or shallower. The percentage of events between 1.1 and 2.0 km for the same set varies between 12 and 34 percent. This is illustrated in Figure 6. Thus, we note that in the period between September 1978 and October 1979, for which we have continuous data: (a) the seismicity was 2 km or shallower in depth; and (b) there was no 1 systematic shallowing or deepening of seismicity. RELATION OF DEPTH AND MAGNITUDE Using the A and B quality data on the Monticello network, we plotted the depth of the earthquakas as a function of their magnitude (Figure 7). Here again we note that most of the seismicity is shal-lower than 2 km. The numbers of events in a particular magnitude range a were examined as a function of their depth (Iable 3). Again, the number of events are grouped more by their depth than by their magni-tudes. For dif ferent magnitude ranges, we have 62 to 79 percent between 0 and 1 'm depth,19 to 35 percent between 1 and 2 'm. Thus, there does not appear to be any systematic association of magnitudes with depth. LOCATION OF DATA RECORDED ON MONTICELLO NETWORK Figures 8a and 8b show the location of events recorded on the I .%nticello network for the periad July - December 1978, and two east-west cross sections through them. We note that the seismicity can be grouped into five clusters (Figure 8c). The monthly locations of seis-micity for the period January - October 1979 are shown in Figures 9a - 9k. Due to the intense activity in October 1979, the seismicity is iI ' 11-6
i i i i shown on two figures (9j and 9k). Figure 10 shows the cumulative loca-4 1 tions between July 1978 and October 1979. We note that the seismicity j can still be defined by the five clusters. i j S L')I'IARY J j 1. After the initial spurt of seismicity following impoundment, there has been a marked decrease in the seismicity. The decreased j level of seismicity was interrupted by discrete swarm episodes, the 4 most prominent occurring in October 1979. l 2. Of the 70 larger events (llt > 2.0) between December 1977 and December 1979, 54 (or 77 percent) occurred in three swarms. j 3. There has been no noticeable change in the magnitude of the largest events over nearly 3 years. The largest earthquake in any
- swa rm i s 51t = 2.8.
- 4. The seismicity began in discrete locations and spread, j ' coalesing into clusters. 31os t of the epicentral growth was completed by the end of the first year. ('Je infer the location of the seismicity l 1s controlled by the local geology (Appendix V) and by the hydrologic i
- regime (Appendix VI).)
- 3. The seisetcity has been c6afined to the top 3 km, with only 2 percent of the events being deeper than 2 km.
- 6. There does not appear to be systematic shallowing or deepening of seismicity with time.
- 7. There does not appear to be any systematic relationship be-tween the depth of earthquakes and their magnitudes.
I I I l _ II-7
TABLE 1 MONTHLY NUMBER OF RECORDED EARTHQUAKES DECEMBER 1977 TO OCTOBER 1980 RECORDED I ML 2.2.0 REMARKS 1977 December (23-31) 64 64 1 1978 January 530 594 5 February 1585 2179 21 1 March 462 2641 0 April 213 2854 2 E May 145 2999 0 5 June 109 3108 1 July 80 3188 0 August 91 3279 2 September 221 3500 I ~ October 196 3696 2 6 November 227 3923 3 December 127 4050 1 1979 January 67 4117 0 DATA ON I February March April 46 28 21 4163 4191 4212 3 0 0 MAGNETIC TAPES May 37 4249 0 June 47 4296 2 July 58 4354 0 August 26 4380 1 I September October 36 700 4416 5116 5282 1 19 November 166 0 l December 70 5352 0 1980 January 37 5389 0 l 3 February 48 5437 0 March 61 5498 1 l April 73 5571 1 May 25 5596 l I 1 June 12 5608 0 I July 152 5760 2 August 70 5830 2 I September October 54 29 5884 5913 0 0 l t i 3 a
i !I i i TABLE 2 J DEPTH 0F EARTHQUAKES ! DEPTH RANGE (KM) > 0.51- 1.01- 1.51- 2.01- 2.51-l YEAR MONTH 0-0.5 1.0 1.5 2.0 2.5 3.0 TOTAL i 2 1978 7 5 1 6 I 8 2 1 5 2 10 9 8 7 5 1 21 i 10 16 12 3 12 1 44 11 20 13 3 7 43 12 12 14 1 4 31 j 1979 1 3 9 2 1 15 I 2 9 10 5 4 28 3 1 6 2 9 i ! 4 1 2 1 1 5
- l i
5 1 6 2 2 11-6 2 7 2 11
; 7 1 3 2 6 8 2 2 1 5 9 7 5 2 14 B 10 40 53 21 13 3 1 131 TOTAL 130 150 47 56 3 4 390 ! % 33.3 38.4 12.3 14.3 0.8 1.0 I
i
- a yo-.---,,,,,e ,yg--, y-wr -cw-,.w--ewe--,w,--y-g --e%-,-,.em----er,.-g--,w ,gwwg> p ,-,e v wg w w --e-w*w-i- - w--
l TABLE 3 l l } I DEPTH AtiD MAGNITUDE OF EVENTS DEPTH RANGE (KM) MAGNITUDE 0-1 1-2 2-3 ; i 0-1 No. 110 27 3 1B : 79 9 2 t B i 1-2 No. 154 67 3 ' !~ 69 30 I
% 1 2-3 No. 16 9 1
% 62 35 4 i
i TOTAL 280 103 7 ! % 72 26 2 !I l lI 1 !I I e i f I i 3 I
a I MONTICEllO NETWORK 81*2 5' 8f20' 81*15' Bl*10' 6 i i i i i i . i.iiii.6 . 4 0 4 34*25' - - 34*25'
.8 .
I I
~
w A ~
- Q3 E C# E RCO 6g _
}z I 34*20' - Duc E 9 10'[E JN GI i
- 34*20' 9 \
I - e. 5 6A g JSC PAH ! 3 4*15' - - 3 4*15' O 5
- 1 - =
~
~
$ PERMAtJEr4T STATIOtJS I _
I E MOBILE STAliOtJS
' i 8
! I i f f f i 1 I f O
2 f f i f I 81*25' 81'2 0' 81'15' 81*10' I I l Figure 1
+ - ---- - - . - - --- - -- --
I SIN 3A3 do ON 3Al1V10'.'.'fD o t I 1
-=
I ;
-e 8
j c I . - , L
~~
I _~~ l ic j .. -s l .. l [~a t- e. I \ [:~ ~ I [ _~ O u
=
.e LA.
~
h gq [-u was - Ty; . _g l .mc w. - 9 w _. I
* =
3 - 9: s (_~ l I ,e w e.s [ lBili!NS$$ k;n. la . ssw ::s s':,wpkswm_ y(it [--U I S o n
/$a
_o-s s
- E
, , , , i . . . , i 4 8 8 8 8 8 8
8 8 89
- 3- 0 S SIN 3A3 SIN 3A3 30 ON 30 CN I
E
,. . 7r3p- 1;. a p-'.Il- ! '/ i .' i fu l y I J 60 0, ,0 C6[j',, @, g ,
-: c::t..: em :::::t;.: t/t-upa 3 ( 22. 2. 22 ;s :s
- to " 22 :: 22. :s Je J s. 1 ,,,
r .. . b. I N ,L \
~
,~ d $~ ish *"
- % fQ .. -
K~').
~ '
1
^ '. N *** s
-- s_
,s - *^
y ," i. A e
)
- r I + )
zS,?,
'. , f.. -
i s z 9
-=
- o. . - Ab s
~~
fe ,;,.. u,. I - e, . , g ( R%j
.s wy
( M ( r"
,u ! ,
* .. - r. s t ..
- .. - r..
- _.. y L~
s .., l" r h I 2 b.;
;3 E *:'e*:"E..* f/l5-31/73 '4 * :C I'.L
- z/t-7/ a se .-
t ' n. 26. 4- :n. :s. .. :s. - 2a. at. :. :s. :n. 2- ;s.' , l
.% g
'A
.F*. -
ag- .
- a. .
, yv f.5 's .
, Tw' cI5
^
'w c,A i.
i
- .'w ,, . -
I
. s. --
4.*'L i- us
'. 5- ,
.- 5s ..
{ .. e L., 6
& st N J.
'GQ ^~~
W - r C. L ~. J t T
&*e
^w
( N I ao .. l C','- J
;" 1 # 3 J r"
,, a w
y~6o p.a l .- .
-- . .. 4 I s -
I 6 L,. m 6.. 1 o - I ls.
-I l
.. - Iu l
, I - s.... u s . ::, q .: s . . . , i n n.::, l5 n, g
l i t i ' *0N' CELL: 2/es.2f/78
- 3 NI . ' E'. . ! Z/3*!*/73
. .. , ,' n . 4 2, ;, s . :n. ;, j a, ,
- s i s. , s. . . ,
j ... J L.. e pb 1 - u
; c.:bu4 ,
C,. -.*. ~ W J7, f5 , 1- E ls. j ,A
,n,V , Eh a.a -
i - > ;
/m > h ls..
h b . m e 1'
- , r I
' b. D. , {n.
D. . - b, o ho f - a* - *\d p G I.e S ,7 oo _. - N'A G C.\ L..
- s . ' C* >
Ln
- s g C
_" - aQ
!g ..
l .
.. - p r -
3 ) sa '
- p. ,
j } >%d t_. . -i
)
g.. l t l.. - I - ' %r - ~
,, V'y, v
M'E :. .. 1 j - i L,., L,.
- 1 j '.
- *
- q I
34,;;
- 3. 00 .
t
**NT::EL,; r/~g
' *3'**'!E- 2/12-29/79 I II 22. < '-. 1: . 22 ,s. je ' ; 3.,
.2 2. 4- .s ., .
.s.
i' ... gD]y L,, - p ! , an. u.
~ :: a. a s e ex
- I L..
.cc _w _.
c,L. , \ G r% s
.- y* -
g/t .-- l
.3( o E'. l WD ,, }.e7o E.'
I y n
- 1~~
~
%jO 1 o i i
,~gg~..
I s
- j. a e' O L.. d -
w o L,. I m
~
h: M 5 i L. 7 i j: , ,. -
~
L.. L..
. w:- u. -
s s 3 A n a- L.. L. , i -i m.. L. ,
'I -
s.. 2 1 l 1_. .
- i. , , ...
r.
,,.= ,
I s mDo..gnmm...aei t ,: o , a u.
" ,'wd~%juut
'a, a
' Figure 3b
t I I E
"0NII CE' ' * ..
t/78
- N'! E. 3/73 a**
ef 4* 18.
- 22. 2t. 20, 2_9
- S *e e... -
P' 22- 4 ;9- is. 17 I... i , j'W_ ,,,-
"" - o f,e'h m
-N, c i O &'
- d. c o u _?c,m~
O 'r. g,_~Q_p c.
.., - .m ..
-; ns _ i
*- i Q_.--
, u' . .
- n.
l.. t
>- 5 : >, 5-
'z-%
E . CD L. ,
$wN 'O # ,w C o s
L.. i ,. 4 - ^ i : ___.,.-a , s.,_ 6 o - -
~.. _> g. (
o l , O L. , i 8 i n 3 1_. . - i 7 l.. t u i.. : # -) p , ,v, ,, (' 3 -c-4' a A,c O,s
- , ;.. i+
s - i . ,e m - ,'l s , i i -
<~a t' w ,
L.. j f,. I i- ,. _t l I - _m 1 L., i 2 2 , .. g.. n . :: 3
**N':E.." **NY::E.. 7[78 6/'~3
; 9. ;S ' 15. "
- 23 . 2; . 2" . II. 29- IT. JS.
I " ' 32. 2t . 23. - _ ... y_, >. s
- . :: i .
o
%s_-9 b:
Q5 MA?c C I q 4% b
/
,u i
I - l k, e l e. .. l
,,a. s -
,s p.
k e
')
y. l ws n
# * / \ ey
- a. E s; ,~ e. .s-,ia f ,
o s. ,v: s - j Q o L. I -
-l i
/ ,- % ' /
- t 4T O
s. i o 3 i-, T- '
.s g'N'%
s O j
,; ; r. J ,_g ; L..
w , I . I l *
-," p"]
, 8, ] rb k q
y g ; q _l 5:4 (". . I h a p. , l _ p. ,
- p. - <.
l..
,! l..
t ' ' 2 , .. " h . ::. 34
~ ,-
4i 1 I <,
@\'l[1,f d 4 W~<
a %. w. . . 9 Figure 3c I i - . -_ ,,-..e _ - 1- - - . - - , , . - . . . . , . . . , , . . , , . _ , _ , , , _ , _ _ _ __ _ __
I
- I
O?$ ~ CELL 3 3NO "CNII Ekk) 78 er g
*
- 22 21. 22 16. I 22- II . 22- +9- iG- 37 Il- I
;9- _ 21 i
-y
{ - La . h ( a vu ' . J e g ]* l La e k[N L,a. . I.. s _S y5s2 E t s% !. ' u C_ f_ . _' ~ . _- f. r
*,s. .
5 5
- ~j '.
s . 1 s.
\ l l
,'s s.,a*,-*p a l l
, 1
.. se
~ - '. '. . -
'W
/
I of f J i l wE A "* t,, ,po, ,/ vj. y* - t.,9 (; ,f," Oc - g., _ t o i
, ~
, ,/ 9 ,5 0 H" l f". . -
N,ps j L.. L.. 3 -
,)
r s' w ..~(";_ .G
. 8
\..
J (-,.F.3 ., 0 %, - Lu
- w i, .
' l l ..
I I I an - p..
- _.. - L..
- ; , ... n -
- , .. .l ." ,
h :" h :: I
# 0'I :CE._* 10/~3 * * ?J :25.L* 11/
- 3 a#
I e' :s.
*
- 22 2t. .' .{ : f .
*t. >
*
- 22 21. 23 g it. 3 i
em
,o_~g
.~
j e ~L4 h~ , t, . O J C
// l.
.[A:7j .
r.
.~.
~m
_ "i C
,. w C. !_,. . ., ,3 , .- 1..
(
- i. i a y 5 ,
, ,c i
~ [', .
~, Y i ' --
- J . -
- r . ; <'i g n ? i p t - w
;,g. i
- /,"~:n ,-- s.. . \\ L,. - c s
O I y, -} ;C ' f l
; 's. 7
'} "..
4 o se - (,, e/,2 3 'r
' % ,5
.::,7 0 j r...
~..
;~
l e
,t7
\s 0 e s,
_3
\r ".
w.
; - i
- . 2
.s .-
, , - so,W-ss,1 c , ;
i las d s W, ,,,.
\..
' I l_.. .
- - Io I.. i I.
I - . i s . . , ie- "n.::.
~
i
.un. :: ,
j gg ; p
^ cs. n Qiasi< . ~ . , s uo.
r' . . I ,, f ! 8 d b lt) . k i '
- O'J D]
4 Figure 3d
I i l
)
= NT:: ELL f2/73 " 'd': ELL:
1[71 l 9 1 ] l e, e' ~ 22 2! . 20. M_ ; t.
' :S !_, *
- 22 21. 23 M J1 1 It g a
.. J i
l'
- Mh L..
jf % l j C - m s
, . J O
s' L.,
! h.'_ h. x-/
I
.M. ,F* L..
i
-\p L.
p !' m .s =
'p.-
s { aa I
.) D I** -
I >
<x $- e=
I,alz69, L ', <<--
. l .h6,:,-1 7 -.
/I % 4 ' "h vg _c i !
s ( g, i l g ,
- (
, /
y ,-g. g.
)
s-O - . . -
> )
C 1 .. . O I l 'A ! 4 0
/i " ~**
O -\ * ~** , _! f,'. j \,/' .. -
+ h ,,g m*
l..
) i )
4 - t s,,7W 'A l
-. J G , '.;-A' '
l ru J' ~ ( L.. . J - L. , l ! 1 l g.. l . l _ u - o l 3 3 s., l..
=d i
, g ess. l..
34.03 34.** i 4 l ,
.:. . :: .2
. ze,
- o ::r.a e, 2, . ,. 2,. ~, . ,.
- 2. 2m 2. ~. r. .
. , m-. a -. -
- am., ,
L.
' e
- l
~
~.
q] v'. n cy = m. r. . t _ yq Ie. -
'u [/. .
4 j %c D_ I ,. .
, i , o& .1
,s
- ,,na -.,
esv 3 L. 1 . 5 _.i u. . e. o . 3 o e v L.. J t J v \ i i 0 r u.. I - t ., 1 s
, i.
cWr,' g t, i Qo ,~':- c i" t,, ; 0 [ i i i- .. I _ J g l I u i ! _i -.. r... .
-i
- ! ' 3 4. :0. I 34. .:
I g ep;, t '.),q n. ,
!c -. r: ' ] ! i , %. n q q{ , ,,
ti Gi,jd
' b. LlNu ', u! u $ !! ^aw _ m l i p!J il u2 Figure 3e i
I I
1 i
- I, 1
*:st::t_t: 4/n -: a::t.t: s/n
' . c
.< n. 's. n. aif 22. ' A is. < :s.~ ,
- n. 22. n :s at i ,, , , , ,
I j b~. sd., L., i i. J k~-
.~a C,
iso L..
,u I at -
i d( # # , yw [ J { .
. 1, . .
J <- cg; * - -. - f c,?.) * - w i
's 3.:
-w 3 N ~c I
[ [ "I '-l O ;$l. O #',hO - ,,
-=
r-4 i - n -- 0 , r-0 ( e
- t I l {
! l v i
_ J
; - p..
4
' _, ,LA] -
k'
),.
, M)-
%A -
L.. i i i ... I _I !_. . - . l l w" , .. L.. J , . . , , n . :: n . :: 1 4 i d
- NT::t : -- 6/n -est::t..: ,'n i
- u. v. n. n: ... >- ,.. ,
- u. v. n. p, :,. ,,; ,
,s e, s sg .
-p.
. i
's" - ,.; .vy L..
i
; s i i
~
uf6 h* I u M~
~
~
w. c-u '.*.
,c%
cr i ..
~% r -
5
') : ls .
(- t - 1 l d i I
>f)n,
,5 fy.c s~s Y
L J. O s. In .. d
-i i
g,A a 3 O 'tw
#' O A A - *
-! U s,.
7
)
r" q I U ) y ; L.. i 'u - m J <P - < L.. ; I ,, I J' k/ ) t L" J' 3
- L..
%o 'p~,iy {)
t m i ~_ *s.r y ()- l.. l . p.. q _ ~ - ... I. w - f 4 I I. - .. 1 t I." r I e , , . . ,
* ! w .:: I 3. ::
- q- 7, m ,sq q q -
a yA ...a w m u& Js.3 2 o
' u I ~
I "- "
, - . +
I , I
-: c Ict- .: . a/n -: c::E t: t/f+-39/7
@f-el
" n.
21.. 2:. n,.. - :... ._, n ai n. n .. 3,.- 3..- , sA s6 L. I
, < i w v.- -.
- yvw ~
1 ,_ $45 I s2 #D -- n e, s C I'. 1 i E r'. I g- p 3% -
,,+ '
L.. s ,..
z- ,
Qa, ~u
- % s* I-.
(, m sn . vc ss ym l-. I
/ -
i. 4
~ /' .' O si
~v gL -
1 ~ . , ~
~ l~"
0 0 -; i s p- -i (. p" 1 r" i p" 5 - 3
,u, . :+ 0 . ;"..
3 u . -p. g' (".,
-l l - u.. - ..
Im
. - I. - '_u j L.,
I.. I " ,.::. i - 3 3,.:3
-: c::t..: to/t-7/ n -: c::t. : :c,/s-*/n
- 22. II . 20 . I I- _
*
- I2 II . $$. ! ... }7 16..
E ... I N.A- L. fS.M . %
'i. I
?' 2*
, pf; -
q 5<VI I
~t:i I - I.
r l- ci t C_- I."~ *. q D J #
<c p..
-f k ,
- s. . .
I
., _.i _.
q... s g. < ,f.,
' t .ca w', ,. I - , .
e g.:a,,
\
s /r
, - s. - a , , ,
u dr o avPt.'O i e s, ,. I (-
/t W.. t ..
; S ,,
- 0 sns y
I
,,\
t, > u.. 8 - 3
'n% s 3
>^
m - I
/
- i. } .
,- f.
y t-j? g "
c-N g I(. g , 7". g - p- - _ , .
-. 9 . -
O . L.. J L., l I. " . 3L:3 I ' 34,00 W
'~p. 3, 3 f.~- '
'Q ~'\[
jb Oau tae3'Sddh% : ., ! b
- s I
Figure 3g I
i I I-
= aN T ; ;I;;.: 10/:5-U/77 "*N!!:ELLO fC/22-3f/79 I , , , < ,< ,, m. . . ,.. ., < , , . .. ,< m. ...
j .M) I. _
' I V L..
* 'I
- 5- p 3 -
e.
! :. C l i cir L, i.
- e. - -
)
- I ) $ ~I 8 ~
l l l Y L.'
~
S v': % f'
- ,L
/*,S. l_ ~'
W J' Jt he i
, ,;,Y ' '
I M[LO' o Il 1 / ,4
.. _ /
<\ u..
l $ s
'r, ! ... . _
l s 'r, I.
"wi l 6 r J'
), ' g
- l l
__pp o [" l i" v y:+ E 0 q _ 7".. _ _ (. . _4 O !.. I.. I. - J s s .. ' -' a s ... I 24.0: 3 I . .; t ; ; E ,,,0 11/i9
- NT*~E LC f2/77 u ,,. v. n. p, ., :.. ._, .
" n. v. n- r? K .: = .
i
, h/h h. J oh
-e
'me i
~f( , t I'
_ "-b- T ~ ;,,* f,* . 1
- . - f-.
C 'N-- l 3-D,. l .. _! Q
.t -
j % l 3
) l fz -- -
,z. ,7 r f'
l l J , c.t> - ' %, r-' , 65
~
8-
,c, dL C i
o N/' t h -; O {". . i 1 l 0 C y I/ s6 > j l" ! ,s m( i t~i s, 3 A - l l 3-i K(w'Aqj0 a .""
> :"s -os 0 '..',
i~ { ] A o o ,..
,i l-.. . - '
I i i n . :e r"w.::. I L. S ( f '* 4 q m
.np ..
- w- m ua' a's I '
Figure 3h
4 iI MONTICELLO Mu >,2.0 12./77 - 2./6/'78 lg j gf l'f2. El. f0. [9. [8. 37.
,1 5 .
l ., 23.O i [ g ) "25 _22.o j _2t.5 2L,O 20.5 f % l 20.3 0 y a s.s
- 9.o
^ q ._:s.s l
~
p " *
- ..s 3
I a 1T.3 a I a.s _.:s.s i
- I i
~
0 5 (KtO 34.00 6 i i i I i I l !I i Figure 4a
l
;I lI l MONTICELLO g 2/10 -16 / ~?8 M u >,2 .0 p2.' 31.' 20.' [9.' [8.' ,J 7. ' 15.'
l ,,
--3 g
~ '5
_ 22.0 l= _ 2t.5 l l l3
- J t.o
_., 20.5 l
,. o E '9.s i I 9.C
( - :s.s _:s.o
- 27.s 17.O I _ a a.s 6.O 5
0 (KM) i ! ' ' ' ' I 34.00 i i i i I i i I I Figure '4b l
I I I MONTICELLC I 2[ts-2G[ 8 M u >f2.0 p2.' pt.' p0.' [s.' ps.' [ 7. ' ,1 s . ' l _m. o I f K . n.s l _ _22.o _2t.s _2..a q I - 2o.s l , t
-l , 2c.o 9.5
-e J 9.0 i
- .6.5
- :e.o l
p !B G J 7.5 l 3 _ J 7.o J s.s I ,,,,, J G.o
~
0 5 IKM) l i ' ' ' ' ' 34.00 i i i 1 i i i I I .
I MONTICELLO lg j , 4/22- 10/27 , 19 7 s Mt >,2.0 g 21 . . . . 7. l P f2. fl. f0. [9. [S. - 16. 53.5 ' i '
- d 3.0 3 l 1 f I; -
% _.22.s i,
b
; - 2 .c
- J t.5 i
- J 1.0
! - f _:e.s iW -
)
_:3.o i ( / j
- a s.s
- 29.o l
$o r
l
-.17.s j - !I - 27.3 I
l'l k l m .s J 8.0 !I
~
0 5 (KM)
**8 34.00
- I I i i i i i i
I
!,I i
Figure 4d
l
- I I
l MONTICELLO
- g 11/24(76 - 9/14lM Me 42 0 s af
- 21. f0. [9. /S. J7. /S.
f2. ' I -l23.5 1 _.23.o
~l c
' :2.s 22.0 I
s _.2t.s lI _
'h ( m ,..
20.5 2c.o
- s.s
, :9. 3
_ x q 2 .., p _._2 s. c 9x i -
! e 4 m >.,
g J T.3 i - 1G.5 lg _ _.i s. 2
" 5 (KM) 0 34.0C ! i i I e i i i i I j
Figure 4e i
I I ~ MONTICELLO 10/3-9['7 9 Mu>f20 IS.' f2.' fl.' 30.' J5.' f S.' 3 7. ' l 22.a 5# :2.s 22.3 J t.5
- 2t.o 4 L2:.s
- 20.s
- 29.s
- :s.o
- / a e.s
/
- s.o I
/
7 J
*7.s J T.0
*s.s
- 8.O I ~
0 i i i i i 5 (KM) a s.s 34.00 l 1 i i i i i i I I Figure 4f
I i
! MONT1CE;_L6 IO/G-21/~F9 Mu),2.O
-l Q* . . . . . . . 3 f'f2. f1. fC. [9. 3S. / 7. ,1 S . l I _ yfzK 23.O
- .2. s l
l _.a Er s.o 1 2t.s i.:
._2o.s f
n ~s. o ' _I c.s I ...o l A l _
/
/} . 1s.s i
iJ / p -:s.c
. 17.5 l - 17.0
- c.s
- 2s.a 0 5 (KM) i ' '
g i e r 3 t! . 0 0 E I i i i i i i I Figure 49
I 1978 I 10l JULY 10 , l G MARCH P I 0 .X L-i i , oi to -
- $1- @ i i 10 -
( AUG APRIL 0, ", 9 I O , ' to - 10 -
" SEPT MAY p[
p; I [i.:Y// i 0- , , 9 0;"' "'] , , 10 -
"W JULY '78 - OCT '79 I 1C -
M' _ OCT 200- 50%l _
~
JUNE G 0' I
.:!.!:3 15 0 - tma V///M
N~~"
" E
'O '
juty 0 i i y m i a ,, I { 20 y , NOV 100 - ,,,, E 25%- O ;""l 10 - p i osc, AUG I ". j o . LL j LT3 /g 6.:.:.:, C
' //gij.-_. O _.,
f'?; 2
= -.
3 Of5 G 3 r 0 1 30 2 0* * ' ' SEPT D DEPTH (km) M I '-g=n DEC 10 - 0 ,m i , I
/
C
~
60-OCT l l Ci , , , 1-19 E CLUSTER I 40- - 1979 CLUSTER II 8 ~ 20 - 10 - JAN ] . LUSTER III CLUSTER E O_i i
$ CLUSTER 3E I
. e
'3 OTHER 40 OCT 10 h FEB 20-
_p I
,!![3bjijki) b ._ -
0, E , ofMD, 0 2 3 I 2 3 1 O 1 DEPTH (km) DEPTH (km) Figure 5
140 - IeZ - I w> 120 - W (i IuO 40 - O ' " _ m lZ" " m 20 - INO F-a E 8 a a
" 8 a
m a g 0 , i i i i i , i i i i , , , , i 100 - I so - . _ g . - 0-1 km 3 . . 60- - l b4 - 40-O O _ . O l 20 - O o O O e O O I
+ .
0 2-skm I I I i I i I I i I i i i i l l l l 6 8 10 12 2 4 6 8 10 1978 TIME 1979 LEGEND: I m TOTAL No. OF EVENTS e 0-1km DEPTH O I -2 km DEPTH
+ 2-3 km DEPTH PLATE 6
4 iI lI 4 !I l 9 i o 20 7 3o 7
- g l3
) .:: .:. .s.. ..- . s
. :g*. }. . .s, .. :. I .
t (
- i3 ,, 1 .. , ,,
15 .' <**
.s .
jl
- . . I-.s..- . :. * * . . ;.
. .. t #
1.0- , .... .. ' ' i v i v * ' *
- I l
I w a t *t
- 4. w .i-l o 2.o _ -
t 1 I t 3.0-1 1 !I i 4 il 1 Figure 7 4
!I
JI MONTICELLO EARTHOUAKES il 4 auo -osc. 7sra,E =
)
1a 20.
; $ 1' f 2 - ?!
i t < Ic u -i ,1 7 . Ic 1-- l'3.5 0 ~, m :1
;I US ,3- -
gr s s {
~
'~
oL
$ . :: s 1
O~ 1 P d N __.2 2 0
~, - '
{J
=
11 \ e , ,.- i w-w c ;-
, um 5 ' )
- _ El . O l
dBh n k A- B =s lI a e w:#
--. : =
=T g3 &
e ,=af ' ac A=, emu
=-= r) 'E -
o (s :- % r~
, 3-
= = C _:e.c
,l - m s' C
- v i .
- 5Se mm s
_1s.s 4 gg,r.m e
- ~ -y eQc~ }e - _is.a e e
,I . _ _ !7.5 l
~ J 7.C
_16.5 I
- _ !6.O I -
0 I t I ! ! 5 fMM) 25 5 M.00 I i ; i I i 1 I 0 I - = e e a m w e = + x e
- o. a. 3. v. o. 2. 3. 5.
h
-i- i. i. ..
MAGNITUDE DEPTH (KM1 l
;9 t. o
4
; I lI , , , _ , . _
,. !g .. i a i
t I B
)
A- t 0 0
',., ' . I
,. I.
0 s 0 *- k:. [ . -i ~.
=
; - -- . 2-. *{ - - - . ,
D
'. ~~,
i 1 - ; -- ; ;-- iA ,'.*_r- %++
? e
.!T I"l l l
. .. r ' (
3-: j i
--az_
i _
~
_:sm, =! U i .. l' -
.l an5 4 .
.: ~ f r
;l ! ,
i ( t w i i g
;A . ~
r l a j 4
! L i
-6 -: -
I c! C3 - 4 i 3 - !I t i
.f eetwe' 81
?. g i j-.
. G+ ,- r- .s -
.,Lb s
p
! L s vL4 I a I
l C ~
.. 0 .%.
ft c C, 3 7 C b 9ay 1[ D
. ,1 s .
i l ."j s !
- p. .
I
.~ .
! 'f ! ! ! l I
e W . 5, *f
&*Fg ,
l ,
*:.~ ~ *.,q . ~. * ~.~.
l * ~ -
. j jj f' ,. ' ;-* 9 .._.;- .^ 1 1 ,
I
,a T ! 't 1 i !
I
~
- S _[~
i, ,: i-:t.. - - -i - '
- ,, d I !
.. "t
*/ -
v t I i 0 ; i
= ===e !
i ! es m
- - t; _.
I O-. 8 s I .. - g a4 u&u - I e49 re 88 I
lI ii '
=
i M uni.c
,,.. , L,u 0 ---..,u,,..,-
-ni c n:
. E 3 .j uLyi-Utt.
ti~1 f. 8f ,1.
; 70. l 9. te. ,t '
,2 2 . , . . i
- l G. ,3 4
y . _ r,
- % _v ..
- E _
ev y ig
- q"~js b,J =zs I n =L g ~
'J G, A
_ y ,r ~* i: a O d
- - I 'NN G v.T
- 2: 5
,.l _ 2 u
%/
- e.
t - a a w L-- 1 - 11 : ::: F@Q , s o-
;;g (u -5..
J ,, ;-$ - U 1, o 1: s :2
-3 507v
_ =_= _~ _,3 j 3i- - ! _ , ; =,c
; mSN ,%.a Wc
- s:- --wZ:. . / % ats 1
n.L
- .x , 25. .r,,=-
i ,. i _ a _A n* N M(,N/ _r: c - -s a { e n- - -
> s ggf 1
su a's i _ -'= __ a s.s g - 3 5 wa -3.. 15 34.00
; i i e i t i
1 : i i - 4 1 4 i . i ,, . l - l ' I as lrr . J l t._J irr
.r
[-
,n
-i i
a'Xlel l
*t 0 1 2 3- * **- i 2- 3 8 S-MAGM!TUDE DEPTH (KM1 iI 4
@ , Figure 8c
'hIh gjg eas -
I MONTICELLO EARTHOURKES JANUARY 1979 TAPt DATR sf . . . . . . - g p2. pt. po. 79. 7s. 77. p s, , h _.23.o 22.s
'"" 22.0
' ~ 2t.s
~' - 2t.o 3
I -
- :0.0
- 2s.s a
- s.o O
I - 3 _ 18.. _ 2s.o J 7.s J7.o 1 _.J 8.s _J s. o 25 5 0 5 (KM) , 3 tl . 0 0 g i , i , , , , I - o e a 6 [D e e a + x o
-i. o. i. 2. 3. .. o. t. 2. 3. ,. s.
, l MRGN1TUDE DEPTH (KM)
, I
9"re 9-
. 3 7028EMBRhk - .
I MONTICELLO EARTHOURKES FEBRUARY 79 TAPE DATA 19.' 18.' 16.' f2.' fl.' f0.' 17. _ 23.o _ 22.s I _ 22.O
# 21.s l
J t.0 I 8 _2c.s
- I -
a
%,=
"oo 4
._.2s.s
_ 39.0 Q J 6.s O . g . se.c s 7.s I _ _17.0
- _.s s s ! _2s.o I -
0 5 (KM) s.s I 1 I I I I I I 34.00 I e + X 0
- o 0 6 6 0 0 A i
l -i. o. t. MAGNITUDE
- 2. 3. .. o. i. 2.
DEPTH (KM)
- 3. .. s.
- I p n Figure 9b f u ) I
I g MONTrCELLO ERRTHOURKES MARCH 79 TAPE DATA g 3r . . . . . . ft, f2. 19. 18, 17. IS. f0. g __23.o
- b _22.s
._2! . 0 2I.s 2L.0 I
J o.s eO c* J 0.0 0 ss.s
) 9. o
_is.s J 6.0 a 7.s
. 17.0
_1s.s a s.0 0 " *** 5 (KM) , i f i i f f f 1 3 tl . 00 1 i l I I i I g
- ci e e [B [0 e e'a + x e
-1. O. 1. 2. 3. 4. O. 1. 2. 3. 4. s.
! MAGNITUDE DEPTH (KM) 'I l Figure 9c I !.I
+ -- o
i MONTICELLO EARTHOUAKES
! APRIL 79 TAjE DATA 31 . . . . .
I p2. pt. po. ,1 9 . ,18 . ,1 7 . I 1s. n.s J .0 22.s __.22.0 E - J a.s 3 J I.0 lI O a .s 2 0.0 u s.s J 9.0 I C + - 18.s J S.0 _ 37.s
._.sv.o
_2s.s _.2s.o .I i 0 5 (KM1 a s.s t i i ' ' ' ' l i l i i i 1 1 3tl.00 !I l
- o O 6 6 @ 6 O A + X 0
-t. o. 2. 3. .. o. 2. 3. =. s.
ll i. MAGt41TUDE i. DEPTH (KM) Figure 9d I .
I MONTICELLO ERATHOURKES I MAY 79 TAPE DATR f2. ft. f0. 19.' 18.' 17.' 16.' l 23.o 2 .s __22.o
=
J n.s I - 0 m
. 21.0
-20.s I -
O g 20.0
- O ._.2 s.s 29.O I -
J 6.s J 8.O 27.s 27.0 J 6.5 16.O 0 5 IKN1 I I I I I I I I 34.00 I
- o a D D D D D A + X 6)
_i. o. i. 2. 3. .. o. i. 2. 3. i, s. MAGNITUDE DEPTH (KM) Figure 9e I
4
- l. MONTICELLO EARTHOURKES I JUNE 79 TAPE DATA
- 17. 16.
31'f2. ft. f0. l9. i8. l J 3.0 g - l O g J .5 2.2. 0 J t.s t.0 J 0.5 J 0.0 1 19.5 1 i J 9.0 _.2e.s O ._.: e . 0 I 4 J 7.5 A7.O 2s.5 I I i J C.0 0 5 (KM) , i ' ' ' ' ! i 3tl . 00
; i i i i g l I
lI 1
-1.
e O. a 1. O 2. O @
- 3. 4. O.
O 1. O 2. A 3
+ X 0
- 4. 5.
" MAGNITUDE DEPTH (KM)
Figure 9f l l I
i MONTICELLO EARTHQUAKES
! JULY 19 TAPE DATA Si . . . . . .
16. f2. ft. f0. ,19 . ,18 , 17, I O
- n.0 E
- _22.s I - _:.2. 0
- _ t.s
- J1.0 jp 20.s l - _20.0
_.2a.s
- _.19.0 e
I _:4.s V !8.0
- n.s
~ J 7.0 a c.s
'i - _16.0 o 5 (KM1 . 34.00 l I . I I I I i i I -
- o e G B C B O A + X 0 t
-l. C. 1. 2. 3. 4 O. 1. 2. 3. 4. s.
I MAGNIfuDE DEPTH (KM) l l g 5 Figure 99 l I
I MONTICELLO EARTHQUAKES I AUGUST 79 TAPE DATA 3f . . . . . . . p2. ft. go. ,t s . ;t e . ,t 7 . j ts. i l J 3.0
'2.5 u.
g _ J .O a _21.5
,_.21. 0 i
l __ 2o.s 3 23.O J 9.5 2s.o 'l _ _ ae.s
-1e.o 17.S J 1.0 as.s l _
_ 2s.o I --" 0 i i i ! ! 5 (KM1 34.00 g i i : 1 i : 1
- o ,a 5 ' [.D @ 6 O A + X 0
- a. 1. 2. 3. s.
g -i. O. t. 2. 3. t .. ! MAGNITUDE DEPTH (KM) t I Figure 9h I
1 l 1 MONTICELLO EARTHOURKES l I SEPTEMBER 79 TAPE DATA 3r . . . . . . . g p2. ft. 30. ,19 . ,1 s . ,17 . ,t s . , I _ f J3. 0 J 2.5 _ 2 2.0 _ . 21.5 _ J 1.0 I . _ S; 2 0.5
- J 0.0 C
_ 39.5
- 4 9.0 m 28.5 2:0
_ J 8.0 e 17.5 l I _ J 7. 0 _ J s.s _ J 6.0 0 5 (KM) , 34.00 I l I I I I I I I - e a O O 6 0 O a + X 0 l .t. O. i. MAGNITUDE
- 2. 3. s. O. t. 2.
DEPTH (KM)
- 3. i. s.
! Figure 91 I
I MONTICELLO EARTHOUAKES
! 1-12 OCT 79 TAPE DATA st . . .
19. f2. ft. f0. 18, 17 16. I 2 .0 _.::2. s I -
.g _ 22.o
- J t.s
- J t.o I ,
e s 20.5
- 0 J o.o
- [ :s.s
- 39.o I
O
- 0 18.5 3,, g e s eE n} O 8 o CYYO \ a e.o g
e% a
- 17.S J 7.o i
_ J 6.5 l _ J 6.0 I ~ 0 5 (KM) 3'4.00 l I 1 1 I I I I
- o e e [0 6 e e A + x o l .. o. i.
MAGNITUDE
- 2. 3. .. o. i. 2. 3.
DEPTH (KM)
.. s.
Figure 9j I -
!l 1 f 1 !I , -,c., i ! ; L [ L ,L u-iL i MI n v ; c) 'I [, ; ' CM]1 6 l n-i *o 7vc , LL is_vvm r 1 . s: -1 : 0 i c o E- CCTC - - : l 3' pa. pi p. ,
- 0 ,: 0 .: 7 i
's.
t 1 s _2 : 1 l N ( u
- _ _n .
~
_ ; ._n c lI _
/ . 2 8 . 'o 2s c l - L, l ... . ,
e, 1 1
-s :
~-
= . n .-
i
- M -= .,W U= .
-- Tb, .ae .c
! m--M c : 2: u 295, - 223 4 4 _ _:c c i Q. , 3.5,n ..
-s ,.
, my r?i-. C -
t 5 m/ l [ 4 __: 5. c
\b) i 5 >
i, 1 '
, - : ?.0 c
___ J 7. C I _ __: s 3 _ -.a s v
-- ass e s inni I
i ' ' ' ' 34.00
; ; i i i ; I Figure 9k a
p w ay uwc p wd ]lj yd A [
, g
i I l i l l MONT.iCELLO EA~ITHOURKES
)I
{ j ' mm O/, e- - - L,,.
.- 4 1 9 / .g -r,m- mer-31 , i S~t uni-E 3
l5 f2. R: {U.
; 7. ;5 i 4 D 3
- - s e
i
% N .
, 3 1
, "q,j
- n. T
%~-
~
V
,==$
J2, L tc.-
\ .~ . & G g%
1 m p
, , , ' ' n %
$DWv 'h Y W 2s 's i
I _ i - tem a~ g_b f n ,. [
%T t- ~
- i . ',
i Tnda: ~ - 3 L G2 ; g$N??QT+ ~ -m c( ~
~
w .
- 4. gsq .,,
, c.f te %m
-s..
f my 4 ~
' 6. w u r~* e
~
Em.sf D' w i
~ ~'
___ ~ g *** m
.wa
. M 4 4h ' tag 1 TP..
~
s-+, W+ --e.1 f.% _Qte m w ., ';" . , i I -
- v~w3' ac gm -
m. L m 4 ( n' j - 4 _ , 3 7.
- U t J i..
i f 5 J '4 ~3 J5 ' t 0 5 i K P. ) I l ! ! ! 3Q b i I I
- I 1
, i.
=
C.
=
a. lorolml J. 3 4 0. m o
- 1. 2.
m!+ x ol 3 v. L. 1 M4GN!TUDE DE oTH (KM) s .- sD ['s , Q
- i. ; . 'W d
% n 'euu!Midu D
j ) ibi:. . a Figure 10 iI r i
_ _ _ _ . . _ _ __ _ ___ _ _______ _____.____.-._.. _ _._. __ _ ___ ___.. .___ _ __ .__-.__ _ . .._.__ .___._.~ l i i , !I APPENDIX III i I IN SITU STRESS CONDITIONS AT MONTICELLO RESERVOIR l l lI l l i p I I l l I . . _ _ . . _ . _ , .
I I IN SITU STRESS CONDITIONS AT MONTICELLO RESERVIOR IN SITU STRESS BY HYDRAULIC FRACTURE Two deep wells, Monticello 1 and 2, have been drilled by the USGS to depths of 1100m and 1203m, respectively, in the epicentral areas of greatest activity (see Figure 1). The results presented here are sum-marized f rom information obtained from Zoback (personal communication). In situ stress was measured at depth using the hydraulic frac-I turing technique (Zoback et al., 1977). The results of these measure-ments are shown in Figure 2 and presented in Table 1. The error bars represent the least and greatest horizontal st..sses. The vertical I stress is represented by the lithostatic gradient (solid line) for a density of 2.7 gm/cm 3. In Monticello 1, the difference in magnitude between the two horizontal stresses is relatively small and there is only a minor increase in stress dif ference with depth. However, at shallow depth both horizontal stresses are substantially greater than the vertical stress implying a thrust fault stress condition. The data in Figure 2 Part A indicate that the tendency for this thrust-type faulting is limited to the upper 300m or so, as the measurement at 486m shows that the vertical stress is no longer the least stress (a requirement for thrust !aulting). Figure 2 Part B shows the observed in situ stresses at Monticello
- 2. At shallow depths both horizontal stresses are greater than the vertical stress. However, at depths from 200m to 320m, both horizontal stresses are approximately equal to the vertical stress. The observa-tion at a depth of 400m could possibly be reopening of pre-existing fractures. The hydrof racture at 508m depth entered a pre-existing I fracture; thus, the stress value lies between the maximum and minimum stress value. No data were obtained below a depth of 650m.
These borehol stress measurements indicate that thrust faulting should be limitem - the upper few hundred meters where the vertical I III-l
I I (11thostatic) stress is the least principal stress. In both deep wells at depths greater than 400m there are zones where the vertical stress I is no longer the smallest. At four locations it lies between the measured CHaax and Chain and in one case it is greatest. If these data represent completely the ambient stress field, we s;uld expect to observe strike slip faulting and possibly normal f aulting at depths shallower than 1 km. However, the observed focal mechanism results give predominantly thrust faulting (Appendix IV). To reconcile this apparent contradiction, we note the following:
- 1. Even if the deviatoric stress orientations are appropriate for strike slip or normal faulting, the levels must be high enough to cause failure. For the data in Figure 2 where strike-slip or normal fault devjatoric stress orientations are indicated, the levels are not high enough to cause failure. Figure 7 illustrates this Mr the deepest observation in Monticello 1 (Figure 2). To achieve failure (Mohr's circle reaching the I failure envelope on this graph) the least principal stress would have to decrease by 50-100 bars or the greatest stress by more than 100 bars. The implication is that in this highly 3 heterogeneous geologic r=tting the stress field changes over small distances laterally and vertically, but that the dominant orientation at places where failure occurs is that for thrust faulting (vertical least principal stress).
