ML19332A009
| ML19332A009 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 09/05/1980 |
| From: | Nichols T SOUTH CAROLINA ELECTRIC & GAS CO. |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8009100320 | |
| Download: ML19332A009 (103) | |
Text
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9' SOUTH CAROLINA ELECTRIC a gas COMPANY eest omes.on re.
COLUMelA.Soutu CAnoWMA 29218 T. C. NicMc Ls,J n.
Septamber 5, 1980 v.......,... o..,,.co'* *
....s,.........
(Nuclear Operations)
Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission Washington, D. C.
20555
Subject:
Virgil C. Summer Nuclear Station Docket No. 50/395 Seismology Questions
Dear Mr. Denton:
South Carolina Electric and Gas Company, acting for itself d as agent for South Carolina Public Service Authority, herewith forwards forty-five (45) copies of responses to Questions (361.13-361.22) as requested by Mr. Robert E.
Jackson's letter of June 20, 1980. These responses are being submitted by letter in order to expedite the Staff review and will subsequently be incor-porated in the FSAR by amendment.
Applicant requests that a meeting be scheduled as soon as possible with the Staff to discuss these responses and address other concerns related to this matter.
Very truly yours, T. C. Nichols, Jr.
RBW:TCN:jw cc:
H. T. Babb G. H. Fischer W. S. Mescher W. C. Murphy W. A. Williams, Jr.
B. A. Bursey T. B. Conner R. E. Jackson W. T. Kane A. e. Murphy Dan Cash NPCF/Dixon File IHIS DOCUMENT CONTAINS POOR QUAUTY PAGES
~
,8009100 3 2 0 O
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r 361.13 In the interest of conservatism of OBE spectra with respect to nearby moderate-sized earthquakes, the Final Report, Significance of the Monticello Reservoir Earthquake of August 27, 1978 to the Virgil C. Summer Nuclear Station for South Carolina Electric and Gas Co., submitted to the Nuclear Regulatory Commission on May 6, 1980, does not fully address the staff concern about the significance of the above-named earthquake.
Spc ifically, the staff wants the applicant to assume that the subject earthquake occurred at the recording i
station and that the station.was near the plant.
In correcting the time series and preparing the response spectra reduction should not be made for geometric spreading or attenuation of high frequencies due to propagation in the soil.
The spectra, thus generated, are to be compared to the Operating Basis Earthquake spectra for Virgii C. Summer nuclear station.
RESPONSE
Figures providing the requested spectra are attached.
The response spectra for the event were completed using the records as digitized at a time-step of 0.01 second and corrected by the Seismic Engineering Branch of the USGS.
The spectra are presented at 2% damping, a level that minimizes oscillations and preserves the general spectral shape. Figure 361.13-1 presents the unattenu-ated spectra, as described above, for the 180', 90', and vertical components.
Figure 361.13-2 presents the spectra of the deconvolved motion for the 180' and 90* components, respectively, compared to the OBE spectrum.
The recorded 4
361.13-1 AMENDMENT 21 OCTOBER, 1980
)
SMA motions were deconvolved utilizing the computer program SHAKE. Average soil and rock parameters were derived from Tables 2.5-30 through 2.5-38 and Figu es 2.5-118 and 2.5-119 of the FSAR after comparison to the log of the Doring drilled immediately adjacent to the SMA instrument site.
As shown on Figure 361.13-2, the motions at bedrock are censiderably less than those recorded at the surface (Figure 361.13-1); thus, it is likely that amplification of motion has occurred in the soil column at the location of the SMA instrument.
With the exception of only one minor excursion, the deconvolved motion spectra for the 180* and 90* components of the August 27, 1978 event are enveloped by the OBE spectrum.
REFERENCE:
Schnabel, Lysmer and Seed -
" SHAKE: A Computer Program for Earthquake Response Analysis of Horizontally Layered Sites;" EERC 72-12, University of California, Berkeley.
362.13-2 AMENDMENT 21 OCTOBER, 1980
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Virgil C. Summer Nuclect Station RESPONSE SPECTRA COMPUTED FROM SMA RECORDS OBTAINED DURING THE AUGUST 27,1978 EVENT F1 GURE 361.13-1 AMENDMENT 21 361.13-3 OCTOBER 1980
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FI GURE 361.13-2 t
5 361.13-4 AMENDMENT 21 OCTOBER 1980
2:
361.14 With respect to FSAR Question 361.7-2, the microzonation of seismicity associated with Monticello Reservoir comprises-three major zones (Teledyne Geotech Technical Report 79-8, pages 11-17):
(a) single zone near the north end of the reservoir; (b) an east-west zone, containing possibly four subzones, across the central part of the reservoir; and (c) a zone near the end of the reservoir consisting possibly of two subzones.
Produce composite fault plant solutions of each zone (or subzone, where possible).
Include first motions derived from MEQ-800 pcrtable seismographs and the six-station USGS network (Talwani, Induced Seismicity and Earthquake Prediction Studies in South Carolina,1979e, page 16).
1
RESPONSE
Before the installation of the six-station USGS network, hypocentral locations were obtained routinely using the four-station SCE&G network together with data from JSC and MEQ-800 portable seismograph (as and when deployed). These data were also used to obtain composite fault plane solutions of events sub-divided according to their locations and depths. After the installation of the USGS network and its incorporation with the SCE&G network, data from 10 channels were recorded on magnetic tape. This forms the Monticello seismo-graphic network.
To compare the hypocentral locations of events recorded on the Monticello netwcrk (tape recording) with those on the SCE&G network (helicorder recording),
about'100 events recorded on both were selected. The epicentral locations obtained from the two recordings agreed wel'1 (latitudes within +0.03' (s500m) 361.14-1 AMEflDMEtlT 21 OCTOBER, 1980
+
for 78% events, longitudes wibin 10.3' (s450m) for 72% events). However only 53% of the depths agreed within 10.6 km.
As the angle of incidence (used in the fault plane solutions) critically depends on the hypocentral depths, for detailed construction of fault plane solutions, only events recorded on the Monticello network were used.
RESULTS For an analysis of data recorded on the 10 station Monticello network (recorded on analog magnetic tapes), visual playbacks are obtained from the USCS facilities in Golden, Colorado. Due to a long delay in obtaining these playbacks, the data used in the analysis presented below covers the period from July to December 1978.
(Playbacks for 1979 are. expected shortly, and it is anticipated that their analysis will be completed in time for the next NRC meeting). However an examination of a small sample of data in 1979, indicated that the basic conclusions presented below are unaffected.
Using high quality data it was possible to divide the seismicity into five clusters (Figure 361.14-1). At least one composite fault plane solution was obtained for each cluster. These are summarized in Table 361.14-1. One 1
solution each was obtained for Clusters I and V, two for Cluster III, three for Cluster II and four for Cluster IV.
Except for solutions 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 solutions indicate that thrust faulting is the predominant mechanism. Some events, especially the deeper l
events (1-2 km) (Cluster IV, Solutions 3 and 4) exhibit a component of strike slip motion.
361.14-2 AMENDMENT 21 OCTOBER, 1980 i
Two predominant orientations of'the nodal planes, NS and NW-SE were noted.