- 2. Due to a large number of f ractures in the wells, very few in situ measurements were made. Haisson (personal communication) in a study elsewhere made 21 stress measurments in a well of comparable depth. He found large aberations from any linear stress gradients, and he cautioned against putting too much significance (for inferring the tectonic state) into a few measurements that showed large inconsistencies.
- 3. One of the assumptions in the in situ stress measurements using the hydrofracturing technique is that the vertical I III-2 I
I I stress is a principal stress and the two other stresses are always horizontal. While this assumption is valid near the Earth's surf ace, it may not be true at depths where the principal stre;ses may not be horizontal. This would lead to an estimate of the horizontal components of the principal stresses rather than the principal stresses themselves.
- 4. Usually, stress measurements are made in unf ractured portions of a deep well. However, it is always possible that some nearby f ractures (which did not penetrate the well) have caused perturbations in the stress field observations.
- 5. Both wells were drilled in the hypocentral regions of the two most active clusters. Well no. I was drilled in July-August 1978, about 9 months af te; the start of reismicity, and well no. 2 was drilled in January 1979, over a year after the start of seismicity. The hundreds of events in these regions have released some of the ambient stored stain energy locally, so I
that the measured stresses may reflect this altered stress state.
- 6. There is a large difference in the stress values in the two wells at comparable depths. This suggests that the stress field is very heterogeneous in the region. It also suggests that the stress associated with the earthquakes may be due to local stress concentrations and the in situ stress measure-ments may not necessari?y represent conditions nearby where the earthquakes have occurred.
- 7. In the deepest measurement in well no. 2 the vertical stress is not too different f rom cFain. If it were slightly less, the inferred stress regime would favor thrust faulting.
Therefore, the apparent contradiction between thrust fault type focal mechanisms and in situ stress measurements is really the result of comparing the stress conditions at failure (focal mechanisms) with ambient stress conditions that are not near failure and that may be t III-3 I
I influenced by both the measurement technique and earlier seismic activity nearby. However, all the evidenca points to a highly variable stress state in this area, both laterally and vertically. STRESS BARRIERS From Figure 2 the naximum stress dif ference in a vertical plane I (maximum horizontal stress minus vertical stress) displays a remarkable trend. With depth this stress difference diminishes. It becomes equal to zero at a depth of approximately 300m in Monticello No. 2 well. Similar conditions are achieved in Monticello No. I well at a projected depth of approximate 1.y 1100m. The depth-dependent decay in this stress difference must indicate similar decay in the distortional strain energy stored in the bedrock. A depth interval for which the distor-tional strain energy is very small defines a stress " barrier." An understanding of the concept of the stress " barrier" can be derived from Mohr's representation of stress in three dimensions. On Figure 3 (Part A) the points, P, (ci ,0); Q, (o2,0); and R, (o3,0) represent I magnitudes of the principal stresses and shear stress. (Note that if ot+c2*C3, ;+0.) is the mgnitude of the Resistance of the fracture plane to frictional sliding can be defined as:
- s = C + tan $. n (1) where: Ts = shear resistance of the fracture plane; c = cohesion; 6 = coefficient of friction; and n = normal stress acting on the fracture plane.
Figure 3 (Part B) shows the complete stress notation in terms of the x , y , a nd : axes. For a given set of circumstances (orientation of fracture plane I relative to the ci vector, magnitude of the f racture strength para-meters, etc.) it can be assuued that T s = constant. In order to define the potential for frictional sliding on a given plane, e.g., the x-y I 1 L L1-4 I
I plane in Figure 3 (Part B), one needs to define the ratio of T3 / Tex, (N), or the ratio of shear resistance of the x-y plane to the shear stress acting in the x y plane. If s is assumed constant, then it follows that for a situation where T zx + 0, N * ". This is exactly what the stress " barrier" defines - it is a point where devi-atoric stresses are at or close to zero, (01*C2*C3), and consequently, where no shear stress is present. The barrier cannot be penetrated by a dislocation plane which has been initiated in a region of high deviatoric stress, (e.g., above or below the stress barrier). Hence, y the barrier is an important factor influencing the cize of a potential seismic source as well as the size of a potential stress drop. It should be noted that such a stress " barrier" occurs at different depths in both hydrofracture wells. Hence, it is anticipated that the boundary representing the stress barrier is a " warped" surface with highly variable depth of occurrence. The occurrence of reservoirinduced earthquakes must be confined to the rock mass above the barrier surface. Important characteristics of the areal stress field pertaining to 11onticello Reservoir can be deduced f rom fault plane solutions. Figure 4 is a stereographic projection of the principal stress axes, specif-ically the 3, T , and 0 axes. The orientations of these axes have been I deduced f rom the well-constrained radiation patterns of the induced earthquakes at the lionticello Reservoir presented in Appendix IV. From Figure 4 it can be noted that the great majority of the kinematic axes n (axes of principal stresses; T= C3, B=C2 and 0= Cl) are not located on the great circle (perimeter of the net) or in the center of the plot. Thus, the principal stress axes are inclined with respect to the Earth's surf ace. This may indicate that the in situ bedrock stress includes shear stress acting in a vertical plane (plunge of 6 and T axes) as well as shear stress acting in a horizontal plane (plunge of the 8 axes). Furthermore, the orientation of the kinematic axes dis-play considerable variabilir.y. For example, the orientation of the O axes varies between E-W to approximately N40E, or E-W to about S30W. I The variability of orientation of the kinematic axes derived from t III-5 I
~
well-constrained focal mechanism solutions indicate that the trends of stress trajectories are not constant throughout the reservoir region. It is concluded that the changes in orientation of the stress trajec-tories are considerable, even over relatively small distances. Hence, the orientation of the stress trajectory must change relative to a W fracture plane with a given orientation. Accordingly, the shear stress acting along the fracture plane must diminish, responding to the rela-I tive change in position of the stress trajectory. At some point the shear stress will become very small. Propagation of a dislocation from regions of high shear stress will be reduced and eventually inhibited. This point , therefore, defines the fracture " strike stress barrier." This barrier, together with the " depth stress barrier," ef fectively limits the extent of the potential seismic source as illustrated in Figure 5. Examining Figure 4 another characteristic of Monticello Reservoir stress conditions can be noted. This characteristic concerns the direction of plunge of a given stress trajectory. For example, the I axes plunge toward either the east or west. direction of plunge of the 3 axes are apparent. Similar reversals in the (The changes in direction of plunge of 8 axes are the more meaningful ones, because this axis is defined by intersection of the nodal planes and thus is not affected by assumptions relating the fault plane (one of the nodal planes) to the axis of greatest principal stress, cl.) The observed directions of pl mga of the kinematic axes indicate that the trajec-tories of principal stress change their plunge direction from one area of the reservoir to another. Regions characterized by constant orient-ation of the slip vector (constant direction of plunge of the stress trajectory), must be separated by another stress barrier. This barrier I prevents propagation of the dislocation plane from one such region to another (Figure 5, Part C). The analysis and discussion above provide additional insighc iato the effects of heterogeneity both laterally and vertically of in situ properties as described elsewhere in this report. All the evidence I available supports the conclusion that only small areas can experience moveme nt in any single earthquake occurrence. i III-6 I
!I I
l 1 IN SITU STRESSES BY OVERCORING i In situ stress measurements were made near the plant site using the overcoring technique. The locations of the two test holes are shown on Figure,6. The overcoring test at the first test site OC-1 was taken to a depth of 11 feet, and the test in OC-2 was taken to a total
~
l depth of 18.5 feet below the surface of rock. During the overcoring . 1 t ' tests where the core broke, the tests were stopped to prevent damage to
, the gage, and new tests we re performed.
1 I
! Using the deformation data obtained in the overcoring tests and from the modulus tests, the secondary principal stresses (the term
" secondary" is used since r zx and T zy are not necessarily equal to zero) in the horizontal plane were calculated. The direction of the I major compressive stress appears to trend in a general north to north-i west direction in OC-1 and NNE in OC-2 (Table 2).
!I !I !I lE
- I I
- 1 I
lI i j III- 7 !I
iI i i REFE RENCES Haimson. personal communication.
- Zo back, M.D. personal communication.
) Zoback, M.D. , Healy, J .H. , and Rolle r, J.C. , 1977. Preliminary stress j measure =ents in central California using the hydraulic fracturing techniques, PAGE0?H, vol. 115, p. 135-152. 4 I !I I !I 'I
.I ll lE
!I !I 1 i 'I j I III-8 ) !I
. . - - _ . . . . _ _ . - . . _ . . , . _ . . . . _ , _ _ , . _ , . , . - - . . . ~ . _ . _ . _ _ . . - . _ . _ . _ . . _ _ _ _ _ . - - _ _ . ~ . _ . - . _ - _ _ - - - . - - - . . . - - . -
I TABLE 1 MONTICELLO HYDROFRAC DATA
; PORE PRESSURE VERT. STRESS DEPTH (M) (BARS) (BARS) S3 S, COMMENTS I
MONT. 1 165 16.6 43.9 86.9 126.8 j 486 49.0 129.4 119.0 189.0 728 73.3 193.5 119.2 167.9 961 96.8 255.8 190.7 292.5 J I MONT.2 85 8.4 22.9 46.3 -> 68.1 S, lower bound
' Estimate
'i 97 9.6 26.1 44.6 96.6 j 128 13.0 34.4 35.0 71.2 205 20.7 55.4 47.7 76.6 298 30.0 79.3 56.7 87.5 312 31.4 84.3 64.8 94.3 - 400 39.8 108.0 82.87 135.8? Possible Pre-e l 508 50.5 136.7 S3 < 212.5 < S1 Existing Pre-Existing 647 64.4 174.0 161.4 294.4-I ~
- I I
I I I
- I
I lI 1 TABLE 2 i e OVERCORING DATA FR0t1 OC-1 AND OC-2 lg VIRGIL C. SUMMER NUCLEAR STATICN HOLE NUMBER TEST P' Q' e l l i !e 00-1 a Data questionable because core. broke i i 5 b 97 bars 44 bars 3250 0 c 49 bars 40 bars 330 !E OC-2 a as bars -20 bars
- Go i 5 -
b 42 bars -0.7 bars 10 0 c 31 bars 0.7 bars 150 i !I !I l
- Negative value indicates tension.
I I I I I I I
t. ll MONTICELLO EARTHOURKE5 1 OEC 77 - DEC 79 MAG GE 2.0
'l n-
- 22. 10.
I"' 21. 15. ,17 15. i i F' C' . i ! i I ,3. 3
!g i
i 3 '
\h ,i.e i
d
- I ~
]
i u
'-hs,Q a 2.:
~-, e
!E *\ %
;E -
5 1s 2t.: 4 7 > a s.0
' L Vl_.,
p2 m CMTICELLO
= 9 e41 q 2e.s u-i I -
A
.h m p WE W Gy
, hO dm-o.e
-r D L
, _. uts t o I 9 a s.c l _.,
3 1E i3 - QC@j N "S' i cm a .- g I
~ '#
l , m [a C' - F ,~ y hg' l - f 3 i; un MONTICELLO #2 I Y C y S.7
'$.0 l
l
? , , ,
f N bC
- I i
6 I I i l i ! ll l-l=!:lEj0lE]
- l. O i. 2 3. 4 lElo!al-!xio!
0 1 2 3. W. 3 I I MOCNUut ut? 7H IKM) I fhc pE 'ntn,3 (2
~
rrn p l!' y i. Figure 1 ,I ((% : , , ~ /6Lsuu @dhjl .i (b .$ 'I - - .- _.
I I a- .
.* ss
~ .
' q\&
I 3
~
E-Y# L
\%.0 d -
osQ I s= .
==
a :;; - t l O . O= 0 em. l e.
. n. -
l ' ' ' . , , , , , , I o
~ = o o
~ S3313M 0111 ' Hid30 I =-
"I I I .
E
<=
6 ct: 5' W O h\
,- o kk
=_m-
~
=
~
T (4
.t
=
m ^ [g8
~
I e 3 - 1
- r .
0s@_s@ yn - 4
==
l' - e-m L.
' ' ' ! ' ' I f 9
, g o ~ - . . 3
- o o a o -
3 S3313M0111
- Hid30 i
lI ! Figure 2 I
-, -. .- - , . - - - - - , , . . - - - , . , - . . . - - - ~ , - - . . . ,
.I I
E E E 60 30 30 E0 15 15 75 3' (U2 ,0) p(g ,o) y g I 3(a) I e E E r I s( E k.,r, xx r-
,,Z y
l S zz ZX
~T 2y 3(b) g I .
g FI GURE 3
I I . s I . I . g A ' I ^ * . i n
, -l-
- E ,
, A
- A I A A
^
I A g , I
- l g ,
n - 33 'I LOWER HEMISPHERE EQUAL AREA PROJECTION
- P-Axis A T-AXIS
. g-Axis I STEREOGRAM: SHOWING ATTITUDES OF p, r, AND $ AXES OBTAINED FROM COMPOSITE FOCAL MECHANISM SOLUTION l
FOR EVENTS AT MONTICELLO RESERVOIR FIGURE 4
i DISTANCE i I ' s
/ DEPTH STRESS BARRIER I s
/ '
\ /
-lNDUCED EARTHQUAKES OCCUR AB0VE ZONE OF SMALL f N(?) s DEVIATORIC STRESS ,
DEPTH s @ h s ' I I BODY 'B l ,TR,
) rpCTuRE\
, m ,, , _ ,E a I @
I DISTANCE STRESS BARRIER #3 I I i l DEPTH I I I FIGURE 5
E I I g,1 g a'y;m. "a ;_:s!~Dpgf\p ts A q qe g g13 I; y I, g I:: gg lill!. g h, .
;q!. .
Mo
#.#. N ESESi S;x.23 E
a Y
-y' R
,' /;
'. / f-n\.c!
V m
,,2 r ' 1
# ' f
/ 7- ._ s 8, - f- . f, ,
,...: \y
,'(' .
;; j
. j / \ s
/
. .. g.:'/[~f^-;
=-
'I ,
e 1 .-_.-)- .6 .
?
( a [ f
~
/ #l I g*
4 j-
,_( .
i
&(
g , 7 . I l l'. w .,4; l .a . i
~. .' ' '
-o < i,
~
ll '-v, g - l \, ,.-s . .
- 1. - . "
L j _~ y I I T~~~
~
h t sj '
$h%
--7 ,
\
^l E $^f-
~
n - {.. h, .s.,
' c'/: ' m' ,.
t o .g
. ' ik s
' ,. '8 ,
s I ,
/ . .
h o I
j 1 I !l I i 1 i E 30o - i iI i \
$ =3o*
7 200 - l g $x28, W ? il4 (9 at'O loo 04 sb lI O I I I f I o too zoo 300 400 500 i ]g eg 3 295 bors !g a3 195 hors 1(bors) i iI i l !.I 1 i iI! Figure 7. Comparison of in situ deviatoric stress conditions in Monticello No.1 lg at h =1 km to failure envelope for two values of '.~.1e friction angle 4 i5 A conservative (low) value of cohesion of 10 bars is assumed.
._-__.-..-.._,__--.,,,__._.-_.,_.,-,_-..,.,.._..,-,__,,.,-.r,
,_--.,,_,...,,,.,,c_ _
l
!E APPENDIX IV FAULT PLANE SOLUTIONS AND
] ASSOCIATION WITH PRE-EXISTING FRACTURES 4 I I I I . !.I I
- I I ,
I I I 11 , I I ! l l
I I FAULT PLANE SOLUTIONS AND ASSOCIATION WITH PRE-EXISTING FRACTURES INTRODUCTION One of the parameters used in the construction of fault plane rolutions is the angle of incidence of the seismic wave. This para-meter depends critically on the hypocentral depth of an event. Co n-sequently for detailed construction of fault plane solutions, only events recorded on the Monticello network for which depths are well constrained were used. For the analysis of data recorded on the 10 station Monticello network (recorded on analog magnetic tapes), visual playbacks were obtained from the USGS facilities in Golden, Colorado. Due to a long delay in obtaining these playbacks, the data used in the I analysis presented here covers the period from July 1978 to October 1979. These data were received and analyzed in two installments (July
- December 1978; January - October 1979). Consequently the results presented below are in two groups, designated as 1978 and 1979 groups.
COMPOSITE FAULT PLAJ. SOLUTIONS OF 1978 GROUP I Using high quality data, it was possible to divide the seismicity I into five clusters (Figure 1). The first motion data for any given earthquake are too few to draw well constrained fault plcae solutions. Typically four to nine first motions were obtained, and it was possible to draw several mechanisms (thrust fault to strike slip faulting) in several directions. Therefore, it was necessary to select events in a certain cluster, usually in a small depth range and obtain a composite fault plane solution. At least one composite fault plane solution (CFPS) was obtained for each cluster. These are summarized in Table 1. One solution each was obtained for Clusters I and V, two for Cluster III, three for Cluster 11 and four for Cluster IV. Except for solu-tions 3 and 4 for Cluster IV, all composite fault plane solutions are for events with depths ranging from 0 to I km. All fault plane solu-I tions indicate that thrust faulting is the predominant mechanism. Some events, especially the deeper events (1-2 km) (Cluster IV, Solutions 3 and 4) exhibit a component of strike slip motion. I IV-1 I .
i Two predominant orientations of the nodal planes, N-S and NW-SE, were noted. The P axes, as would be expected for thrust f aults, are predominantly close to horizontal (Table 1). Howe ve r, their azimuth is not consistent. The slip vectors were obtained for both nodal planes for each of the fault plane solutions. These are summarized in Table
- 2. The individual fault plane solutions are usually well constrained and are shown in Figures 2a - 2k.
COMPOSITE FAULT PLANE SOLUTIONS FOR 1979 GROUP Seismicity in 1979 (Figure 3) was also located in the clusters defined previously and outlined on Figure 1. These data were also used to obtain composite fault plane solutions, with at least one solution for each cluster. There were only two events located in Cluster 1 and these were superimposed on the 1978 composite fault plane solution, I with which they agreed (Figure 4a). We sbtained 4 CFPS for events in Cluster (or Group) II. Most of these events (in solutions 1-3) occurred in February 1979. A flurry of activity lasting about 18 hours (between September 30 and October 1, 1979) occurred to the west of the main cluster. These events were used to obtain solution 4 for Group 2. We obtained 3 CFPS for events in Cluster III. These were located on different (mapped) geologic units (Figure 5). The activity in Cluster 4 occurred mainly in the October 1979 swarm. The seismicity was divided according to the hypocentral depth, the events 1 km and shallower were used in solution 1, and the deeper events in solution 2. The eleven CFPS obtained for 1979 are shown in Figures 4a - 4k. The results are summarized in Table 3. The 1979 group included many i events deeper than 1 km. Here again the fault plane solutions indicate that thrust faulting is the predominant mechanism. Some events in Group 3 exhibit a compo..ent of strike slip motion. Again two prendo-minant orientations of the nodal planes were noted (N-S and NW-SE), although two solutions indicated possible E-W nodal planes. The P-axes are again close to horizontal (Table 3); however, their azimuth is not I
- i l IV-2 lI
I consistent. Slip vectors obtained from both nodal planes for each fault plane solution are summarized in Table 4. For the Monticello network data covering the period between July 1978 and October 1979, we obtained 22 CFPS. The number of events used to construct these CFPS varied from as low as 3 events (Cluster III, solution 2,1978 g oup, or III/2/78) to over 20 events in Cluster IV. We note a preponderance of CFPS. To determine if there were any systematic orientations of the fault planes, the poles of the fault plane solutions were obtained (Table 5). To decide which of the two nodal planes represented the fault plane, and to seek possible causes we examined surface geological and borehole data on the density and orientation of fractures. FOLIATION AND JOINTING IN SITE AREA In this section we excerpt results reported earlier from the FSAR.
"A statistical analysis of foliation and compositional bed-ding planes in the general site area (within a 10-mile radius of the site) indicates a major disruption of country rocks by pluton emplacement. The Areal Foliation Contour Diagram I [ Figure 6) presents a plot of 108 such planes from the entire mapped area, and indicates a shift from the N70E average trend in the general area to be N50E, with dips spread un-I evenly to the southeast and northwest. Both the shift from regional strike and the uneven distribution can be attributed to the disruption resulting from plutonic emplacements.
Plutonic rocks have been mapped as fingers, irregular zones, and as small to moderately large plutons which generally trend in an east-west pattern, indicating a general concord-ance or structural relationship.
"The northwest contact with the country rock east of the Broad River is concordant, while the others are moderately discotAnt, probably reflecting the influence of a joint system. Smaller granodiorite plutons are found north and I IV-3 I
I east of the nuclear plant site. Precise boundaries of the plutons are dif ficult to determine because of the peripheral zones of migmatite. The altered character of the migmatites seems to increase as the plutons are approached.
"The high density zone in the lower right quadrant of [ Figure 6] represents points north of, and adjacent to the nuclear site. This area strikes N43E and dips moderately to the southeast, possibly forming the southeastern limb of a north-east striking anticline. The N43E strike of this area devi-ates approximately 25* from the usual regional strike, and I the isolated density concentration at the etat, south, and west edges of the figure indicate that disturbance is present to some degree. "The density zone near the northern edge of [ Figure 6] repre-sents foliation planes located several miles south of the I site. The general stike of this area is N78E and closely approximates the regional strike. The dip is steep to the northwest indicating the presence of the southern limbs of the synclines. The high density areas of this figure are much broader than foliation contour diagrams of nearby localities prepared by others, indicating a significant pluton influence within the general site area. "[ Figure 7] the Pluton Area Foliation Contour Diagram, repre-sents a plot of 30 planes from areas of known pluton emplace-it. The irregular distribution of high-density zones is very apparent, illustrating the disruptive effect of plutons on older me tamorphic rocks. The field and core data (in-creasing granitization with depth) enhance the probability of subsurface plutons which disturb northeast-southwest trending folds of metamorphic rocks.
I "A well developed joint system was observed in most rocks in
- the mapped area. The Areal Joint Contour Diagram [ Figure 8]
l l IV-4 I
I I represents a statistical ana. lyses of more than 135 joints from the area mapped, and exhibits a system for which the prevailing directions are N30W with an approximate dip of 80* northeast and N67E dipping vertically. T.he secondary set averages N45E and dips approximately 80* northwest. These joints sets probably represent a combination of vertical diagonal shear jointing, with longitudinal and cross-tensior joints, all related to folding. The joint system enhanced development of a trellised drainaged pattern which alters to dendritic where infim aced by pluton emplac< ment." BORE HOLE FRACTURE DATA The de.ca presented in this section were supplied by Zoback (per-sonal communication). Figure 9 shows the distribution of fractures in the wells as determined with a borehole televiewer (Zemanek et al. , 1970). Monticello 2 contains many more fractures than Monticello 1 and the artesian zone between 400 and 500m, stands out as a zone of I particularly dense fracturing. The poles of all the fracture planes in the two wells are shown in Figures 10a and 10b. These figures indicate the presence of fractures in several orientations, there being a much g,reater number of fractures in Monticello 2. The orientation of the f ractures in the wells is shown in Figure I 11 as fracture poles on lower hemisphere, stereographic projections that are contoured in a statistical manner (after Kamb, 1959) to accentuate significant fracture pole concentrations'. Statistically significant pole concentrations are indicated by contours of greater than three standard deviations (3C). Fractures in Monticello 1 are concentrated in two clusters; one striking northeasterly and dipping steeply to the southeast, the other striking west-northwesterly and dipping to the southwest. In !baticello 2 the fractures are very I strongly concentrated in a northerly trending zone dipping steeply to the east. E I IV-5 I
I I Thus, data were in hand to see if the nodal planes obtained in the CFPS were associated with the mapped foliations on the surface and I fractures in the two wells. COMPARISON OF POLES OF FAULT PLANES WITH POLES OF FRACTURES Figure 12 shows the poles of all the fault planes in the 22 CFPS. I To decide which plane represented the fault plane, and which repre-sented the auxiliary plane, the poles were compared with the poles of foliation planes (Figure 6) and of the fractures in the 2 wells. I In Monticello 2, the fracture pole density stereograms used were for dis-crete depth ranges. In Table 5 we have divided the solutions according to the agreement of the poles of the fault planes with the poles of existing fractures. In cases where both the poles f rom CFPS were located at high density portions on the stereogram, both were plotted. The poles of the CFPS were found to be associated with 5 groups of fractures. In Figures 6 and 7 the slip vectors of CFPS in Group A (Table 5) are superimposed on foliation contour diagrams in the general I area. In Figure 6 the high density zone in the lower quadrant repre-sents points north of, and adjacent to the nuclear site. This high density zone is also the location of poles in Group A (Figures 6 and 7). This suggests that the earthquakes in Group A are occurring along N- to NE-striking fault planes if the surface foliations prevail to hypocentral depths. The poles of the nodal planes in group B lie on the poles of the northwesterly fractures encountered in Monticello 1 (Figure 13). I Three events located on CBGN (Table 5, III/2/78) had an average depth of 180m. The pole of the fault plane falls on the poles of i easterly dipping NS fractures encountered in well No. 2 between 0 and 1000 ft (0-300m) (Figure 14). Three CFPS were associated with east-l west fault planes (Table 5, Group C'). The average depth of events used in each of these fault plane solutions is 500m. The poles of these nodal planes lie on the poles of east-west fractures encountered l in Monticello 2 between 1000 and 2000 feet (300-600m) (Figure 15). The 1 !I I l IV-6 lI
I deeper events in Group C are associated with easterly dipping, norther-ly striking fractures seen in the plots for Monticello 2 for depths from 2000 feet to total depth (Figure 16) and from 3000 to feet total depth (Figure 17). Another set of nodal planes, Graup D, appears to be associated wich westerly dipping northerly striking fractures (Figures 17 and 18) . In summary, from the fracture data obtained from Monticello 1 and 2 and previous geologic studies, it is concluded that:
- 1. There are a large number of fractures in the area.
- 2. The orientation and density of fractures varies from place to place and with depth. That is, the region in the vicinity of the reservoir is characterized by a heterogeneous distribution of fractures.
- 3. The striking agreement between the poles of the observed frac-tures and the poles of the fault plane solutions sugbests that the seismicity is occurring along a network of pre-existing fractures which are not continuous in their spatial extent -
either laterally or vertically. This observation and the I borehole data put a constraint on the size of the fractures along which movement is occurring both for single events and the clusters of events. I I I I I I IV-7 I
( I . REFERENCES Kamb, W.B., 1959. Ice petrofabric observations from blue Clacier, Washington in relation to theory and experiment. Journal Geophysical Research, vol. 64, p.1891-1910. Zemanek, J. , Glenn, E. , Nation, C.J. , and Caldwell, R.L. , 1970. f Formation evaluation by inspection with the borehole televiewer. Geophysics, vol. 35, p. 254-269. Zoback, M.D. , Healy , J .H. , and Roller , J.C. , 1977. Preliminary stress measurements in central California using the hydraulic fraturing I technique, PAGE0PH, vol. 115, p. 135-152. 4 I i I I I I I I I I I I t IV-8 I
I - I TABLE 1 1978 I GEOMETRIC DATA FOR CFPS OF EVENTS IN DIFFERENT CLUSTERS CLUSTER ROCK NODAL PLANE P AXIS T AXIS DEPTH (KM) TYPE NO. STRIKE DIP AZIM. PLUNGE AZIM. PLUN5E I 0-1 I Granofels 2 1 N41 W fl180E 48UtlE 60 UW 2600 7 0 160 0 560 Granofels N100W 620E I II 0-1 Solution 1 2 1 N160E 300W 90o 16o 231o 70o Granodiorite 1 N430W 60 NE I Solution 2 Migmatite 2 N28 E 0 600NW 2630 0 0 172 45 U 1 N52 W 42 NE o Solution 3 2 N590W 214 4o 340o 85o 480SW III Migmatite 1 N330W 400NE o 62o 6o 198o 83 Solution 1 2 t!22 0W 50 SW 0-1 CBGN 1 fl170W 600HE Solution 2 N28 E0 92o 10o 201o 63o l 2 40 NW (Set 1) U IV Mixed 1 N06 W 62UE o o I 0-1 Solution 1 fiixed 2 1 N N16 W 0 28 0W 580NE 87o o 17 257o 73 o Solution 2 0 62 11 294o 72o 2 N46 W 36 0SW 1-1.5 0 Mixed 1 N68 W 500flE Solution 3 60 0W 233o 7o 139o 54o I 1.5-2 Mixed 2 1 N060W NO30E 600E 660 80 3250 56 Solution 4 2 N560W 48 SW U V Mixed 1 N43 W 660NE 47 21 227 60' 0-1 CBGN 2 N43 0W 24 0SW I I I 1 I
4 TABLE 2 1978 SLIP VECTORS
~
i 3 S0LUTI0fl fl00AL STRIKE / THRUST , E CLUSTER GEOLOGY PLANE AZIMUTH DIP COMP. I 1 1 108 U 30 0 1.11 j Granofels 2 2280 420 0.81 U II 1 104 600 0.21 l 1 Granofels 2 2600 28 0 0.42 2 1 118 30 1.43 Granadforite 2 227 30 1.43 3 1 31 42 0.11 Migmatite 2 2170 48 U 0.09 III 0 0 1 2 237 40 0.14 Migmatite 1 670 500 0.12 2 1 119 50 0.53 CBGN 2 2530 300 0.81 IV 0 1 1 90 62 0.05
- 2 2640 28 0.11 2 1 44 54 0.31 2 254 U 32 0.47 3 83 U 1 30 1.15 i
(Deeper) 2 2020 400 0.90 lE 4 1 34 420 0.81 (5 (Deeper) 2 2720 30 1.07 j I V 1 1 2 47 0 2270 66 240 0.00 0.00 lI 1 lI - l I l lI l
I I TABLE 3 I 1979 GECMETRIC DATA FOR CFFS OF EVEfiTS IN DIFFERENT CLUSTERS I CLUSTER NCDAL PLANE P AXIS T AXIS DEPTH (KM) ROCK TYPE NO. STRIKE DIP AZIM PLUNGE .1ZIM PLUNGE 0 I Granofels 1 N41 W 480NE o o o 0-1 2600 6 161 57 e 2 N18 E 60 W II 1 N100W 28 E 0 277 0 0 0-1 Solution 1 18 130 68 2 ft160E 65 0W 0 1 N55 W 580NE 0-1 Solution 2 64 0 6 161 56 2 N04 E 50 W 0 600E I 1 N16 W o o o o 0-1 Solution 3 104 5 199 52 U 2 N47 E 52 NW C 1 N15 E 40 E o 0 o 0-2 Solution 4 2500 16 01 54 I 2 N46 W 68 SW III Granodiorite 1 N380W 240E I 0-2 2 N02 W 69 W 257 23 o 107 o 64 o N870W U 0-1.3 Migmatite 1 SO N 0 0 38 20 142 35 0 2 N05 W 50 W 0.5-1.6 C2GN 1 N14 W 0 34 E o a o o 215 22 338 53 U U 2 N76 W 72 S I I I . . _ . . ..
I TABLE 3 (Cont.) i CLUSTER N0DAL PLANE P AXIS T AXIS DEPTH (KM) ROCK TYPE NO. STRIKE DIP AZIM PLUNGE AZIM PLUNGE i IV 1 N12 W 500E 0-1 Mixed 560 0 325 66 l35 2 N56 W 50 SW 1-2 Mixed 1 N16cE 54 E 810 30 347 60 i 2 N340W 50 0SW lI 0 l V 1 N04 E 510E o )' 0-1 Granofel 100 6 230 82 2 N160E 40 0W i !I i i I - iI 4 )I
- I i
i n
~
j !,I i i O '
!I
I l 4 TABLE 4 1979
. SLIP VECTORS 1
{I CLUSTER SOLUTION ututuuf NODAL PLANE AZIMUTH DIP STRIKE / THRUST COMPONENT
! 1 1 108 0 30 1.11 I Granofels 2 229 0 42 0 0.84 lI II 1 2
1 107 0 260 0 250 62 U 0.45 0.21
, 2 1 95 0 400 0.84 0
- 2 215 32 1.04 0
3 1 137 380 1.00 2 253 300 0.81 1 Granofels 1 450 22 1.38 4 285 0 0.67 d 2 500 4 0 III 87 21 0.62 l 1 1 Granodiorite 2 232 660 0.24 I 2 1 85 400 1.19 Migmatite 0 0 2 183 10 4.01 ! 3 1 14 180 1.54 I CBGN 2 2540 560 0.58 IV 1 1 34 40 0.62 Mixed 2 259 400 0.62 0 l 2 1 55 400 0.75 Mixed 0 l- 2 286 360 0.81 l 0 V 1 1 107 0 50 0.16-
- Granofels 2 273 390 0.18 l
!I l !I l I -. .
I - TABLE 5 POLES OF FAULT PLANES CLUSTER PCLE OF AVG SOLUTION GEOLOGIC P AXIS FAULT PLANE FAULT PLANE DEPTH YEAR UNIT AZ DIP AZ DIP STRIKE DIP (KM) 80U 070 0 0 A I/1/78 GF 108 30 N18 E U 60 W 0.69 II/1/78* GF 900 16 0 104 0 60 U N160E 300W 0.63 83 0 0 U 0 0 0 II/2/78 GD 0 118 30 N28 E 60 NW 0.54 0 0 III/2/78* CBGN 920 10 0 119 50 U N28 E 40 NW 0.18 I/1/79 GF 8CU 060 108 0 30 0 0 N18 E 60 0W 0.72 0 II/1/79 9-0 180 107 25 N16 E 65 0W 0.80 0 0 II/3/79* 10$ 050 38 0 0 137 N47 E 52 NW 0.55 100U 060 0 V/1/79* GF 107 50 N16 E 40 0W 0.51 I 0 0 B II/3/78 MG 34 040 31 420 H59 W 48 0SW 0.49 0 III/1/78 MG 620 0 60 67 50 N330W 400NE 0.57 IV/2/78* 62U 110 44 54 N46 W 36 0SW 0.35 V/1/78 4 ' 210 47 660 N43 W 24 0SW 0.45 0 U IV/1/79* 56 00 34 400 N56 W 50 0SW 0.64 IV/2/79* 810 030 55 40 N34 W 50 0SW 1.50 0 C II/1/78* GF 90U 160 260 280 U N10 W 62aE 0.63 0 III/2/78* CBGN 92 100 253 300 N17 W 600NE 0.18 0 IV/1/78 870 170 264 280 N06 W 620E 0.53 IV/2/78* 620 110 2540 320 0 N16 'd 58 NE 0.35 IV/4/78 6 @ 080 272 300 NO3 E 600E 1.74 II/3/79* 0 1040 50 253 300 N16 W 600E 0.55 II/4/79 GF 700 160 2850 500 N15 E 40 E 0.92 I
I TASLE 5 (Cont.) CLUSTER POLE OF AVG SOLUTION GEOLOGIC P AXIS FAULT PLANE FAULT PLANE DEPTH YEAR UNIT AZ DIP AZ DIP STRIKE DIP (KM) i III/3/79 CBGN 350 220 254 0 56 0 N14 W 340E 1.04 0 2590 40 0 0 500E i IV/1/79* 56 0 N12 W 0.64 IV/2/79* 810 030 2830 360 N16 E 54 E 1.50 i V/1/79* GF 1000 060 2730 390 N040E 510E 0.51 C' II/2/78 GD 830 00 2270 300 N430W 600NE 0.54 II/2/79 640 60 2150 320 N550W 580NE 0.55 III/2/79 MG ?80 200 1830 100 N870W 800N 0.54 0 IV/3/78 530 070 830 300 N060W 60 0W 1.14 III/1/79 GD 770 230 870 21 0 N020W 690W 0.99 III/2/79 MG 380 200 850 400 N050W 50 0W 0.54 l
- Both Planes Listed 4
p 1 f I~ I, I . I
)
f -i
! - l 4
'. j vC f; -' . m I
, C --s i . P C C O ~ w [ .l 'u.C. K C C-I I ,,, y - _, v- v --. . - --
- 81'
' 22. 2 *. 70. 19. 18. I' 15.
5 i ! ' < , 23.s ! y;v ,;
~ " ". 21. C
~ s u
> M' r'
s -
% %, 8
_2,, L..,- '. ' 9,v a - B j mkh u a i I s
-. ~
- : ~, w.
- - s - n
=
r i , 3 _a e I
% , ri
_w ^ [' % ll a: - a :.s
- =i==y'2 ,.. ..
., 5: !!! (u
, s -
~, c :
~ ~
'". D ~ :c,C adaw i ::: -
5 ~ !' h 2. "
= >.s
, s ,
\ -
m i -
?, = >.a e vs, a J ? J S.s
-- w =
w r w n.
- '\. - <.
. i
. x cun--(u j
~ f
~ a~ ~ s S_ ,
wA -:e.e
- 3 ~ , . _ IV = ,.s
_.n.o _ :s.s lI n.: i - m "55
,3 , , ,
,s
; : i i
34.00 i i i I I . i r, : , r, : l*l3 l , ! a t L.J ,i m ui; l.m ,v
.c i s i : + l! X j vsl.
.i a. i. 2 t .. 2. i. 2. 3. .. s.
MACt!'TUDE DEPTH (KM) Figure 1 go fiMIN Dj@f(l!D o;oeuw[O -
I g GROUP 1 ih GRANOFELS O-1 km i[ t N i N18E 660 W e Ig N41 W .e
- S48NE 1
0 g0 i i ! e o ![ 90 IO - i P I o T 0 i.I O 6 !l i e i 8 e i 5 e COMPRESSION l I 1 o DILATATION .i Figure 2a i !I
l l5 i GROUP 11 ON GRANOFELS 'l OCT. - DEC.1978 Z 0-1 km .h N10"W N N16 E
- l 662 E E 630 W !
- I o .
. e 00
!I e0
- I ..
o 4 i .% 3 !g 89 se g
+ P -
O T 0 -l
%0 o I .
!I . l jg . COMPRESSION i o DILATATION i5 Figure 2b i ,
I g GROUP ll ON GRANODIORITE - DECEMBER 1978 N Z 0-1 km , N I . g e O O 5 OO Y O - l P O I E . n
.y I . cOmeREssiON g o DILATATION I Figu.e 2c l~
I
. . - - - - _ . _ . _ -~ . - --.._..-_.-__ ___.- _ .-_____ _ ...__.-...-._.... . _.-_ __,-.,_._ __ - - -
h GROUP 11 ON MiGMATITE lg JULY-DEC. 30,1978 Z 0-1 km . N lI a o Nd2 W o ig i 842 NE 00 o C0 0 o e !! 59 W e D ! 8SW o o
- e i
e i e ' !k o
- o T 0
o 0
- s o e e i
o g o
- O l
!l D o e o a h o o P o o gg g I I h e COMPRESSION o DlLATATION h Figure 2d
.I
!g GROUP lli ON MiGMATITE Z 0 - 1 km
]I N33 W N22W 640 N N l , o
, 850 sw lE .
o t o h i Oo o % lI l g OO o o%% !g
! a +T '
0 0 o - I 9 Og o oo OO O 9 i3 - o ig , - a = I, -, I e COMPRESSION O DILATATION 4 Figure 2e i I
8 .g GROUP lli ON CBGN i Z 0-1 km N Il l 860 NE N17 W ' N 28 E l 840 NW CD O ig !W G i O h
- g O 3 OO
- 4 g _
I ' 9e I a l
- COMPRESSION O DILATATION E
g nwe u i - r
!'h i I l SOLUTION 1 GROUP IV Z < 1 km N6 W N 662 E 628 W i8 o !li
- il i
e% o !I 9% !t h i P - g o T I t 5 *
- COMPRESSION
{ o DILATATION Figure 29 i t
- 9 I
ll ! GROUP IV l SOLUTION 2 Z = 0 -1 km ' N16 W N i ' 658 NE i 'I o 0 N N46 W !l l 636 SW 8 O pgo \ d x i l o *P ! dd # I, % %g i T l
~3 lt 6
D
'o lt
- l5 oo i
m o it 1 l$ o COMPRESSION g o DILATATION c 4 Figure 2h j i! 4 _ - . - - _ , . . . ~ - . _ _ _ . - , . - - . _ . . . ~ . - - , - - . - . - . . - . . - . _ _ . - . . - - . - - _ - - - . - - . - ~ .
9 GROUP IV SOLUTION 3 g N6 W 660 W , N i g sE*$E ! I _ _ i i T I cg I . A e COMPRESSION j O DILATATION I ,,, _ 24 I i . - _ _ _ - _ _ _ - - _ - - _ _
lt 5 i GROUP IV
- g SOLUTION 4 Z = 1.5 - 2.0 km N3E ll il iS60 E lh 4 ..a
! N56 W ! 648 SW lI li -
+
l o t % %oo I o l
- 09
%o I i g = COMPRESSION o DILATATION I
g nu I c i
- T*
iI i GROUP V
- g Z 0-1 km N
!I i N43 W i 824 SW i$ 666 NE i O .b ! O i P l, % OO !I # i !I -
+ -
l O T 00 1 I i 4 e i i e COMPRESSION f ,g i O DILATATION !I 4 i Figure 2k !~ iI
i k
!I i
l 1 l i e r3 s r , m 4 j 0 L s '4 I . r 7.b.! L I.97r m '14 L. ycr'r-U F f I. c3 c- ! l
'4 rs. ,cr ,e , , , ,
rem- n c e-
]M 4i ~
UL i. Id LU/U l ~ ~ 7. LM i9 ' q-j j' 22. ,21 .3 ,i 3 l
' fC. -
m L.
,i S .
,3-i "*
./ 1 i.
J , % mr JlL ) - CL, P . K .2,.,>
,,2
- h
- 0,
%v s a2L, mv O
- 1. <
i l, _ g o -Q'!(~- ' y _2a
- o ..
v l r
- DO '".-
u-*
*O "y -,20. C 1 3
%%S -
"':- ".y hmL a u m
0 3 l C
- 19 r
% 4.
r3 y
'\
C#, , l' wz +, - i -
-m m" N g (/ f x:
1 ) ]b " I .
- 17.0 1 -
Js5
=s:
e
~~
'5i C 5 iKMi .
i i ! ! ! E i : ,! t i ; 34 CC l I ,, ! '
! r*, -, 3 !m i ,
I i i
~ '"
lU' L~ , ' - a 3iOjdl ! ! X lI 0 ,!j 1 0. . 2 3 + C. t. 2 3 4 5 Ob. [N [dM]
- 4 ( m l
. _ . ~ _ _ _ _ _ _ _ . . _ . _ _ _ _ _ _ _ _ _ _ _ _ _ . . _ _ _ _ . _ _ _ _ _ _ _ _ ._ . . _ _ _ _ . _ _
l \
\
@@ I I ,g7g GRANOFELS N
fl SOLUTION 1 N 18 E l 60 W f .g f N41W $ ' 6 48*NE l$ o 3
- I
.I
~
i' fk j P T i ! O O l 1 I !$ $ COMPRESSION i lg O DILATATION j!I 4 Figure 4a
- I 4
it
.. .- _ __ _ =_.._.-. -..__ __.- - . . - _ --.- -. .. __. - . - _ - .
!I i
lI GROUP H !l l 1979 i N10*W N ip SOLUTION 1 0-1km 8 28 E 1 N 16 65E,W 11 <o E e e G il \n \\ w Y* _ O I T O I O I I e l I l e COMPRESSION l O DILATATION Figure 4b E 'I
lI i
- I
\ GROUPH
'g 1979 N
ig SOLUTION 2 N 04 E O-1 km ja 50PW O lE l !I ! 858 NE N55 W O O Ig 1 O p O I O lg
- 0 0 -
l O O !I i iI o T jI e ! e lI ii g .* i I ~ i I e COMPRESSION lg O DILATATION Figure 4c i
- I
!I I.-__. _ _ - . . . . _ . - _ _ . . _ _ _ _ . . . . _ _ _ _ . - . . _ ~ . _ . . _ . . _ _ _ . . . . _ _ _ , _ _ . . _ , . . _ _ _ _ . . _ . . _ _ _ _
I !I i GROUP II g 1979 N != SOLUTION 3 N16W q 0-1 km iE B60 E l i I g? 00 8 s47eg l h 852 NW !I
- I @
o cr !I 4 T iI go 1 I - O? lg e l l lI I
$ COMPRESSION ll i
O DILATATION iI Figure 4d !I !l
l !I i 9/30 ~ 10/1 GROUP JI
! ,g g GRANOFELS N N15E ll SOLUTION 4 q m8 40 E O-2km lg N46 W ?
!l 868 SW - l l r o t h o o lE l oo i iI _ ~ ll 9 od 4 lE l h Q lE l l O COMPRESSION ll o DILATATION EVENTS DEEPER THAN 1 km UNDERLINED l 1' Figure 4e !I ..
l !I GROUP H !