The P axes, as would b'e expected for thrust faults, are predominantly close-to _ horizontal. (Table 361.14-1). However 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 361.14-2. A perusal of Tables 361.14-1 and 361.14-2 indicated that the orientations of the P axes
~
and of the slip vectors were related to the rock units the events were associated with. So they were separated on the basis of their geologic association (Table 361.14-3). The geology of Cluster IV is uncertain, and hence the rock unit has been labeled " mixed".
In Group A, i.e., when the epicenters are located on the country rock (granofel, CBGN) or on the intrusive rock (granodforite), the P axes (azimuth measured 0
clockwise from north) are remarkably consistent, and lie between 80 and 92.
Two possible slip vectors are indicated, one set striking 90-120 and the other 225-2650, both indicating a northerly striking fault plane. Other data s
were examined to decide between the two.
U In Group C,. consistent' P axes directions (53-66 ) (indicating a NW-SE fault plane) were obtained, but the calculated slip vectors are not consistent.
Twc solutions included in Group C are for deeper events.
JULY-DECEMBER 1978 DATA; A REPRESENTATIVE SAMPLE AftEr theiimpouldment of the Monticello Reservoir in 1977, induced seismicity 4
l was observed at different locations in the reservoir vicinity. The seismicity spread in subicquent months -(see response to -Question 361.17.2). However, most i
of this spreading occurred in approximately.six months following impoundment.
j-Of the total epicentral area covered in two years after filling, over 90% of it was covered in'the first year, and various clusters had been defined. Thus the AMENDMENT 21 361.14-3 OCTOBER, 1980
,r
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data for the period July-December 1978 comprise an excellent r_epresentative sample of the total seismicity. The epicentral locations for this period cover all observed clusters. The results of analysis for the period July-December 1978, therefore, are representative of the total observed seismicity.
The events comprising Group B are in shallow migmatite and probably reflect local orientation of fractures. The spatial association of different fault plant orientations is summarized in Figure 361.14-2.
SUMMARY
OF RESULTS 1.
The results obtained from an analysis of data recorded on the Monticello network for the period July-December 1978, are representative of the total observed seismicity.
2.
Several fault plane solutions were obtained for the different clusters.
All of them indicate thrust faulting as the mechanism.
i 3.
Although there was some variation in the orientation of the nodal planes, two predominant sets of fault p'ane solutions were obtained. The nodal planes are oriented predominantly N-S and NW-SE.
4.
The P (compressional) axis is usually taken to represent the direction of maximum stress. Here two sets of consistent P axes were obtained. As there should be only one direction of maximum REGIONAL TECTONIC stress in so small an area, the conclusion that can be drawn is that one or both inferred directions of maximum regional tectonic stress are erroneous. This observation has three possible implications:
AMENDMENT 21 361.14-4 OCTOBER, 1980
5A. Because the P axes were obtained from the orientation of the fault planes, the different orientations of the fault planes indicate that the earthqudkes are occurring along differently oriented EXISTING FRACTURES. These are oriented NS in the country rocks (Group A) and NW in the migmatite (Group C).
The NW orientation seems to represent the regional picture. This is because a) the deeper events (Cluster IV, solutions 3 and 4) are aligned in that direction together with those in the migmatite zone.
(The fractures in the migmatite zone could have been more dense as a result of cooling after pluton emplacement due to its location relative to country rock and the plutonic rocks),
and b) the regional direction of jointing is NW-SE.
The events corprising group B are in shallcw migmatite and probably reflect local orientation of f ractures.
- 58. The differences in the orientation of P axes represent a localized difference-in (RESIDUAL) stress directions which possibly result from the effects of rock anisotropy on the regional tectonic stress. This accounts for the seismicity and the orientation reflects the paleostress env1 onments that the rocks underwent.
SC. A combination of both the above mentioned possibilities.
In this inter-pretation, the seismicity is occurring along pre-existing fractures, and is caused by the perturbation due to impoundment of the reservoir on the local residual stress. The seismicity, in this interpretation, will be limited to localized pockets of high in situ stress, whose depth may be restricted by the emplacement history of the rocks. The stress will be relieved by the seismicity.
In this interpretation, there will be a marked decrease in the level of seismicity, as has been observed.
REFERENCE
- Talwani, P., B.K. Rastogi and D. Stevenson (1980) Induced Seismicity and Earthquake Prediction Studies in South Carolina, Tenth Technical Report, USGS Contract No. 14-08-0001-17670, 212 pp.
AMENDMENT 21 361.14-5 OCTOBER, 1980
O O'
TABLE 361.14-1 GE0 METRIC DATA-FOR CFPS OF EVENTS IN DIFFERENT CLUSTERS CLUSTER ROCK N0DA!. PLANE P AXIS T AXIS DEPTH (KM)
TYPE' N0.
STRIKl DIP AZIM.
PLUNGE AZIM.
PLUNGE i
I Granofels 1
N410W 480NE 2600 70 1600 560 0-1 2
N180E 600W U
0 II Granofels 1
N100W 620E 90 16 231 70 0-1 Solution 1 2
N160E 300W 0
Granodiorite 1
N430W 60 NE 2630 00 1720
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N280E 60 NW 0
Migmatite 1
N390W 42 NE 2140 40 3400 850 Solution 3 2
N590W 480SW III Migmatite 1
N220W 400NE 0
0 0
830 62 6
198 Solution 1 2
N330W 500SW 0-1 CBGN 1
N170W 0
60 NE 920 100 2010 630 0
0 Solution 2 2
N28 E 40 NW (SetI) 0 0
IV Mixed 1
N06 W 62 E 0
87 17 2570 730 0
0 Solution 1 2
N 28 W 0-1 0
0 Mixed 1
N16 W 58 NE 62c gio 294 72 0
U Solution 2 2
N460W 360SW 1-1.5 Mixed 1
N68 W 50 NE 2330 70 1390 540 Solution 3 2
N060W 600W 0
0 1.5-2 Mixed 1
NO3 E 60 E 66o 8o 325o 56o 0
0 Solution 4 2
N56 W 48 SW l
V Mixed 1
N430W 660 E 47o 210 2270 N
690 1
0 0
0-1 CBGN 2
N43 W 24 SW l
AMENDMENT 21 0CTOBER. 1980 361.14-6 i
-I
O TABLE 361.14-2 SLIP VECTORS SOLUTION NODAL STRIKE / THRUST CLUSTER GEOLOGY PLANE AZIMUTH DIP COMP.