- I 1979 GRANODIORITE I N O2 W N
)g SOLUTION 1 69 W I O.3-1.7 km j / o !g N38 W 24 E lI O !I i O I O I - __ g7 O _ T $ l O P
+
Q? I I I e I I l 9 COMPRESSION g O DILATATION l ngure u I I i
l 1 GROUP H
! 1979 MiGMATITE
! N05 W N g SOI.UTION 2 50 W I O -1.3 km lI i O O 5 %- o Is O i 'Il N87 W ?c* - O ll 80 N ' b oo I O g O T I ~ O $ O. I
$ $ COMPRESSION l O DILATATION l naure 49 I
lI l.-_. _ . - _ - _ - _ _ - _ . - - - - . - _ - . - ----
i j GROUP E
- ! 1979 CBGN i
N14 W N jg SOLUTION 3 34 E l O.5 -1.6 km 1 4 iI lI 1
# G l8 l T i N 76*W jI 72 S x 1 o ll
'I lI 9 \ lI P 2 'l ) e oo I " I l $ COMPRESSION g O DILATATION Figure 4h I I
1I l i 'I GROUP IE ll 1979 { N12 W N g SOLUTION 1 50 E I O-1 km
- I
! N56 W e 80 l 50*SW g O ' !I o T O 'l 4 i !I l O lI I -
+?
s I n l~ l 9 COMPRESSION O DlLATATION lg l Figure 41 lI I
I 1 i lE l
%@R I$
4 3979 ! N ll SOLUTION 2 N34\y ' N16 E 54 E 1-2 km !a 50*SW i jE * ' Y.
- l,'
.i *. ,.
T C> ll O" I ' - ; O' lE ! O O * 'E . 3 I
$ . COMPRESSION lI O oiLATATION
! Shallow Events Underlined ll l Figure 4j E ,I
- _ . _ _ _ _ _ . _ . _ . . . _ _ _ _ . _ _ _ _ _ _ . - _ . . _ _ _ . . . . _ _ _ _ _ _..________.__.._.s___..__
!g i GROUP 5 ,l 1979 i !a N NO4 E l SOLUTION 1 1 51 0-1 km
- 3 N16E j
40 W i I !I i8 QO OO !l O jl __ O l T g O p i 'I i,I 1I I l $ COMPRESSION g O DILATATION Figure 4k I I
I I GROUP III 1979 I 8 ' 8' 8 8 ' 8' ' "' 3 4*21' ' I I I I 34*21' I GD I - 270 - O 22 ( I 34*20' - O 22 ii. 2' MG GF l _ 34 20' I I 2 0 23"Oh r CBGN 3; soO og 042 I 1 2g,3 CBGN ' ' ' i l i { i - O2 ikm a l 81 20' 81 19 81*18' 81*17' SOLUTION ROCK A 1 GD SOLID SYMBOLS - DEPTH > 1 km O 2 MG D 3 CBGN I I - I Figure 5 I
i !I 1 l
- I i
!l p
- 3 y t
- ::
![;.
W, I I J j .lT T!:
.l:. . . . . .
~
~
.i 8
e*p iI KEY: r o 0% TO 1% )g 1% TO 3% !3 I 4% TO 5% 6% TO 7% B 8% TO 9% South Carolino Electric a Gas Co. J E vi'9" c S"* **' ""c ' S' 'i a Areal Foliation Contour ,I Diagram Figure 6
q.. - l 11 I N r ( 6
!' w- A -E l! (@
i Q i xw g D 6
\ 'N o I o
\
KEY: O*/o TO 3*/o I 3% 4 */o T O 6 */o 7*/o TO 13 */o I i Soutn Carolino Electric & Gas Co. Virgil C. Summer Nuclear Station Pluton Area Foliation Contour Diagrara Figure - 1 I
i l 1 i !I ! N i I } !I lI *) Qy lI to - iI w <f [ i.e i iI t I
;x y g lI J I KEY:
0 % TO 0.8 % I ' " " " " 3% TO 4% 5% TO 7% l South Carolina Electric 8 Gas Co. virgu c. summer nuclear station I AreaNoint Contw Magra Figure 8
'I l, I CUMULATIVE FRACTURES 0 100 200 300 } 0 400 1c0"
- I i
x 200 - I 300 - 400-- c I w& y 500<- I= = \
$ 6 coa-w I 700-I 800 - M O N T- 1 MONT-2 900-1000 < -
sinn. I Figure 9 I from Zoback
i i lI lI I jl MONTICELLO 1 4 i \ t I +
\
'l \ $g + +4 ++ + \E # +
+ + +
1
+ + +
\ 1 + +
+
+
+
' %+ ++
+ +
++ + + +
\{ t \ i
++ + + + * +
+ * +
+ ++
+
+
+ + + ,+ +
+
+ + +
+
1
++ +
+ +
y + ta * .
+ + 0 #
ig + + + j 3 +
+ ++ + +
4 + + +
+ + + +
j ++ + b=
+
+
i
+ + *
- 5 + + +
1
+ +
j + + ++ j . + +
+
!I t i + ++
+
+
+
)a + +
!E ++ +
!I Figure 10a j-lI
t
+ tt ,
++ * .+ + I p++ 4 f+ + + 44
+ .p + ;
+ ++ + +
4+
+ + ;
+
+
+ + + +
+ 4
+ + , w. + +, i c\1 ++ !
4 + +
++ + + ;
O + +
+
+ + + f J + + +
d ++ +
+ + ,
faJ +
+ $ .y
+ g
- 7+ + '
C + + e g
~ g ++ $ +
+ g , +g H ++ + e
' + + 3 4t,+ i Z + + + e 0 + +
t
+ ++ E ,
+
- 7 + + tg +9++ .p ,
++
++ + + +
+4 + +++
+
+ + +++4+ + + +
+
+ ++ ++ - !
+
+ + + ++ ++ + + 4e
+ .+
+ 4p + +p+.
+
t
++ # @ + $+++
+ +
4 ++ + ++ +
+ *
++ 3
#+ j +1' g+ , 4
+ '
%* 4*. 4
+ '*- + + 4.ht,q"i*t-l' ,. ,g++ +$ + , ,y + *
+
+ .
++++
+
E W M g g
--- -- . - - - ^ ^ ^ - - - _ . _
I iI MONTICELLO I L
, s. ,
3 li, .. q -< I .I i i MONTICELLO 2 l E
\\
s
,L 7,,0 g 6 -<
l % 1 I - eb I i
#fom Icback Figure 11
SLIP VECTOP.S OF fiODAL PLAfiES .i j i 'I . n !I . I . .I iI 2 !l l . I e *
- l5 1
i g, l . I % 8 I . !I ,I . - t !I i ) it: ld 1I s . ! N !I i iI j Figure 12 II
l lI lI 'I ] MONTICELLO 1 CUMULATIVE FRACTURE DENSITY GROUP B i N iI i !I \I J
/Q) 0
'I g [ s1 . II jg \ '* v i !,I 1 !I 4 lI Figure 13
- _ - _ _ . - _ . _ - - _ - _ _ _ _ - . . _ _ _ _ _ _ - _ . - - - - . _ . . . - , _ - - - . , . - - - - - . - . - . . - - - - - - , - - - , - - . - . . ~ - - - - - . - , , - . - - - - - , - - , . - - - - - - - -
!l . !I } M0tiTICELLO 2 ] i FRACTURE DEllSITY IE 0-1000 FT (Os300M) 3 GROUP C td !I I !'i i _x iI I ' i I 3 j ## \ 60 I I w I -;';#J I Figure 14
~
l .
ll MONTICELLO 2 FRACTURE DENSITY
- 1000 - 2000' (300 - 600M) i GROUP C'.
\ tl \
\
i \ l ( k LI i \ \ 2cr \S \1 l d' li\ i 4 p\ AT
.%~ 80-
- i e
gg 1 I L- !i N - in i !I Figure 15 1
i i iI MONTICELLO 2 l ! 2000' - TOTAL DEPTH 's FRACTURE DENSITY 'g j GROUP C i !I !I l lI ll \ 1 o 4 t# I l Wlla) 66
\
5 (' N 2cr 4i
'I i Figure 16
'I MONTICELLO 2 ll !5 FRACTURE DENSITY l 3000 - TOTAL DEPTH i GROUP C lI M I ll l t l ls h I.h l i
) I I '
/
I e e i lg N *qk I Ej', j 5' ran Scr
- .g
/ (
,l l ad l Zv / Ie l / l$ lI i ff Ure 9 17 l i ,I
.I 3
'"2!50$i"
!I y I i i N 27 iI
' A
- E
/ ,
!g . c;> Qe
' ""hwd .w q.?,,
d g5 I
\~4 h o l g ,
s Q
- I
*" y
- i Figure 18
- I . . - . _ . .. - _
l_._____ II APPENDIX V i ASSOCIATION OF SEIS:11 CITY WITH GEOLOGY AND ' ! GEOPHYSICAL ANOMALIES I lI !I 11 1 , il l lE I i I ll l I I . .I E I I , I
I I ASSOCIATION OF SEISMICITY WITH GE0 LOGY AND GEOPHYSICAL ANOMALIES In this section we present data that suggest that the observed induced microcarthquake activity is spatially related to discrete i small, shallow bodies. This is done by comparing the observed seismi-city with maps showing the geology, magnetic, gravity, airborne magnetic, and radioactivity anomalies in the area. However, these data do not rule out comparable scale variability at depth elsewhere beneath the area that may be a consequence of the intrusive history in this setting. Figure 1 shows the detailed geologic map of the area (the geology has been described in the FSAR). We note that in the vicinity of the reservoir there are several outcrops of limited spatial extent. Figure 2 shows the total field magnetic anomaly map of the area. This map is characterized by relatively broad (low frequency) anomalies surrounding the reservoir area (box). Within the reservoir area we note the presence of several narrow (high frequency) anomalies. On comparing the two maps (Figures 1 and 2), we note that there is a broad spatial agreement in the location of the broader magnetic anomalies and the regional geologic formations, and the high frequency magnetic anomalies and the intrusive plutons. The depth of a body associated I with a magnetic anomaly is related to the half width of the anomaly. Assuming simple geometric shapes for the anomalous body, we can get an estimate of the size of the body. Simple calculations indicate that the bodies associated with these high frequency anomalies are N1-2 km The positions of well-located earthquakes on the Monticello net-work ( Appendix IV) were enveloped and plotted on the magnetic anomaly map (Figure 2). The seismicity is indicated by a darker stippled pattern. We note that the seismicity is spatially coincident with the high frequency magnetic anomaly in Clusters 1 - 4 (Figure 2). We infer from these observations that the seismicity is associated with the shallow bodies causing the aharp magnetic anomalies. I V-1 i
I Figure 3 shows an aeromagnetic map of the 'Monticello Resevoir area U.S.G.S., 1978b) on which the reservoir is outlined together with the five clu v ers of seismicity. The total field magnetic anomaly is con-toured with a contour interval of 20 gammas. The most prominent I magnetic anomaly is the one associated with seismicity in Cluster 2. Perusal of the geologic maps (Figures 1 and 6) indicates that it is associated with an outcrop of granodiorite (GD). Other clusters of I seismicity (stippled) also appear to be related to discrete bodies, although none so obvious as that associated with Cluster 2. To get some idee of the depth associated with these anomalies, the simplest example is provided by two subparallel high and low anomalies located on the southeast end of the reservoir. On a cross section drawn at right angles to these anomalies the peak-to-trough distance is
%1 km. If we assume that the causative body is like a horizontal cylinder, and is magnetized in the direction of the earth's field, the peak-to-trough distance suggests that the depth to the center of the body (essentially a live element of uncertain radius), trending NNW-SSE is S1 km. The radius is fl km, which indicates that the maximum depth extent is N2 km. The depths to the other shallow perturbing bodies are of the same order, although their depth extents may vary.
Figure 4 is a simple Bouguer anomaly map of the reservoir area. Data were collected on a average grid of about I station per km, al-though in the vicinity of the reservoir the station density is twice that. This unpublished map was compiled by the University of South I Ca rolina. The seismic clusters (heavy stippled pattern), Montic'ello Reservoir, and the Broad River cre also shown. Here again we note that the granodiorite outcrop in Cluster 2 is associated with an elongated gravity anomaly. If we assume that this anomaly is associated with a horizontal cylinderical body, from half-width calcuations, the depth to its center is 0.7 km. As it is exposed, its depth extent would be 1.5 km. Figure 5 shows a map of aeroradioactivity in the reservoir area (USGS, 1978a). The data are plotted in total count contours with a g V-2 1 1
I I contour interval of 25 counts. The low radioactivity in the center is associated with the reservoir. However the seismicity Clusters 1 - 4 I appear to be spatially related to regions of very high gradients on the flanks of sharp radioactivity anomalies. These observations, again, suggest that the causative bodies associated with these anomalies are very shallow N1-2 km deep. In Figure 6 the seismicity is superimposed on a geologic map of the reservoir area. We note that the seismicity is located in the vicinity of the granodiorite (GD) body (Cluster 2), in the migmatite (MG) zones (Clusters 2, 3, and 4) or in granofels (GF) Clusters 1 and
- 3. The presence of tne migmatite zones resulted from the alteration of the country rock when the plutons were emplaced. We infer that some of the shallow bodies inferred from the geophysical maps are not exposed on the surface.
An examination of shallow seismic refraction data contained in
" Geologic and Seismic Report, Parr Hydroelectric Project, FPC Project No. 1894," (SCE&G, 1971) has revealed strong evidence in support of the lateral and vertical heterogeneity of :he subsurface as discussed elsewhere in this report. No correlation was evident, however, between I any velocity contrast on a given set of refraction profiles and the subcrop of plutonic granodiorite bodies.
Summarizing the available geological and geopnysical data and comparing with the observed seismicity, we note the following:.
- 1. Monticello Reservoir is located in an area characterized by high frequency (short wave length) anomalies and small out-I Crops.
2. I The spatial association of outcrops with these high frequency anomalies indicate that they are related, and the causative bodies are shallow and of limited extent ( N1-2 km). I 3. The spatial association of seismicity with the flanks of high frequency anomalies suggests that areas of strong gradients in I physical properties are also areas of strong stress gradients I V-3 I
I where pore pressure changes from reservoir impoundment can cause failure most readily. i
- 4. The spatial distribution and correlation of the seismicity and the geophysical anomalies suggests very heterogeneous material property and stress conditions both laterally and vertically.
These results, coupled with observations of significant heterogeneities in fracture density and orientation (Appendix IV) and hydrologic conditions including permeability ( Appendix , VI) imply that only small zones can experience movement in any single earthquake occurrence. In turn, this implies that there will be a maximum fault dimension, hence maximum magni-I tude for earthquakes in this environment. I I I I
- I I
I I I I V-4 I
l REFERENCES llW South Carolina Electric & Gas Company, 1971. Geologic and seismic Re po r t , Parr Hydroelectric Project, FPC Project No. 1894. Re po rt i Subnitted to the Federal Power Commission. U.S. Geological Survey, 1978a. Aeroradioactivity map of northwestern i South Carolina, Sheet 13, 1:62.500 U.S. Geological Survey, open 5 File Report 78-846. 1 g ! U.S. Geological Survey, 1978b. Aeromagnetic map of northwestern South
- Carolina, Sheet 13, 1
- 62,500. U.S. Geological Survey, Open File 4 Report 78-847.
iI I
- I i
!I !l !I !I 4 iI j V-5
r i' i I
; lr -r i e .-.
I * {4 i l
. l i .
L i. ,h x 7,,- 1
; } j' ,
. ,o
! _ ,o ,'s. .. ,
j_ s y . s g.- o .
.. s ,
- x. '
is
- l' 8 f, .~{ _
't.i.
l q.s ' ,
. /
\; - ,
g i[1 ' :t " %s%'
. :e .
s %yy 7 h
* ~i .-
_ W. m'JM.0 Q (,v:
$ ',.0 !'.MT.. .d) -
/. , l
. f.U~ > V
-f %
o , ,I r Q );:e y.Q*g' J"Ta "$ :M. f N. .' 8 -
'y
. ;, ~" 4, i ,
'lg s[ _ -
yy. ,
,gy;. 8 y -
s
}
Y .q . , r y; l , ,I Y I . '.-
. b * ,. gk ,..
E .
. 4[
k T:,1 g . -
,q '.*4 .n[ }, ' ( $a/;;;;;,.
3l9 - lg ,, '% 3 s(k 1 . h-
~
- < >
- 6
(- ?
.Y.$)'a. . w'6 yib'W
~=. +
, s~ , - y , J}
A , ; . q,. ,1!' , I g?j .' f-g;ND h"g,
. i Yh a [ (, . ,
- k 'gg e. ,,, , -
~
/ * Y
;- I ! /p f. 8 I j,,,
') M!J!'N'f p,
h[
's,. a n, y;Y ..
q -" ' . b *
- ,;/ t.4 - ,i-D ' Jlk. '.. E. . WIN b '
n M [$*/. '
- y. - l
'y y m m pw h :_=w)
.a.
- .4, . g , ;,s 1 3.,
8
* ,s y 3.
L.- p .
,?
'M 4 .y. . -
%' }. > gt gp -
,. p ,,, k-y _, ' ry ,[ Ru
_e a _
. - v ',
- s. %
,s . >..c-
. ' ti 0.
.~
3g Di s
. As- 't .
. y;T*..g D
'$ l _
\
l
- t. Y M .
.- l s.
< P. P l' 4,
= Ja _t z, s E. G...., .
..c d, ... - %,
N - h_ ., .. .. . An. . w' . . .$. b ' Lr r-j .?,fQ#,a' - V f.
'r: )i.. p -..c? .i - i. ' .\..
;4,' '., .+ sw :M y - r ,. ,
} . .
E. y*
&e; (o* ' 'g.
i 2' % f 'N_ , c .;. g , g> g fp ,;j~ [5 ,*,9 , 3' q
,q o
- i. lc,')',
' S. k. ha \
.j g 7
/ . .- ,
3 g f:- g,;,
,g m n: . .s'
,i p . . .' ' '
. & } q.,q) g \ . e.M
~ p, ,- ' I:' l' y;*
$g
*G . . =
\ / q.- .,
- p. . ? .
h n- , - m. ,,
& ^sl, e n ,
*. - 's L v.s , e.,r agg #.: . i,@+ > $ h' 9 , ' * ,,w" .y
- .N..j .
. :,:... %,,1 j -
g%.C .'s, . a e 4.j .- ...
'., , *j .,[ y,%6y rf T .{ f;q, D .
. " 4 "Wy i' 1 ,. ;-
+ 4 ,9 '
* *;[,';ipu K.g:' ~P-( j. : k"
, 1 : s
~,
f jf s ' _ + i g*., p .Q' g
'" ~. f.D' ..M - "
- w. ip . s ,w 3 -
*.' O *. M.* 5
.if m*p;. Y hi; ' ' Y l' tl y'hf*'f.h,,':
,y
.f'
.f j T .'
f;<Q,cn,c. 1
. I a.
. n.o O I s 2
.tr -
,4 r'
" Wc.g+6);W *.*'1
=.
../_~'.,_
c- s.
. ;. ,o I'
ST ATUTE MILES .,- J:*,/' ... ,,. 7 / g?*.y* ,7 ' . , [ ' ff { '& . ' '$
I I PE23 E7MMAl
*~
I i , g, 8 ~
- n. ..
J?g s 3-
' q .
gf;kkN) D
, hh , _ ' '*~
S\k
% Xq t N , w *% w* q*V y
I F~ a i m,
- =g i
' l e 'O f a "
I I I
i i
! h
~:
8 ; s f l 6 1 << 0 O 4 \
'i j
\
N i . C g> i - x
,1 a
'?b. ;
. Y.o-5 53300
/e u:. <3
'" ;A $
O g O O L o c ~ a CO t'E G c O c { O O 0C2 g o i O t
/-
og p gj o o < 'NoW
!- t' y- g o
ua O
/ ;- f f..s l ,I -
r 6 7-33:(Ml/ x' 7 i
\
i
\.,
jQ ,/ '
.. ._. -J'/ ;pf'~,
j' ' ('w. }
, ~ _ -
~
/
, /
,/ l
- ) ' ( -.
? -
a
* 'a
~
$. ~*
a,
'(
Q [ b'~ . 9 E C O C C i r e 0 o 0 0 - -2 o 6 O ,+ m
/e 0 0 s 2 O ik < 0 O >
i 0 1 k e 0 0O J' (/sAa Q') O 0 00 P 9 c6o _
,0 d, XJ e e 0 A M
T
- 0
- t
/
O
- 4 t
/t s0 t i
R U g q 'y J O T j ( O t. N c _ O U C C I T E N G U *a A t t o R O'@ c D a* m A E c 3 3 a PJ &* / e E R U G I o F
]
0 c3 6} C f> C j9 o O n O \ i /
%%'l k1 1 C
( 0
- h ot e
o L 9 l O Cp f o @O%g u a Q7,/j 3 O aC N 6 0 0
- \
q
\2
)
_ . / _ \ ~N _ N \
/
\
\
\ -
x
\
f _ j f \ x
- N ' \
g' ' s \ ,'N y N I
P i :: d $8
.c i .i i
/
, j' /~~~~~
r- l / '%-- J { ' j / '
- x ,
l ,,/~ -_. _s
' / /
, / / /
i l /
,l i
r j % x
\
I I w-I i 9
) ,, ,
x o O Ns oC
\ g \
x c
'Q[h/ m C c
%(
e ~,
-c.
31 o ' e a i O \
\
~~~~., '
s _ 3, Nx c x s \ x x
\,
', 'N s
x
! s '\
. y i
I '
~E
'l '
pM 3d.,IN.Ek-e
y x
? o - -
O C, D
/[ r- .
f l ( c ,
?
/
/ eO a
/ Ace [%o aq a 0 U n.
O O a l A $
, eo o ,
f a O (x a i !
/ '
j '\ ' C? (M
\ \o /
.i 4
x l
\
l e 4 0 iso deirr l , 4 _ ti
\, \
- y'c \\ \
\( ,(
\
g
\ k .
, .z Ot Pin (W tLout it N$)
O O 00 e 00 MAbNifyCL e < 0 00
. \ O I01 200. O O 00-100 g *2 A > 2 00 0 40 200
~
.,.u,or'\s
.- 1
* ," ' ' g, ,. ,,. ,,,,;. ::~
FIGURE 4. BOUGUER ANOMALLY f1AP
, _ . _ _ _ _ _ _ _ _ _ _ .Y
=
f
' ? -
r
! -~
/
t t, ey{O
)
C a
- se
~,
u o O f;m N Q
^
l i-k 6 s I o, I
/
sx g j n\ i g
\
1
;x x
\
K%) j (
xN" O
e:- s
\
- M.~c
(' z
'ss_ ;(
i s. C;' ._
,~ g
)
- ~~ .-
~~-
_.,__ y
/
i 3 .,
~
, f L U
- _ YM ~'rvv ,
g
$ g h c C o oo oa t U 4 ( a ao cb 0
~ o a 3 ,
/ TN \, a
/ ) -q
- o o
S o kn (,a .
, J o m (m
. j m -4 p 675 Q,
, % Q
'I
\!
oe n,, i.itoue nau wacnavut I <ow j a o m noo
,e (g g 3 o i o .z oo o o m ioo $,
,_) a -z oa o n on -t oo -o y -w a > 2 00
( ,, s ,9 o oa..ma
\ 3
\ \ qS O"
\, , o
w,/
- ~ -
j _ . , FIGURE 5. AER0RADI0 ACTIVITY CONT 0UR MAP
\
I i i l MONTICELLO EA9THCURKES l SE i . 7 78 - CCT, i2 1979 TRPE ORTA i
'l p2. p! pc.
-. ;9 .
,u ; 1..
,1 7 . ,3.
,3, l
['~_' . H GD _n e I _ ,' _"I CBGN _ 2/ - MG g
/ // _ _22 c
/- q B GF 2s :
~
~'
_ g \' _ _ , , . ! [;
-s //[/,e _r . ,
l _
- g, ,a s
O- % . *R :
, _. _2.. :
I eg %N
,y.i
'ad k, L ,,,>,.
,V.
6 "
,N a;,
I 4, j **4 ,4, ,
, _4 4 a < '-
, 4 , ", A e e , *. ,, y> - _J o e
" * : '," , 4. ,&!f",'@J
( l
,' ; $ E d r's ., .: ::9 \%::
s
,, %a- 2 - <
s, a _ n.c I N !75 E ~ vo g s
.a.,* . , ' , ' ..j:4
<* . t; . . , ,,&
_a s s E ':; : :;G::U.,',: c^:::: :i::!: :.,g .. C=n
,v; :c:c:n..:<: ., M .c :,::cm:7.
- ,: :::y:.
< , 2 m
~<"..g:: <, <
j ,.. n.: .,. , .
;'< e, 0 5 a. ' '
. .m :v4^.*r,
"'<4
' ' ' * * * ' -"5' 4
*m<
i c . . > , ,, 4_6'**<., *- 34.CC I i i i i ! i I I i i i
-l3 l3lElC;U i i i i 2 O A + X O fl .i. c.
MAGNITUDE
- i. e : .. c. i 2.
DEPTH (KM!
- 3. ..s
, m :m !
c@ fi!
;. Q.
i U! !; Figure 6 f @ d U\\ ( @I h' M i > a ( -
f I . I APPENDIX VI HYDROLOGIC DATA AROUND MONTICELLO RESERVOIR I I l ) i ll l !I 1 i l 1 f t l i. I lI i 4 j i i 1, iI iI 4 1
l I l HYDROLOGIC DATA AROUND MONTICELLO RESERVOIR INTRODUCTION
. aditionally two mechanisms are thought to be responsible for
. induced seismicity. The first is the response of the crust to loading of large reservoirs - as has been suggested for the seismicity at . Kariba (Cough and Gough, 1970). The other mechanism is by the diffu-sion of pore pressures as suggested for the seismicity at Clark Hill Reservoir (Talwani, 1976) and Lake Jocassee and Monticello Reservoir (see for example, Talwani cnd others, 19 76, 19 77, and 19 78) . In the latter case there is a time delay in the onset of seismicity which is governed by the hydraulic character of the medium. THE THRUST FAULTING AND INDUCED SEISMICITY ENICMA Some earlier workers (e.g. , Snow, 1972, Simpson, 1976) have analyzed reservoir induced seismicity using a Mohr circle representa-tion. For a reservolor in an infinite elastic half space, Snow (1972) showed that the filling of a reservoir in a thrust fault environment I would' inhibit seismicity. Even af ter a long time the ef fect of a pore pressure would be inadequate to explain seismicity. In normal or strike slip faulting regimes, i.e. , where the vertical stress is the maximum (c1) or the intermediate (02) principal stress, they suggested that induced seismicity would occur. These workers have ignored the effect of increase in pore pressure at depth which occurs when the water table is raised. Bell and Nur (1976) showed theoretically that the effect of raising the water table causes a rapid increase in pore pressure at depth. .I. Because of the occurrence of induced seismicity in a thrust fault environment at Monticello Reservoir, detailed knowledge of the water table and changes in it are important for assessing possible change in pore pressure as a foreign mechanism. Consequently , we carried out a resistivity survey to obtain the depth of the water table. We realize that the aquifer found in Monticello well No. 2 which may be associated with the seismicity in that area, may not be related to the shallow E VI-1 I
1 I water table. However, its configuration will probably influence the flow of water. This conclusion is based on the vast experience of engineers studying seepage of water at dam sites. In the three cases shown in Figure 1 (Roberts, 1969), the largest seepage occurs when the water table is far below the bottom of the proposed reservoir. SITE AND VICINITY GROUND WATER LEVELS AND FLOW DIRECTIONS Perforated pipes were installed in 19 exploratory borings to observe water levels at the site. Repeated observations of water levels were made to obtain static level information. Analysis of this information indicated that the principal direction of flow at the site
. is toward the northeast into Frees Creek, a tributary to the Broad Rive r. Movement of . ground water is from the ridge axis toward its flanks as illustrated by the water table contours shown on Figure 2.
The ultimate flow of site ground water is into the Broad River. The g estimated rate of flow is expected to be up to 1 foot per day on the R steeper ridge flanks. I Observations of water levels in exploratory borings indicated that the ground water table at and around the site occurs at depths ranging f rom approximately 20 to 90 feet (elevation 350.0 feet to 420.0 feet) I. below the original ground surface, generally in jointed bedrock. Local lenses or perched water in soil occur, indicated by seepage high on the rid.ge flanks. Static water levels and water level contours developed from these data are presented in Figure 2. An evaluation of ground water conditions at the site indicates the following:
- 1. The preconstruction water table slopes downward toward the nartheast in the direction of the sloping land surface.
- 2. The water table gradient is quite flat (0.005 to 0.01 foot / foot) on top of the ridge and steeper (0.02 to 0.07 foot / foot) on the ridge flank.
I I VI-2 I .
- 3. Recharge occurs locally, f rom surf ace infiltration, and dis-charges into Frees Creek.
I CONFIGURATION OF WATER TABLE FROM A RESISTIVITY SURVEY I The depth to the water table was obtained by carrying out a resistivity survey in the region between the & nticello Reservoir and the Broad River. The results are shown in Figure 3. The water table follows the topography, with steep gradients near the tail race (bottom of Figure 3) and near 34*20.5'N and 81'20.5'W. Both these sites are the location of clusters of seismicity (Figure 3).
. These observations suggest that the intrusive bodies that have been mapped geologically, have also influenced the geohydrologic system and possibly the observed seismicity through pore pressure changes in zones of higher permeability.
GROUNDWATER DATA AT MONTICELLO RESERVOIR Water level data in eight wells in the vicinity of Monticello Resevoir are shown in Figure 4. Water levels before, during and af ter the impoundment (Figure 5) indicate that only well no. 3 was immedi-ately affected by the rising water level in the reservoir. Now after 2 years, water in wells nos. I and 2 have also risen to the reservoir height. The location of the epicenters in cluster 3 and a possiale aeromagnetic anomaly accompanying it, suggests the existence of a high permeability fracture system linking it to a ll no. 3. These observa-tions also demonstrate that the ground water level (as in well no. 3)
= rose about 40 feet. They further indicate that the aquifer system is very complex. The '40 fe.t increase in the water table at Monticello I Reservoir would cause an increase in the pore pressure at depth by 1.2 bars which provides the necessary trigger to induce seismicity.
PFinlEABILITY MEASUREMENTS Permeability was estimated in Monticello wells No. 1 and No. 2 (Zoback, personal communication). Permeability was obtained by packing I VI-3 I
I of the upper part of the well at any depth and estimating the perme-ability from that depth to the bottom. When observations were made at I depths from 100- N400m in well No. 2, the permeability values we re of the order of 0.5 mDarcy, and decreased to 10-2 mDarcy at depths of
% 00m. In well No. 1, at a depth of 726 m, the permeabill.y was of the order of 1 mDarcy.
In summary, the hydrolcgical system in the vicinity of Monticello Reservior is complex, with variable systems of fractures and dif ferent system permeability values. The flow of water 1:s related to the ground water table, availability of local fracture systems, and other factors. The seismicity thus appears to be related to the configuration of the complex hydrological system in association with the local variability in geology. It is important to note that the limited observational evidence available as presented here shows a scale of spatial variability in hydrologic conditions both horizontally and vertically similar to other g geological and geophysical anomalies discussed in Appendix V. That is, E there are signifi' ant variations in permeability over distances of about 1 km laterally and over depths of a few :.ci s of meters. I i I I i I I V I-4 I
I REFERENCES Bell, M.L. and Nur, A,1978. Strength changes due to reservoir-induced pore pressure and stresses and application to lake Oroville.
- Journal Geophysical Research, vol. 83, p. 4469-4483.
Gough, D.I., and Gough, W.I., 1970. Load induced earthquakes at 8 Kariba. Royal Astronomical Socie ty, Geophysical Journal, vol. 21,
- p. 79-101.
Roberts, G.D., 1969. Predictions of reservoir leakage. Association Engineering Geologists Bull., vol. 6, no. 1, p. 70-82. I Simpson, D.W., 1976. Seismicity associated witi reservoir loading. Engineering Geology, vol. 10, p. 123-150. Talwani, P., 1976. Earthquakes associated with the Clark Hill Reser-I voir S.C. -- A case of induced seismicity, W. G. Milne (ed.) Induced Seismicity. Engineering Geology, vol. 10 (2-4), p. 239-253. Talwani, P., Stevenson, D., Chiang, J., and Amick, D., 1976. The Jocassee earthquake, a preliminary report. Third Technical Re po r t , Contract No. 14-08-0001-14553, U.S. Geolugical Survey. Talwani, E . , Stevenson, D. , Chiang , J. Sauber, Amick, D. , 1977. The { 5 Jocassee earthquakes (March - May 1977) a progress report. Fifth Technical Report, Contract No. 14-08-0001-14553, U.S. Geological Survey. Talwani, P., Stevenson, D. , Sauber, J. , Ras togi, B.K., Drew, A., Chiang , J . , and Amick, D., 1978. Seismicity studies at Lake Jorassee, Lake Keowee and Monticello Reservoir, South Carolina I (October 1977 - March 1978). Seventh Technical No. 14-08-0001-14553, U.S. Geological Survey. Report, Contract I i i I I VI-5 I
I I , I w47 PROPOSED RESERVOIR I *BLE w s e@ o LV A-SM ALL LE AKAGE q PROPOSED RESERVOIR , WATER TABLE E _. . _- WATER TABLE _ B - PO S S I B LY LARGE LE AK AGE I < II PROPO SE D RESERVOIR
= - _-__ g .
= = = _
I _ - _ . _ _- f l c-PROBABLY LARGE LE AKAGE I From Roberts,1969 I Groundwater indications of probable leakage conditions I I I I Figure 1 I
f.x.:.:'.' 2 VM. .'. : i.'::i:. .: ;: . :. ...:'i.- :i; ' M.i? ?:. : ::'/. '.
. . . . :. .':':4. ... . . : . .;..'.fi.: :.:?.:.. .;Q'.&. ,.\.?&..w..%.
- . 1.:: . .- . . . . .
l n . ,.1:.e.
.:~:.?
. . .- r: . . . . .: .:.. .
. . . . . : )..,. . .;. ..:. ::. : : ': ..:s.._.........;
... . .'L;
- : ;:...::..:.~.-... . . . . .
p ., . . .
. . ... ... ..e...... - .... .
.... s . s.s. - .
. .. .gg :. . y 3p:,. -.
g ec. 's.f.i.di&. .'o.'. 'A E.E. E I.*. R N..
%. r' '
... e. .w. .
. e... .
- i n.t: 6 .: Y: . .
- 3gg :. l .,:<
. :: ..:r. .': ;: .'; : ? .'. ,. . ;
.;;; '*. i(l;. .:
'.. .. ; .: ~. '..
T5 ~ ' - ' '- l d # ,., ___.0,;
, h .,.
o- 3.&.ll 3
.=
3
.~5 5'=h '
..~
_%v , p TE .- 3,% .:.t. ei:,"g. .. :, . , 1 . .: I ,.",..,..- 3 37, . 3,y,,. :, .
-. mo,j . . 7 ,,
/3n 3_._,,3
_.-y. i.. e...
" wee Nc . rij , . . . . . ..... .
BOUND: .f CF .h.::. - INITIAL SITE # '"3- 7' .- y c7 ~ c _ 4- 15 .g i-- warte rastr cenrets t .:. ;* !.', .:t. ( koa " Q f:.. :. .y. ;~ . . . . l . ......- w.. .. . - k ' I o n.i, s A wm
- Tai '
I ELtvation h25 Pet ConstnuC7 ece encuto SheraCE Cc4 Tout
-is .
s .,\,. n W' A fN / j N. c: N 4
.9 I CE' % ster y an en \ ~
soo o soo ioco I n y.w). , r___w -,
*ET:
az_ e N - 19 BORING NUh48ER pj- - -~ 403 ELEVATION OF STATIC GROUND $0y{h CQrQling {lectrIC 6 GoS Co.
-a WATER LEVEL OBSERVED IN BORING.Q "
Virgil C. Summer Nuclear Station . a_na ~ NOTE: (h5 ~.-- @ grr> I 3- contours cF WATER TABLE EVALUATE FROM prior WATER 08 SERVAT 10NS ON 80 RINGS TO FILLING CNLY BO#1NGS WERE TAMEN ARE SHOWN. CF MONTICELLO IN WHICH WATER LEVELS RESERyolR. N*$" g 333 Site Water Table Fir Jre , 2
I 81 22' 21' 20' 19' I r I -
\ . . .... ...
\ .
E4 22,- [::. . :.,. .:.
\ sgo
. y , .; .: .:.
': ..- :. ._ i I
\
\y :. .:: .:. .
$ 4 ;;
- .:5S ::. :i:.;N. .:. ;
h,,,is's. ,. ....: .
. ' .::?P9
.. . ::': ' :.f::
... .. . . y ,s,ss \:~ i , .
n i
.g. .- . . : '.:. . .... . . . .
r l . .:.
' t s t. 1. ..:.. :: *
, \is,,,~i~\ s,,~s :#. '. . . . . ::.
l . - . . v.. . .. . s , a i,, o' 5 .' MONTICELLO ' l
.i: ..
! 21,- c. ::. ,, RESERVOIR -
\,
< -7s; ,>, .::.1.,.:. . . . . . . .. . . . .
m_
. s.,
s
. t
,s .,:s:m. . ..
w, _ , , , , .. . :r.. . :.
.:..u. :.
... s
.*4.
w ... . . . :. ..
/sI Is, i g/g~. :; 5 : .. m. -N.
**=
*:: ,'\g s % t
, s~
~
( ::: . * . . . .
.,s . .
* \
y y \\;-A-Al \ l/ .x, l '1I - ~I s -l~l :- , :gr.s,* *:/+::
- I g- 'tg' t\ .y!.:.,:':M. .M
.. /s g- .t .\ - . . .
,l,. s .........,....
i . e. s I- -.g;fs i N,,,.. . . :: / . :. s/. >\ -
. . . i ,
a.g- ii,s g, :r.s - , .s.%. :rt.~ l v. s . :. . 1 o, p ga /c l s i
. ::n::
..:::a... q
; ::+ :
. .- - s ...q..yQ,::::t:4.
+
r . .. nAys,/M,i L.'sL : ,^ i-MT , . .',45.m.2.1i,s,7 i .. .
!!iM: .M;.~:
.;;r.;;~..
o ..a. .w.&. .g=,. L 20 -px: - .. e-ve/ ,i\ U_,e iT5 !.D.
- . ..H: .. "I.l i i .
. ..c..:.:
.:... . .. s>.sf 1
/. .. 1 -. ; .v::. . .***.
. ' ; ' \ [_
I s' ,1 ,\l t . ;, s. r; :.::. .
'*. s . :+:.::. '* .f;. gl
; \-
,. I _g / g -l f,s\ 'f j\ 1/ '\ ;. f.t;.<,.,:
i f.
.gf
- ...:.. +1 ~! '.:7 , 5.h, :,. .^r L..:. q, , i ,_,s ,.i- E, :i .: <.@:-r .. .: ..r.E . . . .. v . . .
..N I
,s, - . .:: -
. L=.: ::
\
. . .. .. .:;M.;, ..
.-. . . . 4;.:. :
!:r::
.wc. . . . .
w :. . .: . :. :. . . . . . . , 19'- M. . .. s q u~ y i s
?!: k.." .i.:...::.G.i.:.::..:..::.
~ - .. .:. s.: .f -...;.:.
iai2 .
- .n::
,'i,~i,~IIs,s%,m,
;~y .s, ,.
.: Ossg . . . : .. . . ... :: .:...
r.
/ // ..
. s. .
.! 7 ;.:
. ;v.. y,g'7'0s'ys 30 p .. . . . ;: .. -
. . : .:. . -tut ~ c - - .ii+:. , : ...
.:. . j' ..:q:,
. '.:.; y i s'\, i/ 4,1 / , ,.: G . . i,. '.,i.::t s.. '\:
% \ h y.-
/
. ::."4,. .i' I ,/ . . .:./.Y s '\' I .
. r, . . N- *::s . - /-
.:\./: ... *: '..
n.
' ....:.=..
- r . . . .: . M t. * * . * ., ::.::: ;.ss!'s
.. sv ~I/
\.~ :: .. . -
. .\.. a.
. . i - ,<
18 -% 3o
, v' '~
ts
. . . %l.:.r':::: &.w:::=. :. >
- .. .,. ..:.:y;'::::i;,;. .a '
WATER TABLE O . .. 1 2 ELEVATION FT. Figure 3
g N 510,000 Dal / I2 I " y ,, P I 34 0
) , j N 500,000 651 99 I LEGEND:
N 13 OR OO 9 UR G IN S o OENOTES WELLS U I
- EP WELLS O
\
O 5 MON T IC E LLO 7 7 N 490,000 0 4
$ 0 G
y
\
25 \ MONTICELLO RESERVOIR DAM "A"
- 1i I
L f N 480.000 p 16 DAM"B"
's 30 213 le i w l / OAM T b
gl 1 x DAM T 2 18 N cteo w\ ER ] , me ,0 4
e m i m n u l m 5 68# 7 l I Y L u m U m i i J m - u u s l E N e i U n J u s a j i Y e A m i I M a m u L 1 I R a P m 1 A u j 8 5 e 7 r a 9 u g 1 m u l R A i F s l M u s _- l I B a E m u i F O LR j LI W 1/ O u m E V n . CR I I N TE A m NS iJ u m OR E M i s i C 7 tu E 7 s i D 9 1 n e - - - m 0 0 0 5 0 5 i 4 4 3 a m Pu. $ ZO ww o n m m I
APPENDIX VII 1 STRESS DROPS ASSOCIATED WITH MICR0 EARTHQUAKES AT MONTICELLO RESERVOIR I I I I I I I I I I
STRESS DROPS ASSOCIATED WITH ;11CR0 EARTHQUAKES AT MONTICELLO RESERVOIR The data presented in this Appendix were obtained from two studies: the University of South Carolina in July-August 1980, and USGS in May-June 1979. Stress drops and source parameters were obtained fc r 5 events 0.8 < ML < 2.2 that occurred in a swarm in July and August 1980. These events were located by using the S.C.E.&G. network together with data on one portable Sprengnecher DR-100 digital seismograph located at JIM (Figure 1). The source properties of P-waves were obtained by using Brune's model with suitable corrections being made for radiation pattern (Brune, 1970, 1971). >Rndful of the possible inaccuracies in the hypocentral depths, the results are present.:d in Table 1. We note that the stress drops associated with these events are <5 bars and the source radii are of the order of a few hundred meters. In a recent study of joints near Monticello, Secor (1980) observed that most of the joints that outcrop range from 5 to 100 cm although a few joints are longer and extend beyond the outcrops. The source radii calculated are comparable although about an order of magnitude larger than those observed. The calculated source radii are reasonable in light of the observed heterogeneity in the density and orientation of l 1 I fracture planes observed in the deep wells. The stress drops are of the same order as determined by Fletcher (1980) for Monticello events in May - June 1979. (The abstract of a talk to be presented by Fletcher at the Fall 1980 AGU meeting is attached; at this time there is no other written documentation of these l results.) The stress drops are also of the same order as obtained by 1 I lbrion and Long for Clark Hill and Lake Jocassee--two other sites of reservoir induced seismicity (Marion and Long, 1980). i Fletcher obtained a stress drop of 17 bars for the August 27, 1978 earthquake, the one associated with surface ground acceleration of l 0.25g (Fletcher, 1980). 1 1 I Vll-1 I
I The calculated fracture energy is 't104 - 105 ergs /cm2 , Husseini studied the fracturc energy associated with slip on a fric-tional surface and found that it varies from 103 to 109 ergs /cm2 , depending on whether t'te slippage was occurring alc,ag frictional sur-faces (103 to 10 erg'J/cm 7 )2 or breaking new rocks (107 - 109 ergs /cm2 ). Thus the calculated fracture energy indicates that the seismicity is occurring along pre-existing fractures and not breaking new rocks (Husseini, et al., 1975). In summary, the observed stress drops are much smaller than the available deviatoric stresses (obtained from hydrof racture data). Stress drops associated with the largest event are < 20 bars. Using the Brune model, source radii are s100 m and fracture energy 103_ a 105 ergs /cm 2 which suggest that earthquakes are occurring on pre-existing fractures. I I I I I l lI I I VII-2 I
REFERENCES Brune, J.N., 1970, Tectonic stress and the spectral seismic shear waves from earthquakes. Journal of Geophysical Research, vol. 75,
- p. 4997-5009.
Brune, J.N., 1971. Correction to Tectonic stress and the spectra of seismic shear waves from earthquakes. Journal of Geophysical Research, vol. 76, p. 5002. Fletcher, J.B. , 1980. A comaprison between source parametes determined f;om body-wave spectra and in-situ stress measurements at Monticello, S.C. In. press, American Geophysical Union Fall 1980 meeting. Husseini, M.I., Jovaniovich, B.B., Randall, M.J., and Freund, C.B., 1975. The fracture energy of earthquakes: Geophysical Journal, vol. 43, p. 367-385. I Ma rion, G.E. , and Long, L.T. , 1980. Microearthquakes spectra in the southeastern U.'S. Seismological Society of America Bull., vol. 70, p. 1037-1054. Secor, D.T., Jr., 1980. Geological studies in an area of induced seismicity at Monticello Reservoir, South Carolina. First Technical Report to U.S. Geological Survey, Contract No. 14-08-0001-19124. Talwani, P., 1976. Earthquakes associated with the Clark Hill Reservoir, South Carolina - A case of induced seismicity. In: W.G. Milne (ed.) Induced Seismicity. Eng. Geol., Vol. 10 (2-4), p. 239-253. Talwani, P., Stevenson, D, Chiang, J., and Amick, D., 1976. The Jocassee earthquake, a preliminary report. Third Technical Report, Contract No. 14-08-0001-14553, U.S. Geological Survey. Talwani, P., Stevenson, Chiang, Sauber, Amick, 1977. the Jocassee I earthquake (March - May J77) a progress report. Fifth Technical Report, Contract No. 14-08-0001-14553, U.S. Geological Survey. I Talwani, P., Stevenson, D. , Sauber, J . , Ras togi, B.K. , Drew. A. , Chiang, J., and Amick, D., 1978. Seismicity studies at Lake Jocassee, Lab "eowee and Monticello Reservoir, South Carolina I (October 1977 March 1978). Seventh Technical Report, Contract No. 14-08-0001-14553, U.S. Geological Survey. I VII-3
TO BE PRESENTED AT FALL 1980 AGU MEETING I A COMPARIS0N BETWEEN SOURC ARAMETERS DETER-MNED FROM BODY-WAVE SPECTRA AND IN-SITU STRESS
.NEASUREMENTS AT MONTICELLO, SOUTH CAROLINA Jon B. Fletcher (U.S. Geological Survey, 345 Miccler1ela Rd., M/S 77, Menlo Park, CA 94025)
Five 3-component digital seismographs were I deployed at the Monticello Reservoir, S.C. during May and early June 1979 to determine the stress drops of reservoir-induced earthquakes and to compare them with earlier hydrofracture I stress determinations of Zoback (1979). Records for 326 events were obtained; 9 of these events, each recordea at four or more stations, were I analyzed to determine the stress drops. Depths for the nine events ranged from 0.5 to 1.4 km. Focal mechanisms were determined from body-wave I displacements, using a new inversion technique; individual radiation-pattern corrections were applied in calculating the moments at eacn station. Final estimates of the moment, source radius and stress drop were obtained by averaging the values for each event. Source parameters were also calculated frem the strong-I motion accelerogram of the ML =2.7 event, which snowed a peak acceleration of 0.25 c. Mcments of the nine digitally recorded eartnquakes I ranged from about 1.8 to 7.2 x 1018 dyne-cm for tne P-wave data and from 2.2 to 14 x 10 18 dyne-cm for tne S-wave data. The stress drops generally ranged from 1 to 2 bars. The Mt =2.7 event had a moment of about 7 x 1020 dyne-cm with a stress drop of about 17 bars. In compari-son the in-situ stress data give a level of I shear stress of about 40 bars at 0.2-km depth and 20 to 40 bars at 1-km depth. The variation in shear stress at 1-km depth indicates an I inconsistency between the thrust-faulting mechanisms of the earthquakes and the state of in-situ stress which is consistent with thrust I faulting at 0.2-km depth and strike-slip or normal faulting below. Thus, the level of shear stress from in situ measurements is of the same I order as the stress drop of the M(=2.7 event but about one order of magnitude larger than the stress drops obtained for the smaller events. l l t
M M M M TABLE 1 SOURCE PROPERTIES OF 5 MONTICELLO EARTilQUAKES DEPTil r no f Mo R 'ao C DATE TillE (Kft) (Kf1) fit . (CilSEC) (ilZ) Q (DYilECM) (M) (BARS) (ERG /Cl[] LAT(fi) LONG(W) 800727 0433 2.78 3.83 2.16 -6 19 C1 2.00 10 14.8 4.1 10 156 4.7 1.C J 340 19.09' 81U20.47' 800728 0707 1.88 3.69 B1 1.34 2.95 10-7 36.3 4.2 10 18 76 4.2 6.1 10 4 34 18.73' 81020.49' 1.80 5 800801 0525 3.06 B1 2.05 2.82 10-6 13.8 7.7 10 19 192 4.8 2.0 10 34018.99' 81 20.06' 800801 0636 1.80 3.76 B1 1.32 4.79 10-7 23.4 2.5 10 18 99 1.1 5.7 103 34 18.60' 81 20.37' 800805 1238 3.60 5.08 B1 0.82 1.58'10-7 38.0 6.7 1018 100 2.9 3.8 104 34 18.52' 81 20.28' r: distance between epicenter and recording station Ao: stress drop Q: quality factor for event location: A is the best C: fracture energy M't: magnitude n: o lon9 period level of displacement spectra f: o corner frequency M: o seismic momnt R: source radius Note: Depths obtained from SCE&G 4-station network, JSC, and 1 Portable. From previous experience relocation using USGS tape data will reduce above depths.
l
! l l
l MONTICELLO ! l ' m
- 22. E1.' f0.' 9.' 7.' 15.'
[8.' l 1 I - 6 n.o
- .2.s 22.0
_.2t.s _ 2i.o
- 20.s
~
20.0 _ 29.5 C 39.0 I - O^2- _:s.s 2e.o 27.s I q ._.2 7. o as.s a s.o 0 5 (KM) " g , ', 3 tl . 00 Figure 1 I I
1 l l
- $3 APPENDIX VIII i
i ASSOCIATION OF AREA 0F FDi INTENSITY VI l { AND MAGNITUDE OF INDUCED EARTHQUAKES - ll lI ii l { ' II i lE i. lE i I
- 8 8
8 4 t -
. . _ . , _ _ _ _ _ . . . , . . _ _ . . _ . ~ . . _ _ , _ . . . - _ _ _ . _ . . _ . . . _ _ _ _ . . _ _ _ _ _ . _ _ _ _
I I ASSOCIATION OF AREA 0F MM INTENSITY VI AND MAGNITUDE 0F ' INDUCED EARTHQUAKES I SEISMIC MOMENT AND AREA 0F MM INTENSITY VI SHAKING I The seismic moment (M o ) of an earthquake is a more consistent and more physical measure of source strength than its magnitude. Hanks determined the seismic moment for 47 larger Southern California earth-quakes by conventional seismological means and found a relationship between oM and the areal distribution in FDil VI (Ayi) (Hanks et al., 1975). This relationship is log Mo = 1.97 log Ay1 -2.55 Herr = ann made a similar study of some events in eastern North America. The Ayr versus M o data for eastern North America (solid i dots) are compared to the Hanks Southern California data (open symbols) in Figure 1 (Herrmann et al., 1978). There is, of course, considerable scatter, which is not unreasonable since a qualitative effect, inten-sity is being compared to a quantitative measure of earthquake size, seismic moment. However, it is seen that the eastern North America and Southern California data sets form two distinct populations. A similar separation is observed if felt area is plotted versus magnitude fer the two regions (Nutt11 and Zollweg, 1974). The attenuation characteris-c':s in the sottheast U.S. more closely resemble those of central U.S W earthquakes tnan those in Southern California. Two lines (nos. I and
- 2) were drawn through the solid dots to represent reasonable fits through the data.
INTENSITY OF INDUCED EARTHQUAKES IN THE PIEDMONT I Severy examined the seismicity at 59 reservoirs which were con-structed in the Piedmont Province between 1891 and 1974 (Severy et al., I 1975). Of these, 12 were noted to be associated with seismic activity. This list has been updated to include i Jocassee and tbnticello Reservior (Table 1 and Figure 2). Altaough the depths of the reser-voirs and their capacity varied by as much as a factor of 10, g reser-voir in the Piedmont was associated with an earthquake of FDil greater 4 VIII-1 i I
I E than VI. This record is complete and, although the years elapsed be-tween the impound =ent and the observation of eismicity vary widely, I the reported intensity is MMI VI. This observation suggest that MMI VI is the largest estimate of an induced earthquake in the Piedmont rrgion. MOMENT MAGNITUDE RELATIONSHIP I Hanks and Kanamori demonstrated that for earthquakes from many different areas the relationships between seismic moment and the I magnitudes M L , M3 and My were nearly coincident and of the form M
= 0.67 log Mo -10.7 which is uniformly valid for 3 f ML f 7.5, 5 f Ms f 7.5 and Mg > 7.5 (Hanks and Kanamori, 1979). This obser-vation implied that the relationship of M ato ML obtained empiri-cally by Thatcher and Hanks for Southern California earthquakes (3 f ML f 7) log Mo = 1.5 ML + 16.0 is valid for different regions also (Thatcher and Hanks, 1973).
Thus, we have a method of estimating the magnitude of an event if the areal extent of MMI VI is known. Log Mo is estimated from Figure 1 for any value of Ayr and using the moment magnitude relationship ML is estimated. AREAL EXTENT OF INTENSITY VI The largest event in the Piedmont, the Union County event of January 1, 1913 was associated with an MMI VII. The areal extent of MMI VI shaking is %660 km2 (or log Ayr = 13.82). This cor-responds to log Mo values of 23.65 and 23.25 from curves 1 and 2 (Figure 1), respectively, or magnitudes 5.1 and 4.8. For the Keowee event (7/12/71), considered by Talwani (Talwani et al., 1979) to be induced, USGS assigned it an intensity of VI based on the observation of a cracked chimney and movement of furniture at Newry. The intensity VI reports were from a small crea (< 5 km2), This event was reassigned as MMI IV by Sowers and Fogle based on a I VIII-2 5
l I personal survey and the observation that the chimneys were already in a state of disrepair (Sowers and Fogle, 1979). For the largest event at I Lake Jocassee, 1011 VI was observed at just one location. From the other cases of induced events the value of Ayt in uniformly small. The data gathered by Severy suggest that the largest induced earthquake in the Piedmont is >Dil VI, and from available data, Ayt is small (Severy et al., 1975). To obtain an upper bound, one can assume Ayr to be 100 km2 and apply the data of Herrmann et al. (1978) to the Piedmont. In this case the estimated values of log Mo are 21.1 and 20 for curves 1 and 2, respectively (Figure 1). These' correspond to ML = 3.4 and 2.6 respectively. These calculations sug-gest that from the empirical analysis based on all existing data, the s, largest induced earthquake is conservatively estimated to be I ML% 4.0. I I I I I I. I I I VIII-3 I
E I REFERENCES I Cof fman, J .L. , aand Von Hake, C. A. , (ed.), 1973. 1971. U.S. Earthquakes in U.S. Department of Commerce, Washington, DC., 176 p. Gupta, H.K., and Rastogi, B.K., 1976. Dams and Earghquakes. Elsevie r , Amsterdam, 229 pp. Gutenberg, r., and Richter, C.F. , 1944. Frequency of earthquakes in California. Bulletin of the Seismological Society of America, vol. 34, p. 185-188. Hanks, T.C., and Kan',.tmori, H., 1979. A moment magnitude scale. Journal of Geophysical Resea rch. vol. 84, p. 2348-2330. Hanks , T.C. , Hileman, J . A. , and Thatcher, W. , 1975. Seismic moments of larger earthquakes of the Sour.hern California region. Geological Society of America Bull., vol. 86, p.1131--1149. Herrmann, R.B., Cheng, S., and Nuttli, 0.W., 1978. Archeoseismology I applied to the New Madrid earthquakes of 1811 to 1812. gical Society of merica Bull., vol. 68, p. 1751-1759. Seismolo-I Mogi, K., 1967.
- p. 35-53.
Earthquakes and Fractures. Tectonophysics, vol. 5, Nuttli, 0.W. and Zollweg, J.E., 1974. The relation between felt area and magnitude for central United States earthquakes. Seismologi-cal Society of America Bull. , vol. 64, p. 73-85. Severy, N. I. , Bollinger, G. A. , and Bohannon, H.W. J r. , 1975. A seismic comparison of Lake Anna and other Piedmont reservoirs in the eastern United States. First International Symposium on Induced Seismicity, Banf f, Alberta, Canada, September 15-19, 1975. Sowers, G.F., and Fogle, G.H., 1979. Intensity survey of the Seneca, South Carolina, Earthquake, July 13, 1971. Earthquake Notes, I vol.50, p. 25-36. Talwa ni, P. , Stevenson, D. , Amich. D. , and Chiang, J. , 1979. An earth-I quake swarm at Lake Keowee, South Carolina, Seismological-Society of America Bull. , vol. 69, p. 825-841. I Thatcher, W., and Hanks. T., 1973. Source parameters of Southern California earthquakes. Journal of Geophysical Research, vol. 78,
- p. 8547-8576.
Yoshikawa, K., and Nishi, K., 1966. Seismic observations at volcano Sakurajima, 5. Disaster Prevention.Res. Inst. Asoc., vol. 9,
- p. 47-54.
VIII-4 I
M M M M M M M' M M 8 M M & M M N M M M TABLE 1 PIEDfl0flT PROVillCE RESERVOIRS WITil SEISillCITY RESERVOIR CAPACITY SEISillCITY YEARS tlAl4E/ DAll 6 ELAPSED REr4 ARKS llEIGitT(fl. ) (ll.3x10 ) DATE/li.M. DATE LOCAT10fl 30 132.0 3-5-14/Vi 4 (IllDUCED ?) 1910 LLOYD Sil0At.S JACKS 0ll, GA.
~
370.0 1919/VI 0 (IllDUCED ?) 1919 .BRIDGEWATER 50 11AR10fi, it.C. 21.9 83.4 7-8-26/VI 1 (Irl0UCED ?) 1925 Rl!0DillSS GRAtlITE FALLS, fl.C. 151.0 9-9-70/V - 42 1921 OXFORD 35.4 tilCKORY, fl.C. 63 2,600.0 7-26-45/V1 15 (FAULT PRESEllT) 1930 SALUDA IRl10, C.S. 334.0 12-73/ FELT 34 1940 BtlZZARD'S ROOST 25 GREE!!tl0OD, S.C. REPORTS Iti CO. 3,096.0 1960/ FELT 8 (FAULT PRESENT) 1952 CLARK lilLL 67 REPORTS AUGUSTA, GA.-S.C. 22 8-2-74/VI 407.0 3-12-64/V 11 1953 SillCLAIR 32 I4ILLEDGEVILLE, GA. IIARTilELL 73 3,145.0 10-20-68/V 7 1961 IIARTilELL, GA.-S.C.
M M M M M M M M M em M M M M M M M W 4 1 TABLE 1 (CONTIfiUED) ! l l RESERV0lR l flAftE/ DAll CAPACITY SEIS!!! CITY YEARS ! DATE LOCAT10tl llElGitT(fl. ) (f t.3x1]}6) DATE/fl.t1. ELAPSED REftARKS l 1 1963 StilTil 1-100ilTAlti 69 1,357.0 --/V -- ALTAVISTA, VA. 1969 KE011EE S3 1,179.0 7-31-71/V 2 tilDUCED SEllECA, S.C. 1973-74 J0CASSEE, S.C. 133 1,490.0 ll/75-PRESEllT 2 IllDUCED 8/25/79/VI 1977 fl0flTICELLO RESERVOIR, S.C. 35 493.6 12/77-PRESEllT 0 ItIDUCED 11L < 3.0 1 i i i l 1
I ; I I I I I co o " I . I - e 0 ig I g O E cn I
$C D
O m 8 - 0 0 ! l - O O O - g O oo E
. o 0
I -
.o -24 -
O E m - c I a [ I >. -
- m g m I
8 e Z W it - 8
=
I O N g I l l l N D # 7 v- r-l (g ua) '^ y % oi I I
E
,_ MICH g PENN. /
INO. osso 40* ! '
^
i I I N.J. 7r, Y l A s ,,. wts.) / ., "W "^^/g'"~ E
/ ,_
K Y.
} y% /
sj A x=A
> 1 OC SEE A TENN. /
/ ![ A A N+
4 ^ A Y) { 34, ll /
/
A
~
A,
, 4 7-i M WJONTICELLO g j MISS. g g# CJ ny r 1 ALA. GA. -
32* I {
% FLA.
3co D ^
" N \
I 88* 86* 84* 82* 80* 78* 76 f r MILES
, n , . - ,ER, y
L EG END: A RESERVOIRS h RESERVOIRS WITH SEISMICITY I - nye 2
I i i APPENDIX IX PROBABILITY ANALYSIS FOR MONTICELLO RESERVOIR - INDUCED SEISMICITY
- I
.I l l I I I !I I I I I I
PROBASILITY ANALYSIS FOR MONTICELLO RESERVOIR - INDUCED SEISMICITY I INTRODUCTION I In order to evaluate the hazard associated with ground motion due to induced seismicity, it is appropriate to conduct several probability analyses using available data. These are reported here in two sec-tions. The first addresses magnitude - frequency relationships; the second reports probabilities associated with various levels of ground acceleration at the site associated with induced earthquakes. MAGNITUDE - FREQUENCY RELATIONSHIP The magnitude-frequency distribution of earthquakes are conven-tionally represented by the Gutenberg and Richter (1944) relationship log N = a-bM (1) where N is the number of earthquakes having magnitude greater than or equal to M, recorded during time interval T and M is the magnitude (such as ML , are Ms or mbLg). The fit of the equation to a plot of log N versas M is a straight line and yields a value for the slope, b. The b-value varies with source region, focal depth, type of earthquake process, stress level and in some cases with time. For tectonic earthquakes (as opposed to reservoir induced earthquakes), the Gupta and Rastogi (1976) I b-value is generally between 0.3 and 1.5. have noted that the b-values of reservoir induced earthquakes are usually greater than those of tectonic earthquakes in the same region. In trying to use the b-va.ues 1 to obtain recurrence rates (see, for example FSAR Question 361.18, Amendment 21), it is important to realize I the assumptions involved. The fundamental assumption is that the fre-quency distribution is exponential and that the same tectonic forces will continue (a steady-state condition). The nature of induced seismicity is non-steady state and thus extrapolations of magnitude-frequency relations have to be interpreted carefully. However, the apparent b- /alues can be instructive in understanding the nature of the I IX-1 I
I observed induced seismicity and some of the controlling factors. With this in mind, we have examined data for Monticello Reservoir. To obtain the magnitude-f requency relation for Monticello Reser-I voir, events recorded at JSC (a permanent seismographic station of the South Carolina network) were used. These events are cataloged according to their duration (D) in seconds, which is related to the local magnitude ML by the relation ML = 1.83 + 2.04 log 10 D (2) It should be noted that the ground motion estimates used in this I study are consistent with the ML values described below, as evidenced by comparison of the etimates with observed peak acelerations recorded during the largest (ML = 2.8) induced earthquake. Therefore, argu-I ments about whether the duration-defined ML scale is consistent with other ML scales for the southeast U.S. are immaterial for the present study. The N(D) versus D data for the period December 1977-December 1979 are shown in Figure 1. (The corresponding magnitude scale is also shown.) These data are based on the duration of the events. The data can be defined by the relation log N(D) = a*-b* log 10 D (3) where b* = 2.04b or b = 0.5b* The b-value estimate is thus about 1/2 the b*-value obtained by plot-The duration accuracy as measured by the I ting log N (D) versus D. reading error (2-3 seconds) divided by the duration is smaller for the larger events. Thus, the error in duration for the largest events that have occurred is less than 2 percent (or .01 in ML units). The long-term variation in cuculative numbers may be estimated by I assuming that events above each magnitude level occur as a Poisson process. In this case the variance of the process is equal to the estimated long-term rate; alternately, the variance of the estimate of I IX-2 I
I the long-term rate is equal to the estimated rate. Thus, the standard deviation of the estimated rate is equal to the square root of the number of events used in the estimate. Error bars corresponding to plus and minus one standard deviation in the estimate are shown cn Figure 1. The observation of non-linear frequency-magnitude behavior such as shown in Figure 1 has been noted by other workers. Mogi (1967) has studied the frequency of shocks in laboratory experiments. In these experiments it is assumed that earthquakes are caused by brittle fracturing of the stressed Earth's crust, and that the fracturing can be simulated in the laboratory. He noted that if there is a regular network of cracks distributed in a region, "the log n (number) - log a (trace amplitude) relation is expressed by two straight lines broken I downwards at a discontinuous point." Mogi's data for such a case resembles those shown in Figure 1. Mogi also noted some examples of non-linear relation between log n and log a for earthquakes. One example quoted by him was an observation by Yoshikawa and Nishi (1966) who observed such non-linear relation between log n and log a for shallow volcanic earthquakes. This observation was attributed to the fact that the maximum seismic volume was limited to a narrow region underlying the volcanic crater. There are several possible matbematical interpretations of the data shown in Figure 1. First, it can be assumed that the underlying ! log-number ve rsus magnitude relation is log-linear and untruncated. Thus, departures of data from linearity are ascribed to statistical If this is assumed for the data of Figure 1, and values I variations.
- below ML = 0.87 are excluded, a b-value of 1.34 is obtained.
1 Alternately, a bi-linear distribution can be assumed. In this
" case, it is appropriate to fit two straight lines as shown in Figure 2, with b* values of 2.24 and 4.0 (or b-values of 1.12 and 2.0). The l
lower b-value is for events with a duration <120 see (Mt < 2.3); the higher b-value is for events with 2.3 < Mt < 2.8. l I g IX-3
I I A third mathematical interpretation is that the Gutenberg-Richter relationship described above is appropriate for the number of events equal to some magnitude level (a log-linear frequency interpretation), rather than greater than some cagnitude level (a log-linear c.mulative interpretation). In any seismic environment with an upper-bound magni-tude the frequency intepretation is more logical physically: the traditional log-linear plot based on the cumulative number of events implies a log-linear frequency plot with a delta function at the I upperbound magnitude, which is physically unrealistic. Using the frequency interpretation implies a complementary cumulative probability G(m) of I G(m) = 1 - k + k exp ( -b in10 (M-M o )) mo I a f m1 where so is a convenient lower bound, mi is the upper bound, and k = (1 - exp (-b in10 (mi - co )))d Fitting the data of Figure 1 to this form, with mo = 0.87 indicates that the data imply an upper bound of about 3.0. This is shown in Figure 3. With this interpretation, magnitudes greater than 3.0 are not possible. A fourth interpretation (Merz and Corna11,1973) is that the log number versus magnitude relation is of a quadratic rather than a linear form: log 10N=a-bi (m - mo ) - b2 (m2_2 02) mo < m An upper-bound can also be inposed on this interpretation, although this is not done here. The fit to the data is shown in Figure 4. The recurrence intervals for various magnitudes can be cd culated for reservoir-induced seismicity using each of the interpretations discussed above. These results for ML = 4.0 are shown below: I Recurrence Interval Model for Mr. = 4.0 Log-linear, untruncated 14 years Log-bilinear, untruncated 130 years LX-4 I
Log-linear, truncated = Log-quadratic, untruncated 1600 years All of these interpretations except the log-linear, untruncated model indicated that an earthquake with ML = 4.0 in the vicinity of I the Monticello Reservoir is a rare event. I As further evidence that the log-linear untruncated model is in-appropriate to describe reservoir-induced seismicity, all the lines of evidence presented in this report (local geology, potential field anomalies, seismicity patterns, in situ stress and fracture density, focal mechanisms, hydrologic observations, etc.) lead to the observa-tion that there is a great heterogeneity in physical properties and stress conditions on a spatial scale of about 1 km or less. This litaits the maximum extent of any single f ault movement to approximately I km and implies a limiting magnitude for induced earthquakes in this specific tectonic environment. This limiting magitude is estimated to be ML = 4.0 or less. GROUND MOTION PROBABILITY ANALYSIS FOR INDUCED SEISMICITY To determine an upper-bound on probabilities of exceedance as-sociated with various levels of ground acceleration and spectral ampli-tudes, the reservoir induced seismicity was modeled in the following manner. The area of influence of Monticello Reservoir was taken to be 9 km from its center, this being approximately the largest dimension of the reservoir. The area so described includes the Virgil C. umme r I site. The energy associated with seismic events was assumed to come from depths of 0.5 and 1.5 km, with equal probability. The static stress drops of earthquakes at those shallow depths can be represented as 25 bars for the purpose of estimating ground motion, as has been shown elsewhere by comparison to the ML = 2.8, August 27, 1978 earth-quake record. Using the Brune model of seismic sources, which is basic in the method used to estimate peak accelerations, the seismic corner fre-quency can be determined theoretically. Using a stress drop of 25 I IX-5 I
bars, the following corner frequencies f oand durations Td are associated with various magnitudes (the relation between magnitude and seismic moment Mo being log Mo= 1.5 ML + 16): ML Mn r, km fo,hz Td , sec. 0 1019 .056 21.3 .05 2.5 5.6 x 1019 .099 12.0 .08 3.0 3.2 x 1020 .178 6.7 .15 3.5 1.8 x 1021 .316 3.8 .26 4.0 1022 .560 2.1 .48 On this basis it can be seen that magnitudes below 2.5 will not generate substantial energy, nor spectral amplitudes of ground motion, at frequencies above 12 hz. It was argued in an earlier submission (FSAR Amendment 21, Question 361.18 response) and it is argued f rom other points of view in this Appendix that the use of "b" values (from observed frequency of occurrence vs magnitude) to infer probable rates of occurrence for larger events is not valid. Instead a maximum cut-off magnitude is likely in this setting; considering the spatial scale of geological, geophysical, and stress tensor variations at the site and experience at other reservoirs in this tectonic province, this cut-off is estimated to be approximately ML = 4.0. Despite this fundamentally different set of physical condition. as l compared with those for regional tectonic activity, we have made a calculation assuming the usual log-linear, untruncated model of earth-quake recurrence. 1 For the probability analysis, a b-value or 1.34 was used; this has l been documented by microseismic network results, as described above. A maximum magnitude of 4.5 was assumed, and an exponential distribution was used to describe the relative frequency of different magnitudes up to that limit. With these standard assumptions, the observed occurrence of 35 events per year with ML > 2.0 translates into 7.5 events per year with ML > 2.5. lI I LX-6 1
It should be emphasized that the events being discussed here are all short duration, impulsive motions which generate high f requencies. An example is presented in the Figures 5 and 6, the 180* component of the ML = 2.8 event recorded August 27, 1978, and its calculated response spectra, respectively. The motion is extremely short in duration and contains only high frequencies; it is substantially I different from typical response spectra used for design, as Figure 6 shows. The probability analysis yields the following probabilities associated with various acceleration levels at the Earth's surface: Digitized Peak 0.10 0.15 0.20 0.25 0.30 Acceleration, g Annual Probability 0.18 .075 .037 .020 .012 Return Period, Years 6 13 27 50 83 Although the probabilities indicated are high, it does not follow that the motions corresponding to these accelerations will be damaging to the facility. The duration of these events is extremely short, less than 0.5 seconds as previously shown. The ability of these motions to induce damage in the building and equipment is much less than for a MM1 VII event in the far-field with the same peak acceleration, for which l the facility was designed. The latter motion may last for 5 to 10 seconds, with 10 to 20 times the damage potential as events induced by the reservoir. To illustrate, the August 27, 1978 event ground motion, as recorded (Figure 5), consisted of a single spike of acceleration with a frequency of 25 hz, followed by relatively lower level motion. Such high frequencies may produce high spectral amplitudes of response l l I but do not contain enough energy to induce damage. 1 It is important to note that the exact mathematical representation of the recurrence relationship is immaterial because none of the induced events, have any effect on structures or equipment at the l l I IX-7 I
a s _,, __ _,, _A _ 1, _ e- -se J -- - -e ---_m- m-a. -,_a ,,_ , m - .s a_-, _. m ,_, _m am a . -2 m aa I !
- I v1<s11 c. e.mme,s c1..,e<.<1. 1, <mex . e 11 1<.e c. xs < .o. ..
concluded from al1 the site-specific evidence in this report. l i lI l i I '1 1 l l l l I I I I I IX-8 I
l ( t REFE RENCES l Gupta, it.K. , and Ras togi, B.K. , 1976. Dams and Earghquakes. Elsevie r , Amsterdam, 229 pp. t I i Gutenberg, B., and Richter, C.F., 1944. Frequency of earthquakes in California. Bulletin of the Seismological Society of America, I vol. 34, p. 185-188. Me rz , H. , and Co rnell, C. A. , 1973, Seismic rate analysis based on a quadratic magnitude frequency law. Seismological Society of America, Bull. , vol. 63, no. 6, p. 1999-2006. l Mogi, K., 1967. Earthquakes and Fractures. Tectonophysics, vol. 5, a p. 35-53. Yoshikawa, K., and Nishi, K., 1966. Seismic observations at volcano Sakurajima, 5. Disaster Prevention Res. Inst. Asoc. , vol . 9,
- p. 47-54.
I l I I I LX-9
I I M L 0 t 2 3 4 io4 I f M l l 1 I I I I I I o
. DEC 77 - DEC 79 MONTICELLO RESERV0IR 103- _go3 E
I f
- OBSERVED RATE 102 - 1 t ONE cr _io2 I = 8 o
10- [ _;o 3 1- -l l I 0.i , , , . o, i O to 10 0 3000 30,000 i DURATION D(SEC) I I Figure 1. Cumulative number of events versus duration and magnitude. I
I I Mg o i 2 3 4 5 104 io4 l l l l l l l l I I I
- I "
DEC 77 - DEC 79 MONTICELLO RESERVOIR 103 - I b : 1.1 I 102 - I - e z W
,. -'o g 8=20 g
. I I ., s ,
,1,
\ ,&
DURATION D(SEC) I Figure 2. Log-bilinear, untruncated model . I I .
I I Mg I t04 O i i I i i 2 i I 3 I I 4 i I 5 I is l DEC 77 - DEC 79 MONTICELLO RESERV0IR l 103 - -10 3 b = 1.1 I -102 IO2 - I - e z I - 10 - -10 I I -1 I O.I , 0.1
; i O 10 10 0 1000 10,000 DURATION D(SEC)
I Figure 3. Log-linear, truncated model . I I
i I M 0 1 2 3 4 5 104 104 I I I I I I I I I I I I o
..o = 0. 87 DEC 77 - DEC 79 MONTICELLO RESERV0IR 103- -103 log N : 3.1 + .49 (m -mo) 2
.50 (m2-mo) 102 - -102 a_
I Z W IO- -lO I I I- -1 I O.1 - , , O.1 O 10 10 0 1000 10,000 DURATION D(SEC) I I Figure 4 Log-quadratic, untruncated model . I
l M M M M M M M M M M M M 4 5 4 1 l i i l
! d I 8/27/78 MONTICELLO DAM RECORD.180 COMPONENT. RAW DATA, SO S 100 PTS /SEC i
12 SEP 79 NEY:
- RAW DATA (.02 SECONDS CORRECTED) ; m
- 8 SE POINTS PER SECONO DIGITIZATION
! s
$ d o' ---- 100 POINTS PER SECOND DIGITIZATION
- c. cm $
8
. . s i
2 mEEe vi 78
~
4 8 e ' _.8 h . A w NW i Ed i.4 - , If . 3 ! E*9* N , j B
- s
.)
j I $
- N
" 8 -
i IQ i d I 8 8 8 8 8 8 8 ' i
- O.00 0.10 0.20 030 Q40 0.50 0.60 0.70 0.80 0.90 1.00 TIME-SECONDS
} 3
I e o . . . . . . . . . . . . . . . . . . ..... . i 3 : : l g . - I I 6 r m : I e_
^ [i G -
;.gdi -
3 '}!'~g; h, i
$ !(; -b,\' C.24 *45 a %.4 s t
[3.D 'S ] t ee et* 6i),
.~.l - -
I
;l,t' ll .
, E - , , , , , , , , if $? , , ,,,,, , , , , . E o.I i. f.i.pp' 80. 3oo . yhNEOUENCY I I NRC SPECTRA FOR PEAK ACCELERATION OF 10 PERCENT GRAVITY l ACCELERATION = 0.10 DAMPING = 0.52.05.07.010.0
- - - MONTICELLO DAM. AUGUST 27, 1978 COMPONENT 180.100P0lNTS PER SECOND DAMPING = 0.5 2.0 5.0 7.010.0 I Computed Response Spectra for 0.10g - 1800 Component i Response Spectra at 100 pts /sec Compared to OBE Figure 6
.uh-. _. me -ae __J _ A____-a_ --maa==---..a_mma_- 4As__A_h .;.--a__-=_C_a .a2 , aa sA -a- .e. _ _ _ , .w-_ , ___ -_ -_._ _- _-------- -
g APPENDIX X 7
'l errects or usia-eteto tiariiquaxt caouso nortos
[ es 1mc1e m - zoetPxes, ee tcs ( ll E l I i
- I l
18 I
- I 1g I
1 I I
'l
I I. EFFECTS OF NEAR-FIELD EARTHQUAKE GROUND MOTION ON STRUCTURE AND EQUIPMENT DESIGN The extensive body of site-specific observational evidence pre-
.. sented elsewhere in this report shows that in this highly heterogeneous environment there is a limit of approximately one kilometer to the maximum f ault dimension for any single earthquake occurrence. This corresponds to a limiting magnitude of ML = 4.0. The calculations of ground motion presented in this Appendix were carried out both for this I limiting magnitude event and for a larger assumed magnitude of ML=
4.5 which provides a conservative assessment of ground motion effects on the Virgil C. Summer Nuclear Station. The ML = 4.0, stress drop of 25 bars, and R = 2.0 km case gives a zero period acceleration (ZPA) value of 0.14 g, which is less than
= the Virgil C. Summer SSE ZPA value of 0.15g for structures on rock.
Therefore, this maximum induced seismic event has no effect on struc-I tures or equipment. I The ML = 4.5, stress drop of 15 bars, and R = 2.0 km case gives a ZPA value of 0.22 g, which is hj,her than the Virgil C. Summer SSE
- ZPA values of 0.15 g for structures on rock and is lower than the SSE ZPA value of 0.25 g for structures on soil. In the original seismic analysis, a very conservative 2 percent damping value was used. NRC l
Regulatory Guide 1.61 allows a 5 pe.rcent damping value for prestressed ( concrete and 7 percent damping value for reinforced concrete structures in the SSE analysis. Thus, the Virgil C. Summer (0.15 g) SSE spectrum at 2 percent damping is compared with the Mt = 4.5 (mean value plus one standard deviation) event spectra at 5 percent and 7 percent damping in Figure 1. As shown in the comparison, the ML = 4.5 event Summer (0.15 g) SSE spectrum only in the i exceeds the Virgil C. frequency region higher than about 9 Hertz. The dominant frequencies . of all Seismic Category I structures are lower than 9 Hertz except for the interior concrete structures of the Reactor Building. However, since the original seismic analysis used the artificial time history l ( ATH) as input and the the ATH's response spectrum always exceeds or I X-1 I .
I I equals the Virgil C. Summer SSE spectrum, additional conservatism of the ATH method can be used to justify the original seismic design of the interior concrete structures and the equipment contained therein. To remove the conservatism of the ATH method, the Oroville I accelerograms were used in statistical studies. Four horizontal components of two ML = 4.0 aftershocks of the 1975 Oroville, California earthquake, recorded at rock sites, were modified to match the 5 percent damping target spectrum in the mean as shown in Figures 2 to 5. Four Oroville aftershocks were extended into 36 components by adjusting the time increment, which achieves the effect of shifting the frequency content of the accelerogram (Tsai, 1969). The original Oroville af tershock accelerograms have time increments of 0.005 second. Each component was extended into 9 components by using time increments I at 0.0038, 0.0041, 0.0044, 0.0047, 0.005, 0.0053, 0.0056, 0.0059, and 0.0062 second. The 36 accelerograms were used as input to the Reactor I Building seismic analysis. The rean values of the 2 percent floor response spectris were compared with the original Virgil C. Summer floor response spectra used in design (Figures 6 to 20). As shown in the I comparison, the Virgil C. Summer spectra envelop the ML = 4.5 mean l value spectra in the resonance peak region and almost all other frequency regions. Thus, it is concluded that the original Virgil C. Summer seismic design is not exceeded by this ML = 4.5 event. j Some NSSS equipment are designed to floor response spectra generated at 5 percent structural damping, for loading combinations containing SSE and LOCA. This set of Virgil C. Summer floor response spectra is also compared with the ML = 4.5 mean value floor response spectra (Figures 21 to 35). As shown in the comparison, the original Virgil C. Summer seismic design is not exceeded by the ML = 4.5 event i and continues to be valid and adequate. 1 1 8 lI I X-2 I
B i REFERENCES I Tsai, N.C., 1969. Transformation of Time Axes of Accelerograms. Proceedings, Engineering Mechanics Division, vol. 95, no. EM3. ASCE I i 1 I E I i I , I l t I I i X-3 I _ _ _ _ _ _ _ _ -_ -_
.I _
I 10.00 E \ \ I _
- Y
- #<g
#g g R s 27-b1.00
~
t
%b' s
* - N' #.j I 0 7%
a - [ N o g. 0.1C N 4
~ T
.4
+#
/ x 0.01 //m ! Ne ee! ' e i i eee e e e eee 0.01 0.10 1.00 10.00 UNDAMPED NACURAI. PERIOD (SECONCS) 25 SSE Spectrum vs. 5% and 7% M + c for Mg = 4.5 R = 2 km l
I I I n eere i I .-
I E I c 3 4 4 & a 4 r .' I . E r . I " F P I'
$ - A u
a Y-E d,
.1 f.
I - I a d- ,, , , , , , ' ' ' - - C. 01 a. ; 1- 10. E P[gfgg CRCVILLE SCCE RFTER 1 ITERATICN - 4. 5 MAG SPEC DRMPENG = 5. O SC.9LES = 1. oc 1. co E Figure 2 10 NCV 80 1
I I I d I r. I : I : r I. .
~
m 1 t . A s . D lf ~ %) KN C . C I O "7c r / h 1 l g C 3 l 5 r'. - i I : :
\
-I E I d '.01 C
'.1 C
PERICD 1 10. CRCV!LLE NS5E RFTER 1 !TERRT!CN - 4. 5 MAG SPEC DRMP!NG = 5. O I SCALES 1.00 1. CC
=
I Figure 3 I 10 NCV SC I
i I I ei
~ , .. .. . . .. .. . i . .
.q r- 1 i
i [- l
.}
!I ", , r r -
,I t :
m y . T _ /N .
- I TV
~
s, . N - .I 9,o \-~ r ' 1 r 1 !l .
)
N .I
~; , ,l C. 01 C.1 1. 10-l PERfCQ CRCV!LLE N9CH RFTER 1 liERRT!CN - 4. 5 HRG SPEC
}I 'DRMP!NG = 5. O SCALES = 1.00 1.00 1
- I Figure 4 I 10 NOV 80 I
4 l i
~
lI I d E 'F ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' '
'i
- g [ 1 1
1
- I 1 i
J
;I t-- .
I n c. f - t
.' s n I
i b u - [W - C
" O 1 >l u d,
i
;E r -,
r
- u r ]
.i r
j u - I
, ~ .
j C d' + r > , . , , , , , , , , , , , , , , C C1 C. 1 1. 1 C. }
' PERICD CRCV!LLE S55E AFTER 1 ! TERAT!CN - 4. 5 MRG SPEC e DRMPING = 5. 0 SCRl.ES = 1. CC 1. CC i
i I Figure 5 t, I
am uma em man mum man ums sum amo en amm em num man amm uma ama em one L c' .. W c=; I I III as NaDE 1 RfnCTOR BLOG. Sht:L L l l l ll I l I l I l es, , E. L F.V A T I O N 58 2 f- T 11 IN l l l ll l l l
. _8 I^ j _ . _. _ _ _ _ __ _ .. . _ L _ _ _ __ _ ._ _ _. .__._ l_. _
_ ._ }l Ms l I III I I I I I III I I I EE ,, I I III I I I I I J 4 - W " >#$ 1 I I p2; _ . . ._I__ .__ _ _. ._ _ l _ _ _ . _ _ . . _ _ l . _I_ _I _ _ I_1._ L _ _ _ _ _ l_ .__ _ } _.
, I I III I I I I I I III I I I :
- n. , I I III I I I I I III I I l
- ; .. _ _ . l _ I _. .. . _ _ _ _. ._1._
_ ._ y _. L_ _ Ij_y_j._:q_.L_____________ 1 I III I I I I I I III I I I i:{ , I I III I I I l l l l l l l l l l M. _ . I _ L. I _ L. L __ .. . _ ._ _ L ._ _ l _ _ I_ L _t _. l_ L L L _ _ _. _ _ L ._ __ _L_ _ L _ l I III I I I I I III I I I I I III I I I 4 I I I !l $="$2ar,, I I l . I I III I I I I III I I l _. . L _L . L L L _ - . . _ _ L _ _ _ _L _ _ . _ __. L _L _ L L L L _ _ _ _ _ . _ L _ __ _L_ _ L _
, / I I I -
0 / .i q 1 ti a 4 a9 a y 3 S [ ti 7 3 5 ,. I til.ljllf.Ilf. T COMPARISON OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 6
ma t.c.-:7+ Ow '" l I III I I I M nonE a nEnCl0B BLUG. S H E Ll_ l I III I I I g ,, t.ti vn T I ON Sil6 F T 0 IN l l l ll l l 1 Eg; in ETAe= 2% _ _[ _ l_ { { l_ _ _ _ _ _ [_ _ l _ _ l_ _ E I I III I I I I I I III I I l E*b I I III I I I I I I III I I I
>=4 ,, I llI 1. I I ! I I III vcs,,,= 2 s ' l I l-f __ _.l_I I_ _ _
. _ _ _ I _ _ _ _L._ _ I_ ,_L.. _
l _. _ _ . L ._ _ I _ _ l l III ! I I I I III I I I Et i I III I I I I i l l II I I I
?". _ l _ I _ L L L _ __ _ _ _ L _ _ _ L _ _ I ___ L _1 _ L L L L _ __ _ .__ _ L _ _ l _ _ L _
F I I III -l l I I I I III I I I I I III I I I I I III I I I I III I I I I I III I I l (l;;. y __l l.__..l_.l_]_.__.___.___l._____.]._._.______L__._1.__I__ l l lll l __l._I__LLL_.._____L__._._1.-__[._\. l l ' i
.__l._.l_
& M '"
, l L L L____ _ _ _ L _.__l _ _ L _
l I I III I ! I I II I I I l l l 6 l lA1
/s 9 10" a 1 l
il S l ,f!L l1l 6 7 ao 10' 2 i it
- t F Bf.uuENC Y COMPARIS0N OF V C SUHHER SSE FLOOR RESPONSE SPECTRUM AND H = 4.5 FLOOR RESPONSE SPECTRUM Figure 7
mum um aus sum uma sum uma mas som nas amm som em nas uma em amm mem aum
- ff, . . . . . _ _ _ .. . __ _ . .. . .._ _ . . _ _ _ . . _ . . _ . _ _ _ _ _ _ _ _ _ _ . . . _ - . _ _ _ . . _ _ . . . . _ - _ _
W C=s; nooE 3 nEnc Ion aum. SnEl t l I III I I I El.EvnTION 523 FT 0 IN l l l ll l l l Ba u BETAe= 2 % __{ _ l _ [_ { l _ _ __ _ _ __ _ { _ _ ] _ _ _ l _ _ m i I III I I I I I I III I I I , Fn i I III I I I I I I III I I I i MG ,, I I III I I I I I I III I I I g _.___ _ _ _ _ _ _ _ _ _ _ 1. _ _ l _ _ L _ _ . _ I _ . l _ _ L _ _ _ _ _ L _ _ _ _ _ _ I _ _ , h, G I I III I I I I I I III vcs,as=2s l I I m I I III I I l I \ I I l [2"1 _ _ L ._I_ L L L __ _ _ _ _ _ L _ _ J. _ _ L _I_ L L L _ _ _ _ _ L _ _ J._ _ L _.
. ._L _ {AM F
l I III I I I I l l l II I I I "2 I I III I I I I I I III I I I l l III I I I I I I III I I I (t.t"
*" _ J _L_ L L L _. _ _ _ _ L __ _ J. -_ _. _ _ L _L J_ L. L L _ _ _ _ _ L _ _ J. _ _ L _
l I III I I I I I I III * = C5 I I I I III I I I I I I III I I I
K ,
l I l I III III I I I I I I I I bl-M I III
"'# $=" '" l I
I I I I . [j; _._J._ _.l_ _ _ _ _ _ _ l_ _ ._ _ __ _ I _ _ l _. _ _1_.l_L_ _ _ _ _ __I___ _ _ _ _l_ _ l l l ll l l l l l l 'l l l l l c, I I III I I I I I I II 1 I I
._ I _ I_ L L L _. _. _ _ _ L __ _ J. _ _ L _ L J L-L L R=j-l,a7.,,L.___L__L_
f\I l I III I I I el ilI I I I l'l Ill / N g I I I I I I ly'J-lj -I l I l\lsl_5P I I l I r; s 'i S /n 9 10" 2 1 in /a 3 10' 2 1 is s
- InEQUENCY COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND H = 4.5 FLOOR RESPONSE SPECTRUM Figure 9
em amm uma sum man ams mum uma men mum man sum em amm amm num sum uma mum
' gp III ! !
pj N000. 5 BEHCIOR BLOG. SilELL I I I LLLvnT10h 401 FT. I I IIl l l l M BETAe:2% _ _ _ _ _L _ L L L L _ _ _ _ _ L __ _ _1. _ _ L _
@T W
Mf5 l I I I III III I I I I I I I I I I I I III III I I I I I I ii I III I I I I I I III I I I t@$.S. _ _lL _L_ L L L _ ._ _ _ _ L _ _ _L_ _ L _ L _L _I_ L L L _ _ _ _I _ L _ y=e"' I I III I I I I I I III I I I I III I I I I I I III I I I 5 I I III I I I I I I III I I I _ I_ L _ _ _ _ _ L _ _ _ _ _ _ _ _ __ _l. __I _1_1_ L __. .__ _ __ _ L _. _ _L _ _ l_ _ _ .]. _l d l l l l' - l l l g _ _lj. _llql l ll_ _ _ _ _ _ lg _ _+_l -___ _ _ -_l4_g _ + _ g I I III I I I I I III uv+s l I I I I III ! I I I I I I #s = 5% a7% I I I M.