0 300 1.11 I
1 1
108 0
Granofels 2
228 42 0.81 II 1
1 104 600 0.21 0
0 0
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260 28 0.42 2
1 1180 300 1.43 Granodforite 2
2270 300 1,43 0
0 3
1 31 42 0.11 Migmatite 2
2170 480 0.09 III 1
1 2370 400 0.14 Migmatite 2
670 500 0.12 2
1 1190 500 0.53 CBGN 2
2530 30 0.81 0
U 0
IV 1
1 90 62 0.05 2
2640 280 0.11 0
2 1
44 54 0.31 2
2540 320 0.47 0
0 3
1 83 30 1.15 (Deeptr) 2 2020 400 0.90 4
1 340 420 0.81 (Deeper) 2 2720 300 1.07 0
V 1
1 47 66 0.00 0
0 2
227 24 0.00 AMENDMENT 21 0CTOBER, 1980 361.14-7
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TABLE 361.14-3 GE0 LOGIC ASSOCIATIONS WITH FPS DATA CLUSTER GE0 LOGIC P
SLIP VECTOR AZIMUTH (SOLUTION)
. UNIT AZIMUTH DIP 1.
2.
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Granofel 800 70 1080 2280 U
U 0
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CBGN 92 100 1190 2530 IV (1)
Mixed (?)
870 170 900 2640 B
II (3)
Migmatite 340 40 310 2170 V (1)
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Migmatite 62 6
67 237 IV (2)
Mixed (?)
620 110 440 2540 U
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Mixed (?)
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83 202 IV (4)
Mixed (?)
660 80 340 2720 AMENDMENT 21 OCTOBER,1980 361.14-8.
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46 AMENDMENT 21 w
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AMEN 0 MENT 21 OCTOBER 1980
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i 361.14-10
361.15 Discuss the relationship between the stress field determined from the fault plane solutions and (a) local (<150 km) structural / tectonic geology and (b) the stress field determined in the two USGS deep wells located west and southwest of the reservoir (Talwani, Induced Seismicity and Earthquake Prediction Studies in South Carolina,1979e. page 21).
RESPONSE
As discussed in response to Question 361.14, the inferred direction of maximum
. horizontal stress (aj) at hypocentral depths is NE-SW. The local ( 150 km) structural / tectonic geologic framework ha; a pronounced NE-SW orientation.
The NE-SW geologic grain would suggest that, at the time of formation of those rocks, the direction of maximum horizontal stress would be NW-SE. Thus, the stress field inferred from fault plane solutions is representative of the present day stress field, whereas that inferred from the local geology probably would represent a paleostress field.
In the two USGS deep wells no stress orientations are available frora the hydrofracture data (Zoback, pers, comm.). Other features of these measurements are addressed in response to Question 361.21.
Regionally, orientation of the principal stresses has been obtained from fault plane solutions at Lake Jocassee (about 170 km to NW of Monticello Reservoir). Stresses have also been measured at Bad Creek (about 10 km NW of Jocassee Dam) by hydrofracture (Haimson,1975) and by overcorirg (Schaffer et, al.,1979). These results are summarized in Table 361.15-1.
361.15-1 AfiENDMENT 21 OCTOBER, 1980 e
t Table 361.15-1 4
s Average Principal Stress Values Depth Below ogMin oHMax location Surface bars Direction bars Direction Method 0
U Bad Creek 236 159+25 N30 W 228+55 N60 E Hydrofracture 0
0 Bad Creek 181 184 N32 W 293 N57 E Overcoring NE Fault Plane
.e Jocassee
< 2 km NW Solutions Monticello
< 2 km NW NE Fault Plane Reservoir Solutions Thus, the stress directions inferred from fault plane solutions at Monticello Reservoir are in agreement with those in the vicinity of Lake Jocassee "'..ch were inferred from fault plane solutions, and with those obtained from over-coring and hydrofracture measurements in the Bad Creek well. All these data indicate that oi (the largest principal stress)is horizontal and oriented NE-SW, is also horizontal and oriented NW-SE, and o3 (the least principal 2
stress) is vertical.
REFERENCES Haimson, B.D., (1975) Hydrofracturing stress measurements, Bad Creek Pump Storage project, Report for Duke Power Company,19 pp.
Schaeffer, M.F., R.E. Steffens and R.D. Hatcher (1979).
In situ stress and its' relationship to joint formation in the Toxaway gneiss, Northwestern South Carolina, Southeastern Geoloay, 20, pp.129-143.
361.15-2 AMENDMENT 21 OCTOBER,1980
361.16 On the basis of the more recent seismicity reports by Teledyne Geotech-(1978a,b,c,d; 1979a,b,c) and Taiwani (1979a,b,c) for Monticello Reservoir, update the discussions of the spatial and -temporal distribution of hypocenters and their relationship to the local (<150 km) structural / tectonic geology.
RESPONSE
The monthly epicentral locations have been provided in the various quarterly seismicity reports and are not presented here.
Details of epicentral riaration are discussed in response to Question 361.17.2.
The pattern of seismicity noted in the earlier reports appears to be continuing.
The st ismicity appears to be related to the local (<10 km radius) geology rather than to any regional feature. The seismicity still appears to be related to existing fractures as inferred from an analysis of fault %ane solutions. Most of the seismicity is still located in 3 broad bands (Figures 361.16-1 and 361.16-2).
After the initial spurt of seismicity fcllowing impoundment, there has been a marked decrease in the seismicity.
The decreased leval of seismicity was interrupted by discrete swarm episodes, the must orominent occurring in October 1979. The bulk of the larger (ML > 2.0) events occurred in 3 swarms,
-1.e. 26 events in January-February 1978, 9 in October-November 1978 and 19 in October 1979. That is, of the 70 large events between December '77 and December '79, 54 or 77% occurred in 3 swarms. After the initial seismicity associated with the impoundment, the cause of the other swarms is not very clear. However, there has been no noticeable change in the magnitude of the largest events in any of the swarms (<ML 2.8).
AMENDMENT 21 361.16-1 OCTOBER,1980
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1 AMENDMENT 21 OCTOBER 1980 361.16-3
361.17 Re: The maximum earthquake potential under Monticello Reservoir during the lifetime of the Virgil C. Summer Nuclear Station.
(1) In response to FSAR Question 361.7, the applicant presents arguments that earthquakes associated with Monticello Reservoir cannot be very large because of the shallow focal depths, 0.5 km.
1 Two problems exist with that argument:
(a) the assumption is made that the vertical extent of the fault plane equals the focal depth, and (b) calculated focal depths are as much as 4 km (Talwani,1979c,d). Justify assumption (a) in view of focal mechanisms calculated by Talwant that indicate nodal plane dips between 300 and 600 (Talwani, e_t_ al_.,1980), 'or propose a different assumption that can be justified. Determine whether new evidence exists that the focal depths are really different from those published (Talwani,1979 and Talwani et al.,1980).
Using the justified assumption and the most recent estimated of focal depth, evaluate the ' maximum earhtquake potentail under Monticello Reservoir. Present and justify the limitations or uncertainties of that estimate.
RESPONSE
The argument that earthquakes associated with Monticello Reservoir cannbt be very large because of shallow focal depths (0.5 km) cannot be justified in light of recent data. However, a case can be made that some of the depths calculated from helicorder data (SCE&G network) are in error.
1 In the period July-December 1978, '173 events were recorded on analog tapes (Monticello network). Of these,170 or 98% were shallower than 2 km (66% with AMENDMENT 21 OCTOBER, 1980 361.17.1-1
^
a depth < 1 km).1When these depths are
,ared with those obtained from helicorders (SCE&G network), only 53% agree to within 10.6 km, whereas for 47% of the events, the depth differences range between 10.6 and +2.6 km.
These observations indicate that although the SCE&G network gives adequate etlicentral locations, the depth estimates are unreliable.
4 Discussion of maximum earthquake. potential under Monticello Reservoir is contained in response to Question 361.18.
i AMENDMENT 21 OCTOBER, 1980 361.17.1-2
361.17 (2) In several recent reports (Talwani,1979a,1979b, and Talwani, et al.,1980), the applicant's consultant has mentioned that the areal extent of the seismic activity has increased with time since the initiation of activity with the initial impounding of the reservoir in December 1977.
Is the seismic activity continuing to spread horizontally and to deepen?
If it is still spreading, present graphically, with a clear unambiguous description, a representation of the seismically active area and the rate of the spread with respect to time.
If it is not still spreading, present the above mentioned representations up to the time of cessation of spreading and estimate the limitations on the maximum undetectable rate of spreading. What etfects would continued spreading of the seismicity have on the maximum earthquake potential under Monticello Reservoir? Explain and justify selection of the effects.
RESPONSE
As pointed out.in response to Question 361.14, the epicentral locations obtained from SCE&G network agree quite well with those obtained from the Monticello network (recorded on analog tapes).
Meaningful data from analog tapes wc obtained from July 1978.
Consequently, to make a study of the temporal behavior of seismicity, only data recorded on the SCE&G network has been utilized.
The temporal variation in seismicity is shown in a series of figures (Figures 361.17.2-la through lb).
Events were located by using HYP071, and only those events located with an RMS error < 0.1 sec and ERH < l.0 km were considered.
The periods chosen are weekly (during swarms in February 1978 and October 1979),
biweekly (January 1978, September 1979), and monthly.
An envelope surrounding AMENDMENT 21 0CTOBER, 1980 361.17.2-1
the epicentral locations for a particular period is shown by a dashed line; an envalope showing the cumulative 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 (as compared with the cumulative seismicity plot shown in Figure 361.16-1).
Similar plots are made for the larger events (Figures 361.17.2-2a through 2 ).
9 1
RESULTS The initial seismicity (December 1977 - January 1978) began in unconnected locations (probably at locations where 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 the lake are natural or are quarry blasts.
In Febraury 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 reservoir.
In March-April 1978 there was further spreading of the seismicity, but most of it occurred in filling the holes in between the previously defined periphery of epicentral locations.
The same pattern of small epicentral growth continued until the end of 1978 (Figures 351.17.2-lc through le) by which time many of the holes had grown and coalesed into three ' puddles'.
In this period the larger events occurred to the SW and NW of the center of the reservoir.
In 1979 there was very little further growth (Figures 361.17.2-l e through l b).
This growth occurred in February, August and the first week of October 1979.
The October 1979 swarm was also associated with the la ;er events located to the west and southwest of ~the reservoir. After the first week of October 1979, there has been no further epicentral growth.
AMENDMENT 21 361.17.2-2 OCTOBER,1980
7 Die hypocentral depths obtained from the SCE&G network are not accurate enough to document any changes.
However, qualitatively, there does not appear to have been a marked increase with time of the hypocentral depths.
Qualitatively, we can see from Figures 361.17.2-la through lb, 'that the epicentral growth rate was fastest in the period December 1977-March 1978.
During that period the epicenters covered approximately 70% of all the epicentral area occupied (through 12/79).
During the next nine months, (4/78 - 12/78) a further increase of about 20" of the total area occurred.
In 1979, the growth rate was much less (as was the seismicity).
In summary, a?ter rapid epicentral growth associated with the initial impoundment, fur.ther growth has been extremely limited. A possible explanation is that initially the seismicity was associated with the heavily jointed regions--predominantly in the migmatite units.
Since then seismicity has spread up to the less permeable country rocks.
The country rock is associated with fewer and possibly tighter joints (Secor, pers. comm.).
Contin;ed spreading would indicate the presence of permeable jointed rocks, and would not (in the absence of through-going faults) suggest an increase in the maximum earthquake potential.
AMENDf1ENT 21 OCTOBER, 1980 361.17.2-3
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3'l.17.2-10
l 1
361.17 (3) Prior to 'the most recent technical report (Talwani,1979d) nearly all the seismicity under Monticello Reservoir has occurred in previously jointed or fractured rock.
At a recent meeting with the applicant and his consultants.(2/21/80 at the plant site), Talwani indicatrd that earthquakes were beginning e
l to occur in the more competent plutonic rock, similar to the rock which is under the plant.
Does this indicate that the reservoir i
l is inducing more energetic earthquakes (breaking fresh or unfractured rock) 1.e., earthquakes with greater stress drops?
Explain consequences with respect to maximum earthquake potential.
a
RESPONSE
The presence of seismicity in the nors competent plutonic rocks does not suggest that the reservoir is induci.ng more energetic earthquakes.
It merely indicates that seismicity in these rocks occurred after that in the more fractured rocks, because these rocks are associated with fewe. fractures.
~Due to lower permeability, these rocks would not be associated with seismicity, when more permeable rocks are available for fluid flow. However, after seismicity (and presumably fluid flow) in the heavily fractured rocks has stopped, seismicity will be observed in the less permeable plutonic rocks.
The seismicity is still associated with joints and fractures (as indicated by an analysis of fault plane solutions). Thus, the presence of seismicity in f
the plutonic rocks does not increase the maximum potential earthquake, i
l 4
361.17.3-1 AMEND?iENT 21 OCTOBER, 1980
~
161.1 7 (4) What peak accelerations and what response spectra would be expected at the Virgil C. Summer Nuclear Station for a Magnitude 4.0 earthquake and a Magnitude 5.3 earthquake (two non-simultaneous events) if the earthquake occurred at a distance of 10.0, 3.0, and 1.0 km from the plant?
(The staff is asking for peak acceleration and response spectra estimations at three distances for each of two earthquakes - 6 cases).
Consider the 1.0 km cases at 0.0 horizontal range and 1.0 km depth.
Compare the resulting spectra to the V.C. Summer OBE and SSE response spectra.
Discuss the effects of the above calculated spectra exceeding the OBE and SSE response spectra.
RESPONSE
The dearth of-strong ground motior data in the near-field of earth-quakes, and.in particular for the shallow depths specified by NRC, require examination and use of theoretical methods for estimating ground motion.
This response examines one such method for estimating peak accelerations which has been verified by comparison with California strong notion data obtained over a wide range of source-to-site distances.
To estimate response spectra for -the associated ground motions, use is made of available response spectra primarily from California obtained from records in the magnitude and distance anges most closely resembling those specified by the NRC, and standard methods for scaling those spectra to estimate response spectra for the six
' designated events.
AMENDMENT 21 0CTOBER, 1980 361.17.4-1
O A. - PEAK ACCELERATION ESTIMATES To estimate amplitudes of peak acceleration, the Brune (1970, 1971) model of seismic sources is used to determine the root-mean-square acceleration a and duration Td of direct shear waves at the site of rms interest, and a simple application of random vibration theory is used to estimate the peak acceleration a from a and T.