. _ _L _I_ L L L _ _ _ _ _ L __ _ _L _ _ L _ L_1 _LL__ __ L _ _ _L _ _ L _
~
l l IIl l l l l l l ll l l l l l l ll U ) ' I I III I I c
, I I III f l #
I/ I I i-*;' l 'l
s 6 /ao jg 2 1 4 5 6 7 8 9 1 O' 2 'l 4 5 FBEQUENEY COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 10
imme una amm aus sus num num amm um ums - num man sum see - amm mum um N I I III I I I . Noot. o itEncion llLilG. SilELL I I III I I I , @@ $ LLLVHIIUN tica l-I. l l l ll l l l fj __.DE{AL: {/. _-_ _ - - _ _ _ - _ l_l_{__ _ _ _ ,- _ l --_-_{__ m i I III I I I I I I III I I I M I III I I I I I I III I I I
- N$ l ._ _IL_ L L L L ._ _ _ ._ ._1 ' _.._ L _ L _1 _I_ L Ll _ _ _ _ _I L _ _ lI _._ L__
@= e"' I I III I I I I I I III I
- W' I IIII I I I I I I III I I I
( l i III I I I I I I F2"'. _ l ._L L L L _ ..... _ _ _ L _ _ _l. _. _.. L _ L _l. _I_ L L L _ _ _. _ _ L _ _ l _ _ L _ III I I I lP I I III I I I I I I III I I I io l I IIi i I i l l I III I I I 4, I I III I I I I I I III vcs.a s=as l I I
- y. ._ _I _ l_ l_ I _ _ _ _ _ _ l_ _ _ _ __ _ I _ _ _1. _ _ _ _ _ _ I _ __ _ _ . _ _ I_ _
I I l l l III I I I I I
\,4III l l l
. c, I I III I I I I l Ip_l_%==by,a7%_
; .__1._ _l_l.__ _ _ ___l__ __. __ ___1__
.___l.____l__
l l III I I I I III I I I a I I I I ly-i A kl s l% I it s
's 6 7 a 9 10 2 1 il s 6 7 ri o t o' a 3 f- RE QUE NC Y COMPARISON OF V C SUHHER SSE FLOOR RESPONSE SPECTRUM AND H = 4.5 FLOOR RESPONSE SPECTRUM Figure 11
- sum amm uma seu sus em uma man som e num e e nem nas som em som num q
19 -
--3 'd
~
JJ <a l l l ll l l l h59 NODE ~/ REREFOR BLDG. SilE U. l l l ll l l l g[g , Eu.vnTION ilm; FT. I I Il l l l l
';'. __ P l^1=_'_._ _ _ _ _ _ _ _ __
_ L - _ _ _ , _ _ l _ _ _ _l. _ _ l _ _ _ _ _ ._1. _ ] _ 2E I I III I I l l I I III I I I F I I III I I I I I I III I I I _ _1. _ l_ l_ _ L __ __ _ _ _ _ L _ ___l___j_______l____1.__j__ _ . _ _ l_ _
, I I III I I I I I I III I I I Eic, I I III I I I I I I III I I I i2"1 _ _l. _L. L L L _ _ _ _ _ L _ _ _l. _. _ L _ L_ _L _I_ L L L _ _ _ _ _ L _ _. _l. _ _ L _ -
L'C' I I III I I I I I I III I I I
"> I I III I I I I I I III I I I I III I I I I I I III I I I
$g.__I _ _ I_ _ . _ ..l _ . l _ l _ L _ _ _ _ _ L _ _ _l _ _ I _ _
_]_l_]_l______L__ l l l l1 I I I I I I III ves,o ,=ay. I I I I I III I I I I I I I I I I
] _]_ l _ l_ I__ _ _ _ _ _ l_ ._ _ _l. __ _ l _ L _ _ q _ _ l_ _
, l l l l l/ l - d 7 _l_ _. ..l;. _l2LiwMwl
~ ~ - - ~~ -
s o 7 a a g (f a 3 is s 6 7 aa 1 o' , a in 5 fREQUENEY COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE j SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 12
mum en su nas use amm man mas uma ese su num amm ese ese som nos ions um m @@ n gg , . __ _ _ _ - . . gg l I l ll l l l NUDE 9 REflCIUB OLOG. INTER 10B l l l ll l l l' @ a) E l l. V fl I l u N 11 7 5 f I . M fd DETAe -2% l l l ll l l l _ _ _ _ _ _ _ _ _ _ _[ l__l_ { { __ _l ___[__{_ 2E2 I I III I I I I I I III I I I I I III I l ~l I ' $a I I I I I I I l l III I I I I I I III M # ' "* ?" I u J'. _ _L_ L L L L _ _ _ _ _ L _ _ _l _ _ L _.L _L _ L L L L _ __L___L__L_ e"' I I III I I I I I I III - 1 I I I I III I I I I I I III I I I 4, I I III I I I I I I III I I I
;2*.
__ L_L L L L _ _ _ _ _ L _ _ _t _ _ L _ L _L_ L L Li_ _ _ _ _ _ L _ __1 _ _ L _ E I I III I I I I I I III I I I "j l l l ll l l l l l l l II I I I os i I III I I I I I I III I I I S. _ _ L _ L L L L_ _ _ _ _ _ L _ _ ._ L _ _ L _ L _1_L L L I _ _ _ _ _ . _ L _ _ _L _ _ L _ l I III I I I I I I II
- I I I I I III I I I I I I I 7 2: Nary. I o
i l III I i l I I I l I)l If l I I _ _ L _L L L L _ _ _ _ _ _ L _ _ _ L _ ._ L _ L _ L _ L l f_ L _ _ _ . _ L _ _ _L _ _ L _ I I III I I I I I I -1 ml4lJ1,V IlllI I ll'-
'd 6 l l 7 a 9 1(f i~
J 1
~
81 I) 6 7 a 9 10' 2 l 4 3 'l 'd I-REQUENEY COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 13
aus me e sus aus lie m um m em um aus M M M W EMI ums
,? '-i ;=s in EP l l l ll l l l E N00 f. 10 llERC[OB BLUG. INIEBlOR I I III I I l y, ELE vn T I ON 8162 I I . l l l ll l l l n<,
g ;5 i BETA e = 2% _ _ _ _ _ _j_ _ l _ l__,[ _ { _ _ _ _ _ [ _ _ _ _ [ _ _ {__ _ I I III I I I I I I III I I I Ks l I III I I I I I I III I I I
- E I I III I I I I I I III I I l-D f<, _ _L_ L L L L _ _ _ _ _ L _ __I _ _ L _ L _L_ L L L L ._ _ L _ _.. L _ _ L _
b e"' I I III I I I I I I III I I I
, I I III I I I I I I III (i V'S' # $= (" l 4
;2"'.
I I III I I I I I I III I I I _ _L ._ !_. L L L _ _ _ _ _ L _ _ _I _ _ L _ L _L _L L L L _ _ __ ._ _ L _ _ _ L _ _ L _ [" l I III I I I I I I III I I I "a III l l I I I I I I III I I I i bM l i llI I I I I I I III I I I
"' _ _L.L L L L _ _ __ _ _ L _ __ _1. _ _ L _ L l _L L L l _ . _ _ . _ _ L _ _ _ L _ _ L _
I I III I I I I I I II I a= s ' I I I III I l- 1 I I I II as = 5% a 7s l c, I I III I I I I I I III I I l
._ _l.
_1_Ll____._____.].___I__I__._]_ ____.__p___1.__I__ c. l I l I l ll II17- [ (( l i
~l'il-lJ'!
l l l I I II II I J bi l
'i 6 /8 9 c! 3 il S 6 / fi 3 10' 2 i 'l 5 FREQUENCY COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM i
figure 14
l l'E
- g=:
c. e if "" l I III I I I NUDE !1 HIRCIOR BLDC. INIFRIGH I I Il l l l l ' g. , ELE V A T I ON litDi FT. 11 IN. l l l ll l l l "J _ _ _ _ _ _ _ _j_ _ _ _ _ _ _[_ _ _ _ _ t_ _
... . . 8 2^f a 9_ _ _ _ _ _ __ _ _
ng i I III I I I I I I III I I I EM I I III I I I I I I III I I I (2 . _ _1. _ I _ _ l_ _ _ . _ _ ____l.__I____L_]_I_I________I____L__l___
, I I III I I I I I I III I I I 4 I I III I I I I I I III I I I
.T"' _ _l. _I _ L L L _ _ _ _ _ L _ _ ._L_ _ L _ L J. _I_ L. L L _ _ _ _ _ L _ _ _.L _ _ L _
T' I I III I I I I I I III I I I e lll l l l l l llll R~ ~4 l _ l _.ll_ L L L_ __ _ _ _ _ L _ _ _L _ _ L _ L 1 l I III I I I I I I III I I I I I III I I I I I I III I I I l c, I I IIl l l l l l l l ll l ut = 4.5 l _ l. _l._ L L L _ _ _ __ _. L _ _. L _ _ L _ L J. _L_ L 1 _ _ _ _las= 5% a 7% l_ _ -
/
If l l I III I I I I I I l I I I I III I I I I I I I I
; <, I I I I ly- lxl7 Ip'I 1-1 I I i x l-
~ -
"S ti /H 9 10 0
2 1 il S G 7 8 9 10' 2 3 11 S lREQUENCY COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 15
I I I I I I J J l I _.
; i 8
~ I I I
--__y___#
i I cp __ m l l I l ~ l +c l I l '
' l (A l 11 te l
{ I ! 8 i % i J e I h ___.____.d l o l l _/ y l { l l I i i I l l l l l l l
~l l I I _
o I I l _ _ _ _l _ _ _ _l _ _ _ J l i a___ l l i c m M N gg m w g i i l l l l ' l--- $ 0; i
\ "w
'j _ _ _ J _ _ _ _l _ _ _ _I _ _ _ _l .
l t l ~ w m a z l ' i O C
!___________s_______g__}
i e c u eg I I
, 1
{ l i l. leZ
'd m
W% M w g e l I
}
= o i l C
l ' d " d e i l= l l i I ! -l
~"
- E i l l l l #
C ! ! l l g
! I i l ! l i >
I r
; g _-_a___ ___ ___ ___ m g fl s 5 i ; i .
z i i l I l I
- i
- :c 2
E E 3 C I l l l
<+
- m,n ,
i wd i l , A U l l I hg d TJ C e l OM w 1 'l C '_ l l l l ' c ; :
! - 'O l ! l l !
1 ' UC
'l l l [ (
\
! Cr , e i l W
I l l ! !
., C2 . , , i I' .
c_
! = l l l i k "-
cu i
-w 4ll o l l l lm I
Nt___ _ C II ! g
! I I ! _.,
d> el - 1 i C-o _: 0l w
! i 4
l i i j s l l 2i C l 1 l I i [ t
}
l
--- --- _-_ ___ ___ g
! ! I i i I
*2 '*
'e - 1C
-- =
' l J' 's ,( V,': ,___
J 4 VO T. 1's i) J
' '$ + 1 UD3 IOdd
- SMii.m I
i il (Wnm
- n. ihib'l u ilf It' 31
, [ h((b ' h((b;hS.lbh l bNa l l . - , - . -
i l l i i l l l g e l l l SI N l I N CD 1 . . 1 l i 8 I I i I se 4_ e 4. To t [4 - - l 3-
; I l I g" li es es i J
f f l cl J e j
' I I I >1 2 %. I
_ __ __ e; i - I l l l l t i i !
; i l [
. l<
I I I I \. l
- I I I i l
I l I I i _ o I I I. i i i l l 1 ' W g l l_ E$" i _ _ _-;_ _ _ - _ _ J _ _ _ J_ _ _ _ a _ _ _ _l _ _ _ _l I _ _ _ _I _ _ __ _I _ _\ . i
& lt' t
e
. i i i i i i w w I I I i I l l 2
l < i j
. i
- e o z N
i l i l i
~___--__ '___ l_ _ ' i i c
,, e o J
w E
- _ _ _ -.,, !
I
- _ _Il _ _ _ m[i _,,,,,.,,a__
i
. , , , .p
' de,
=
J w% m g g l l I' l C e o Q - I I I l I L ,. 1 ,-.- y a N f f f I h
', $ I r , ,
- " 6 3 m in. 5
! C 3 I i !
U e [ W l l l I I n i l l % { ,
.- m g
i 2 ' i i o g 2 , , z < I _ i 4 i i
! I 1 6 o j
l e M T e f f w"""' l c m"'3
- - . < r l _ _ _ _, _ _ _ _._ I
'"'". i ) l C. O
_ _ _ _ _ _ _ _ _ _ - ca x .w
.I = 8m i
! I I l j t t &
l c I
-r ! l l l l ;
4 , um i . i ; C7 h ! i i , I I i Z l c_ C n_ gl l l l ]___ ! -
-ms--_1-
-------____I__--I_---
I i o I
*H I ! l l
#l l d>
p I ha 2'dO ! ml l i l l I l r
' ' f I i I ! e l - _ _ .,,,4,,, - - - - - - - _ _ _ a - _ _
i i I e i I l I A 4 3 .i a '- , .. ,..,,, ,a .q . U '..
. J3 - 0 - Gg .- J - oc s 7 .'. G IUJ N ' ) 1.'. U b : _I : JJC 5 ;'. ', : , in !
g % d. tv,u au
me sus an mum uma sus ses an an :en mee an ame sue sus som aus sua sus m <, J. . __. ._.---_..- - .. - _ ._.- . - . . - - - - - . _ . _ . . . - - - . . _ - . _ _ . . - _ . . -
- NonE i ii af ac ron HLUG. INTEftl0B I l l ll l l l
. g7 m ELEVRIION 1130 FI. S IN. l l l ll l l l ' 2%
3 . _ .. . B E{A _____-___ _. _ - _ _ __ __ _ l _ l _ l _ - - __ - _ _ { _ _ _ [ _ - [ - _
- sn l I III I I I I I I III I I I .
E I I III I i l l I I III I I I f _ _1. _I _ I_ _. ._ __ _ _ _ l _ _ _ _1. __ _ l_ _ l_ _ . _l _ l_ _, 'l I III I I I I I I III I I i 8m I I III I I I I I I III I I I
'l;. _..
_l_l____.____I___.__1.__l_..._l__L_l_I______I_____l..__I__ l I III I I I I I I III I I I i l l III I I I I I I III I I ) EM
"^ _ _ l. _ L. L L. L __ _.. _ _ _ L __ _. _1. _ _ L _ L _I _I_ L L L_ _ _ _ _ _ L _ __ _l. _ __ L _
l l I I lll III l I l l l l l l I I llll I III A ~+ I m. = 4i l l M.
~ _ _l. _L. L L L _ _ _ _ _ L _ _ l _ _ L _ L J _L L L L _ _ _ zi' # $ = " ^ 7 '- L_
l l III I I I I I I II / \ l l I o a I l I I III I I ly I i xl/l - I I l# t(Jgl I I 1II (l x!h L I "S ti /a 9 l([ 2 1 11 S 6 7 8 9 10' 2 '$ 'I 5 I:flEQUENEY COMPARIS0N OF V C SUMMER SSE FLOOR RESPMSE SPECTRUM AND H = 4.5 FLOOR RESPONSE SPECTRUM Figure 18
ms aus aus um seu as aus aus en sus as oss nas seu ans ses seu um me il 2 U 'q I I III I I I NUDE IS HF.hCTUR Bl.0G. INlERIOR I I III I I I gp m El.EVHT10N 1126 FI. G IN. l l l ll l l l ff
- gs
[ _. _. B E T A e = 2 %__ l I III I I I I _ ]_ ] {U__ I I III _ _ _ [_ _ _ _ { _ l_ _ I I I J=8 I I III I I I I I I III I I I l l l l l l l' I I I I III I I I [G= e'" _ J.l _I_ L L L _ _ _ _ _ L _ _ _l. _ _ L _ L J. _I_ L L L _ _ _ _ _ L _ _ _L_ _ L _ III I I I I I I I III I I I ! ., I I III I I I I I I III I I I t G,c I I III I I I I I I III I I I
?" ._ _ L _L L L L _ _ _ _ ._ L _ _ J. _ _ L _ L _l _I_ L L L _ _ _ _ _ L _ _ J. _ _ L _
lE' I I III I I I I I IIII I I I "2 I I III I I I I I I III I I I
- $ I I III I I I I I I III ti9. _ J _l _ L L L _ _ ._ _ _ L _ _ _L_ _ L _ L_ _ L _L. L L L _ _ _ _ _ L _ _ _L _ _ L _
I I I l I III I I I I I I III I l I I III I I I I I I III /Ivcs,a,=as i 1 M l l III I I I I I I III / I M'= 4 5 l
~
_ L _I_ L L L _ _. . _ _ _ L _ _. _ L _ _ L _ L ._L _L. L LL _ _ _ _,/1'_#' ' i'* " '*_L_ . l l III I I I I I I II I I o S l ti l 7 fi 9 l l-f08 n lh hi %,/ 2 3 il S fi 7 F8 9 10' IWl 2 3 't S FREQUENCY COMPARISON OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND H = 4.5 FLOOR RESPONSE SPECTRUM Figure 19
l o i I i l l I I I I J 1
=
I I I l l I I I I I I g ___q___q___q___q___q__ - 1 I I I I I I I I I I I J I l I ca l I I i ! I l
. i I I *<,,
~ 8 s
I m # l l l l g i I I q I I l l l l [ I i g m-c,
*8
, o,,
J # 2 ~o I _ _ _ _!___J___l I
\_a* i -
w E _ _ _ _. _ _ _ J _ _ _ _i _ _ _ _.I _ _ - _ _ _i_i a 1 i = = ge I i l i i I w I ' n' & I i _ _ _ _i _ _ _ _l_ _ _ _I _ _ _ _I _ _ _ l _ _ ii i l=~ "m m i t
> o z i
j___m___ , i I i I 1 I 1 l o u-o o eaw i i : i i W wm m i i l i l l m___] l og m g I I i j j j m o o L ,j w J N I -
- l I i J c E" .
= = m u O I
, I= W
- 5 I l l l '
- l a s I .
i
=
e l i i l l l g, z
? -
z.
-= ;___q___a,___q___q__/,
I I I I I z o 3 I a i eE i i i l I gs E__ l l J I ! Id I c o m
=- 1 I i l
"=
- I
== l l- l ! I a .
I ! l i
-e uc c _- i i
I i I l I I I I I ~, I I I I I I s ,i _ _ i A___w__ i i e a x_ f l___
-- l i _J _ _ _ l I c
I , u>c
=w s ,
ca m
". l i
i i. I. I i i i i i: . s Zu m i [ i l _ _ _ _l _ _ _ _l _ _ _ ql _ _ _ ql _ _ _ i ! I e t i a m,. c ..., ao =, v= --
;o ,m i
J NU1y.=CDO .,. , IOJJC
, , s,s .-
I F 0, ,7 , 'a : n ljii..j!; ') bh $ h b. L , '!Of i
;,,,L'..'
' , M10 i> 4 o; [
[3 'l Ib
em aus aus esa su aus sus ame aus em am aus mas ama as aus aus em um C) -gn c4 l I III I I i 9E w r io t a. i. ni ric r ilit ir.t uc . ',s it i. i l l Ill I I I
, it.Evarion Saa iT 11 i ti l l l ll l l l
;7 .. SEP ~A.._.___.___.._... _. __.._ .. ._. _ ..l_ _ _ _ _ _ .
_ .].._. _
=1 I I III I I I I I I III I I I y$$f
~ c., I I III I I I I I I III I I I l . l _ l _ I _ _ _ _ _ _ l _. _ .. _ l. _ _ _ _ . _ _ _L _I_ _ _ _ _ _ L - _ __L_ u l ; l l; l l l ; l l l l; i i ; 5 I III I I I I I I III I I I
- j. . I. . _ _ _ _ _ _ ._ _ I _ _ ._ . ._ _. _ _ l_. _ _
_ l _. L_ _ _ _ __ _ _ _ _ _ _ _ . _
"' # = "*
d l I III I I I I I 'i i Ii $ I I I
% I I III I I I I I I III I I I M".
.l.._.l_l... Ll.._.._._._ _ L _ _.L___. _ _ L _I_l_ L L L._ _ _._ _ L _ _ _l._ _ L.._
l I III I I I I I I I I I a = 4.3 I I I I i Iil l l l
' o, = sw a 7s l I I e i I III I I I I III I I l
. .I . L L L L _ _ _ _ _. . __ L _ _ _ l. . _ _l _l. l._ .L L.. _ ___ _ L _ _..I _ _ L _
l I III III g I I I I III IiI f fl'i I l y l l l l l l I I lYl~ I I I III S #x' I t I I L_ 1' I I S 6 7 d 's t Q" 2 3 8 S 6 7 6 0 10' 2 3 'i s f-llE ullEllC l' COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.S FLOOR RESPONSE SPECTRUM Figure '21
i 2 l l ll l l l 3 ggs ,., noni: a al:nt run nun;. u.l.va r I ON Sil 6 IT 0 IN rinu i l l l l III l ll I l l I [ I
- Fs "j. _ .
.B E ((e 2*g _. _ _ _ _ _ _ _ _ _ __ _ _ _l_ _l_ _ __ _ - - { _ _ _ _ _ l_ __
I I I,1 1 , , I l l l ll , , I III III m@ggq. o l I I I I I I I
.. _ _ l. _ I _ I _ L L _ _ . _ _ _ _ L _ ._ _ I _ _ L _ L J. _J _ L L L _ _ _ _ _ L _ _ J. _ _ L _
I I I gge l I III I I I I I I III I I I Ro'; . . ll. ..l_ lll l l l l l llll l l l _ I_ _ _ _ _ _ _ _ _ .]. _ _ I_ _ l_ . . _j_ I_ I_ _ _ ._ _ _ _ I_ _ _ _ _ _ I_ _
"j i l l II I I I I I I J__. vcs, g ,= sv. I I I
% I I III I I I I 'TT~ l I I I Q,',, . _ . _ _ _ _ _ _ _ _ ._ _._._ .__J.._._._1. _ _.l_l_l_ _ _ _ _ _ L _ _ _ _ l_ _
l l III I l- 1 I I I III ut = 4.5 I I I o l I III I I I l P i ~#5=5'87"'1 I I
._._I_ .. _ I _ __ _ _ _ _ l_ _ _ _ l. ._ _ _. _ _ _l. _ . L_ l_ L _ _ _ _ _ L _ _ _1. _ _ I_ _
l l l ll l l I fxl lU \ l l l g I I III f r Ipl l I I P'H-C ~- - l 4 b g - - __ ._ _ _ __ _ _ S 6 7 8 9 [O 2 3 S 6 /0 3 11 10' 2 .3 ii 5 f at.0Ul.NC r COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE i SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 22
mas em use em nas amm aus as mas em aus me sus em aus an ums um em g I I III I I I 2; noin.1 nEnr. l on ni.rm. Sul i i. I I III I I I gg ii.i.vorion saa Fi o in I I III I I I g u BETAe = 2% ___,_ _ _ ____. _ _ _ _ _ l _l_ { { l_ _ _ _ _ _ _ { _ _ _ _ l _ __ _ l _
- c. I I III I I I I I I III I I I L' I I III I I I I I I III I I I EEE o l I III I I I I I I III I I I b f'. _. _ I _ L I .. L L _. .. _ _ _ _ L _ _. J . _ _ L _ L _ l. . _I _ L L L _ ._ . _ _ _ L _ _ I _. ._ L _.
Mi e "' I I III I I I I I I III I I I 15 3 I I III I I I I I I III I I I r "~ q, I I III I I I I I I III I I I
,j'.; _ . _ _
_ . l_ I_ _ I_ _ _ _. _ _ l_ _ _ _ _ _ _ l _ _ I__.____L___[__L_ _
- 'i l l III I I I I I I III I I I i
W. l l III I I I I I I Il vcs as=5% i l I
. J . _ I _ L L L _ __ _ _ __ L _ _ ._ L. _ L --L . _I _ ._ L _ _ _ _ _ L _ _ J . _ _ _ L _
I I III I I I I I III a=3 I I I I I III I I I I I III a = 5% a 7% I I I c.
'~'
I I III _...l _ L L L _ . _ __ _ _ L _ _ _l..._ I I I I I M I I I _I l /_ . . _ _ . _ _ L L L _ _. _ _ _ L _ _ J. _ _ L ._ r
- I l l ll l l I I III I I I I I III l -t l I I I II I I I o
u i I I I ly 1l I I MJ11 Mmh
".. ii 7 a o 10 2 i 'l S 'i ? a 'J 10' 2 1 'l S f al~.0UENC T COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND H = 4.5 FLOOR RESPONSE SPECTRUM Figure 23
ame uma amm aus amo ses sua sus sua en aus me aus ma me uma sus an amm a 9 '" I I III I I I K:=s noot it nEnc ron ei.tm. nuru. I I III I I I sg
~
t u.vo r IGN 500 F f. l l l ll I I l L! DETAe= 2 % _ _,_ ___ _ ___ _ ___ __ _ j _l_l__l l_ _ _ _ _ _ { _ J, _ _ l_ _ I I III I I I I I I III I I I N=J l l l ll l l l l l l l ll l l l
@3 c I I III I I I I I I III I I I gp- g,. . Ll_1_LL__._ __I___.__
1 I II __[__.._I___.___L___l..__I___ g- .. I I III I I I I I I III I I I G <, I I III I I I I I I III 1. I I r:"t _ _l. _I_ L L L _ .. _ _. ._ _. L. _ _ _._ J _ _ L ._ L _L_I_ L L L _ _ _ _ _. L _ _ J _ _ L _ LE ~' I I III I I I I I I III I I I "2 I I III I I I I I I III vcs es=5% l I I Jl l f,{. _ _ lI _l l _l l_ l _ l _ ._ _ _. ____ l l l l l l If I I _ _ L _ _ _1._._ L _ I ___I__.L]._I_/f_l_ K,
, , , ,1 1 I I 1l . = ~ . I I I I III I I I I IM '
I _. ! _L. L L L _ _ ._ _. _ L __ _ J. _ . . L _ L . . r_ L L L _ _ _ _ _ L _ _ J. _ _ L _ I I
/ - [
S 6 / 's 9 1 (( ! '1 il S 6 ,
7 3 9 1(j' 2 1 il S lnEQUENCY COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE j SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figitre 24
um em ums uma ame sus sus mas em aus sus me aus amm aus em uma ama em my I I III I I I y=% nooE s nEncton BLoG. SilEl L I I III I I I y ,, ELEvnTron inni 8 tI. I I III I I I slD '
,j . ._ L^L=27_ _ _ __ _ __ ___ _ _ _ ___ _____ ___l___ ._
. l_ _ t_
3 I I III I I I I I I III I I I U$3 m I I III I I I I I I III I I I
@iR ._ '. __I _I_ L L L _.._ ___ L _._ _l._ _ L _ L l _ L L L L _ ,_ L__ _..I___ L _
a e"' I I III I I I I I I III I I I 2 950 I I III I I I I I I III I I I b Bo l I III I I I I I I III I I I
; '; __I_ _ I _ _ _ __ _ _ _ _ _ _ . _ _ _ ._ _ _ l _ ._I_I_ _I_ _ _ _ _ _I_ _._ ._ __I_ _
yj l l III I I I I I I III ycs, ,,.3s l I I io l I III I I I I I I II I l l M _ l _ L L L L _ _._ _ _ L _ _ l _ _ L _ L _L_I_ LI _____L__.l__L_ I I III I I I I I I / I M'= 4 5#' " ** " '* I I I I I III I I I I I /I l l I a l l III I I 1 7 i / I I I I __1 _ L L L L _ _ _ _ _ L _ _ l __ L _ L _1 _ L LL _ _. __L__l__L_ I/I l I III I I III I I I m l l l l lxl JJ'tJ i_ u M l 6 7 1 9 to a 3 ii , n i 10' a 3 in s FnEQUENEY COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM l Figure 25
aus sus aus une aus uma ma sus sua amm aus mas aus ma em en am uma ame l I III I I l CF nonE 6 nEncion utoc. sorti. I I III I I I y m tit.vn T 10N 8162 0 f . l l l ll l l l g ;-) ._. DE{A{: /o _ _ _ - _ _ _ - - _ _ _ - _ - _ _ _ _ _l_-_---l_____l__ _ & I I III I I I I I I III W ,, I I III I I I I I I III I I I I I I l;. ._ _. _I_l_ _ ._ _ _ __I__. _ _1. _ _.l_ _.l_ _ _I_ l_1_l_ _ _ 1 ._ _ L_ _ _ _.l.__ _ _ _ g , I I III I I I I I I III I I l u I I III I I I I I I III I I l "j. _ _ . _l _ l_ I_ l_ _ _ _ _ _ I_ _ _. _l. _ _ I_ _. III ______L__].__l__ _ _ l. _ _ I_ I I I I I I I I III I I I LM I III I _ l...I_ L_ L L l_ _. _ _ _ _ _ L. _ _ _1_ _. L_ _ l_ _ L _I_ L I. I I I I I III "' # $ * * '" l I I
"'" _ _ __ _ _ L _ _ _ L 1 I III I I I I I I I I M' = 4 5 I I I I I I I 1. I I I I I I II # * = ""'" " 7 '" I I I
<, I I III I I I I I I II I i 1
','] ___ _ ._._ _ __ __ __..__ _...._ -___.__ _ ___ _ _ _ __ == ==_ l. . _ _ _l__. _ _ _ __ _ _ _. _ ___ l__ __
u
"* 6 l l l
/a 9 llJj%LUU %\i 10" 2 't il S G ? 8 0 1 O' 2 ~1 il S FHEOUENEY COMPARISON OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 26
me sus aus == mas uma sus ame sus amm aus me aus em as as ame uma amm u q , , g+a (i ~ ' Nlll)E 7 flEflC IOfi f3 LOG. Sill.l.l_ l l l ll l l l g I.t i v n i I a n 4 :v i I T . I I l1I I I I BETAe =2% l ~l- l~{~l~ ~ ~~ ta
~ ~~l l l_ ~
99 I I l ll l l l l l l lll l I I iWj l l l ll l .I I I I I III I I I Ma> l l l ll l l l l l l l ll l l .I Ns._S - . l _ L _ L L L _ _ ._ _ _ L ._ _ _I _ _ L _ L J _I _ L L L _ _ _ _ _ L _ _ _I _ _ L _ ge e?"' l l l l l l ll l l l l l l l l l 11 b I I l ll l l l l l l l I I I 5 I I Ill l l l l l l l ll l l l
;$. __L _ L L L L _ _. _ _ _ L _ _ _L ._ _ L _I_ _ L _I_ L.. L L _ _ _ _ _ L _ _ l _ , L _
f I I l ll l l l l l l l ll l l 1
>i I l l ll l l l l l l l ll "' # '""* lI l l ! .-l l l l ll l l l l l l l ll I I ME.
_ J _I_ L L L __ __ _ _ _ L _ _ _ I _ _ L _L J _I_ L L _ _ _ _ _ _ L _ _ _L _ _ L _ l I Ill l l l l l l l l l l l l 1 III I I I I I I II m=45 I I I o l IIII I I I I I l ll \ I l l
~ _ _L _L L L L _ _ _ _ _ L _ _ J _ _ L _L l/L. L LL (___L__l__L_
i l I IIl l l l l I III I l l 1 a
!!II! - -
' !yIMb b_
"* 3 ei /' 9 l (( a 1 il S 6 / is 9 10' 2 1 'l 5 FflEQUENCY COMPARISON OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 27
4 men sum uma mas sus amm amo mas sus sus aus sua men am num en aus uma mas 4 0 I I III , e=a:, I I I i+ 3 NODE 9 REntlHit BLOG. I N I E ft l ull l IIII I I I
'y ,,
Eu.va r i oN it /s i I . I I III I I I ! ;: .. . __3 8
. _ _ _ _ _ _ _1. __l ___i ___ _ _ I _ _ _ _j. _ _ L _
' sM I I III I I I I I I III I I I L3 g i I III I I I I I I III I I l gg _q. __l._.l_ L L L _ ___ _ _ L _ _ _1._ _ L _ L _1._.l_ L L L _._ _ _ _.L _._ L_ _ L _ go e i I III I I I I I I III I I I
- r. I I III I I I I I I III I I I
- em I I III I I I I I I III I I I
?". ._ _1_L. L L L _ _ _ ._ ._ _ L _ _ _ ! _ _ L .__ L _ l _I_ L L L_ _ _ _ _ _ L _ _ _1. _ _ L _
~
T' I I I I III III l I l I I I I I I I I I III III O vcs.d=ss I I m.=h.s l R, I I III I I I I I I III ! - as= s% a 7%I d" _. .. l _ L_ L L L _. _ _ _ _ L _ _ _1. _ __ L _ L _L _I _ L L L . _ _ _ _ _ l _ _l. _. _ L _ . l I III I I I I I I II I I I I I III I I I I I I II I I l I I III I I I I I I I l.1 1 I I
~
_ _ 1_I_ L L L _ _ _ _ _ L _ _ _1. _ _ L _ L _1 _I_ L _ L _ _ _ _ _ L _ _ _1. _ _ L _ l l III I I I I I I 'II !ml I
<, I I I I I ; c
- "S 6 7 >s 9 ((f a 1 it u 6 /a 9 t o' a a it s lIlEQUENCY i
COMPARISON OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 28
am amm amm mes em aus ens ama amm som aus me amm sua me ma sus sum aus M . l':] l l l l l l l l @M NOUF 10 REACIOR m_0G. IIH F R I OR I l l ll l l l 3k '03 m Eu. var inn inca r r. I I III I I I ni B E TAe = 2% _ _ _ _ _ _l _ l _ l_ { {- _ _ _l_ ____j__ _ { _ e I I III I I I I I I III I I I , fid l I III I I I I I I III I I I g <, I I III I I I I I I III I I I esi J. _ l _ L. L L L _ _ _ _ _ _ L __ _. J. _ _ L ._ L _L _I_ L L L _ _ _ _ _ L _ _ _ _L _ _ L _ Z5s e'" l I III I I I I I I III I I I D ., I I III I I I I I I III I I I b Go l I III I I I I I I III I I I l '". ._ l _ I _ L L L _...._ _ _ _ L _ _ _ L _ _ L _ L _L _ L _ L L L _ _ _ _ _ L _ _ _1. _ _ L _ lE' I I III I I I I I I III I I l "j l l l l l l l l l l III f v S 8$="% l Q I III I I I I I I III * = 4.s I _ .l! _ I _ L L L __ _ _ _ _ L _ _ J _ _ L _ L .l _I _ L L L - '[_#' = "*
- 7* L. _
l I III I I I I I I III I I I I I III I I I I I I II I I I
- c, I I III I I I I I I II I I I
~
__L _I_ L L L _ _ _ _ _ L _ _ l _ _ L _ L l _I_ L _ L _ _ _ _ _ L _ _ l _ _ L _ l I III I I I I I I II I I I s, o I II I III I -_ l -
" 'a A /a 9 1 [f' a 1 si '3 6 7 a 9 10' 2 3 'l '3 FREQUENCY COMPARISON OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 29
man sum nas uma em um aus som amm aus som num sua sum um aus mas uma em a l I IIl l l l N NOME 11 111:11L illll ULI)G. I N 'I E ll I Ull l l l ll l l l $W ,,
^
Et Evn f 10N 11 :1 11 lI. I1 IN. l l l ll l l l @ ,'_ . . . . B J^L= 2 /._ _ . _ . _ _ _ _ _ _ _ _ _ _ _ . _ _ . . _ _I______ ___l__L_ TT I I III I I I I I I l ll 1 I I M ,, I l l ll l l l l l l l ll l l l GD J' _-._I _l_.L L L _._ _._ _ L _._ I _._.L _ L I_ L L L L._ _ _ _ _ L _ _ l ._ _ L _ 09 2" l I l ll l l l l l l l l l l l l Ei G ., I I III I l l l l l l ll l l l cs = ' 2> ., I l l ll l l l l l 1 l ll 1 l l
.._._j_ _I_ _ _ _ _ _I_ _ _ l.._._ _ _l_ _1 _ _I_I_ L _ _ _ _ _ ._ _ _ ._ _l_ _
I I Il l l l l 1 l l l 1I I I I [3", I I i ll l l ! I I I III I o _ _ l _l._ L L L _ _ _. _ _ L _ _ _1. _ _ L _ L _1. _I_ L L L_ ._ _ _ _ _y_ _ves, $ ,=s f. I __i__L_ l l l l1 l l l l l l l ll fl u i. = .i . 's l l l ll l l l l l l l ll Ps=sT a'% c, I I l ll l l l l l l l ll l l l
"', _._ _ _ ._ _ L _ _._ _ _ L _ _ __ . _ __ _ I _ _ _ _1. _ _ _ _
...___q__I__._L n
S l l l 6 7 l l/mlJMM'l l il 9 [(J"
\
2 1 81 5 6 7 a9 [0 3 2 iL 3 11 5 f IlE OUE NE Y COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 30
me sus m uma em uma amm sua uma amm aus mas sum uma en as mas em em l I III I I I suoi. ia ni ni: r on et.uc. iniiiiion l I III I I l
@TJ .
i.u.vn i i on ina r I . u in. I I III I I I
!M [j .
. 8EJAg: */a
_ _ _ _ - _ - _ - _ _ _ _ _ _l_ _[____-__ ____-l__
@~? I I III I I I I I I III I I I h <, I I III I I I I I I III I I I Fq c _ ._ I _. l _ L L I. _. _ _ _ _ _ L _ _ l _ _ L _ L .I _I _ L L L _ _ _ _ _ L _ __ _l. _ _ L _
@Lgge";. l I l l l l l .i I I I III I I I p , I I III I I i l l I III I I I
.c o m i l III I I I I I IIII I I l _l._l__l____.____p__.._.____I___ _l_l_ _ _._ _ _l_ _ _ _ _ _ L _ I I III I I I I I I III I I I ii!, , I I III I I I I I I III I I I
& __ l ._L L L L _ __ _ _ _ L _ _ l _ _ L _ L l _I_ L L L _ _ _ _ _ l vcs. e s=s v- L_
~
l I III I l l I I I III ye = 4.s I I I III I I I I I I III - a, = s v. a 7 v, 1 l I III I I I I I I III l l 1
", _ _ l _ _ l ._ __ __ .__ _ __. . _ _ _. _ _ __ _ _ _. _ _ _ . _ _._ _ _ _ ._ _ __ __ l_ _ l# _ _ _ _ _ _ _ _ _ . . _ _ . _ _ _ _
l l l ll I I I i 1/n, l aw_n l l l 1 1 l I I m I I I
S u / .1 9 ju ? 1 il 5 ti /: 9 l (j' 2 1 il 5 I lil OUl.UC Y COMPARISON OF V C SUHHER SSE FLOOR RESPONSE SPECTRUM AND H = 4.5 FLOOR RESPONSE SPECTRUM Figure 31
mas sum uma mas som amm uma amm aus uma em aus sus aus em een mar uma man 5: l l l ll l l .1
'5 99 noot: l a at:nt: i cit ut.nc,. I n f i:.it i tut l I III I I I v: I:U VH f ION 81381 II. IN.
II l l l ll l l l [j y . 8j?^r "f_ _._._._ _ _ _.... .._ .._ _ _ __
._ I_ _ _ _ _ _ I _ _. _ ]. . _
50 l I III I I I I I I III I I i MW I I III I I I I I ! III I I I kir$ f,,. . J. _I_ L L L _ _ _ ._ _ L _ _ J _ _ L _ L J _I_ L L L _ _ ._ _ _ L _ J. _ _ L _ gja e"' I I III I I I I I I III I I I g _,G I I III I I I I I I III I I I o o l I III I I I I l l III I I I
;2"'. _ . J _ L. L L L _ _ _. _ _ L _ _ J __ _ L _ L J. _ L. L L L _ _ _ _ _ L _ _ J' _ _ L _
i' ! I III I I I I I I III I l l I III I I I I I I III I i 1
-I i I III I I I I I I III I I I ti"m.. _ J _L. L L L _ _ _. _ _ L _. _ J. _ _ L _ L _L _.l_ L L L _ _ _ _ _. L _ _. J. _ _ L _
l l iiiii lll l i l i l i l l llll i iiiii X' a& 8
- l m .,.
._..L_l_LLL_____L_._J.__I__L._L_I_LLl/_.____L__J.__L_
III f l I I I I I I I r I I I
'~
e!. l l l i l lxH lJ:W, il 'di_
"S ti Ia9 ) (( 2 1 11 5 6 /d 3 l [J' 2 3 4 S l-I1EQUENCY COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM . ND H = 4.5 FLOOR RESPONSE SPECTRUM Figure 32
m m m m m m m m M M M M M M m m m m m sb Mc9 '"
@@ l I III I I I
@f3 NUlll: 1 11 111Ill 1 Ull Bl_ (Jli . INIllllIlift l I III I I l EES E LI Vil f l ull 11 3 0 17 f. S l lJ . l l l ll l l l 8
J3 = /o _ ___ _ __ _, _,,__ _ _ _ _ _ __ _ __ _ _ ,_ l_ _ [ _ _ , _ __ _ - _ _ _j__ _ _. E3 I I III I I I I I I III I I I EM I I III I I I I I I III I I I Ets _m".._ l ._L. L L L. _. _ -__ _ _ L _ _ _1_ _ L. _ L _ L ._ L. L L L _ _ _ __ _ L _ _ _1 _ _ L _ J2 e"' I I III I I I I I I III I I I
, I I III I I I I i l III I I I B ., I I III I I I I I I III I I I
_ _1_l_ _ ._._ _ _ __ _ __.. _ _ l__ _ l_ _l _ _1_I_ _ _ _ _ _I_ _ _ _I _ __l_ _
" III l I I I I I I I III I I I i l I III I I I I I I III I I I
[j,[. _
. _ . l_1_1_ _ __ _ _ _ I_ _ _ _ _ _ _ . _ L _ L _ _ I_ I_ I_ _ _ _ _ _ I_ _ _ _L_ _ I_ __
l l l ll 1 I I I I I III " ' # 'i " " '* : o l I III I I I I I I III I 7,==15 ' , a 7.f.
" _ . _L_I_ L L L _ _ ___ _ L _ _ l _ _ L _ L l _I_ L L L _ _ _
_ L __ l I III I I I I I I Ily - 1 I I I I I I I -I I y ' l III l I I I l~~~LIL-if#l I II i xlyO I I I
*a 6 7 a 9 1(f a 3 in s 6 7 ao 10' 2 3 ii a fflLOUENCY COMPARIS0N OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 33
mis ame uma amm num uma uma uma mum amm amm nas ums um num uma man amm uma I.'
~
n , ,,\ , , Fu I i i i i I I !SS N(lOE I S llt.fl E I u lt (11.ll G . I fl II:lt 1011 l l l ll l l 1
@g ,, liu VH I I ON 8126 I I. G IN. l l l ll l l l A u BETAe = 2 %
III ____--__________l_((((_____{_-_l_-{_ < t r, I I I I I I I I III I I l as ' I I III I I I I I I III I I I E <, I I III I I I I I I III I I I ':+E S W e"' _ _I I _I I _ L L L ._ _ __ _. _ l_ _ _ L .... _ L __ L _I _I_ L L L _ _ _ _ _ L __ _ _I _ _ L _ l ll l l l l l l l l l l l l F3 I I III I I I I I I III ( l l III I I I I I I
- "'. ._._l._.l_ L L L _ _ _ _ _ L _ _ _L _ _ L _ L._t_I_ L L L _ _ _ _ _ L._ _ _1 _ _ L _
III I I I I I I L'f' I I III I I I I I I III I I I "i I I III I I I I I I III I I I i i l il I W' - _1 I I I I I I III I I I
"~'" l ._ L L L _ ._ _ _ _ L _ _ _1. _ _ I_ ._ L . l _I _ L L L _ ._ _ _ _ L ._ _ ._ L __ _ L _
1 I III I I I I I I III I vcs, g ,'= 5% ! I I III I I I I I I III I i I c ., I I III I I I I I I III a=d '
~ - _. _ L __ I __ ! _._ b._. l__ _ _. _._ _ _ k_ __ _. _ __ l. _ _ l _ _ I __ __ l. _ I _ f_ l_ k___ _ ___#s =_5_%_8 7% ._ _
l' IIII I I I I I I I I l- 1 I I
^
g I I y - I - -
"S li / .1 9 l([ i' 3 il S G /s 9 10' ? '$ 'l 5 l-REQUENET COMPARISON OF V C SUMMER SSE FLOOR RESPONSE SPECTRUM AND H = 4.5 FLOOR RESPONSE SPECTRUM Figure 34
amm nas ums aus mas amm amm amm um amm mas mum uma sus aus amm uma amm amm Ni
,y I I III I I I d=3 Nout: ta nuicron HLUG INTEnlon l l l l l l l l ggg c, Eu.vn T I ON 1106 F T . O IN. l l l ll l l l ni BETAe=2%
sir =9 _}_l_(({_____{ _j__{_
. I I III I I I I I I III I I I M*
# I IIII I I I I I I III I I l l l III I I I I I I III I I I M
. _ _. _ _ _ _ _ l _ _ _ _ __ _ _I_ _ _ _ _. L_ _
._}__ .__j__I_._I__
se ... I I III I I I I I I III I I I L M <, I I III I I I I I I III I I I
;2"'.