This method has been shown to be p
rms d
appropriate by comparison to recorded ground motions in California (McGuire and Hanks,1979; Hanks and McGuire,1980) and is applied to the project site with certain modifications as discussed below.
The model used here to estimate characteristics of motion begins with a description of the Fourier amplitude spectrum of displacement u, at source-to-site distance R, caused by shear waves in the far field.
The initial description given here is for ground motion in all directions at a site located in a uniform, isotropic full space. A correction factor to account for free surface amplification, vectorial partitioning of energy into instrumental components, and radiation pattern will be introduced below so that predictions can be compared with observations.
l The salient characteristics of the spectrum u are shown in Figure 361.17.4-la.
The long period level n is given by (Haskell,1964):
o Mo Go " 4no Rs3 Rg (1) where M is seismic moment, Rg is the radiation pattern of the shear excita-o tion, p is density, and S is shear wave velocity.
In Figure 361.17.4-la, the corner frequency f is inversely proportional to source radius r and can be o
estimated with the relation (Brune,1970):
fg = 2.348 (2) 2:r AMENDMENT 21 OCTOBER, 1980 361.17.4-2
High frequency (f>>fo) spectral amplitudes fall off as f-Y, y = 2, and this is an important feature of the assumed model. Hanks (1979) has argued from an observational basis that the Y=2 model is the one generally applicable to crustal earthquakes.
The Fourier amplitude spectrum of acceleration a can be obtained from u by multiplying by (2nf)2, leading to the typical spectrum shown in Figure 361.17.4-1. Amplitudes for all frequencies are decreased i:v the anelastic attenuation factor ka = exp (-nfR/QS)
(3) where Q is the specific attenuation. Anelastic attentuation for typical values of Q and 8, and for distances of interest, is only important for frequencies >>f.
o The earthquake stress drop ao is related to M and r in the Brune g
(1970, 1971) model by:
Ac =
(4) 16r3 Relations (1) Through (4), together with Figure 361.17.4-1, constitute the model used to estimate shear wave Fourier amplitude spectra of accelera-tion.
To estimate Fourier amplitudes of single components of motion at the ground surface, we account for the free surface effect (a factor of 2),
vectorial partitioning of energy into three components of equal amplitude 1/ 8 and the root-mean-square (rms) of the double-couple radiation pattern (0.6), or a combined correcMon factor of 0.7.
The division of energy into three equal components of motic.0 is considered appropriate for the near-source distances of interest in this Ody, in light of recent strong motion data obtained in California which show high-frequency vertical motions to have AMENDMENT 21 OCTOBER, 1980 361.17.4-3
amplitudes as large as horizontal motions at small source-to-site distances.
The Fourier amplitude.of-single horizontal component acceleration at frequency-
.f is given by:
I j + (fgff)2)
(5) a (f) = (0.7) exp (-n fR/QB ) (
PRS 1
Alternately, a(f) can be expressed in terms of source parameters Ao and r r
using equations (2) and (4):
1 I
a-(f) = (0.7) 0 exp (-nfR/QB) (
1
)
(6) g 1
1+(Fo/f)2 ihe method'used here to estimate peak ' acceleration a requires p
of shear waves.
-calculation of the root-mean-square acceleration arms i
Following Hanks (1979), and McGuire and Hanks (1979), we estima:e arms using Parseval's theorem. The estimate is valid for a time window equal to the faulting duration'Td beginning with the direct. shear arrival; in Hanks (1979) is taken equal to fo-l.
'In terms of spectral parameters Ao and r the Td result is g @ 2.34 rms = (0.7) (2n)2 2Qr a
(7) ao a
u 106 p
where-
/
~(
(-2nfuR) s %
1 (8) 1-exp gg a =
u (
Note, from equation (4), that r is proportional to M l/3; thus the rms o
'is acceleration is only very weakly ' dependent on the earthquake size (arms proportional to M l/6) but is directly proportional to stress drop.
The o
factor a accounts for limi_ts on spectral amplitudes, and hence on arms' u
resulting from the maximum frequency f which can be recorded by the instrument u
~and maintained with accuracy-by the _ strong motion r'ecord filtering and proces-l
. sing procedures.
JAMENDMENT 21
-OCTOBER, 1980 361.17,.4-4._
To estimate a from arms, a simple result is adopted from Vanmarcke and p
Lai (1977), which assumes that ground acceleration time histories are stationary, random, and Gaussian in a time interval T:
al 2 in (
)
(9)
=
arms a
where T is the " predominant period of the earthquake motion" and a is the o
p peak acceleration which will be exceeded once, on the average, in time T.
In this study T is equated with Td and for T the reciprocal of f is used, that is, o
u the reciprocal of the highest frequency preserved in the processed strong motion recording.
In contrast to ground motion studies made for longer source-to-site distances, the small distances-examined here imply that the highest frequencies in the (digitized version of the) strong motion record will be limited by instrument characteristics and processing procedures, rather than by anelastic attenuation or other physical phenomena.
To calculate peak acceleration for a specified earthquake magnitude, it is necessary to determine source parameters for the above model.
(It is
~
assumed here that magnitudes specified by NRC are local magnitudes).
Seismic moment can be determined using the relationsh;p of Thatcher and Hanks (1973):
log M = 1.5 ML + 16 (10) o where ML is local. magnitude.
For static stress drop associated with crut?.al earthquakes, the result of Hanks and McGuire (1980), a value of 100 bars, is appropriate to estimate a for the high frequencies of interest.
This has been shown to rms be the case for a large number of events in California, and is adopted here for the R = 10 km event to be considered in this study.
Values for the other earthquakes are discussed below. With M and Ao specified, the source o
dimension r can be calculated from equation 4.
AMENDMENT 21 OCTOBER, 1980
To illustrate the application of these ideas, Figure 361.17.4-2 shows peak horizontal acceleration data recorded at surface distances (either to the fault trace or to the epicenter) of 10 km or less. The mean distance of the data is 6.4 km. Also shown is the estimate for R = 6.4 km based on 3
equations 7, 8, and 9, using p = 2.8 g/cm, Q = 300, B = 3.2 km/sec, and ao=100 bars, values appropriate for California which constitute the majority of data shown.
The value used throughout this study for f is 20 bz.
This u
is appropriate because instrumental records are low-pass filtered at 23 or 25 hz, and instrument characteristics and the digitizing process typically reduce energy content in the frequency range of 16 to 25 hz.
For California earthquakes the largest amount of seismic energy typically is released at some depth; if a 5 km depth is assumed, the appropriate source-to-site distance for comparison is 8.1 km.
Figure 361.17.4-2 also shows a curve for estimates based on this distance. The comparison shown in Figure 361.17.4-2 for near-source data, and other comp arisons for farther distances (McGuire and Hanks,1979; Hanks and McGuire,1980) indicate that the method described above for estimating arms and a is appropriate for shear waves in the far-field when the source is p
specified only by earthquake magnitude. The far-field is taken to be distances
~
greater than several source diameters, or 4r.