T _.l_LLLL_____L__l__L_Ll_LLLL_____L__l__L_ l l l ll l l l l l l l ll l l l I -l l II I I I I I I III 3". I I III I I I I I I III I I I I I I
" "' __l._ L L L L _ _._._ _ L - _ _L_ _ L _ L l _ L L L L _ _ _ _ _ L _ _ l _ _ L _
I I III I I I I I I III ves, o ,= ss l I I I I III I I I i I I Iil i I I o l I l- 1 I I I I I I I II ' I I
" _ ll _ L L L L _ _ ___ _ L _ _ l _ _ L _ L l _ L L _ L _ _ut=4.3asaurs__l__L_
I III I I I I I I II I I l III III f l QI 1 I I I I I III I I I g, I I I l A l-i W i~ xin xl_ _I "5 6 /a 9 i t/ ? S 6 a9 1 in 7 t o' 2 1 it s fBEOUENCY COMPARIS0N OF V C SUMitER SSE FLOOR RESPONSE SPECTRUM AND M = 4.5 FLOOR RESPONSE SPECTRUM Figure 35
\
I l i !I t 1 l l I i l i l REVIEW AND EVALUATION OF TRE 1886 EARTHQUAKE AT t CHARLESTON, SOUTH CAROLINA !I 1 i !I iI
- I J
l i SOUTH CAROLINA ELECTRIC & GAS COMPANY i l DECEEER 1980 i
- I l'
l 4 I s I _mr=wmw"**am-w-.-wwwwwawa---w+veemp-w*wrw-*e*r- g y q e-ww w -P ew y e+-49wep-me w- w- t q e rgw+w w
I I TABLE OF CONTENTS I Section Page List of Plates . . . .. . . . . . . . . . . . . . . . . . . 11
1.0 INTRODUCTION
. . . . . . . . . . . . . . . . . . .. . . 1 2.0 ORIGIN OF CHARLESTON EARTHQUAKE . . . . . . . . . . . . 4 2.1 GENERAL . . . . . . . . . . . . . . . . . . . . . . 4 2.2 GEOLOGIC EVOLUTION OF THE SOUTHERN APPALACHIAN OR0 GEN . . . . . . . . . . . . . . . . 5 2.2.1 Formation of the proto-Atlantic Ocean . .. 7 2.2.2 The Taconic Orogeny . . . . . . . . . . . . 7 2.2.3 The Acadian Orogeny . . .. . . . . . . . . 8 2.2.4 The Alleghenian Orogeny .. . . . . . . . . 8 2.2.5 Formation of the Atlantic Ocean . . . . . . 9 2.2.6 Cretaceous and Cenozoic Tectonics . . . . . 10 2.3 HISTORIC SEISMICITY OF SOUTH CAROLINA . . . .. .. 12 2.4 ORIGIN OF THE CHARLESTON EVENT .. . . . . . . . . 14 2.4.1 General . . . . . . . . . . . . . . . . .. 14 2.4.2 Mechanism : Deco 11ement Reactivation . . . . 14 2.4.3 Mechanism : Stress Amplification (at Margins of Mafic Plutons) . . . . . . . . . 18 2.4.4 Mechanism : Reactivation of Steep Base-ment Faults . . . . . . . . . . . . . . . . 21 3.0 EARTHQUAKE PROBABILITIES ASSOCIATED WITH VARIOUS HYPOTHESES CONCERNING EASTERN U.S. TECTONICS . . . . . . 26 3.1 GENERAL . . . . . . . . . . . . . . . . . . . . .. 26 3.2 MES0 ZOIC RIFTING .. . . . . . . . . . . . . . . . 26 3.3 DECOLLEMENT HYPOTHESIS . . . . . . . . . . . . .. 27 3.4 MAFIC PLUT0N - STRESS CONCENTRATION THEORY . . . . 29 4.C CONCLUSIONS . . . . . . . . . . . . . . . . . . . . .. 31 4.1 GENERAL . .. . . . . . . . . . . . . . . . . . . . 31 4.2 EARTHQUAKE MECHANISM: UNCERTAINTIES . . . . . . . 31 4.3 DETERMINISTIC CONCLUSIONS . . . . . . . . . . . . . 34 4.4 35 I PROBABILISTIC ANALYSES CONCLUSIONS . . . . . . ..
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . 37 I I i
I LIST OF PLATES Plate 1 Tectonic Units of the Southern Appalachian Orogen. Plate 2 Tectonic Map of South Carolina. Plate 3 Buried Triassic-Jurassic Basins in South Carolina. Plate 4 Map of Cretaceous and Cenozoic Deformation in Coastal Plain. I Plate 5 Historical Seismicity of South Carolina and Surrounding Region. Plate 6 Isoseismal Map of the Charleston Earthquake of 1886. Plate 7 Isoseismal Ebp of Earthquake of November 22, 1974 (MMI VI). I. Plate 8 Generalized Structure Section and Detailed Segment Showing Inferred Attitude of Appalachian Orogen Decollement and Associated Eastern and Westernmost Thrust Be lts . Plate 9 Behrendt et al. (1980) Model. Plate 10 Simple Bouguer Gravity Map of South Carolina. Plate 11 Relationship of Bouguer Gravity and Aeromagnetic Anomalies I to Area of Ma:cimum Intensity for 1886 Earthquake, Charleston, South Carolina. I Plate 12 Conceptual Model of Failed-Arm Though of Triple Junction in Charleston Region Southeastern Appalachians. g Plate 13 Seismogenic Zones Associated with Mesozoir Rif ting 3 Hypothesis. Plate 14 Seismogenic Zone Associated with Decollement Hypothesis. Plate 15 Site MM Intensities versus Distance for the 1886 Charleston Earthquake. I !I I 11 1
REVIEW AND EVALUATION OF THE 1886 EARTHQUAKE AT CHARLESTON, SOUTH CAROLINA I
1.0 INTRODUCTION
I Earthquakes constitute a major geologic hazard which has attracted the attention of geologists, seismologists, and geophysicists since the study of earth sciences became a systematic discipline. Because of the potential of earthquakes to cause both heavy loss of life and extensive property damage, the ability to adequately predict their occurrence would represent a major breakthrough in earth science, and much effort in the past 10 to 15 years has been made toward this end. The basis for earthquake prediction is embodied in the understanding of the origin of the seismicity. In the United States, the character of contemporary seismicity is not uniform, being different in the western U.S. from that in the cen-tral and eastern U.S. because the tectonic regimes of the two regions are distictively different. In the We s t , seismicity is typically associated with major geo-logic faults which have undergone recent slip at or near the ground surface following intermediate to large magnitude earthquakes which are commonplace. Such events are not commonplace in the historic record of the eastern U.S. Consequently, studies related to earthquake predic-tion are greatly facilitated in the West where scientists know a_ priori geologic structures which are seismically active. Recent work has shown that major geologic structures in the East do have associated weismicity (Zoback et al., 1980; Aggarwal and Sykes, 1978; Ratcliffe, 1980). However, the origin or cause of many seismic events in this region is not understood, for as yet no unified theory of earthquake generation in intraplate regions has been generally accepted. For this reason intensive study of tne 1886 Charleston, S.C., event (among others), the largest in historic time of the southeastern U.S., has been ongoing for most of the past decade. It is thought that with the I understanding of the origin of this event may come the key to the constitutive relationships of the origin of seismicity of the South I I I Carolina-Georgia Seismic Zone (Bollinger,1973b), and perhaps other areas of the intraplate regions of the U.S. Development of seismic design criteria for important engineered f acilities situated within the South Carolina-Georgia seismic Zone I would be f acilitated if the cause of seismicity in the zone were known. As it is not, then the criteria typically employed are based upon empirical relationships based upon observations of the distribution of earthquake epicenters, the range of their intensities, and the fre-quency of their occurrence. This information is used to estimate the maximum earthquake intensity or magnitude (1) which can be expected in a given time interval at a site of a particular facility and (2) which constitutes the seismic design basis for the facility. Following s.ch an evaluation, construction of the Virgil C. Summer Nuclear Station near Parr, S.C., was begun and selection of the required Operating Basis and Safe Shutdown Earthquakes (OBE and SSE respectively; South Carolina Electric & Gas Company 1971 and 1976, Preliminary and Final Safety Analysis Reports) was made and compatible design parameters were implemented during construction. Since that time, scientists in government, academics, and industry have intensified their efforts to resolve a fundamental question regarding the Charleston event: is the seismicity related to the Charleston event limited to an isolated source area or not? The design for the Virgil C. Summer facility was predicated on the premise that it was an isolated source, a premise that was justified by the work of Hadley and Devine (1974). The following sections cf this report review the current state of knowledge with respect to the origin of the Charleston event in light of the foregoing question. This re-view considers all recent published material available regarding the I Charleston studies, as well as studies of regional geology, seismology, and tectonophysics of the southeastern U.S. near Charleston, S.C. The results of this review are used to assess the annual probability of exceedance of an earthquake at the Summer site based upon stochastic analyses applied to possible mechanistic models of a Charleston-type I event. This report also concludes that there is no significant new ( knowledge which would identify the origin of the Charleston earthquake, and that earlier decisions re3ative to the design basis for the Virgil C. Summer Nuclear Station continue to remain valid and provide
- an adequate margin of confidence.
I l l ( I I I I .
-----.ess--- ew,--e **ww-ww-wuee-4-- en wgww e %-we w - eee- r eew wmw-w----mmeenweNs-e-e-w ar-N w g >we-- W-ew we w w ' r w*-----re -- e- *----e" * -=*T T T'*
I 2.U ORIGIN OF CHARLESTON EARTHQUAKE 2.1 GENERAL To evaluate the origin of any earthquake such as the Charleston I event of 1886, it is necessary to have certain data with regard to the local geology, seismology, and tectonophysics. Studies of the evolution of the Somthern Appalachian Orogen (e.g. Fisher et al. (1970) have provided much data at the surface, but have until recently there have been insufficient geologic and geophysical data to help provide indications of the structure and composition of the upper 10 km of the crust in the southern U.S. (Rankin, 1977). Now only a broad general knowledge of the deep crust in this region has been gained, principally f rom geophysical measurements such as gravity analyses, and high resolution seismic profiling (Long, 1979; Cook et al., 1979). These models of the structure of the lithosphere can help, where sufficient, to define the geometry and extent of the potential source region for the Charleston event. Additionally, knowledge of the geologic evolution of the region is important because the crust contains relict structural features which evince past deformation, that could be re-activated. Moreover, the Tertiary and Quaternary histories of a region contain indications of I the earthquake periodicity in terms of geologic time ( Allen, 1975). This information is. vital to the understanding of the neotectonic environment and its relation to sudden releases of stored strain energy. Seismologic data, naturally, provide indications of the occur-rence, the nature, and the potential limits of strain energy release. Seismicity is a measure of the extent of tectonic activity of a region. The locations and depths of earthquake foci help to identify regions of the cruse which are unstable. Focal mechanisms provide information regarding the attitude of causative faults and the orientation of the stress tensor at failure. The record of historic seismicity can be used to estimate recurrence relationships describing the frequency of l l
I I earthquakes of different maximum intensity; these relationships are used in estimating the maximum future earthquake inten-ity in a given time pariod at a given location. Knowledge of neotectonics and tectonophysics of the region is a necessary part of understanding the origin of recent seismicity, which is the celease of strain energy stored in the crust. I The source of this strain energy has been attributed to lateral and vertical crustal movements in response to internal driving forces of the earth (Sbar and Sykes, 1973; Bollinger,1973a). An assessment of the origin of an earthquake must address the questions of the distribution and veloci-ties of differential crustal movements in the source region, and ultimately the rate of change of translation, uplift, or subsidence in time. The impacc of these processes on the overall structure of the crust in the Charleston region is not well known and has been the objective of few investigators. 2.2 GEOLOGIC EVOLUTION OF THE SOUTHERN APPALACHIAN OROGEN Charleston, S.C., is situated in the physiographic province termed the Atlantic Coastal Plain, which borders on the southeast side of several other physiographic provinces which are collectively underlain by various crystalline and clastic rocks comprising the Southern Appalachian Mountains. The Atlantic Coastal Plain is underlain by an eastward-tM :kening wedge of weakly indurated sedimentary rocks and unconsolidated sediment ranging in age from Early and Late Cretaceous I to Pleistocene and Holocene. The wedge thins out 125 km inland from the Atlantic Ocean along a boundary called the Fall Line (see Plate 2), and extends far eastward onto the Continental Shelf. The rocks of the Appalachians also comprise the basement upon which these units were deposited. The present configuration of the Appalachians la the product of a long evolutionary process of the continents over the past 1100 million years. The geologic history of the continents is best understood in
, terms of the theory of plate tectonics (Wilson, 1966), which is I
l l
generally accepted by the geologic com= unity. This theory argues that the continent!s consist of rigid slabs or plates comprising the litho-sphere and the upper solid portion of the mantle, which " float" on the more fluid part of the earth's mantle called the asthenospnere. The apparent continental " drifting" during millions of years in the past is a function of the creation of oceanic crust at midocean ridges, causing How-I the continental crust (plates) to spread away from the ridges. ever, continental collisions have occurred, where it is found that oceanic crust is consumed as it is subducted or underthrust to depths of hundreds of kilometers, where it is progressively heated and melted partially, and where it gives rise to lighter, more siliceous rocks of intermediate composition that feed volcanoes on the ov e hrust plate. The wedge of upper mantle overlying the deeply subducted oceanic crust, but underlying the older continental crust, also partically mcits to yield new mafic rccks. The remaining dense, ultramafic rock gradually sinks deeper into the =antle. These newly formed bodies of molten rock, o r =agma , slowly crystallize into batholiths or stocks of coarsegrained igneous rock, whereas only a small fraction intrudes to shallow levels in the crust to emerge as lava or to form diabase dikes and sills. During the last half-century, many models of the evolution of the Southern Appalachian Orogen have been proposed (Fisher et al., 1970; Rankin , 1975, 1976; Hatcher, 1972, 1978) and it is beyond the scope of this discussion to compare and contrast all the models. However, I recent findings make at least one model seem to be the most promising (Cook et al., 1979; Cook et al., 1980). The southern Appalachians have evolved from a sequence of collisional episodes involving the eastern edge of the ancestral North A=erican continent and fragments of continental or island are material between the western edge of Africa or northern South Africa. This series of collisions gave rise to several stages of mountain building (orogenesis) that are termed (from oldest to youngest) the Taconian, Acadian, and Alleghenian Orogenies. These orogenic stages destroyed ocean basins and strongly metamorphosed I I I and foreshortened the basin sediments which had been ddposited during quiet periods prior to the collisions. The culcination of the final orogeny ( Alleghenian) was marked by the formation .f the present-day
. Atlantic Ocean.
I The configuration of the southeastern North American continent at present is shown on Plate 1. The tectonic units of South Carolina are shown on Plate 2. 2.2.1 Formation of the proto-Atlantic Ocean Approximately 820 to 750 million years ago (Late Proterozoic), a single =egacontinent began to rift apart and an ocean basin known as the proto-Atlantic (Wilson,1966) was born, separating two large con-tinents: Laurentia and Gondwana. The former represents ancestral North America whereas the latter was probably proto-Africa. As a con-I sequence of the rifting, fragments of contental crust which included a frag =ent combining part of the Blue Ridge and Inner Piedmont provinces and one represented by the Carolina Slate Belt (Rankin, 1975; Hatcher, 1978; Harris and Bayer, 1979; Long, 1979; Cook et al., 1980) (Plate 1) were separated from the continental =argins. Sediments and volcanics were deposited in the intervening basins between the continental frag-ments. The occurrence of volcanism signaled the impending collision representing the Taconic orogeny (500 to 450 million years ago) in Early Paleozoic time, and hence beginning the demise of the proto-Atlantic Ocean. 2.2.2 The Taconic Orogeny Tne first episode of mountain building in the southern Appala-chiana, the Taconic Orogency probably resulted from the closure of the ocean basin rift between the proto-North American Continent and the Blue Ridge-Inner Piedmont fragment (Cook et al. ,1980; Rankin, 1976) and the subsequent collision of the land masses. The effects of this collision were marked throughout the entire orogen, namely, progressive regional metamorphism, folding and faulting, plutonism, and the I development of a new sediment source area for geosynclinal deposition for the next 250 million years (Rankin, 1976; Cook et al., 1980). At I I -'~
I i this time sediments were shed westward from the rising mountains ir the east. This mountain belt was actually part of a large allochthonous thrust sheet which was starting to move westward onto the continental margin (Harris and Bayer, 1979). 2.2.3 The Acadian Orogeny Between 400 and 350 million years ago, a very intense episode of mountain building, known as the Acadian Orogeny, affected the Southern Appalachians. The orogeny represented the probable closing of the ocean basin between the Inner Piedmont-Blue Ridge fragment and the fragment represented by the Carolina Slate Belt (Cook et al., 1979; Ha tche r, 1978) . Extensive metamorphism, plutonism, and deformation of the Piedmont province rocks occurred. It is likely that the Inner Piedmont-Blue Ridge fragment was accreted to the margin of proto-North America following the Taconic Orogeny, and that the Acadian Orogeny f urther developed extensive westward overthrusting. By the close of the Acadian event, the margin of proto-North Amer-ica had been apparently extended as the' Carolina Slate Belt fragment was accreted and transported westward onto the continent along the g large decollement which had begun to develop previously (Harris and N Bayer, 1979). 2.2.4 The Alleghenian Orogeny The final collision was a continent / continent collision (Dewey and Bird, 1970), from 300 million to 250 million years ago. This event is known as the Alleghenian Orogeny, and sometimes referred to as the Appalachian event. At this time, the proto-Atlantic ocean was finally closed as proto-Africa collided with proto-North America (Rankin, 1975; I 1976; Hatcher, 1972, 1978; Cook et al., 1979). Extensive overthrusting and plutonism characterized this deformational event, which strongly atfected the Paleozoic sedimentary rocks in the Valley and Ridge Province (Plate 1). The Brevard Zone (Plate 2), part of an extensive fault system in the Blue Ridge-Inner Piedmont fragment (Hatcher,1977), I I I
I probably surfaced 'during the Alleghenian event. By 250 million years ago, another mega-continent, called Pangaea, had been formed. 2.2.5 For=ation of the Atlantic Ocean The period from 250 to 200 million years ago marked the beginning i of the demise of Pangaea, for the continent began to rif t apart, much as it had nearly 500 million years before. During the Triassic and Jurassic periods, a large rift basin plus smaller basins, were de-veloped in the Southern Appalachians, filling with continental sedi-ments and intermittent basalt flows. These basins were typically fault bounded on one or both borders. The largest basin, currently buried beneath younger rocks of the Coastal Plain extends from South Carolina to Alaba=a and Florida (Gohn et al., 1977; Gohn et al., 1978). These basins, in South Carolina, are shown on Plate 3. Also, during the Meso::oic opening of the Atlantic Ocean, the crust was intruded by many I diabase dikes (Place 3), which exhibit northwest trends in the Southern Appalachians. However, these trends swing to north-south in the cen-I tral Appalachians, and to north-northeast in the New England Appala-chians. This has been explained (May, 1971) as being indicative of a compressive stress field induced in the crust during the early opening of the Atlantic, one in which the crust in the southern part of the orogen failed in pure shear (maximum principal compression oriented northwest), as opposed to simple shear in the northern part (sinistral shear along faults parallel to northeast Appalachian structural trend). It can be argued that the separation of the continents was probably divergent. Thus, it follows that transcurrent faul. ting along the rift margins was likely pervasive. With the advent of the Cretaceous period, the portion of the eastern margin of North America began to I subside, and the Atlantic Ocean began to transgress toward the west commencing the deposition of clastic and carbonate facies which com-prise the deposits of the Atlantic Coastal Plain. The source of the I sediments being deposited was the uplands of the continent which were being uplifted in Early Cretaceous time. l I I
I 2.2.6 Cretaceous and Cano=oic Tectonics The record of Cretaceous and Cenozoic deposits of the Atlantic Coastal Plain indicates that a series of many marine transgressions and regressions occurred during this time. These sedimentary rocks are weakly indurated and largely undeformed, indicating that depths of burial were shallow to moderate and that major structural shortening has not occurred since the end of the last orogenic event 250 million years ago. The cause of marine transgression and regression has been under study for many years, and the consensus is generally that more
.han one factor is involved such as - changes in global ice volume, changes in the volume of ocean basins caused by plate tectonics pro-cesses, changes in the geoid, and differential vertical crustal move-ments (local or regional) related to isostasy, plate cectonics, or I other processes. In terms of the Southern Appalachian region, eustatic sea level changes caused solely by changes in global ice volume, and perhaps solely by changes in the earth's geoid would result only in sedimentation and erosion of Coastal Plain sediments. In other words, there would be no presistent evidence of tectonic straining of the sedime nts. However, this is clearly not the situation because investi-Sators have reported the occurrence of fractures, faults, folds, clastic dikes (Dillon, 1974; Howell and Zupan, 1974; Inden and Zupan, 1975; Zupan and Abbott, 1975; Heron et al., 1971; Colquhoun and Comer, 1973; Zoback et al. ,1978; Behrendt et al. ,1980) and warped strand lines (Winker and Howard, 1977) in the Charleston region. Moreover,
's the region has experienced several' major earthquakes in the past two centuries further indicating that strain energy is stored in the crust, I and geodetic studies have revealed that differential crustal movements are taking place today (Meade, 1971; Holdahl and Morrison; 1974) (Plate 4). Bollinger (1976) suggested that there may be a relationship between recent seismicity and these verticM v.cvements, and it can be seen from the post-Triassic sedimentary record (Owens,1970) that vertical tectonics played the dominant role in the final stages of the evolution of the Appalachians to the present. I I
I The early Cretaceous period of the Charleston region was charac-terized by large scale warping and uplift. This phase appears to be the most intense compared to all of the others which followed (0 wens, 1970). Consequently, no early Cretaceous sediments occur in the Charleston region (Owens, 1970; Hazel et al., 1977). Late Cretaceous to Oligocene time was characterized by gradual marine transgression, reflecting overall decreased vertical tectonic activity for the Southern Appalachians. However, in the Central Appalachians, uplift was apparently note intense (0 wens, 1970), and it was during this time that the youngest-known intrusives of the Appala-chians were formed in Virginia ant'. We.t Virginia - namely liiddle to I Late Eocene (Dennison and Johnsoc ,1971) . This further indicates that differential vertical crustal covements were shaping the ancient I Appalachian Orogen in the Tertiary Period. I A second episode of vertical uplif t is indicated for Miocene to Early Pliocene time in the Central and Southern Appalachians causing reactivation of older structural basins. I A major marine regression occurred, and clastic sediments were shed off the uplands, being thickest in the basins and thinnest on the arches. Large sr. ale epeirogenic uplift of the entire Atlantic Coastal Plain, whie.n was greatest in the southern part, occurred from Late I Pliocenc through Quaternary time. Eustatic changes caused by glacia-tions resulted in further emergence of the continent during this time. Recent analysis by Matthews and Poore (1980) proposes that a global ice budget has existed at least since Eocene time and perhaps even through much of Cretaceous time. They interpret geochemical data to suggest l that a sea level drop of 50 meters is conceivable for the period 38 to 40 millions of years ago (Early Oligocene) due solely to the accumula-
- tion of ice at the poles. Nevertheless, the distribution of clastic dikes (Plate 4) in Late Cretaceous and Early Tertiary rocks of South Carolina, can be used to infer that earthquakes occurred causing fluidization when these sediments were only a few meters from the sea l
5 bottom. One can see, therefore, that vertical tectonic processes have l 1E
I caused straining of the crust since the Mesozoic, and are clearly continuing in the Charleston region today. 2.3 HISTORIC SEISMICITY OF SOUTH CAROLINA The compleue, recorded seismic history of South Carolina and neighboring areas, with the exception of the Charleston zone outlined on Plate 5 and the recent microseismicity recorded at Lake Jocassee and I Monticello Reservoir, are shown on Plate 5. The numerous small events attributed to these locales precludes graphic presentation; however, all historic events greater than Intensity IV have been shown for these aren so that the record of significant seismicity is complete and presents a diagnostic picture for the total study region. For the well-instrumented zones, records and analyses hr.ve been studied in detail and reported in the literature. Outside the Charleston-Summerville area the significant historical seismicity has been generally moderate and areally diffuse, with no definitive relationship to known or suspected structures. The in-creased density of significant events in central North Carolina (Inten-sity VIIs), and, to a lesser extent, extreme western South Carolina (Intensity Vs) can be generally attributed to characteristic Appala-chian seismicity. The postulation of a northwest-trending zone, based on the rather diffuse distribution of epicenters l'n central South Carolina and its aligament with the Charleston and North Carolina zones, has not been structurally defined. The Charleston event of 1886 (Intensity X) generated the most widespread damage ever imposed on the region in recorded history. As shown on Plate 6, intensities at the site of the Virgil C. Summer Nuclear Station were probably as high as VII. The noted anomalies in I the distribution of intensity (Bollinger,1977) suggest a possible wave guide effect engendered by stratigraphic conditions, basement character, and/or surficial geology, as well as possible variance brought about by cultural differences (construction) and population distribution. It is interesting to note that the recent, smaller event I I of November 22,1974 (Plate 7) resulted in a porportionally similar I distribution of intensity ac,malies. The general elongation of zones for both events in a northeast-southwest direction can be attributed to the pervasive structural grain associated with the Appalachian Orogen and evolution of the continental margin. The Charleston-Summerville zone is presently under detailed geo-technical investigation, but, to date, no reliable conclusions as to the character and extent of possible source structures can be drawn. Analyses of compressive stress orientations based on in situ stress measurements and available earthquake focal mechanism solutions do not necessarily agree. Two regional studies (Talvani et al. ,1979 and Nishenko and Sykes,1979) have postulated various (specific) intersecting structural anomalies at depth as likely source zones for the observed seismicity at Charleston, but no definitive data are cur-rently at hand to corroborate the postulations. Tarr (1977) has pointed out that the "b" value for the Charleston-Summerville seismic = zone is much higher than for the surrounding area of South Carolina, exclusive of Charleston, which again points out the anomalously high I rate of seimic flux for .:he Charleston area. I It is apparent from the spatial and temporal distribution of seis-mic activity about Charleston that Charleston is anomalous in regard to
.other areas of the East Coast of the U.S. Section 4.4 of this report describes a statistical assessment of annual probabilities of ex-ceedance of a " Charleston Type" earthquake in conjunction with three general hypotheses of tectonic setting which are described in the fol-loving section. It can be seen that until sufficient facts are known which would support a hypothesis for the cause of the Charleston earthquake, if indeed they are ever known, probabilistic assessment of its recurrence may well be the only manner in which it can be treated in design applications for critical facilities.
I i I
I 2.4 ORIGIN OF THE CHARLESTON EVENT 2.4.1 General I The foregoing discussions have provided a simplified and gener-alized overview of the seismotectonic environment of the Charleston, I S.C., region - both past and present. Many data are available and elly the most representative have been cited above. It is difficult to know whether or not any given data are (a) significant and (b) meaningful, and not misleading. An exhaustive survey of the literature reveals that agreement as to the origin of the Charleston earthquake does not exist. In fact, it will be shown that the current range of views is broad, with some overlap, and in some cases conflicting. An adequate understanding of this significant seismic event is not yet at hand. There are three principal mechanisms which have been proposed to explain the occurrence of the 1886 seisaic event. Each is discussed in the following sections, and their proposers are cited. In each instance there are two variations of the hypothesis pointing toward the fact that there are unexplained questions and inconsistencies associated with each hypothesis. These will be mentioned for the sake of thoroughness.
2.4.2 Mechanism
Decollement Reactivation la Section 2.2, it was shown that the collisional events between continents and fragments of continents resulted in the formation of a large allochthonous (detached) overthrust sheet which currently under-lies part of the Valley and Ridge Province, the entire Blue Ridge Province, and possibly the entire Piedmont Province (Plate 1),cand may even extend beneath the Coastal Plain cover (Cook et al. ,1979; Plate 8). I It is not known how laterally extensive this feature is. Harris and Bayer (1979) suggest that it may be a feature characteristic of the entire orogen whereas Hatcher and Zietz (1979) and Long (1979) suggest that it stops in the inner Piedmont. The predominance of large-scale overthrusting is well known f rom the New York City area southward; hr.nc e , it is inferred that the decollement extends throur,hout the I I
I central and southern Appalachians from the western margin of the orogen to the western portion of' the Piedmont and perhaps even f arther east. Behrendt et al. (1980) have identified a northeast-trending :one of high-angle faulting near Charleston (Plete 4) based on seismic reflection profiling. They have ter=ed the zone the Cooke Fault, and have identified 50 meters of downdrop on the southeast side; they I tentatively interpret it as being a Cenozoic reverse fault. By extending the fault upward it coincides with an area where a cluster' of earthquakes occurred form 1973 to 1978. They suggest thtt this fault aay be causally related to those earthquakes. Moreover, Behrendt et al. (1980) has interpreted northeast-striking Cenozoic low-angle thrust faults offshore, at depths of 10 to 15 km, which are in the range of hypocentral depths. For this reason Behrendt et al. suggest a causal relation to seismicity. Finally, they identify the Helena Banks f ault, 12 to 25 km offshore; this is a high-angle reverse fault, striking northeast, 60 km in length, and displacing sediments within 10 meters of the sea bottom. Plate 9 illustrates the Behrendt et al. (1980) model, wherein they interpret that the northeast-striking, high-angle reverse faults are produced as second-order conjugate shear faults in response to slip along the decollement of Cook et al. (1979) and Harris I and Bayer (1979). They further interpret that this slip is caused by active regional compression in the Charleston region based upon the stress provinces defined by Zoback and Zoback (1980). Behrendt's model (Plate 9) presents some ambiguity in terms of the I mechanism of failure. Hubbert and Rubery (1959) demonstrated that un-less significant pore pressures develop at the sole of a detachment block whose length is greater than 30 km, then slip along the sole would be impossible. However, at present geopressured zones similar to those reported from drilling data in the Gulf Coastal Plain, are un-known in the Charleston region. In addition, the hypothesis of Harris and Baye.r (1979) suggests that the length of the detachment zone is considerably greater than 30 km even within the coastal stress province of Zoback and Zoback (1980). Furthermore, the lateral compressive I - 15 - I
I forces which give rise to this form of crustal failure are not kown in a neotectonic environment such as Charleston's, but are instead typical of zones of plate-to-plate collision. Very recently, Seeber and Armbruster (1980) have also explored the hypothesis that the Charleston event was caused by slip along the master decollement of Cook et al. (1979). They performed a re-evalua-tion of the seismic intensity data and coseismic phenomena from the Charleston event and compared the results to similar data and analysis from the Himalayan detachment earthquakes. They conclude that the distribution of coseismic phenomena, indicating both extension and compression, suggests a southeastward slip of the detached sheet during the 1886 earthquake. I They state that stress data available indicate that there is a transition in stress orientation with decreasing dip of the detachment toward the ocean, namely minir.um compression (T axis) is I parallel to the dip of the detachment near the northwest end, and maxi-mum compression (P axis) is parallel to the dip near the southeast end of the structure. This relationship is used to infer that stress reversals may occur within the decoupled sheet, and that this is indi . cative of " widespread, ;ravity-induced backalip of the Appalachians" (Seeber and Armbruster, 1980). Seeber and Armbruster's (1980) hypothesis, by their own account, compares the seismic intensity fall-off distributions and structural geometries of two tectonic regimes which are vastly different in terms of stress distributions, relative strain-rates, and seismic mechanisus. They propase that the seismicity of today near Charleston occurs above the detachment during interseismic periods between great earthquakes which occur along the decollement. Thefr only evidence, as stated before, is the nature of the change of stress orientation across the Jppalachian orogen, which is based upon the work of Zoback and I Zoback (1980). Stress provinces ha - :en interpreted, utilizing focal mechanism solutions, recent fault movements, and hydrofracturing in deep walls (Seeber and Armbruster, 1980). These stress i avinces are highly I I
I interpretive because of the mixture of types of information relative to the stress field of North A= erica. Moreover, the hypothesis of Seeber and Armbruster does not address the question of =echanics of the backs 11pping. They indicate that the decollement " constitutes very extended continuous layers of low strength in the crust" at depths of approximately 13 km near Charleston based on seismic reflection data of Cook e.t al. (1979). If backslip originates where the decollement is steepest, say the Brevard Zone or the Great Smokies Thrust, then gravity-induced slip must propagate hundreds of kilometers eastward to the Charleston vicinity. This is difficult to envisage for the same reasons stated previously. Near Cnarieston the effective vertical normal stress of the decollement surface at hypocentral depths would be of the order of approximately 2 kilobars, and the excessive fluid pressures characteristic of active I overthrusting are not known to exist at this depth near Charleston. For these, and other reasons, there is difficulty invoking gravity-induced backslip along the detachment as the likely mechanism for the Charleston event. The reactivated decollement mechanism, whether of thrust or back-slip sense, =ust also overco=e asperities of the detachment in order for slip to occur. Mesozoic rif ting post-dated the for=ation of the decollement, and Rankin (1976) propcsed that failed-arm troughs along the Appalachian salients are deeply rooted, transect the Appalachian structural trend, and are typically reactivated during later episodes of rifting. Moreover, Hatcher and Zietz (1979) and Long (1979) suggest that the development terminates at the Kings Mountain and Charlotte Belts based upon the aeromagnetic and gravity signatures. The presence of the asperities, and possible lateral boundaries, represented by aulocogens or suture zones, would segment the decollement, thereby I providing resistance to low-angle slip. I I l lI
I 2.4.3 Mechanism: Stress Amplification (At Margins of Mafic Plutons) A second hypothesis is that the amplification of regional in situ stress in the crust around the margins of mafic /ultramafic plutons ultimately leads to failure, thereby generating earthquakes (McKeown, 1975; Long, 1976; Kane, 1977). More than a few investigators have proposed this, and differences are largely based upon the cha_acter and manner of deformation of the igneous pluton. This mechanism is suggested to explain the Charleston event (Long and Champion, 1977) because earthquakes there occur within or near a large mafic /ultramafic crustal intrusion as evidenced by a positive Bouguer gravity anomaly (Plate 10). Deep coreholes were drilled up to 1150 m at the center of the anomaly in an effort to detcrmine the source of the anomaly, but the holes penetrated 257 m of basalt under-lain by 121 m of terrigenopa, red clastic rocks of probable Triassic-Jurassic age (Gohn et al., 1978). The actual source was never reached. McKeown (1975) suggested that local high stress concentrations occur within the mafic bodies and at their contacts with the country rock. He further suggested that the earthquakes are the result of release or reduction of these stress concentrations. He compared I (McKeown, 1978) this to the development of similar concentrations developed around rigid inclusions of various shapes in an elastic plate, and suggested that stresses could be amplified around the intrusives rather than around faults or fractures, depending upon the ratio of Young's modulus of the inclusion to that of the host rock. Kane (1977) also recognized the spatial association of major eastern and central U.S. earthquake epicenters and positive Bouguer gravity anomalies, interpreted .co be caused by mafic plutons. He con-tends that neither isostatic effects nor intrusive activity seem likely, and that the most probable cause of the earthquakes is stress amplification caused by lithologic contrast. Kane (1977) further cited published laboratory data which he believes indicate that mafic / I I
I ultre. mafic rock f ails by creeping rather than stick-slip, and that if the plutons causing the high gravity anomalies have been serpentinized rels.tive to the more siliceous hest rock, then they probably deform continuously by creep as changes occur in the regional stress. This situation would be analogous to a hole in a two-dimensional elastic plate >Ic.f. McKeown, 1975). When uniform stress is applied to the boundaries of the plate, it can be shown that the normal stresses are I concentrated at the margin of the hole, and attain values which exceed the applied stresses by several times. campbell (1978) analytically investigated the validity of both McKeown's and Kane's proposed mechanisms. A two-dimensional model of a I vertical circular or elliptical cylinder normal to the plane of hori-zontal uniaxial stress was considered for different assumed effective rigidity moduli, to represene a stiff intrusion versus a plastic one. Equal Poisson's ratios were assumed, these having little effect on the results. Campbell's calculations revealed that if the inclusion is stiffer than the plate, the factor by which the differential stress in-creases as caused by the inclusion, is generally greatest within the inclusion itself, but locally also within the plate at the margin of the plug. Conversely, if the inclusion is weaker than the plate, this factor is highest in pockets just outside the plug, in a direction normal to the applied stress. In general, elliptical bodies increased the tendency for failure at the margins of the body. Campbell (1978) concluded that unserpentinized mafic bodies would likely give rise to seismic events only if they have very sharp edges, and/or only when the I regional differential stress is very high. Conversely, for serpentized intrusions, he concluded they would give rise to seismic events for lower levels of regional applied stress. He concluded further that: (1) the chemical composition of the intrusion, (2) the depth, radius of curvature, and orientation of the intrusion, and (3) the volume of the intrusion, all affect the probability of large earthquakes occurring in proximity to mafic /ultramafic intrusive bodies it the crust. I I
Inherent in the cechanism of stress amplification are a number of uncertainties, although the anisotropy of the lithosphere, both local and regional, has generally been recognized as a major influence upon the strain pattern of the bedrock. These uncertainties (as recognized by Kane, 1977) are due principally to the paucity of data with respect I tc: (1) the precise cause of the gravity anomalies, although most likely they are mafic intrusions; (2) the state of stress in proximity to the anomalies; (3) the three-dimensional geometry of the anomalous masses; (4) the boundary conditions of the host rock containing the I anomalous masses. Kane (1977) also recognized that regions of eastern North America I which exhibit positive Bouguer anomalies do not have associated seismic He speculates that this could be due to the lack of serpen-I ac tivi r.y. tization of the mafic body, to an insufficiently large or variable regional stress field, or ineffective geocetric relations between the body and the associated stress field. Two principal questions not treated by either Kane (1977) or Campbell (1978) are: I (1) What is the nature of changes in the regional stress field in the central and eastern United States which lead to stress concentrations? I (2) Why do these changes occur? A plausible answer to these two questions would indicate that this mechanism of stress amplification is not one of regional significance with respect to the occurrence of large earthquakes. Although the information at hand lends support to the general validity of this I hypothesis, very little data have been obtained to indicate that this mechanism acted to trigger the 1886 event at Charleston. These data I I
I are principally circumstantial and conceptual in nature (Plates 10 and 11).
2.4.4 Mechanism
Reactivation of Steep Basement Faults A third hypothesis for the origin of the Charleston event, dif-ferent aspects of which are supported by many investigators, is that I the earthquake and recent seismicity in the eastern and central U.S. were the result of reactivation of steeply dipping basement faults. Investigators prefer causative faults of different tectonic origins in the history of the orogen, but believe essentially that slip along faults of this nature is the ultimate cause of the strain energy release in this region. This mechanism, in the view of some investiga-tors, conflicts with that of a causative master decollement in that Cook et al. (1979) and Harris and Bayer (1979) indicate that the detachment is apparently through going and not displaced vertically by later faults, such as those formed during Trio-Jurassic rifting. I Indeed, Harris and Bayer (1979) interpret that normal faults of this age are listric into the decollement (Plate 9). By contrast, there is a great deal of evidence from the Charleston region indicating vertical displacements along faults since Cretaceous time (see Section 2.3), and it is beyond the scope of this report to I provide every example. Consequently, Plate 4 illustrates these occur-rences only for South Carolina, including the Continental Shelf. This third alternative mechanism for the Charleston event seems to be divided into two broad areas. One school of thought indicates that the Charleston event was the result of strain energy release along a north-west-trending zone of weakness whi.ch is a tectonic inheritance from the evolutionary development of the Appalachian Orogen, namely a remanent of a failed-arm trough (aulocogen) of a triple junction (Plate 10). The other school of thought seems to favor vertical tectonics affecting either fault-bounded basement blocks, or old faults along borders of Traissic or younger basi:s. I I
I Rankin (1976) has postulated that the salients and recesses in the structural trend of the Appalachian Orogen are inherited from the initial break-up of the proto-North A=erican continent by the intersec-tion of rift valleys radiating from triple junctions about 820 million years ago. He presents strong evidence for the failed-arm trough of the triple junctions, striking across the orogen in the Appalachian I recesses from the geologic record of the salients (Plate 10). et al. (1978) and Rankin (1978) further indicate that the Charleston Rankin event meisoseismal area is siteted within a major Late Triassic-I Early Jurassic rift basin that connect the ancestral Gulf of Mexico with rifts that paralleled Appalachian structural trends but were located on the Atlantic Continental Shelf. The northern boundary of this trough is east-northeast in South Carolina (Plate 2). Ha further states (Rankin, 1978) that the Blake Spur Fracture Zone, which strikes N55*W (Plate 2) and " offsets" the Atlantic Shelf Edge Magnetic Anomaly, occurs withn 200 km of Charleston and represents a probable Jurassic transform fault as the extensional zone evolved into a sid-ocean ridge. Rankin (1978) and Fletcher et al. (1978) suggest that this evidence points toward a possible explanation for earthquakes along northwest-I trending South Qirolina-Georgia seismic zone of Bollinger (1973b). Dillon (1974) obtained evidence from offshore seismic reflection pro- , filing for Cenozoic northwest-trending high-angle faults along strike l from the Blake zone. Recent work by Behrendt et al. , (1980) indicates that only one very small such fracture exists, but mat northeast-striking faults are recognized. Furthermore, Nishenko and Sykes (1979) interpreted the Blake zone to be related to the Gecrgia-Florida rift zone which intersects it at the Shelf anomaly. Hence, they suggest that the Charleston event is related to this intersection, and is somehow related to the South Carolina-Georgia seismic zone. To date the only evidence of a northwest-trending structure in the crust near Charleston is based on geophysics such as: (1) distribution of seismicity (Bollinger,1973a,b); (2) focal mechanism solution from Summerville, S.C. (Tarr, 1977); I I
I (3) northwest-trending gravity contour irregularities superimposed on diabase-caused anomaly near Bowman, S.C. (McKee, 1973), where a microseismic event has been recorded (Tarr, 1977); (4) predominance of N20*-30*W induced hydraulic fractures in Cenozoic rocks in two deep coreholes near Charleston (Zoback et al., 1978), plus possible evidence of northwest-trending normal or reverse faults in the Coastal Plain near Charlestot. (Zoback et al, 1978; Inden and Zupan, 1975), and folding of similar age (Colquhoun and Comer, 1973); (5) the seismic zone coincides with a major Appalachian salient (Rankin, 1976) implying a remanent failed-arm trough nearby. Many investigators believe that reactivation of basement faults from Precambrian to Mesozoic age resulted in slip which produced the 1886 Charleston event. Sheridan (1974, 1976) proposed a model of base-I ment structure of the Atlantic continental margin in which he believed that sedimentary basins of the geosyncline are fault-bounded troughs with alignments more or less parallel to the continental slope. Funda-I mental basement faults were inferred from =agnetic anomalies and linear hinge zones in the basement from seismic reflection data (Sheridan, 1974). Reinhardt et al. (1980) have identified post-Paleocene high-angle reverse displacements of tens to hundreds of meters on diversely oriented fault boundaries of a Paleocene continental basin. Wentworth and Mergner-Keefer (1980) have interpreted that most Cenozoic reverse faults of the Atlantic margin "probably follow older discontinuities, especially early Mesozoic normal faults...". Thay infer (1980) that the Charleston event probably had a reverse-fault origin and cite Behrendt et al. (1980) evidence of the Cooke and Helena Banks Faults, discuused previously (Section 2.4.2). The concept of remanent aulocogens which may be re-activated according to the Wilson cycle (Wilson, 1966) as proposed by Rankin (1976) has gained supportive evidence in recent years. Root and Hoskins (1977) discovered a major fault zone, the Transylvania fault, I I .