For the earthquake ground motions to be studied here, the following 3
parameter values were used:
B = 3.2 km/sec, o = 2.8 g/cm, and Q = 1000 which reflects the low anelastic attenuation characteristics of the eastern U.S.
The exact value of Q is of secondary significance since anelastic attenuation has only a small effect on ground motions at the distances of interest.
AMENDMENT 21 0CTOBER, 1980 361.17.4 s i
For ao a value of 100 bars was assumed for earthquakes which occur at R = 10 km, since the source of these events could be as deep as 5 or 10 km, i.e. they _could be similar to California shocks.
For evants at R = 3 and 1 km, which must necessarily occur at shallower depths than 5 km, it is reasonable to assume that the appropriate values of Ao are lower than 100 bars because lithostatic pressures at these shallower depths will De reduced.
To determine an appropriate value-of Aa, comparisons were made to the ML = 2.7 earthquake which occurred in the vicinity of the site on August 27, 1978. A 2
value of ao= 30 bars gives estimates of a of 115 cm/sec for a source-to-p site distance of 0.8 km. This compares well with the digitized peak accelera-tions of 130 and 106 cm/sec2 (for the two horizontal components) and 40 cm/sec2 (for the vertical component). The specific parameters used for this calcula-tion are shown in Table 361.17.4-1.
Note that, since the theory accounts for truncation in frequency content due to record digitization and processing, it-is consistent only to compare theoretical estimates with peak accelerations obtained from the digitized, processed record. The assumption of 0.5 km depth for this event is conservative,-and a shallower depth would imply a closer source-to-site distance and a smaller value of ao.
From this comparison we conclude that a value of Ao of 30 bars is appropriate for very shallow earthquakes (depths < 1 km).
Therefore, this sslue is used below for estimating ap for events with R = 1 km.
For source-to-site distances of 3 km, a value of ao of 50 bars is thought to be appropriate.
This event is not specified to occur directly under the site at a depth of 3 km, but may occur away from the site at a depth between 0 and 2 km, and presumably at a depth closer to 1 or 2 km. The values of Ao assumed here are based on only one earthquake and on substantial judgment.
AMENOMENT 21 0CTOBER,1980 361.17.4-7
However, they do form a preliminary basis for estimating ground motions at the small source-to-site distances requested by NRC.
Estimates of peak acceleration for the six combinations of magnitude and distance requested by NRC are shown in Table 361.17.4-2, along with the parameters used to obtain these estimates.
For several cases (indicated by parentheses) the distances involved are less than several source diameters (R < 4r) so that the estimates are actually made for near-field conditions where the theory is not strictly applicable.
In these cases the estimates are conservative.
B.
RESPONSE SPECTRA ESTIMATES Response spectra were estimated based on a procedure developed by Newmark and Hall (1969) and later modified by Mohraz (1976) and Johnson and Traubenik (1978) to incorporate site conditions, and magnitude and distance respectively.
A general descriptien of the procedure used by Johnson and Traubenik (1978) to obtain ground motion ratios and amplification factors is as follows:
2 1.
Ground motion ratios, v/a and ad/v, for various percentiler using a normal and a log-normal distribution were calculated.
2.
The selected response spectra were normalized to obtain amplifi-cation factors. The procedure is to obtain, at each frequency point, the ratio of the spectral response to the maximum ground motion (i.e., amplification factor) for acceleration, velocity, and displacement in the corresponding frequency ranges. Averaged AMENDMENT 21 OCTOBER, 1980 4
361.17.4-8
amplification factors are then calculated for the low frequency or displacement region, the intermediate frequency or velocity region, and the high frequency or acceleration region.
Ampli fi-cation factors for 50 and 84.1 percentile values using both a normal and a log-normal distribution were calculated.
The accelerograms used in the Johnson & Traubenik (1978) study were chosen for their magnitude (greater than or equal to 4.5), the local geologic conditions at the accelerograph site (rock), peak acceleration (approximately 0.10g or greater), and source-to-site distance of 20 kilometers or less.
Six near-source events ranging in magnitude from 4.7 to 6.5 recorded at eight stations (horizontal components only) were used in the study.
The spectra shown on Figure 361.17.4-3 were estimated based on the results of the recent study by Johnson and Traubenik (1978).
Ground motion 2
ratios (v/a and ad/v ) and appropriate amplification factors are shown in Table 361.17.4-3.
Values are estimated for a magnitude (M )
f 4.0 and 5.3.
Ground L
motion val as in Table 361.17.4-3 are normalized to 1.0 g.
Estimated values are believed to be representative of source-to-site distances of less than 20 km and rock site conditions.
Spectral amplitudes (S. S, and S ) are shown in Table 361.17.4-4.
A V
D The values of S SV and SD were computed by multiplication of mean ground A
motion parameters by the appropriate amplificaticn factor (both of which are shown in Table 361.17.4-3. Two sets of spectra were constructed as shown on Figure 361.17.4-3.
Shown on Figure 361.17.4-3 are average spectra (5% critical damping) for two ranges of magnitude based on the data set used in the Johnson and Traubenik (1978) analysis.
Comparison of the curves indicates that the estimated ML = 5.3 spectra are similar in shape to the average spectra in the 5.4 to 5.6 range and that the ML = 4.0 spectra are appropriately narrow-AMENDMENT 21 OCTOBER,1980 361.1 7.4-9
banded and contain less long period energy than the ML = 4.7 to 4.9 average spectrum, as should be expected.
To make comparisons with design spectra, these earthquake response spectra were scaled by the peak acceleration values shown in Table 361.17.4-2.
These scaled spectra are shown in the next section, along with OBE and SSE spectra.
C.
COMPARISON WITH SSE AND OBE SPECTRA For comparison of response spectra estimated for the hypothetical events and response spectra corresponding to the Safe. Shutdown Earthquake (SSE) and Operating Basis Earthquake (OBE), we show here only spectra corresponding to 5% damping.
Comparisons for other dampings would be similar, and the conclusions drawn here would be the same.
Figure 361.17.4-4a shows the 5% damped response spectrum for the ML = 4, R = 1 km event, obtained by scaling the 5% damped spectrum in Figure 361.17.4-3 to a peak ground acceleration of 0.289 Also shown are the SSE and OBE spectra for horizontal motion and 5% damping.
Figure 361.17.4-4b shows a similar comparison for this event (ML = 4.0, R = 1 km) for vertical motions.
It should be emphasized that the spectrum for the hypothetical event is conservative because the peak acceleration estimate is conservative, as discussed previously.
I Figures 361.17.4-5 through 361.17.4-9 show similar comparisons for the other hypothetical earthquakes:
ML = 4 and R = 3 km (Figure 361.17.4-5),
j ML = 4 and R = 10 km (Figure 361.17.4-6), ML = 5.3 and R = 1 km (Figure 361.17.4-7), ML = 5.3 and R = 3 km (Figure 361.17.4-8), and M = 5.3 and L
i AMENDMENT 21 OCTOBER, 1980 1
361.17.4-10
R = 10 km (Figure 361.17.4-9). Similar comments regarding conservatism apply to Figures 361.17.4-7 and 361.17.4-8, because a for these events are conser-p vatively estimated as noted in Table 361.17.4-2.