I - which strikes approximately east-west along the 40*N latitude line, and
~
appears to be an extension of a fracture zone farther east along the line of the Kelvin seamounts. Farther south, in the Virginias, the 38th parallel fracture zone has been shown to be a zone of recurrent igneous activity from Precambrian to Late Eocene time (Dennison and Johnson, 1971). Other similar examples exist in the northern Appala-chians. The significance of aulocogens is that there is geological evidence supporting their existence within the continental crust, and I evidence of their relation to the Wilson cycle, namely that aulocogens are centers of tectonic reactivation with respect to crustal transla-tions and dislocations and igneous activity. I In the instance of the Charleston event, however, only circum-stantial evidence exists that permits identification of a northwest-I trending zone of weakness along which earthquake-inducing slip occurred near Charleston in 1886. Furthermore, the distribution of seismicity along the northwest zone is diffuse (Bollinger,1973b), and there are clearly aseismic segments of the zone in terms of the record of historic seismicity. The evidence of post-Cretaceous deformation in South Carolina indicates that tectonic strains are not limited to a northwest-trending fracture zone. There is also evidence that basement blocks, as sug-gested by Sheridan (1974, 1976) may have been active in Cenozoic time. The boundaries of these blocks are diversely oriented steep faults, from Precambrian to Mesozoic age, which have experienced principally vertical displacements. Owens (1970), Winker and Howard (1977), Winker (1980), Heller et al. (1980) and Zimmerman (1980) all have shown that differential vertical crustal movements are recorded in the geologic record, rupporting the geodetic findings of Meade (1971), Holdahl and I Morrison (1974), and Lyttle et al. (1979) tehich indicate areas of rela-tive subsidence and uplift in the southeastern United States. It seems I very possible, therefore, that the Ce'nozc.ic high-angle reverse fault displacement in the Charleston vicinity, offshore from Charleston, and , along the Fall Line may be the result of complex differential crustal I I
I movements, of which the dominant component is a vertical one and, thus, seismicity in the Charleston vicinity may reflect these movements. For this reason, it is questionable that the Charleston earthquake of 1886
) was governed solely by strain energy release along a northwest-trending zone of crustal weakness.
I I I I I I 1.I I I I I 1 j 25 - I
l I 3.0 EARTHQUAKE PROBABILITIES ASSOCIATED WITH VARIOUS HYPOTHESES CONCERNING EASTERN U.S. TECTONICS 3.1 GENERAL I To provide a basis for compariscn and analysis, probabilistic seismic hazard results were obtained at the Virgil C. Summer Nuclear Station for several hypotheses on eastern U.S. tectonics. These seis-mic hazard results are described in the following paragraphs. For all probability calculations, the following attenuation func-tion was used to . describe the variation of site intensity Is with epicentral intensity Ie and epicentral distance a: Is " Ie Of. 10 km Is = 3.08 + eI - 1.34 in a a> 10 km This equation is based on the observed attenuation of intensities during the 1886 Charleston earthquake. Uncertainty in the predicted intensity was described by a normal distribution with a standard devia-tion of 1.19 intensity units, which is typical of observed scatter in I ground motion estimates. 3.2 MES0 ZOIC RIFTING Se tsmogenic zones were drawn corresponding to the hypothesis that earthquakes in the southeastern U.S. are associated with Mesozoic rifting. These zones are shown in Plate 13. Historical earthquakes which have occurred in each zone were used to define seismic activity rates (for MMI > IV) and maximum earthquake size. The relatively small seismogenic zones in North Carolina and' South Carolina did not encom-
- pass any substantial historic seismicity; hence these zones were given l an activity rate of 5 percent of the historical rate observed for the large zone in South Carolina, Georgia, and Alabama. All zones were assigned a maximum possible intensity of X - XI, and a b-value of 0.5 l was used to describe the log-number versus MM Intensity relation.
I I i I _-_
I The background seismicity in the area was accounted for in the analysis using historical earthquakes in the southeastern U.S. which have not occurred in the above-defined seismogenic zones. This back-ground seismicity dominates the seismic hazard at the Virgil C. Summer Nuclear Station. The primary effect of the seismogenic zones as-sociated with Mesozoic rifting is to restrict large earthquakes to occur some distance from the facility. For background seismicity a maximum possible intensity of VIII-IX was used, along with a b-value of 0.5 and a rate of activity per unit area which is consistent with historical seismic events. The following table describes annual probabilities and return periods associated with various levels of MM Intensity at the Virgil C. Summer Nuclear Station: I Probability Results for Mesozoic Rifting Hypothesis MMI V VI VII VIII Annual Prob. .73 x 10~ .17 x 10~ .30 x 10 -3 .33 x 10 I Return Period, Years 140 590 3300 30,000 As discussed above, the background seismicity accounts for a large part of the calculated hazard, on the order of 70 percent. l 3.3 DECOLLEMENT HYPOTHESIS The decollement hypothesis previously described in this report was examined to determine the effect on seismic hazard, if this hypothesis ( is adopted to describe eastern U.S. tectonics. The seismogenic zone l used to represent this hypothesis is shown in Plate 14. All historical earthquakes were used to determine the rate of activity in this gener-l ally broad zone encompassing the Piedmont and Coastal Plain. The maxi-mum earthquake possible in the zone was assumed to be MM Intensity I X-XI, and a b-value of 0.5 was used. I
I The decollement hypothesis described by Seeber and Armbruster (1980) associates large earthquakes with large zones of backslip along the decollement. These zones may have dimensions of several hundreds of kilometers; the release of seismic energy within these large zones is not uniform, as evidenced by the variation in seismic intensity observed within several hundred kilometers of Charleston during the 1866 earthquake. To estimate the seismic hazard accurately, several mathe=atical models may be conceived to represent the source of energy release and the resulting ground motions at the earth's surface. The most logical seismic hazard model would represent the source of seismic energy as a horizontal plane of rupture along the decolle-ment, account for the uncertainty in ground motion intensity above this plane, and estimate ground motion intensity at more distant locations (away frce the area of the plane) using available attenuation functions for ground motion. This would involve a straightforward application of seismic hazard analysis. The plane of rupture would be moved to all possible locations on the decollement, and the seismic hazard at a site would be accu =ulated from all possible rupture locations and all pos-sible sizes of rupture. An alternative method of analysis is to treat the ground motion intensity at the earth's surface as if the seismic energy emanated from a single point source (at the earth's surface), and to attenuate the ground motion intensity from this hypothetical point source. The seis-mic hazard analysis would consist of moving the point source over all possible locations consistent with the decollement hypothesis, and I varying the maximum intensity in size. This is a viable alternative method of analysis because che available intensity data from the one large historical earthquake in the area do not suggest any great dif-I ference in intensity attenuation between observations close to (within several hundred kilometers of) the maisoseismal area, from observations farther away. This is evident in Plate 15, which shows intensity ob-servations during the 1886 Charleston earthquake (as reported by Bollinger and Stover, 1976), plotted versus epicentral distance. There I I
is no indication that intensities within several hundred kilometers of the region with highest da= age, attenuate less or are less scattered than intensities at farther distances. Therefore, even if the earthquake is caused by rupture along the decollement, the ground motion hazard can be calculated by using the meisoseismal area as a point source. This is a great advantage computationally since available computer programs can be utilized to perform the analysis. The attenuation equation shown in Plate 15 is that described in Section 3.1; it correctly predicts the average M31I at a given distance (not the mean distance for a given M311), as it should (Bollinger, 1977). The following table describes probabilities and return periods associated with 101 intensities at the Virgil C. Summer Nuclear site. These were calculated using the above-described point-source analysis, with parameters and an attenuation function as previously discussed. Probability Results for Decollement Hypothesis , MMI V VI VII VIII Annual Probs. .91 x 10 '
.25 x 10 ~
.60 x 10
-3 .12 x 10
-3 Return Period, 110 400 1700 8300 Years 1
3.4 MAFIC PLUT0N - STRESS CONCENTRATION THEORY I l I The theory that mafic plutons cause crustal stress concentrations was examined as a hypothesis for eastern U.S. tectonics. Available maps of mafic plutons in the area of the nuclear site were examined and I I seismogenic zones were drawn around these intrusions. With the excep-tion of the Charleston earthquake, which may be associated with mafic j plutons in the area, these seismogenic zones exhibited no distin-guishing features in terms of historical seismicity, from that observed in the adjacent region. As a result, any seismic hazard results would I be similar to the hypotheses already examined. That is, seismicity would be represented by a large region of background seismicity with a
- I
i i i
~
i i 1 l l maximum intensity of VIII-IX (as in the Mesozoic rif ting hypothesis) or } of X-XI (as in the decollement hypothesis). For this reason we do not present results here specifically for the mafic pluton-stress concen- ! tration theory. i l 4
+
1 lI i i i
- I i
i k I i h 1 i i I ) J l 4 r l b. I i _ . . _ . _ . ~ . . . _ _ _ - _ _ . _ _ . _
4.0 CONCLUSION
S 4.1 GENERAL The likelihood of occurrence of another event such as the 1886 Charleston earthquake is being considered, and the question of its possible impact upon the Virgil C. Summer Nuclear Station depends upon the mechanism governing the event. A thorough review of available data and literature has been performed regarding the geologic cause of the earthquake, and probabilistic analyses based upon the three most prominant possible scenarios were made for comparison to the current design parameters at the Su=mer facility. This section presents the principal conclusions of this review and the probabilistic assessment. 4.2 EARTHOUAKE MECHANISM: UNCERTAINTIES This review identified three principal hypotheses for the origin of the Charleston event, with variations possible for each one. None of the three is without weakness, and none can be considered to be unequivocal. It is concluded that the cause of the earthquake is still not known. The three major hypotheses which have been reviewed are: (a) re-activation of a master decolle=ent either by active thrusting or by gravity-induced backslip, (b) stress amplification at the cargins of mafic or ultramafic plutons, (c) reactivation of steep basement faults
~
of diverse orientation and age of development. It can be concluded with a high degree of certainty that a master decellement is a fundamental structure of the Appalachian Orogen, beneath the Blue Ridge, Piedmont, and perhaps at least part of the Coastal Plain Provinces. From some data, it has been inferred that Late Cenozoic movements occurred on the detachment toward the continent, whereas from other data, it has been interpreted that Late Cenozoic movement away from the continent is occurring along the detachment as induced by gravitational backslip. iI l !
There are several uncertainties regarding the ability of a decol-lement in the neotectonic environment of the southeast U.S. to slip at depths of 12 to 18 km:
- asperities need to be overcome to accomplish slip;
- the normal stress of the overburden must be overcome (i.e.,
roughly 2 kbars);
- presumably, nor=al fluid pressures occur, as opposed to over-pressures in active overthrust belts.
In short, the shear strength of the decollement would be grr.ater than that of other zones of weakness. Moreover, if slip towara the conti-nent is proposed, then : .s difficult to reconcile the following:
- indications of possible dip-slip faults with northwest strikes I both onshore and offshore; whether lateral shortening due to sea-floor spreading is suf-ficient to cause low-angle thrust faults splaying from the decollement (listric faults).
Conversely, if the slip is away from the continent on the decolle-ment, then it is difficult to explain: the nature of the eastern boundary of the allocthonous sheet;
- propogation of slip from the steep portion of the detachment in the Blue Ridge Province, to the Charleston vicinity nearly 200 km away;
- whether the interpretations of the state of stress in the litho-sphere from three different types of widespread information l (focal mechanism solutions, hydraulic fracturing tests, and orientations of geologic deformation features) provide an adequate knowledge of the stress distribution; whether comparison of the Appalachians today with the Himalayans
, today is reasonable. I l I l l
~
It is further concluded that stress ar;11fication at the margins of safic/ultramafic plutons because of lithologic contrast or anisot-ropy is mechanically feasible. Analytical modeling for both rigid and plastic plutons in an elastic plate indicates that plastic plutons with I angular shapes are more likely to fail at the margins than are rigid plutons. The principal weaknesses of this model, with reference to the Charleston event are: it is based upon 2-D linear elastic analysis, and therefore simplistic; the paucity of data regarding the nature and geometry of sources of positive Bouguer gravity anomalies; the state of stress in proximity to the anomalous masses; the assumptions of changes in the regional stress field, but not specific as to the nature or cause of these enanges; the absence of knowledge regarding the boundary conditions;
- the absence of data in the Charleston area confirming that seismic events are always restricted to margins of plutons (interpreted from gravity anomalies).
Abundant evidence of the reactivation of steep faults in the base-ment has been presented. It is possible that reactivation of an ancient aulocogen is causing the seismicity of the South Carolina-Georgia seismic zone. Moreover, the borders of basement blocks and buried Triassic basins also have been reactivated, and may also contribute to the recent seismicity. However, in the Charleston area, it is uncertain:
- whether any northwest-striking zone of crustal weakness exists; what the relation of the aulocogen is to the Appalachian decol-lement;
- why Cmozoic faults near Charleston have diverse orientations I
and slip senses; I I
- why certain segments of the South Carolina-Georgia seismic zone are aseismic.
4.3 DETERMINISTIC CONCLUSIONS I The foregoing discussions show that several explanations for the Cht rleston earthquake of 1886 are possible. Analyses of geology, geophysics, and seismology are not in all cases compatible, and clearly more data are needed to answer such generic questions as: (1) What are the geological and geomechanical meanings of the I recent and historic seismicity in the Charleston region? (2) What is the relation of crustal straining during the Cenozoic Era to evidence of Holocene crustal straining such as seismicity, differential vertical movements, and lateral I sea-floor spreading? (3) What is the geologic structure of the crust near Charleston? (4) What is the current state of stress in the crust near Charles-ton, and how does it relate to.the stress and strain history of the crust there? (5) At what rate (s) have these strains changed in the past, and what, if any, is the current change in strain rate? (6) What is the source of strain energy stored in the crust near Charleston? It is conceivable that answers to some or all of these questions could either eliminate or refine some of the three hypotheses regarding the origin of the Charleston event, and permit a more accurate assess-ment of the probability of a recurrence of the event elsewhere in the southeastern U.S. Until now, current evaluations of this probability have been such that the Charleston event of 1886 has been fixed to the meisoseismal area with respect to the seimic design of the nuclear facility. At the current state of knowledge, it would appear that the present day reactions of the earth's crust (the dense historical seismic activity restricted to the Charleston-Summerville zone) may be I l the most diagnostic constraint in assessing the distribution of ' significant events in the i==ediate future. This revier has revealed I no significant evidence which indicates that an event of this intensity should be transferred to the site from Charleston. The spectrum of interpretations is so broad that for any given hypothesis, arguments I can be =ade as to why such an event would occur again only within the Charleston area. These arguments, of course, would be entirely specu-lative because the data at hand are possibly misleading as to their true meaning regarding the origin or cause of the 1886 event. Conse-quently, there is no justification based upon the review presented herein, for overturning the commonly accepted conclusion of restricting the locality of a recurrent event of similar intensity as the 1886 Charleston earthquake to the same meisoseismal area. 4.4 PROBASILISTIC ANALYSES CONCLUSIONS The following table summarizes the probability results in terms of return period for the hypotheses described here and for results pre-sented previously: Return Periods (in years) for various Seiscogenic Zones MMI V VI VII VIII
- F S AR zo ne s wi t h max.
intensity = largest 200 830 4500 32,000 I hist. intensity + 1
*FSAR zones with max.
intensity = largest 270 1400 10,000 67,000 I hist. intensity
*Algermissen-Perkins zones with =ax. 180 714 3100 15,000 I intensity = largest hist. int.
Mesozoic Rifting 140 590 3300 30,000 Deco 11ement I 110 400 1700 8,300
- FSAR Table 361.19-1, Amendment 21 I
These results indicate approximately the same return periods for mi Intensities of V through VII at the site. Thus, even if any hypotheses I I I
1 presented here were adopted to describe tectonics in the eastern U.S., it would not substantially change the perception of the seismic hazard I at the Virgil C. Summer Nuclear Station resulting from tectonic earth-quakes. It is therefore concluded that the seismic design basis for tectonic earthquakes is adequate regardless of which current hypothesis is used to explain the distribution of seismic activity in tectonic pminces in the eastem U.S. I I I I I I I I I I I I
I I REFERENCES Aggarwal, Y.P. and Sykes, L.R. , 1978. I Farthquakes, Faults and Nuclear Power Plants in Southern New Jersey. Science, vol 200, no. 4340,
- p. 425-429.
Allen, C.R., 1975. Geological Criteria for Evaluating Seismicity. Geological Society of America. Bull., vol. 86, p. 1041-1057. I Behrendt, J.C., Hamilton, R.M. , Acke rmann, H.D. , Henry , J.V. , and Bayer, K.C., 1980. Seismic Reflection Evidence for Cenozoic Re-activation of Older Faults on Land and Offshore in the area of the I Charleston, South Carolina 1886 Earthquake, Geological Society of America. vol. 12, no. 7, Abs t. , M d An. Mtg., Atlanta, Georgia. Bollinger, G.A., 1973a. I Seismicity and Crustal Uplift in the South-eastern United States. American Journal of Science, Cooper Volume 173-8, p. 396-408. Bollinger, G. A. , 1973b. Seismicity of the Southeast United States; Bull. Seismological Society of America, vol. 63, no. 5, p. 1785-1808. Bollinger, G.A., 1975. A Catalog of Southeastern United States Earth-quakes - 1754-1974. Virginia Polytechnic Institute, Department of Geologic Sciences, Research Div. Bull. 101, Blacksburg, Va. Bollinger, G.A., 1977. Reinterpretation of the Intensity Data for the 1886 Charleston, South Carolina Earthquake. I Society Prof. Paper 1028-B, p. 17-32. U.S. Geological Bollinger, G. A. , Langer, C.J. , and Harding, S.T. , 1976. The Eastern Tennessee Earthquake Sequence of October through December, 1973. Seismological Society of A= erica, Bull. , vol. 66, no. 2, p. 525-548, April 1976'. Bollinger, G. A. , and Mathena, E. , 1980. Seismicity of the Southeastern United States. Southeastern U.S. Seismic Network Bulletin No's 1 through 5, July 1, 1977 through December 1979. Bollinger, G.A., and Stover, C.W., 1976. List of Intensities for the 1886 Charleston, South Carolina Eartnquake. I Society. Open-File Report 76-66, 31 pp. U.S. Geological Campbell, D.L., 1978. Investigation of the Stress-Concentration Mechanism for Intraplate Earthquake. Geophysical Research Letters, vol. 5, no. 6, Paper no. 8LO481, p. 477-479. I I I I Cof f=an, J.L. and Stove r, C.W. , 1975, 1976, and 1977. United States Earthquakes. U.S. Dept. of Commerce and U.S. Dept. of Interior, annual series. Colquhoun, D.J. and Comer, C.D. , 19 73. The Stono Arch, a Newly Dis-covered Breached Anticline Near Charleston, South Carolina. South I Carolina State Development Boards; Division of Geology, Geologic Notes, vol. 17, no. 4, Co_umbia, S.C. Cook, F. A. , Albaugh, D.S. , Brown, L.D. , Kaufman, S. , Oliver, J.E. , and Hatcher, R.D. Jr., 1979. Thin-Skinned Tectonics in the Crystal-line Southern Appalachians. C0 CORP Seismic Reflection Profiling I of the Blue Ridge and Piedmont; Geology, vol. 7, no. 12,
- p. 563-567.
I Cook, F.A., Brown, L.D. , and Olive r, J.E. , 1980. The Southern Appalachians and the Growth of Continents. Scientific American, vol. 243, no. 4, pp. 156-170, October 1980. Dennison, J.M. and Johnson, R.W. Jr. , 1971. Tertiary Intrusions and Associated Phenomena Near the Thirty-Eighth Parallel Fracture Zone in Virginia and West Virginia. Geological Society of America. Bull., vol. 82, p. 501-508. Dewey , J.F. , and Bird, J.M. , 1970. >buntain Belts and the New Global I Tectonics. Jour. Geophysical Research, vol. 75, no. 14,
- p. 2625-2647, May 10, 1970.
I Dillon, W.P., 1974. Faults related to seismicity off the Coast of South Carolina. U.S. Geological Survey Open File Rept. , 74-145, 8 p. Fisher, G.W., Pettijohn, F.J. , Reed, J.C. Jr. , and Weaver, K.N. , 1970. . Studies of Appalachian Geology - Central and Southern; Inter-l science Publishers, 1970. i Fletcher, J.B., Sbar, M.L., and Sykes, L.R., 1978. Seismic Trends and Travel-Time Residuals in Eastern North America and Their Tectonic , l l Implications. Geological Society of America Bull. vol. 89 pp. 5 1656-1676. l l g Gohn, G.S., Higgins, B.B., Smith, C.C., and Owens, J.P., 1977. !E Lithostratigraphy of the Deep Corehole (Clubhouse Crossroads Corehole 1) Near Charleston, S.C. U.S. Geological Survey, Prof. Paper 1028-E, p. 59-70. l I Gohn, G.S., Lanphere M.A., and Higgins, B.B., 1978. Regional Implica-tions of Triassic of Jurassic Age for Basalt and Sedimentary Red i Beds in the South Carolina Coastal Plain. Science, vol. 202, pp. 887-890, November 1978. lI Hadley, J.B., and Devine, J.F., 1974. Seismotectonic Map of the Eastern United States; U.S. Geological Survey MF 620. I Harris, L.D. and Bayer, K.C. ,1979. Sequential Development of the Appalachian Orogen Above a Master Decollement - A Hypothesis. Geology , vol. 7, p. 568-572. Hatcher, R.D. Jr. , 1972. Development Model for the Southern Appalachians; Geological Society of America Bull. , vol. 83, no. 9, pp. 2735-2760. Hatcher, R.D., 1978. Tectonics of the Wesern Piedmont and Blue Ridge, Southern Appalachians: Review and speculation. American Journal of Science, vol. 278, p. 276-304. Hatcher, R.D. Jr., Howell, D.E., and Talwani, Pradeep, 1977. Eastern I Piedmont Fault System: Speculations on its Extent. Geology, vol. 5, pp. 636-640. Hatcher R.D. Jr. , and Zie tz , I. , 19 79. I Thin Crystalline Thrust Sheets in the Southern Appalachian Inner Piedmont and Blue Ridge: Interpretation Based upon Regional Aeromagnetic data. Geological Society of America, Abst. with Progr., vol. 10, p. 417. I Hazel, J.E., Bybell, L.M., Christopher, R.A., Frederiksen, N.O., May, F.E., McLean, D.M., Poore, R.Z., Smith, C.C., Sohl, N.F., Valentine, P.C. , and Witmer, R.J . , 19 77. U.S. Geological Survey Prof. Paper 1028-F, p. 71-90. Heller, P.L. , Wentwo rth, C.M. , Poag, C.W. , and Me rgner-Keef er, Ma rcia, l l 1980. Episodic Post-Rif t Subsidence of the Eastern U.S l 5 Continental Margin. Geological Society of America, vol. 12, I no. 7, Abst. 93, Ann. Mtg. Atlanta, Georgia, 1980. Heron, S.D., Judd, J.B., and Johnson, H.S., 1971. Clastic Dikes i Associated with Soil Horizons in the North and South Carolina i Coastal Plain. Geological Society of America Bull. , vol. 82,
- p. 1801-1810.
Holdahl, S.R. and Morrison, N.L., 1974. Regional Investigations of. l Vertical Crustal Movements in the U.S. Using Precise Relevelings l and Mareograph Data. Tectonophysics, vol. 23, p. 373-390. Howell, D.E. and Zupan, A.W., 1974. Evidence for Post-Cretaceous Tectonic Activity in the Westfield Creek Area North of Cheraw, South Carolina. S. C. State Development Board, Division of l Geology, Geologic Notes, vol. 18, p. 98-105. 1 I lI t
I - Inden, R.F. and Zupan, A.W., 1975. Normal Faulting of Upper Coastal Plain Sediments, Ideal Kaolin Mine, Langley, South Carolina. South Carolina State Development Board, Divisionof Geology, Geologic Notes, vol. 19, no. 14, 1975. Kane, M.F., 1977. Correlation of Major Eastern Earthquake Centers and Mafic /Ultramafic Basement Masses. U.S. Geological Survey Prof. Paper 1028-0, p. 119-204. Long, L.T., 1979. the Carolina Slate Belt - Evidence of a Continental Rift Zone. Geology, vol. 7, p. 180-184. I Long, L.T., Talwani, P., and Bridges, Gravity Map of South Carolina. Map Ser. 21, 27 p. S.R., 1975. Simple Bouguer South Carolina Division Geology, Long, L.T., Talwani, P., and Bridges, S.R., 1976. Simple Bouguer Gravity Map of South Carolina; South Carolina Development Boa rd , Division of Geology, Scale 1:500,000. I Long, L.T. and Champion, J.W. Jr. 1977. Bouguer Gravity Map of the Summe rville-Charles ton, South Carolina, Epicentral Zone and I Tectonic Implications,. U.S. Geological Survey Prof. Paper 1028-K, pp. 151-166. I Long, T.E., 1976. Speculations Concerning Southeastern Earthquakes, Mafic Intrusions, Gravity Anocalies, and Stress Amplification. Earthquake Notes, vol. 47, no. 3, p. 29-35. Lyttle, P.T. , Gohn, G.S. , Higgins, B.B. , and Wright, D.S., 1979. l Vertical Crustal Movements in the Charleston, South Carolina-Savannah, Georgia Area. Tectonophysics, vol. 52, p. 183-189. Marine, I.W. and Siple, G.E., 1974. Buried Triassic Basin in the Central Savannah River Area, South Carolina and Georgia. Geological Society of America Bull. vol. 85, no. 2, p. 311-320. Matthews, R.K. and Poore, R.Z., 1980. Tertiary 3180 Record and Glacio-Eustatic Sea-Level Fluctuations. Geology, vol. 8,
- p. 501-503.
May, P.R., 1971. Pattern of Triassic-Jurassic Diabase Dikes Around the ) North Atlantic in the Context of Predrif t Position of the Con-l tinents. Geological Society of America Bull. , vol. 82, l p. 1285-1292. lI l I I I McKee, J.H., 1973. A Geophysical Study of Microearthquake Activity Near Bot. ::an, South Carolina. Unpublished M.S. Thesis. Georgia Institute of Technology, McKeown, F.A., 1975. Mafic Intrusives and Their Contact Zones are Source Zones of Many Earthquakes in Central and Southeastern United States. Earthquake Notes, vol. 46, no. 53, Abstract. I McKeown, F.A., 1978. Hypothesis: Many Earthquakes in the Central and Southeastern United States are Causually Related to Mafic Intrusive Bodies: Jour. Res., U.S. Geological Survey. vol. 6,
-I no. 1 p 41-50.
I Meade, B.K., 1971. Report of the Sub-Commission of Recent Crustal Movements in North America. International Association of Geodesy, 15th General Assembly, Moscow, USSR. Nishenko, S.P. and Sykes, L.R., 1979. Fracture Zones, Mesozoic Rif ts and the Tectonic Setting of the Charleston, South Carolina Earthquake of 1886. Transaction American Geophysical Union, vol. 60, no. 18. Overstreet, W.C. and Bell, H., 1965. The Crystalline Rocks of South Carclina. U.S. Geological Survey Bull. 1183. Owens, J.P., 1970. Post-Triassic Tectonic Movements in the Central and I Southern appalachians as Recorded by Sediments of the Atlantic Coastal Plain: M Fisher, and others, eds., Studies of Appalachian Geology - Central and Southern; Interscience Publishers, John Wiley & Sons, 1970. Prowell, D.C. , and O' Conner, S.J. , 1978. Belair Fault Zone: Evidence of Tertiary Fault Displacement in Eastern Georgia. Geology, vol. 6, p. 681-6S4. Rankin, D.W., 1975. The Continental Margin of Eastern N >rth America in E the Southern Appalachians: ,the Opening and Closing of the Proto-5 Atlantic Ocean. American Journal of Sciences, col. 275-A,
- p. 298-336, 1975.
Ra nkin, D.W., 1976. Appalachian Salients and Recesses: Late Pre-cambrian Continental 3reakup and the Opening of the Iapetus Ocean. Journal Geophysical Research, vol. 81, no. 32, p. 5605-5619. Rankin, D.W., 1977. Studies Related to the Charleston, South Carolina Earthquake of 1886 - Introduction. U.S. Geological Survey Prof. Paper 1028-A. I I I
i I Rankin, D.W., 1978. The Charleston, South Carolina Earthquake of 1886 and the Blake Spur Fracture Zone. Geological Society of America, vol. 10, no. 4, 27th Ann. Mtg. Southeastern Section - Abs. of Programs. Rankin, D.W. , Popeno, Peter, and Kiltgord, K.D., 1978. The Tectonic Setting of Charleston, South Carolina. Geological Society of America, vol. 10, no. 4, 27th Ann. Mtg. Southeastern Section Abs. of Programs. I Ra tclif fe, N.M. , 1980. Brittle Faults (3amapo Fault) and Phyllonitic Ductile Shear Zones in the Basement Rocks of the Ramapo Seismic Zones of New York and New Jersey, and Iheir Relationship to I Current Seismicity: g Fiel.i Studies of New Jersey Geology and Guide to Field Trips, 52nd Ann. Mtg. of New York State Geol. Assoc. , Rutgers Univ. , 1980. Reinhardt, J. , Prowell, D.C. , Christopher, R. A. , and Ma rkewich, H. W. , 1980. Evidence of Cenozoic Tectonics from an Isolated Sedimentary Basin Near Warm Springs, Georgia. Geological Society of America, vol. 12, no. 7, Abst. 93rd Ann. Meg., Atlanta, Georgia. Roots, S.I. and Hoskins, D.M., 1977. f at 40'N Fault Zone, I Pennsylvania: A New Interpretation. Geology, vol. 5, 719-723. p. I Sbar, M.L. and Sykes, L.R., 1973. Contemporary Compressive Stress and Seismicity in Eastern North America: An Example of Intra-Plate Tectonics; Geological Society of America Bull. , vol. 84, no. 6,
- p. 1861-1881.
Seeber, L. and Armbruster, J.G., 1980. The 1886 Charleston, South , Carolina Earthquake and the Appalachian Detachment. In press. Sheridan, R.E., 1974. Conceptual Model for the Block-Fault Origin of the North American Atlantic Continental Margin Geosyncline. Geology, vol. p. 465-468. Sheridan, R.E., 1976. Sedimentary Basins of the Atlantic 4fargin of North Anerican. Tectonophysics, vol. 36, p. 113-132. Talwani, Pradeep, Amick, D.C. , and Logan, R. ,19 79. A Model to Explain I the Interplate Seismicity in the South Carolina Coastal Plain. Transaction American Geophysical Union, vol. 60, no. 18. l Tarr, A.C., 1977. Recent Seirmicity Near Charleston, South Carolina, and its Relationship to t i August 31, 1886 Earthquake. U.S. Geological survey Paper 1028-D, p. 43-58. I I
I Wentworth, C.M. and Mergner-Keefer, Marcia, 1980. Atlantic Coast Reverse-Fault Domain: Probable Source of East-Coast Seismicity. Geological Society of America, vol. 12, no. 7, Abst. 93rd, Ann. Meg., Atlanta, Georgia. Wilson, 1966. Did the Atlantic Close and Then Re-Open? Nature, vol. 211, no. 5050, p. 676-681. Winker, C.D. and Howard, J.D., 1977. Correlation of Tectonically I Deformed Shorelines on the Southern Atlantic Coastal Plain. Goelogy, vol. 5, p. 123-127. I Winker, C.D., 1980. Rates of Regional Quaternary Deformation, Atlantic and Gulf Coastal Plains. no. 7, Abs t. Geological Society of America, vol. 12, 93rd Ann. Mtg., Atlanta, Georgia. Ziccerman, R.A., 1980. Patterns of Post-Triassic Uplift and Inferred Fall Zone Faulting in the Eastern United States. Geological Survey of America, vol. 12, no. 7, Abst. 93rd Ann. Mtg., Atlanta, Georgia. Zo back, M.D. , Healy , J.H. , Roller, J.C. , Gohn G.S. , and Higgins , B.B., I 1978. Normal Faulting and In-Situ Stress in the South Carolina Coastal Plain Near Charleston. Geology, vol. 6, no. 3,
- p. 147-152.
Zo back,M.D. , and Zoback, 1980. In: Eve rnden, J. F. , 1980. Magnitude of deviatoric stress in the Earth's crust and upper mantle. U.S. Geological Survey, Proceedings of Conference IX, Open File Report 8.0-625, p.353-433. Zo back, M.D. , Hamilton, R.M. , Crone , A.J . , Russ , D.P. , McKeown, F.A., I and Brockman: S.R., 1080. Recurrent Intraplate Tectonism in the New Madrid Seismic zone. 971-976. Science, vol. 209, no. 4460, p. Zupan, A.W. and Abbott, W.H., 1975. Clastic Dikes: Evidence for
?ost-Eocene (?) Tectonics in the Upper Coastal Plain of South I Ca rolina. South Carolina Development Board, Division of Geology, Geologic Notes, vol. 19, no. 1, Spring 1975.
I
~
I l i I ig ; i i g i j IND. 0H,0 U"',f k 0 I ytt*
/ .
\
',\
W.VA. j # f VA. -
- / ,/* I l yf 1
l % xy' sA
\' ,' /
' i. ,
' 1 ,J? jed* '*?)'/ Ill 9
-- - -.-. . .~. . .d.s . . - . ,- - .~.-/-
/
~~
p t - A
~
s+ /h ' TENN T s, N.C.
/' ? 's J
$ >s .
d' _,_ l
, ,/
.c.....
i
..j -
j a! g\' *+ / s
, e '.,-s.
hf e' co*
~
lI ,,-
,7. . . ,, r . ,
ATLANTIC OCEAN I l Am. I f.)^>'/ 'h I !l TECTONIC UNITS OF THE SOUTHERN APPALACHIAN OROGEN o 125 4 KILOMETERS
!I
<Ams .. esemN. - ou-a. tem PLATE 1
I ! k Y p** o/ Y 3 _ _ ~r ~ Yh ' g Y ' 4 o ' I e ys e s + X X
/ %
Q n.s t w +'
,Y*o-%
.s # c0 5
' e /* o*~
A"$ yD 8 Y N g T O t 50+ l y w D 9 # O o+ X X X d 0
- i !$ ,% 1 o-e-s s w s
smy gv. 9 X N+ f a xw Xo a o
+ e 0 25 5,0 7,5
]00 MILES 0 100 KILOMETERS X
2,5 5,0 7,5 l TECTONIC MAP OF SOUTH CAROLINA (INCLUDING OUTER CONTINENTAL SHELF)
, RE FE REN CES : G.V. C0 HEE, chm., 1962. 'i E CTONI C MAP OF U.
OVERSTREET AND BELL, 1965 i HATCHE R ET AL. , 1977 FLETCHER ET AL.. 1978
._.g 1
l $ D o ' j XY . [ 9(k s
\\,
\ .n. .
o
\\
i e ] X X ! \.\.X > 1- c. . . 1
.I \. \\
o f Sp0R ggggE 9 -
\, N,*N FRACTURE z0ut
[&
\. g N- g N.
p . W-\. .n. @ [) #)(-/^NI__. [# o ( X X \ _- .\. [*%. %-
.J u- j N.
- ./ .
x
./
{
/
'I j fX X l k N: -
.- ! s"
/
L.T. N .X ( \. 4 Xo. X. Q ' {
)
0g
'(
\.~. ., s v
EXPLANATION: T - TERTI ARY SEDIMENTARY ROCKS f I K - CRETACEOUS SEDIMENTARY ROCKS i 9 - GRANI TE K3 - KIOKEE BELT l 1 '$ . (AAPG) SB - CAROLINA SLATE BELT CB - CHARLOTTE BELT KM - KINGS MOUNTAIN BELT f IP- I NNER PIE DMONT sammens a neooms PLATE 2
O
- # +
^
& *?
X ep c7 X ' N s5,'
. r-k- :
;4, ~ ' l. .) . ..-
%O s.
_P -
,.-~ ..
_. 4 .- O4 ~.. p
+ ,O -
- ~-
)
g ?.*.
...- ^7
, X'C .
..- /,
' , "e X -
'"' {j ...;
j~ - s afs u # 4 G l-4 Ws N e' ..- l -1 5m 8j ~ : 0 {y ' s - 9 s, & e y' y~ e % %,. OT
%* .9 :
\ aN' \ .'l : O' W' c- %'s h ~ F yv. '
\o e /'e 0 25 50 75 100 MILES t e i i a f
'A 0 25 5,0 7,5 100 KILOMETE RS BURIED TRIASSIC-JURASSIC BASINS IN SOUTH CAROLINA SHOWING RELATIONSHIP TO PRINCIPAL
, IGNEOUS ROCKS IN PIEDMONT RE FE REllCES : G.V. C0 HEE, CHM., 1962, TECTONIC MAP 0F MARINE AND SIPLE, 1974 r
G0HN ET AL., 1978 McKEE, 1973; UNPUB. M.S. 6 OVERSTREET At!D BELL, 1965
l w sz \. 1
.s4 .
's \ \\
j I j /( ,
\ \.
o
}
s' l
~\ \.
/
\\x l 'g' %.
o
,5 s (o
X 8 spur-(f' , . -g-s, N- . p SgCbt Io"' N. s. - s e o Xo P' ( ~ R \' &
/
X (/,e.N --
.. '.\
- .s - u./, ) s
/ .
x i ) [
./ ,
x kN \. I N. ,. C I
\ '\ N*
'Xj - '\. y$'
?@
\. '
O og '\ - N.q s. \ EXPLANATION: D DUNBARTON BASIN F FLORENCE BASIN U UNNAMED BASIN DR DEEP RIVER BASIN p @ GRANITIC PLUT0NIC ROCKS me MAFIC PLUT0NIC ROCKS e
- MAFIC Dl KES , TRI ASS I C-JURASS I C l ommesamoo&m PLATE 3
p Y f- *g pCAPEF p y* ARC 5 s .. .
\ ~p '
/ .. f
' N .. ._._ .g d _ _ q - j X>%,#' X j p
- .. e U j i
x
/ &
W. TY.-. N.. N ' g.p
,fkW"'+2.o jf ..pK$ b X
,O d
~X 9 \l o 0 c.. - 7g A A .# 7, n .
y,,
- r
.* % , e1 yE sA o y
- u' * % . . N , ../e $+
' \\oi %
n:.'
- o*
\ ,% ,- -
e- . Q* .k ' URTON-BEAUF4 HIGH (mid Tl l v- ,, n g s.- . g
\s#go
\
o t s v .fa Xo p e >
.m o
. ' I =w e fr
o 25 5,0 7,5 100 MILES I 0 2,5 5,0 7,5 100 KILOMETERS MAP OF CRETACEOUS AND CENOZOlC Exel DEFORMATION IN COASTAL PLAIN SHOWING DISTRIBUTION OF RECENT VERTICAL CRUSTAL MOVEMENT IN mm/yr RE FE RE NCES : G.V. C0 HEE, CHH., 1962, TECTONIC MAP 0F U.S. (AAPG) 20BACK ET AL.,1978 LYTTLE ET AL., 1979 DILLON, 1974 VINKER AND HOWARD, 1977 1 HOWELL AND ZUPAN, 1974 COLQUHOUN AND COMER, 1973 HERON ET AL., 1971 ZUPAN AND ABBOTT,1975 PR0WELL AND O' CONN 0R, 1978 BEHRENDT ET AL. ,1980 HUSTED, 1970 MCADE, 1971 INDEN AND ZUPAN. 1975 HOLDAHL AND MORRISON, 1976
l l 1 *\ w . - N
.e A R l
I ' k i. 5.\. < 1
\ '\ .I t..
I i \. \. . .
\\ t I
\.NX X
x - l g w. .
\. s j ! .
; %.\- .
- m. 9
\ '
'g.-,
g \\ X .8 se seon
,,oca to, s s.s.s ,#
N. .~ e s .
$ )
- . y.
p c l. . y ^*\. - m X
;', - (/. - .\ - T l
j
.s
-Gfs- -
) \.s.
y
.x
\N
/
I l 1 N X X l. s. y N. 3
. f
! s.
3 ./ . I L'\\ l - Xf (X\v l Xj- \.\ ,
., 'I O
o
\.
! \. N I fm l LANATION. T TERTI ARY SEDIMENTARY ROCKS h DOUBLY PLUNGING ANTICLINE l K CRETACEOUS SEDIMENTARY ROCKS MON 0 CLINE STRIKE AND DIP 0F REVERSE FAULT
,_2 . 0 IS0 BASE (m/yr) (MEADE) p STRIKE AND DIP OF NORMAL FAULT TREND OF VERTICAL CLASTIC DIKE
.. 0.g ... I S0 BASE (m/y r)
(HOLDAHL + MORRISON) x LLASTIC DIKE, ORIENTATION UNKNOWN j APCH, ANTICLINE ! j PLATE 4
l 84 83 82a 81
+
H e 36 - - g 4 a
+ ++4 NORTH CA o+ 0+ ~~
+ +
+e *+
4- ,/~~- - - - - - - . - - - ,
+
3s. +
=
1_ x
\ i..
*+ }
\ /
xN . .g
+ ++
\ +g o+++
{ { 34' 'a , a
\ L 5
O+
'( '
SOUTH CA I
+ .
\ g 33
++ ;
', \
+ '.
GEORGIA
'\., .
(
- SAVANNAH ,
32 1 1 9 f
'~
J I k l 3 HISTORICAL SEISMICITY OF SOUTli CJ
l 1 80' 79 78 ; ~
++ +
4 ROLINA
++
+
+ +
+O N s s o 4 N *
+ \ [
's 4
)
+
SYMBOLS: .R OLIN A
+o + 1E AND LESS (AND MAG. < 3.0)
SEE TEXT O e 3E (AND MAG. 2 3.0) re -,m I ' (gQ l ,- y/ ar
+ I We e G 3EII L__ _J [
0 0 Om
- O1 (Ca^ ales 10s, S.c. Os'v)
REFERENCES:
eBOLLINGER, 1975 e BOLLINGER AND MATHENA, 1980 o C0FFMAN AND ST0VER,1975-1977 \ROLINA AND SURROUNDING REGION a PLATE 5
84 83 82 81 a N 36 -- -- NORTH CA [~ 39 -- '
~
] yn)s
\
/ V i
t, s EI g
\ '
\i, 34' \lk 0A g [
' i SOUTH C/ t
\
t
~, m /
33
/
N f s, 's GEORGIA r 32 $ q I 4 i. l SOUTH i ISOSEISMAL MAP OF THE CH/
\
i 80' 79 78 i ,ROLIN A m "' ' N r A s s, G~ n
^ J (ROLIN r / f% /
oc #
/ /
ADAPTED FROM BOLLINGER,1977 CAROLINA (RLESTON EARTHQUAKE OF 1886 PLATE 6
- 81 84 83 82 N
36*_. . _ . _ - - J NORTH CA
~
7 --- _ _ _
\s
__ _ _ . 3 c,o __ ,#
\. \
/'
- NOT FELT
. 's n
3L
\ '
I I COL
\g
\' .u-m i
'( SOUTH CA
( ' 1 [ 33 - ( ,
's i.
( f - GEORGIA
)
i ( 32 SAVANNAH s q fl 1
, 1 4- \
l 4 t ; i ISOSEISMAL MAP OF EARTHOUAN
l 80' 79 78 ROLINA
~--
's, x 's g
n-12 s I 's \ UMBIA ) (_
,ROLIN A H -I
)
W
'<y l CHARLESTON Y
'L I
e# / i ADAPTED FROM TARR, 1977 l l !E OF NOVEMBER 22,1974 (MMI 3ZD l PLATE 7
- . _ . . _ . . . - - . -_. _ . - . . _ _ ~_ .-
M M M M M M M M M M M M M M M M M l 1 GREAT SH0KlES BREVARD FAULT IGNE0US
, FAULT MULTIPLY-DEFORMED 0 VR I 50 BR 100 150 F 200 P 250 F 300 KM y
'" $ $lY Y.Y!Y.l0/, ' "I' 0///////h///////////////h'/Slllk{h ? Yf 0CKS
* # l ~
*~
. z 15 - # v v v v v
%- \ .-
-~v- STRATA v g { { y y i "
4 H
~
< , v v v v+- CRYSTALLI NE y
v y y v v v y v
,/ BASEMENT 45 s N -*--- MANTLE s
N
,/,-
s ,/ N s , - s
- N s '
N s N BREVARD ZONE s , AP VR l BR l ,P' l CP 3l CS A% 'i pzw- ' YM; .; , * :' * . ' . -' . . . . . .. .. .
,q'*,.,.ge",*'__*
.1 ,,
... . .v:.n . . : . . . c. ,
, _ - ~ -
* . i. . . . '. : -
GENERALIZED STRUCTURE SECTION AND DETAILED SEGMENT SHOWING INFERRED ATTITUDE OF APPALACHIAN OROGEN DECOLLEMENT AND ASSOCIATED EASTERN AND WESTERNMOST THRUST BELTS. 2 = PROTER 0 ZOIC: EE = CAMBRIAN AND PROTER 0ZOICJ Pz = PALE 0ZOICJ pPz = POST-PALE 0ZOICJ AP = APPALACHIAN PLATEAU) VR = VALLEY AND RIDGE; l BR = BLUE RIDGE; P = PIEDMONTJ CP = COASTAL PLAINJ CS = CONTINENTAL SHELF. 8 I a
-4 0 4 m e
, ADAPTED FROM:
COOK AND OTHERS,1979 COOK AND OTHERS, 1980
I I CHARLESTON, S.C. (DEPTH km) i x .- -0 x k 6 i I y
-50m($7 \%\ }6 =150m(
3 MAX JAR^/^N~' 9 5+
"w^he "w DECOLLEMENT
'<'"""E"A
%E w^m - 10 i
- I ]
L,. 7 a
.y
^
r= MOHO (?)
- a ig =h __ --
~ 30
- l:
- \
IE
.I
- BEHRENDT ET AL.(1980) MODEL l
l l I lg PLATE 9 ( . . _ _ _ _ . - . . - - .
lI .I 83* 82* Bl* 80' 79' 'I y ,ht;i
#/*', ,,h,;. e.,n n
- a
$ V. $p5 ires 4,,,.:
m+a:a,.,G
,, p -Q ~ ,
g ,
- I 'I @ * .. [ O ef'w.
_,y g .\(p E
- -o -
"- / 32*
- I l SIMPLE BOUGUER GRAVITY MAP OF SOUTH CAROLINA (AFTER LONG, et al,1979 AND LONG,1979)
- I I I , 'i
'g xitOM ET E R S g e m A ~ ATl_ CONTOUR INTERVAL IS IOmgol I e M.R. - MONTICELLO RESERVOIR e C - CH ARLESTON SC.H.R.- CLA R K HILL R ESERVOIR
@ ANOMALIES GREATER THAN + IOmgot hh i i L ., = ^ = " * * " * * " "
PLATE 10
E ) 4 80'30' 80* I i I i
- I l gg - ~ ~
of>'g 's
. SUWWERVILLE ;
+4'/
-33' / d ) -
' Do I
. % ,,, i
.3 ! ,I l ,/
,. / ...
l 0,800 ' . j
,/ ~ s.' i+0 . -
l
/ I j 300 37 f C RLESTON r . _
.j \ ,-
2
's ~~_ 0,' N W e*
n s 4 o 5 10 i,5 KILCMETERS
! 32*30' i RELATIONSHIP OF BOUGUER GRAVITY AND AEROMAGNETIC ANOMALIES TO AREA OF MAXIMUM INTENSITY FOR 1886 EARTHOUAKE, CHARLESTON SOUTH CAROLINA I
EXPLANATION
--- -+10- BOUGUER CONTOURS (mgals) sob 0RE HOLES 200- AER0 MAGNETIC CONTOURS (gammas)
- AREA 0F MAXIMUM INTENSITY AND DAMAGE (AFTER RANKIN,1977)
I MlH 2Hid2_ - - . . . - - . . PLATE 11
E s a N 4 i t-N b g >N N F -l Ny
'~
>x%,
a ^q_ ; 3 1 3 '
1 i .
.. /s J-S . e . . Q'sg iSEISMIC ZONE 1 4 y
\,;.
-M, d Y
. / %./ m I %
N
'N, Y
a M N N 8 a $ *
' = 4#
i 5 04
]
.L 1 ACPO g
b d g#g 0 I NOTE: ARROWS INDICATE RELATIVE RATES AND DIRECTIONS OF SPREADING. 4
&g#
l g CONCEPTUAL MODEL OF FAILED-ARM TROUGH . OF TRIPLE JUNCTION IN CHARLESTON REGION SOUTHEASTERN APPALACHIANS I I
REFERENCE:
RANKIN, 1976 , , . . . , , , PLATE 12
M M G N 1 3 TI N E E T S A M E R P L P E R
' S R S M E T E G NN
- O I M Z T o Ot F
%E
- CI 8
I R M C. o s M N E CI N N G O O OZ T 0 S 0 MO SS M 8
- E I E E
' L SM G A
R : N
' A Y I V '
' i l
E T K F M I R M y c N C I O Z
~
j % E T s
\
0 S E I
~
S
, M
~
e G M .
' N I
l -
~
T N E S
. E M '
- v. R
'f. .
s\ i
~
_ P E R f /, ~ S M -
- ~
E N O f~
. ~ .
Z
~ A _ C c I M N E
G 0
. M
' . . , S M ' t, I I E
- S M N N
E T A x\N M
' L A N A
' ~
~
-},s c}
M - .
*(\
'8
[ - M M M lil; , 'll l I . } l l:
I 75*w 85*W 80* w E F E 7 I 40* N E E E 35*N SITE e CHARLESTON E E 30*N E t E E I E SEISMOGENIC ZONE REPRESENTING DECOLLEMENT HYPOTHESIS 1 E PLATE 14 f _ __ ~ ~ - - - - . . - _ _ _ _ _ _
)
M M 5 0 1 0 0 E 01 T A L P M M E
'o "i
m K A
"T U a Q u H M E
! T R
A _' E N y e ! O m b i T
, 2
- m
_o S _ L mm o E d s L p u 'a i R N k A M _ d. l E J H C w , 3 N L _ r 6 8 i d \ a, _ 8 1 M .
=\
u 4 J. 3 _ m E k H T
._ L E R i O M
C
- . t
- . d. s. N F
a 4 . s.
. A E
- . e T C
- S N
. . I A M E ;
. _ D T S
L_ a. t I
- _ i ? D t t
. I
= S U
M
=
?
o S o R N s
. E
- N. V N S E
M N - . I T
\ I S
M \ N E T x s N I M M M E T I S M O x i n v v i iI M e v n v vi [GzUz s2 a a t s M W
i j 4 l-i 1 i j i l l l EVALUATION OF DESIGN EARTHQUAKE REPORT l GEOLOGICAL AND SEISMOLOGICAL
- EVALUATION OF EARTHQUAKE HAZARDS I
l AT THE RICHARD B. RUSSELL PROJECT i I 1 I I SOUTH CAROLINA ELECTRIC & GAS COMPANY DECEMBER 1980 I I I
TABLE OF CONTENTS Page I List of Tables. . . . . . . . . . . . . . . . . . . . . . . . 11 List of Figures . . . . . . . . . . . . . . . . . . . ... 11
1.0 INTRODUCTION
. . . . . . . . . . . . . . . .. . . .. . 1 2.0 EVALUATION OF MAXIMUM INDUCED EARTHQUAKE . .. . . ... 2 2.1 SSTIMATE OF STRESS DROP . . . . . . . . . . . . . . 2 2.2 ES?IMATES OF LARGEST PLAUSIBLE FAULT PLANE . ... 6 2.3 FINDINGS . . . . . . . . . . . . . . . . . . . . . 7 3.0 EVALUATION OF DESIGN GROUND MOTION . . . . . .. .. .. 9 3.1 des 1GN mTs20m ................. e 3.2 DESIGN GROUND MOTION 9 g . . . . . . . . . . . . . ..
3.3 GROUND MOTION ESTIMATES FOR MMI . . . .. . . . . . 10 3.4 COMPARISON WITH AVAILABLE ATTENUATION FUNCTIONS . . 11
.I 3.3 COMPARISON OF DESIGN METHODS . . . . . . . . . . . 12 3.5.1 Nuttli (1973) Attenuation . . . .. ... . 12 3.5.2 Schnabel and Seed (1973) . . . . . . . . . . 12 3.5.3 Page et al. (1972) . . . . . . . . . . . .. 12 3.5.4 USGS (1975) . . . . . . . . ... . . . .. . 13
[ 3.5.5 Trifunac and Brady (1975) Intensity l Correlations . . . . . . . . . . . . . . . . 13 3.5.6 Ambrayses . . . . .. . . . . . . . . .. . 13 3.6 I 4.0 FINDINGS CONCLUSIONS 13 15 REFERENCES . . . . . . . . . . . . . ... . . . . . . . . . . 16 I l I I I l 1
. I
- LIST OF TABLES Table 1 Statistical Analysis of Peak Acceleration Data Shown on Figure 1 I LIST OF FIGURES Figure 1 Acceleration versus M>il in the "Near-Field" (taken from Figure 24 of the Russell Project report).
2 Accelerations for MMI VI and VIII Plotted on Log-Normal Probability Paper. 3 Velocity versus >DiI in the "Near-Field" (taken from Figure 26 of the Russell Project report). 4 Velocity versus M:11 in the "Near-Field" (taken from Figure 17a of Krinitsky and Chang, 1975). I 5 Lower Hemiphere Equal Area Projections of , , and from Focal Mechanisms Solutions at Reservoir Sites. Axes l I I I 1 l I \ I l I 11 I
y EVALUATION OF DESIGN EARTHQUAKE REPORT GEOLOGICAL AND SEISMOLOGICAL EVALUATION OF EARTHQUAKE HAZARDS AT THE RICHARD B. RUSSELL PROJECT
1.0 INTRODUCTION
I The purpose of this review is to examine in an objective manner the data, observations, and interpretations presented in the Design I Earthquake Report " Geological and Seismological Evaluation of Earth-quake Hazards at the Richard B. Russell Project," (U.S. Army Corps of Engineers, 1977) in view of the location of the project relative to the Virgil C. Su=mer Nuclear Station wtich lies in a common geological province. This evaluation presents an interpretation of the methodolo-gy used for defining a design earthquake at the Richard B. Ruscell Project. Inevitably, comparisons will be made between the seismic ground motion selected for design at the Russell Project, with that designed for the Virgil C. Sum =er Nuclear Station. These are two separate projects with somewhat different seismic environments and somewhat different considerations for seismic design. Understanding these differences requires understanding and evaluation of the method used to determine seismic design =otions at the Richard B. Russell I Project. This review focuses on two specific issues which are considered very relevant to the present induced seismicity evaluation of the Virgil C. Summer Nuclear Station site. The first part examines the i maximum induced earthquake as presented in Appendix 1, Section B of the Russell Report, while the second part evaluates the design ground i motion selected for the Pussell Project. ifter a thorough review of that report, it was concluded that I these two specific issues are the only matters with relevance to the Virgil C. Su=cer Nuclear Station. I I _t_
I 2.0 EVALUATION OF MAXIMUM INDUCED EARTHQUAKE 2.1 ESTIMATE OF STRESS DROP Based on the data available at the time of his analysis (1976), Long established a range of values for stress drops associated with Piedmont earthquakes (Long, 1976). The range of data available included: a study of eastern U.S. intraplate earthquakes by Street et al. (1976); an analysis of the 1974 Clark Hill earthquake [ variously esti=ated magnitudes from 4.3 to 4.9; see Jones et al., (1976)] and associated stress drop by Bridges (1975); and spectral analysis (from which stress drops were calculated) of Clark Hill microearthquakes by Marion (1977). From this data base, Long concluded that a stress drop of 6 bars was plausible, since the data were very consistent and none I were available for the Piedmont which indicated that higher stress drops existed. Recent studies at the University of South Carolina have shown that for events at Monticello Reservoir of =agnitudes of approximately 2.0, average stress drops are approximately 5 bars, which is consistent with Long's arguments for Clark Hill events. However, the August 27, 1978, ML = 2.8 earthquake has been shown to be associated with an average I stress drop of 17 bars (Fletcher, 1980) at very shallow depth (s100m), which is not consistent with data that were available to Long. [An argument can be constructed to demonstrate that, at Monticello Reser-l l voir where focal mechanisms indicate thrusting, the greater the stress drop, the shallower the seismic event should occur, and conversely, the lower the stress drop, the deeper the seismic event should occur.] In l situ stress =easurments at Monticello Reservoir by Zoback (FSAR Figures 361.21-1 and 2) show that deviatoric stresses are low or nonexistent in the upper 400 meters of Hole No. 2. In Hole No. 1, they are approxi-mately 70 to 80 bars in the upper 0.5 km, apparently approaching hydro-l g static conditions near a depth of 1.1 km. This shows that the devi-l atoric stress varies both laterally and with depth, and, at some point, equals zero. With zero deviatoric stress, there is a barrier to the l l l lI l I occurrence of earthquakes. If this stress barrier were mapped in three di=ensions, it would be seen that the available volume for dislocation, hence the size of the earthquake, would be small. Long continues to discuss another important parameter in relation to stress drop - rigidity. He argues correctly that, the higher tne rigidity, the smaller the displacement required for a constant stress drop and fault dimension. Also, because tne near-surface rocks in the vicinity of Clark Hill have P-wave velocities on the order of 5.8 to 6.1 km/sec (high velocity, suggesting high rigidity), this implies that for displace =ents computed for Clark Hill events of similar stress I drops, these displacements should be equal to or less than displace-ments of other small earthquakes occurring elsewhere. Conversely, stress drops for Clark Hill events should be equal to or greater than I stress drops of other shallow earthquakes. Monticello Reservoir data show that this is not the case, suggesting that it is inappropriate to use behavior at one reservoir to infer behavior at another. A proper assessment of the potential impact of reservoir-induced seismicity at the Rich &rd B. Russell facility may not be attained if the environ =ent of in situ stress and seismic flux are considered to ba equivalent to those at other major reservoirs in South Carolina or elsewhere where seismicity has been monitored such as at Lake Jocassee, Lake Keowee, Clark Hill, and Monticello. The reason for this is tne: the local geologic and environmental conditions as well as the state of stress at each site are different, and such differences are manifest in the expression of seismic energy release as illustrated by the compo-site focal mechanism solutions. Figure 5 presents solutions for five different reservoirs in South Carolina and Virginia. In South g Carolina, Monticello Reservoir has been affected by microearthquakes E which represent thrust faulting. The data from Lake Keowee and Clark Hill Reservoirs indicate that nor=al faulting dominates, whereas at Lake Jocassee strike-slip is occurring at shallow depth, changing to normal slip (not shown in Figure 5) at greater depth. I i I
I In general, one would expect that for normal faults to occur, releasing distortional strain energy stored in the bedrock, that the events would occur when two general conditions were met: (1) The component of vertical normal stress at the point of failure exceeds the magnitudes of horizontal normal stresses; (2) That the stress difference is sufficient to permit slip along a failure plane according to some criteria of shear failure. Condition (1) would be unlikely to obtain at very shallow depth, hence it may be inferred that the normal fault solutions at Lakes Jocassee, Keowee and Clark Hill probably represent slightly deeper seismic events than the thrust events at Monticello. It follows, therefore, that in areas where normal faulting predominates, the magni-tudes of earthquakes would increase as a function of depth, but would decrease as a function of depth for thrust faults. I Similarly , the stress drops associated with the normal and thrust type mechanisms for seismic events respectively would show similar variations with depth, I namely, an increase of stress drop with depth for normal faulting and a decrease with depth for thrust faulting. For all of the data available on stress drops for Clark Hill and Jocassee Reservoirs (Bridges, 1975; Marion, 1977; Marion and Long, 1980), values of 6 bars or less have been evaluated for these sites affected by normal and strike-slip faulting. However, the August 27, 1978. ML = 2.8 earthquake at Monticello (the largest recorded to date) yielded a stress drop of 17 bars (Fletcher, 1980) at a very shallow depth (%100M). These data, to the present day, seem to confirm the normal / thrust faulting relationships relative to earthquake magni-tudes and stress drops in the Piedmont of South Carolina. Figure 5 shows that composite focal mechanism solutions for the North Anna nuclear facility in the Piedmont of Virginia indicate thrust faulting as at Monticello. A survey of sei.smic monitoring results there for the I .. I
I period January 22, 1974 to June ~23, 1977 indicates that five of the seven earthquakes with the largest local magnitudes (Mt = 1.5 to 2.1) occurred within the upper 1900 m, and four of those five within 700 meters of the surface, where the majority of events occurred to depths of up to 5 km. Moreover, approx 1=ately 60 percent of the total number of events occurred within the range of 0 to 2 km depth. The largest event recorded at North Anna, the ML = 2.1 event of September 4, 1975 occurred at a depth of 600 meters. Consequently, this seems to agree with the coatention that magnitudes and stess drop of earthquakes where thrust faulting predominates in the Piedmont are greatest near the I surface. Simply stated, therefore, the origin or cause of seismic releases of distortional strain energy at a given reservoir reflects (a) the I state of stress in the immediate area of the reservoir, and, thus, its heterogeneity, and (b) geologic, hydrologic and man-induced environmen-tal factors at the specific reservoir. For this reason, it would be erroneous to define (a) and (b) above for one reservoir based upon a mixture of data about earthquakes and regional stress characteriza-tions, and geology-hydrology at other sites. Instead, the site-specific data which af fect decisions regarding seism:.c design para-meters of a given facility must provide the principal basis for the understanding required to make the most meaningful decisions. While rigidity is important to the ar3ument of stress drop avail-able for an earthquake in the Piedmont, the most important parameters are those of source dimension, available volume of stored elastic l strain, and the disposition of the stress censor in time and space. These parameters are different for each area, and one cannot compare them on a one-to-one basis, especially .4 the absence of site-specific data. l lI I lI
I 1 2.2 ESTUiATES OF LARGEST PLAUS1BLE FAULT PLANE The crux of Long's argument for a magnitude 5.5 earthquake being the maximum plausible earthquake associated with Clark Hill or Richard
- 3. Russell Reservoirs is dependent on available f aul t plane dimensions.
H. argues that, because of a lack of confirmed active faults in the region, other sources of data are required to determine the maximum fault plane dimension. He then describes the "aftershock" zones of'two reservoirs - Clark Hill and Jocassee. By assuming that the locations of microearthquakes at Clark Hill Reservoir defined the limits of the
" highly streased" region around the reservoir, and knowing the maximum depth among these events, Long simply enveloped the area (5 km length I by 2 km depth = 10 km2 ) as representing the maximum plausible fault plane available for rupture. Ibr Jocassee, the area of "af tershocks" I is significantly larger, some 9 km long by 4 km deep, giving an avail-able =aximum plausible fault plane of 39.5 km2 (after correction for " dip" of the fault plane).
The argument that Long creates may very well hold true for areas I where tectonic elements are well defined (by their ground rupture during seismic events or by distribution of seismicity along their lengths) and applied stress is uniform over large distances of the earth's crust. Such is very rarely the case in the intraplate regions such as the eastern United States and clearly not the case at the Virgil C. Summer Nuclear Station. There is a distinct heterogeneity. in the stress field, both laterally and with depth in the eastern United P a.tes which is most likely a result of a number of factors such as verti crustal movements (Sbar and Sykes, 1973; Bollinger, 1976), weathering (unloading) and stress amplification (Long, 1976). As a result of this heterogeneous stress field when applied to non-homo-genous crustal rocks such as are present in the eastern United States, seismic activity is diffuse with no widespread coherent patterns. By simply enveloping "aftershock" zones at Clark Hill Reservoir and Jocassee Reservoir to present the maximum plausible fault plane, I Long has implicitly assumed that: I _ -
I
- 1. the events referred to as "af t shocks" are truly a coherent sequence of events following a main seismic event;
- 2. the "aftershocks" define a single, tectonic element along which all of the microearthquakes have occurred;
- 3. there is a : single, unified stress field acting upon this hypothetier : plane, and the reservoir would act as a catalyst 1 g it to failure;
- 4. Clark Hill -ir earthquakes are the same as Jocassee Reservoir ,aakes in terms of source mechanism and stress field and constitute a basis for the design analysis for the Richard B. Russell Project.
2.3 FINDINGS I Long has not substantiated any of the above assumptions, and, fo r I the last three, the assumptions are not applicable to the eastern United States. Specifically, the " calculation" of a magnitude 5.6 earthquake as representing the maximum plausible " induced" earthquake is entirely dependent on the initial assumptions made. More important-ly, even if Long had substantiated his assumptions for Clark Hill and Jocassee Reservoirs, it is not appropriate to extrapolate those results to other locations (i.e., to the Richard B. Russell project). Each case da unique to its own environment. While Long's analysis may be considered appropriate to the R.B. Russell study '(where no relevant site specific data existed), it is not appropriate to export that analysis to areas where site-specific date. exist. In the case of Monticello Reservoir where: 1) variance of the stress field over short distances, both laterally and with depth, creates " stress barriers" which effectively limica the seismicity to shallow depth (<2.5 - 3.0 km) and to small e :tective source areas; and
- 2) the rate of seismicity is decaying with ties, suggesting relief of stored elastic strain that is not being replenished; it is more likely and indeed probable that the presence of Monticello Reservoir does not I
I . I
l l
)
1 increase the risk of localizing a seismic event which would be equiva-lent to what the largest event has been in the site tectonic province.
!I I
I I
'I ;
l I lI I I I 1 l I , I I 1 l I -g-lI
I . 3.0 EVALUATION OF DESIGN GROUND MOTION 3.1 DESIGN EARTHOUAKE I The design earthquake for the Richard 3. Russell Project is I assumed to be a Modified Mercalli Intensity (MMI) VII event which occurs at and is induced by, the Richard 3. Russell Reservoir. The validity of the assumption that such an event can be induced is ad-dressed in Section 2.0 of this report. The ground motiona are esti-mated for this event in the near-field, as discussed below. The =agnitude estimated for the design event is 5.5 (Richter magnitude, which for this size event is equivalent to local magnitude). The reference given for this esti= ate is a personal communication by Nut:11 (one of the authors) in 1976. Since that time, Nutt11 has published more recent work (Nutt11, 1979) which indicates that a body-wave magnitude of 5.3 is appropriate for representing MMI VII; this body-wave magnitude is used below to estimate ground motion. 3.2 DESIGN GROUND MOTION We examine here the method used to select the design ground motion for the MMI VII earthquake. Considered elsewhere is evidence uhich suggests that a =agnitude 5.5, MMI VII event occurring at the reservoir is an overly conservative assumption for seismic design. Rather, we evaluate here the methodelogy for quantifying ground motion for a specified design event, without consideration of the method used to select that event. Values of peak, single-component horizontal acceleration, velo-city, and displacement are selected based on the presu=ed occurrence of a MMI VII in the near field. This event indicates the largest values of those parameters, of the several earthquakes considered. The values obtained are compared to estimates available from other researchers, which are primarily in the form of peak parameter values for various magnitudes and distances. With this comparison, the report justifies I the values selected on the basis of MMI. I
.9
I I The distinction between how parameter values are selected, and how they are compared with available functions, is an important one. The reason is that distances considered to be "near-field" for M>1l inten-sities are different from those distances considered to be "near-field" when =agnitude and distance are used to estimate peak motion para-meters. Recognizing this difference is important in understanding the justifications given for the design ground motion levels selected. 3.3 GROUND MOTION ESTIMATES FOR MMI The data used to select a peak horizontal acceleration in the near-field of an MMI VII event are shown in Figure 1, taken from the Russell Project report. To compile the "near-field" data shown in Figure 1, records of M111 VI within 5 km of the source were used, and records of MMI VII within 15 km of the source were used. There are several misleading characteristics of this figure. First, the straight lines represent " increments" (they are also called " percentage bands") between the mean values and the limit of observed data. These "incre-nents" do not represent. confidence levels of tne data, and have no basis in probability or statistics. The " limit of ooserved data" is determined by only two points: the acceleration recorded at Pacoima Dam (M>1l X), and that recorded .t Melendy Ranch (M>il V). To represent those " percentage bands" as confidence levels, and to use them to I select peak acceleration levels, is without mathematical justification. I Secondly, the hor'izontal bars in the figure purport to represent mean plus one standard deviation (M + c ) levels. A cursory statistical I analysis of the data indicates that the levels shown are not correct. l Table 1 indicates means and standard deviations of acceleration for intensities VI and VII, both for the raw data as shown in the figure and for the data assuming a lognormal distribution. In either case the M + c levels are substantially below tMae shown on Figure 2. No reason for this discrepancy is given. I l E5 l
I I The lognormal distribution is appropriate as a first approxima tion to represent the distribution of peak acceleration for a given >Dil in the near-field. This is demonstrated in Figure 2, which shows data from Figure 1 for MMI VI and VII plotted on lognormal probability paper. If anything, the log-normal distribution is conservative in that it would indicate, by extrapolation, larger tails than the data. Using this distribution calibrated to the data for 5011 VII, the levels of acceleration selected in the report, 0.4 and 0.5 g, correspond to probabilities of exceedance of 0.063 and 0.034, re s pectively. This is a factor of five greater safety than is indicated by the " percentage bands" which have no cathematical basis as discussed above. Peak velocities and displacements for FDil VII in the near-field (30-45 cm/sec, and 20 cm, respectively) are established in the same arbitrary manner as for accelerations. In fact, the arbitrary nature I of the method can be demonstrated by comparing the velocity plot from the Russell Project report, with a similar plot published by Krinitzsky and Chang (1975), the first of whom is an author of the Russell Project report. These two plots are shown in Figures 3 and 4, respectively. They portray essentially the same data; the differences are that Figure 3 uses an arithmetic velocity scale while Figure 4 uses a logarithmic scale, and the " percentage t, ands" have been drawn higher in Figure 3 than in Figure 4. Thus, the 75 percent " increment" in Figure 3 is about 38 cm/see for MMI VII, whereas it is about 23 cm/see for 5D1I VII in Figure 4. Such arbitrariness demonstrates that ground motion levels chosen by these procedures have no mathematical basis. 3.4 COMPARISON WITH AVAILABLE ATTENUATION FUNCTIONS Values of peak horizontal accelerations, velocity and displacement chosen for MMI VII in the near-field are compared to values predicted l by several available attenuation equations, and this comparison is used i as a justification for the values selected in the Russell Project l report. A critical element for a consistent comparison is to define "near-field" for the attenuation functions examined. Unfortunately, this is not done in the Russell Project report. In fact, because of I 11 -
I the high peak motion amplitudes which have been chosen, the attenuation equations must be examined for ML = 5.5 at very close distances. In particular, the near-field definition of 4115 km must be abandoned. For the purposes of discussion we will assume here that a = 5 km repre-sents near-field conditions for ML = 5.5. Within this distance, ground motion will not vary appreciably, in the mean, because the energy producing seismic waves comes from some depth in the earth's crust, typically 5 km for California earthquakes. In the following discussion it is assumed that unspecified magnitudes discussed in the I re ierences cited are Richter (ML ) values. 3.5 COMPARISON OF DESIGN METHODS 3.5.1 Nutt11 (1973) Attenuation The Russell Project report gives several reasons why Nuttli's early work is not appropriate for comparison. More recent work published by Nuttli (1979) indicstes a peak acceleration of about 0.4 g for ab = 5.3, a = 5 km, when Nuttli's sustained acceleration is modified to account'for (a) an average peak-to-sustained acceleration ratio, and (b) a factor to predict an average horizontal component rather than the maximum of two. 3.5.2 Schnabel and Seed (1973) The Russell Project RBRP report compares the selected peak ac-celerations to the range of values shown by Schnabel and Seed at a distance of 2 miles (the closest distance indicated by the curves), where the accelerations are comparable. A more appropriate comparison would be for a distance of 5 km (3.1 miles) where the Schnabel and Seed graph indicates a peak acceleration of 0.30 to 0.40 g. 3.5.3 Page et al. (1972) The values used for comparison from this study are data obtained at 0.08 km (station No. 2 of the Cholame-Shandon array,1966). This station indicates an acceleration and displacement which are comparable to the values selected in the Russell Project report. The velocity I from chis record is dismissed as anomalously high. Rather than use data from a single station, it is more appropriate to make comparisons wita the range of data for ML = 5.0 to 5.9. Extending these data by e a to an average value at 5 km indicates 0.30 g acceleration, 20 0.a/sec velocity, and 10 cm displacement. 3.5.4 USGS (1975) The Russell Project report states that the peak acceleration from this ref erence in the near field is 0.6 g, but this is erroneous; the curve shown for ML = 5.6 does not exceed 0.45 g. A more appropriate value to use would be ML = 5.5 at 5 km. Since the USGS curves are equivalent to Schnabel and Seed (1973) at small distances, the predicted peak accelerations are 0.30 to 0.40 g, as discussed above. 3.5.5 Trifunac and Brady (1975) Ini:ensity Correlations These are discarded by the Russell Project report because they do not distinguish between far-field and near-field conditions. 3.5.6 Ambrayses The graph shown in the Russell Project report indicates a velocity of 30 cm/sec for ML = 5.5 at 10 km fault distance and the report claims this substantiates the ground motion velocity range chosen for design (30 to 45 cm/sec). This velocity, however, is a peak particle velocity; it must be multiplied by 0.7 to approximate a single-component peak velocity. Estimating a particle velocity for ML = 5.5, and multiplying by 0.7, indicates a value of about 21 cm/see at 10 km. Values for closer distances are not available. 3.6 FINDINGS The design ground motion for the Richard B. Russell Project has not been established with any mathematical arguments. Even if the design seismic event is accepted as appropriate, which is open to question, the corresponding values of peak motion parameters were selected at an arbitrarily conservative level. An attempt was =ade in the Russell Project report to justify these values by compariso.. with I I
I I other available attenuation functions. To do so, "near-field" distances ranging from 0.05 to 10 km were used to obtain estimates I which =atched the selected values. A more consistent selection of distances representing near-field conditions indicates that an Mt = 5.5 in the near field will produce a peak single-component horizontal acceleration of about 0.3 to 0.4 g, a peak velocity of about 20 cm/sec, and a peak displacement of about 10 cm. The selected design values are substantially more conservative than these. I I I I I I l I l I I I 14 -
I I
4.0 CONCLUSION
S
- 1. The calculation of the maximum induced earthquake is based upon inappropriate assumptions, therefore an induced earthquake of ML = 5.6 is without justification.
- 2. It is not appropriate to extrapolate results from one location to another without detailed knowledge of site-specific data. In the case of Monticello Reservoir, it is indeed probable that the presente of the impoundment does nothing to increase the risk at the site of the occurrence of an earthquake equivalent to the largest tectonic earthquake.
3- The design ground motion for the Russell Project has not been established with any rational scientific argument but rather was selected at an arbitrarily conservative level.
- 4. Theoretical estimates and assumptions =ade to calculate maximum earthquakes as described for the Russell Project are inappropriate I for the tectonic setting of the southeast U.S. Even if the esti-mates were correct, there is no basis, nor is it appropriate, to extrapolate results from one reservoir to another. Each is unique I to its own environment.
I I I I I 1 I I I REFERENCES 1976. The Ecstern I Bollinger, G.A., Lauger, C.J. , and Harding , S.T. Tennessee Earthquake Sequence of October through December, 1973. Seismilocigal Society of America Bulletin, vol. 66, no. 2,
- p. 525-548.
Bridges, Samuel, R., 1975. Evaluation of Stress Drop of the August 2, 1974, Georgia-South Carolina Earthquake and Aftershock Sequence. Georgia Institute of Technology, Unpublished Master's Thesis. Fletcher, J. B., 1980. A Comparison Between Source Parameters Deter-I mined f roc Body Wave Spectra and In Situ Stress Measurements at Monticello, S.C. g press E.D.S. American Geophysical Union. Jones , F.B. , Long, L.T. , and McKee, J.H. , 19 72. Study of the attenua-I tion and azimuthal dependence of seismic-wave propogation in the southeastern United States. Seismological Society of America, Bull., vol. 67, no. 6, p. 1503-1513. Krinitzsky, E. L., and F. K. Chang, 1975. Earthquake Intensity and the Selection of Ground Motions for Seismic Design, State-of-the-Art I for Assessing Earthquake Hazards in the United States. U. S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Mississippi, Report 4. Long, L. Timothy, 1976. Speculations concerning Southeastern Earth-quakes, Mafic Intrusions, Gravity Anomalies and Stress Amplica-tion; in Earthquake Notes, Eastern Section, Seismological Society I _ of America, vol. 47, no. 3, p. 29. Marion, George E. 1977. Spectral Characteristics of Microcarthquakes in the Southeastern United States. Georgia Institute of Technology, Unpublished Master's Thesis. I Marion, George E. and Long, L.T. ,1980. Southeastern United States. Microearthquake Spectra in the Bulletin of the Seismological Society of America, vol. 70, no. 4, p. 1037-1054. Nutt11, O. W., 1973. State-of-the-Art for Assessing Earthquake Hazards in the United States; Design Earthquakes for the Central United States. U. S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Missippi, Miscellaneous Paper S-73-1, Report 1. I Nuttli, 1979. The Relation of Sustained Maximum Ground and Velocity to Earthquake Intensity and Magnitude. Acceleration U. S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Mirsissippi, Report 16. I I
Page, R. A., et al., 1972. Ground Motion Values for Use in the Seismic Design of the Trans-Alaska Pipeline System. U. S. Geological Survey, Circular 672. Sbar, Marc, and Sykes, L. R. , 1973. Contemporary Compressive Stress and Seismicity in Eastern North America: An Example of Intra-Plate Tectonics. Schnabel, P. B., and Seed H. B., 1972. Accelerations in Rock for I Earthquakes in the Western United States. Earthquake Engineering Research Center, University of California, Berkeley, Calif. , Report No. 72-2 and Seismological Society of America Bull., vol. 63, no. 2, p. 501-516. , South Carolina Electric & Gas Company, 1980. Amendment 21 to the Final Safety Analysis Report. Response to Question 361 .21 Figures I 361.21-1 and 361.21-2. Street, R. L., Herrmann, R. B. and Nuttli, 0., 1976. Spectral Characteristics of Low Waves Generated by Central United States Earthquakes. Geophysical Journal of the Royal Astronomical Society, vol. 41, p. 41-63. Trifunac, M. D., and Brady, A. G., 1975. On the Correlation of Seismic Intensity Scales with the Peaks of Recorded Strong Ground Motion. g Seismological Society of America, Bull., vol. 65, no. 1, 3 p. 139-162. U.S. Army Corps of Engineers, 1977. Geological and Seismological I Evaluation of Earthquake Hazards at the Richard B. Russell Project. U.S. Army Corps of Engineers, Savannah District, Savannah Georgia. U. S. Geological Survey, 1975. Chart of Accelerations in Eastern United States. Earthquake Engineering Research Istitute, Newsletter, vol. 9, no. 4, p. 29. I I I 17 -
l Table 1 Statistical Analysis of Peak Acceleration Data Shown in Figure 1 1 MMI VI MMI VII Raw Data Mean 0.128 g 0.161 g 0.101 g 0.115 g M+0 0.229 g 0.276 g Data Assuming Mean 0.135 g 0.167 g Lognormal Distribution 0.158 g 0.148 g M+c 0.293 g 0.315 g I I I I I
------_-.-------------------r-~~ n- - . - , , - , - - - , , , -,,,,-----ww-, - - - e,e , , e-. - -
I I l.2 - p
/
M E AN + 7 o\o / l.0 /p ! NE AR FIELD / f O'* %- o\of /
/
0.8 l , l 0 / e I , 0'7 4 / / / i / / / / I 5' i
/ /
ih b / ,l / / I U O.5 N t
/ /<
/
/
l
) / -
ol0 / 04 l ' '
, >/ 4/
0~2 " , 0,
/g
$ 5 e 0
m 11 1 m m mr xx x MM I Figure 1. Acceleration versus fell in the "near field" (taken from Figure 24 of the RBRP report). I
E I.oo 0.98 x 0 95 o.90
,/, / - -n E e *~"ME d 0.80 i
o o.7o F w o.60 o.50 o.4o
,I" }
g O 0.3o :P
)r E 0.20 0.t o 0.05
/
o.02 0.or
.02 0.03 0.04 0.05 0.06 008 o.;o *'"*
ACCELERATION, g I I I Figure 2. Accelerations for MMI VI and VII plotted on log-normal probability paper. E I
I I 120
/
-- M E AN + e /
100 I NE A R-FIE LD '
/ /
l 90
/ / /
80 ! l l
$f #l f I O 70 h O
/
/
o
~
of / gp@l7
>- 6 0 Qj' ,'
% A
~ _ / , ,
$ 50 j '
/ /
e / y/, / / s)- o f' // ,/ :/ g / (' 30 / ' (/ u
/ V 20 ' ! !
// v v 10 0
III IE I YI 3ZII 3ZIII 2X I. MM I Figure 3. Velocity versus MMI in the "near-field" (taken I from Figure 26 of the RBRP report).
1 I I I 200 j 1 l00 '* 80 ch @G y sf /4V 40
$ 30 --
8 / " X./ / I " v " I to 0 " 3 o. v/ a / 6 / I 4 5
/f AL JT
/
/ I/I l 2
- f. o T/.
o 3 H m cr 2 2I En m:I a x i l
- I I
I Figure 4. I Velocity versus fiMI in the "near-field" (taken from Figure 17a of Krinitsky and Chang,1975). l l
, 8
i t 4 JOCASSEE A s~ -- -- - -- - -- - .. 7-
.. # 'gj __ ____
i O [ f C )" hE , 35 y' /
- O\
/
N
- g f,e f %
# 1 KEOWEE
\ *
\
\ .
, + -
82 34 CLARK HILL e e
. _l 7 s
/
,e 1
1 1 i I ...
-, - . - - . - - . . ~ -. -. . . . , .
l 1 1 l
/
/ j
/ u.anwa N
/ 89'\
v s .".. s
/
35 \, . ,
/
).
. 4' 4 l----------------- 2..s.- ,- '
. ." s MONTICELLO ,
= y ,
/
=."
s 4 j'
/ \ m_l
/
/ b h 8 -
/
A
. ss j N
- 8
/
N.
/~
/ . p-AXES a T -AXES ah-AXES l _-
r 1 /
/
l LOWER HEMISPHERE EQUAL AREA PROJECTIONS OF p, r, AND $ AXES l FROM FOCAL MECHANISM SOLUTIONS 9 5 1p 1,5 2,0 2,5 AT RESERVOlFt SITES l MILES i \ FIGURE 5
I I I I I STATUS OF WATEREE CREEK FAULT INVESTIGATION l 8 I I
- 5 I
I I II 1 l I I E lE
I _ TA3LE OF CONTENTS I i Section Page I GENERAL . . . . . . . . . . . . . . . . . . .. . . . . . . 1 I EVIDENCE OF FAULTING POSSIBLE NORTINARD EXTENSION OF FAULT . 2 3 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . .. 5 I i l i II I ,I lI lI I I I I R I
I STATUS OF WATEREE CREEK FAULT INVESTIGATION GENERAL Subsequent to the impoundment of Monticello Reservoir and the ensuing increase in local seismic activity, the United States Geologic Survey (USGS) acquired by contract (No. 14-08-0001-19124) the services of Dr. Donald T. Secor, Jr. , Department of Geology, University of South Ca rolina , to conduct an intensive geologic investigation of the general vicinity surrounding the reservoir. The purpose of the investigation is to provide increased and detailed geologic information which, it is I hoped, will enable a better. understanding of the causes and relation-ships of the observed variations in the local seismicity. This geologic investigation encompasses an area considerably beyond the area investigated by the South Carolina Electric & Gas Company during Preliminary Safety Analysis Report studies. The investigation, as presently conceived, ccasists of the following tasks:
- 1) Geologic field mapping of the Jenkinsville, Chapin, Pomaria, and Little Mountain 7-1/2 minute quadrangles.
- 2) Extensive study of fracture orientations within the four quadrangles.
- 3) Magnetometer survey of diabase dikes within the aforementioned four quadangles.
The investigation was initiated in March 1980, and is scheduled to be completed in February 1982. The first technical report of the progress I of the investigation was submitted on September 30, 1980. The reporti,
" Geological Studies in an Area of Induced Seismicity at Monticello Re servoir, South Carolina, by Donald T. Secor, Jr., Principal Investi-gator," contains a description of the work accomplished, findings, and tentative conclusions. Dr. Secor has emphasized that the conclusions presented in his report are tentative and subject to revision during progress of the investigation.
During the course of the investigation to date, Sec.or has mapped a previously unrecognized fault within the Chapin quadrangle which he has I I
I I named the Wateree Creek Fault. The document presented herein " Status of Wateree Creek Fault Investigation," summarizes the work performed to date which pertains to the Wateree Creek Fault, and discusses the pos-sibility of extension of the fault from the Chapin quadrangL northward into the vicinity near Monticello Reservoir. EVIDENCE OF FAULTING The fault has been mapped along Wateree Creek in the central Chapin quadrangle area for a distance of approximately 8 Kilometers. The fault extends from near the southern edge of the quadrangle north-ward to approximately 2 kilometers southeast of Peak, South Carolina. Secor cites the following e"idence as indicating the presence of the fault:
- 1) The occurrence of fault breccia at two points.
- 2) Apparent offset of stratigraphic coatacts and of tne Charlotte I Selt/ Slate Belt border.
- 3) Apparent discontinuity of the local magnetic anomaly pattern.
- 4) Apparent drag phenomena associated with the fault zone, as indicated by anomalous orientations of 50 (compositional layering) and S1 (foliation) data and rotation of L1 (lineation caused by slaty cleavage intersection with So or I St ).
I 5) Apparent offset of older east-west oriented silicified breccia zones.
- 6) Possible offset of a diabase dike.
- 7) Association of open partially quartz-filled extension fractures with the zone.
The relative strengths and limitations of these findings are de-scribed in detail in Secor's report. Secm' believes the dip of the fault to be steep to vertical, with the eastern side down relative to the western side, and has mapped an g
l I I apparent strike-separation change f rom right lateral to lef t lateral from south to north where the fault crosses a synclinorium. The rela-I tive minimum age of the fault may be established during the investiga-tion if it is determined whether a diabase dike of probable Jurassic I age is offset. One exposure exhibits a layer of surficial colluvium that apparently has not been displaced by the fault. Although Secor's final opinion regarding the Wateree Creek Fault will not be presented until the conclusion of the investigation, it appears probable that the fault exists in the Chapin quadrangle and that it pentrates a short distance into the Charlotte Belt. I POSSIBLE NORTHWARD EXTENSION OF FAULT Recent communicetion with Dr. Secor has established that he places I a high priority upon determining if the fault extends northward across the Broad River and through the area situated between Monticello Reser-I voir to the east and the Broad River to the west. Investigation of that portion of the Mayo Creek stream valley that crosses the projected fault area did not produce evidence of the fault. Additional field work in this general area has been performed under Secor's direction, and continues. To date, no field evidence of faulting has been found beyond Secor's northernmost control point, located approximately 2 kilometers southeast of Peak, South Carolina. Secor notes that a theoretical projection of the fault approxi-mately coincides with a narrow topographic drainage divide located between the Broad River and the west side of Monticello Reservoir. Should the fault actually extend t.hrough this area, the topographic depression could be an expression of the fault's location and orienta-It is pertinent to note that similarly oriented drainage I tion. features occur throughout the general area, many of which are probably related to prevailing joint trends. Secor (personal communication, 1980) does not feel that the coincidence of the topographic feature is sufficient to establish a projection of the fault. I I I Information has been requested redarding the apparent strong N-S linearity of aeromagnetic and gravity maps of the Monticello Reservoir area and the significance of these geophysical linears to the pre-dominant oriertations discussed in the Site-Specific Sc2.smicity section of tnis report. The linearity may be apparent (i.e. non-structural in nature), caused by the disposition of known granodiorite bodies, or the I linearity may be ralated to diabase dike material (encountered in boring DHP-3 near the Fairfield Pumped Storage Powerhouse); therefore, it is not known what significance the N-S lineation has to the orienta-I tions of nodal planes for composite fault plane solutions other than what has been reported in the Site-Specific Seismicity section of this report. There is an apparent N-S to NW linear pattern, observed on Figure 5 of Appendix V ( Aeroradioactivity Map), to the west of Monti-cello Reservoir. The linearity is interpreted to represent the masking effects of the Broad River on radioactive emissions from underlying rock bodies, and is not considered to be indicative of site structure. Secor comments that the Wateree Creek Fault has dip-slip displace-ment (where mapped in the Chapin quadrangle south of the site) along a nearly vertical plane, while focal mechanism solutions for events at Monticello Reservoir indicate reverse thrust faulting along much shallowe r dipping planes. The fact that Talwani's epicenter clusters 1, 2 and 4 (Site-Specific Seismicity section) line up with a projection of the Wateree Creek Fault is likely coincidental. Close examination of the composite fault plane solutions for those clusters reveal that the solutions (predominantly thrust with some strike-slip component) are not only I different as to the orientation of the nodal planes from one cluster to another, bet that they also. change vertically and laterally within any given cluster. This supports the arguments made for the high vari-j ability (both laterally and with depth) in stress trajectories l ( Appendices III and IV of the Site-Specific Seismicity section) throughout the reservoir area. I I I
I I CONCLUSIONS Substantial evidence exists indicating the occurrence of the Wateree Creek Fault in the Chapin quadrangle as presently mapped by Dr. Secor. The fault has been traced northward to a point app roxi-mately 2 kilometers southeast of Peak, South Carolina. The progress of the field work to date has not provided evidence of northward continua-tion of the fault, and intensive efforts to resolve the limits of the fault have been given a high priority by Dr. Secor. A theoretical northward projection of the fault apparently coin-I cides or closely aligns with a topographic drainage feature west of Monticello Reservoir, and possibly with general areal geophysical linear patterns. Dr. Secor and consulting geologists familiar with the I site geology do not believe these associations to be sufficient evidence of faulting to extend the northern limit of the fault beyond the northernmost control point presently mapped. Dr. Secor has conducted and participated in geologic mapping projects in the general vicinity, especially in the Slate Belt materials, since 1963. His previous work provides him with explicit knowledge of the geology of the area south of the site, and he has f ollowed the various earlier geologic investigations conducted for the Virgil C. Summer Nuclear Station throogh examination of core samples and site visits to observe rock exposed in the excavations. The scope of Dr. Secor's present investigation is thorough and comprehensive, and it is highly probable that his intensive efforts to define the northernmost extent of the fault will produce conclusive field evidence I on whether the fault continues across the Broad River towards Monticello Reservoir. 1 l l I l I
-- S -}}