Figure 361.17.4-10 is presented to show the comparison of a magnitude 4.0 earthquake at R = 1.0 km and a magnitude 5.3 earthquake at R = 5.8 km to the desigr, OBE and SSE spectra (where R is the significant distance from the focus of energy release to the plint site).
These two events are singled out as representing in our opinion, the worst case events ever to occur. The ML = 4.0 event as representing the maximum induced earth-quake has been discussed in response to Question 361.18. The ML = 5.3 event was chosen as being representative of the occurrence of the SSE (MM Intensity VII).
The choice of 5 km depth to represent the effective minimum energy release during tectonic earthquakes is based on observations and comparisons of theory with data obtained during California earthquakes. Recently obtained strong motion data near the fault trace of earthquakes suggest that peak accelerations do no t attenuate in the range 0 to 5 km from the fault, suggesting that the energy-causing strong ground rr stion effectively is generated at a depth of around 5 km rather than at the surface.
Hanks and McGuire (1980) illustrate this with observations of peak acceleration obtained during earthquakes with ML = 6.5.
A further reason for using a depth of 5 km is that this depth gives esttnates of acceleration using the Brune seismic source model which are consistent with strong motion dau.
Figure 361.17.4-2 illustrates this point.
It shows peak accelerations obtained within 10 km of the fault trace, plotted versus magnitude.
The average d'. stance of the data is 6.4 km; when this is 361.17.4-11 AMENDMENT 21 OCTOBER 1980
1
! 10 used as the significant distance for estimating ground motion, a'slightly conservative estimate is obtained.
If an energy depth of 5 km is used, resulting in.a significant distance from source-to-site of 8.4 km, a more appropriate, mean-centered estimate is.obtained as shown.
i Other considerations influencing the choice of 5 km source depth
-are physical:
the release of energy representing a ML > 5 earthquake requires significant lithostatic pressure, as represented in our model by a stress drop of 100 bars.
Such pressures cannot be obtained, and thus cannot release large stresses across a fault face, near the ground surface.
Time histories of these events have been prepared using four com-ponents of the Oroville, California 1975 earthquake for the Mt = 4.0 earth-quake and two components of the 1957 Golden Gate Park earthquake for the M := 5.3 event. The time histories will be used to generate floor response L
spectra for the OBE and SSE, respectively, so that a check on certain systens j
and components in the reactor building can be made.
These results will be forwarded at a later date.
(
t I
b
^
361.17.4-12 AMENDMENT 21 OCTOBER, 1980
REFERENCES Brune, J.N. (1970), " Tectonic stress and the spectra of seismic shear waves from earthquakes",
J_. Geophys. Res. 75, 1997-5009.
Brune, J.N. (1971), Correction, J_. Geophys. Res. 76, 5002.
Hanks, T.C. (1979), "b-Values and y-n Source Models:
Implications for Tectonic Stress Variations along Crustal Fault Zones and the Estimation of High Fre-quency Strong Ground Motion",
J_. Geophys. Res.
Hanks, T.C. 'and McGuire, R.K. (1980), "The Character of High Frequency Strong Ground Motion," (in preparation).
Johnson, J.A. and Traubenik, N.L. (1978), " Magnitude-Dependent Near-Sourra Ground Motion spectra", Proc. A.S.C.E. Conf., Ear'thquake Engineeri e and Soil Dynamics, Pasadena, June, p. 530-539..
McGuire, R.K., and Hanks, T.C. (1979), "RMS-Accelerations and Spectral Amplitudes of Strong Ground Motion During the San Fernando, California Earthquake,"
submitted to Bull. Seis. Soc. Am.
Mohraz, B. (1976), "A Study of Earthquake Response Spectra for Different Geological Conditions," Bull. Seis. Soc. Am.~, v.66, No. 3, June.
Newmark, N.M., and Hall, W.J., " Seismic Design Criteria for Nuclear Reactor Facilities," Proc., 4th World Conf. on Earthquake Eng., Santiago, vol. 2,
- p. 85-1-.85-12.
Thatcher, Wayne, and Hanks, T.C. (1973), " Source Parameters of Southern California Earthquakes", Journal of Geophysical Research 78, 8547-8576.
Vanmarcke, E.L., and Lai, S.P., " Strong Motion Duration of Earthquakes", M.I.T.
Dept. of Civil Eng., Report R77-16, 32p.
AMENDMENT 21 OCTOBER, 1980
TABLE 361.17.4-1 PARAMETERS USED FOR COMPARISON TO AUGUST 27, 1978 EARTHQUAKE l
ML 2.7 M
1.12X1020 dyne-cm o
depth 0.5 km R
0.8 km ao 30 bars r
0.118 km f
10.1 hz o
'1 Td 0.099 sec Q
1000 ap/a 1.66 rms 2
i a
69 cm/see rms i
2 a
115 cm/sec p
Observed
- ap (180 comp.)
130 cm/sec2 0
Observed
- ap ( 900 comp.)
106 cm/sec2 Observed
- ap (vert. com.)
40 cm/sec2
- from digitized, processed record, as explained in text.
AMENDMENT 21 361.17.4-14 0CTOBER, 1980
TABLE 361.17.4-2 PEAK ACCELERATION FOR SPECIFIED EVENTS, FOR BOTH HORIZONTAL AND VERTICAL PCTIONS*
M 4.0 5.3 L
R km 1
3 10 1
3 10 depth, km 1
<3
>5 1
<3
>5 M, bars 30 50 100 30 50 100 M, dyne-cm 1022 1022 1022 9x1023 gx1023 gx1023 o
r, km 0.53 0.44 0.35 2.4 2.0 1.6 f, hz 2.3 2.7 3.4 0.50 0.60 0.75 o
T, sec 0.44 0.37 0.29 2.0 1.7 1.3 d
Q 1000 1000 1000 1000 1000 1000 a /a 2.4 2.3 2.2 3.0 2.9 2.8 p rms rms.cm/sec2 (116) 58.2 29.1 (246)
(124) 61.7 a
2 a,cm/sec (281) 135 64.6 (734)
(358) 174 p
- Values in parentheses are calculated for distance in near field (i.e.,
R < 4r) for which the theory is not strictly applicable; these values are therefore conservative.
AMENDMENT 21 361.17.4-15 OCTOBER,1980 L
TABLE 361.17.4-3 ESTIMATED GROUND MOTION RATIOS AND AMPLIFICATION FACTORS Mean M
Mean Mean Damping Amplification Factors t
v/a ad/v2 d(ft)
P V
A (ft/sec)/g 0.5 4.6 1.5 3.4 2
4.1 1.4 2.6 4.0 0.42 0.8 0.004 5
3.3 1.2 2.2 10 3.3 1.0 1.7 0.5 2.0 1.5 3.4 2
1.7 1.4 2.6 5.3 1.67 3.3 0.29 5
1.5 1.2 2.2 10 1.3 1.0 1.7 AMENDMENT 21 361.17.4-16 0CTOBER, 1980
TABLE 316.17.4-4
SUMMARY
OF SPECTRAL AMPLITUDES M
Damping S
3.4 0.63 0.018 2
2.6 0.57 0.01 6 4.0 5
2.2 0.50 0.015 10 1.7 0.42 0.01 3 h
3.4 2.5 0.58 2
2.6 2.3 0.50 5.3 5
2.2 2.00 0.44 10 1.7 1.67 0.38
)
AMENDMENT 21 OCTOBER,1980 361.17.4-17
mu r
tce p
y S
2 c
)
n o
n f
e o
7 u
7 i
q t 2
e
(
a o
r r
f n
e l
g e
o c
ol lIII1II l i 1 l1 I f
c A
L
)
b ED
(
O I
M 2
E CR 1
UO 4 S 7
- ** w*-
M 1
O R
1 F
6 3
E E P R Al U i G S FI LAR TC E
P S
mur 2
tc y e c p I
n S eu t q n 3 e e
!lll gi1I\\)3
<3!
f r m f e c g a ol p
l s
o i
H D
)a
(
4 m
" % e o-i i yE A" 8h9 $
w$ LNf-Q
I000 ~_
-e-g ESTIMATE FCR R 6.4km 500 ESTIMATE FOR R 8.lkm A
N
+
o ese m
8 w
4
(
T o
2 A
e S
y o
+
A z
o E
r e
o a
g.
4e 100 de o
"i w
3 50 c.
3 e
- 4 1
I I
I I
I I
10 3
4 5
6 7
8 M AGNITUDE(M )
- OTHER C ALIFORNI A EARTHQUAKES o COYOTE L AKE (1979)
APAN
+ 11..F E RI A L V A L LE Y (19 7 9 )
EOTHERS FI GURE 36'.17.4-2 PEAK HORIZONTAL ACCELERATIONS RECORDED AT SURFACE DISTANCES < l O KM,
AND THEORETICAL ESTIMATES AMENDMENT 21 OCTOBER 1980 361.17.4-19
I O.0 0 Z
[ t=5.4 - 5.6 M
~
(5%)
n 0
/
Wn W
ML - 5. 3 -
p g
N w I.00 M =4. 7 - 4. 9 t
b
~
( 5'% )
[
M -4.0
///#
t o
O W>
W 2
F-jO.IO W
0:
l 0
~3
~
y NOTE: SPECTR A SHOWN FOR I / 2,
{
5.AND I O PERCENT CRITIC AL DAMPING O.0I i
t i iit ii I
i I iitiIi t
i i I I I
O.01 0.10 1.00 10.00 UNDAMPED NATURAL PERIOD (SECONDS)
FIGURE 361.17.4-3 MEAN ROCK NEAR-SOURCE HORIZONTAL ELASTIC RESPONSE SPECTRA FOR A M AGNITUDE (M ) 0F 5.3 AND 4.0 t
AMENDMENT 21 OCTOBER 1980 361.17.4-20
Io.o
\\
i nO w
v3 SSE w
I.0 M
OBE 5
O
.Jw y
I 2
l-
.Jw CC lo 0.1 c
aw M =4.0 t
y R=1km 0.01
,,,,it
,,,i'.I 0.0 I o.I
!o i cr.o.
UNDAMPED NATURAL PERIOD (SEC) l FI GURE 361.17.4-4a HORIZONTAL SPECTRUM FOR M =4.0, R =
1 km t
AMENDMENT 21 OCTOBER 1980 361.17.4-21
IO.0
)
l mo w
m i
N
[
SSE v
l.0 f
OBE o
O Iw w
2 l-Iw a:
Mu=4.0 o
0.I,-
R=lkm ao w
m c.
,,iiil i, i-,
0.01 t
0.0 l O.I I.0 i O.0 UNDAMPED NATURAL PERIOD (SEC)
I FI GURE 361.17.4-4b VERTICAL SPECTRUM FOR M = 4.0, R =
l km t
AMENDMENT 21 OCTOBER 1980 361.17.4-22
I O.0 O
w m
N
[
SSE l.0 v
M OBE 5
N O
Jw w
F-
!wc:
O.1 Q
3w m
Q.
Mu=4, R=3km 0.01 i
, - i i -
0.01 0.1 I.O I O.O UNDAMPED NATURAL PERIOD (SEC)
FIGURE 361.17.4-Sa HORIZONTAL SPECTRUM FOR M =4.0, R =
3 km t
AMENDMENT 21 OCTOBER 1980 361.17.4-23
l 3_
8 w
m N
[
SSE v
l.O U
OBE 5
o J
w W
_H
~
Iw Z
d, O. I o
ow m
c.
M,=4, R=3km 0.01
,,,,et
,,,,,t 0.01 0.I I.O I Cr.O UNDAMPED NATURAL PERIOD (SEC)
FI GURE 361.17.4-5b VERTIC AL SPECTRUM FOR M = 4.0, R =
3 km t
AMENDMENT 21 OCTOBER 1980 361.17.4-24
l I O.0 mow to N
[
SSE v
l.0
[
OBE 5
O
.Jw w
2 l-Iw CC 1
o 0.1 o
3 w
toc.
fg =4, R=10km 0.01
.,,,,1
,,iet i
i 0.0 i O.I I.O I O.O UNDAMPED NATURAL PERIOD (SEC)
FIGURE 361.17.4-6a HORIZONTAL SPECTRUM FOR M =4.0, R = 10 km t
AMENDriEllT 21 OCTOBER 1980 s
361.17.4-25 6
I O.0 8
w m
N
[
SSE v
I.0 C
OBE 5
O Iw w
2 Jw o
0.I o
3 W
mc.
M,=4, R=10km 0.01
,,,,,i 0.0 i O.I I.0 10.0 UNDAMPED NATURAL PERIOD (SEC)
FI GURE 361.17.4-6b VERTICAL SPECTRUM FOR M
=4.0, R = 10 km t
t AliEllDriEllT 21 OCTOBER 1900 361.17.4-26
i 10 0 8
w m
N
[
SSE v
I.o
[
OBE 5
O_:
R =5.3 y
R=lkm w
2 H
<Iw CY.
o.1 oo w
mo.
o.0
,,,,,t i
,,t 0.01 o.
1.0 l o.o '
UNDAMPED NATURAL PERIOD (SEC) 4 FI GURE 361.17.4-7a l
HORIZONTAL SPECTRUM FOR M = 5.3, R =
1 km j
t i
AttENDriEllT 21 OCTOCER 1980 361.17.4-27
I O.0 O
~
W co N
[
SSE I.0 v
[
f OBE 5
O M,=5.3 J
R=lkm W
2 l-
.Jw Qll c
O.1 a
D W
CD a.
1 0.01
,,,,1 i
,il O.01 0.1 I.0 10 0 UNDAMPED NATURAL PERIOD (SEC)
FI GURE 361.17.4-7b VERTICAL SPECTRUM FOR M =5.3, R =
l km t
Af tEllDME!!T 21 OCTOBER l')00 361.17.4-28
10.0 8
w M
N
[
SSE I.0 v
f
/
b x
0 OBE o
Jw w
2 F-M =5.3, u
d R=3km x
0.1 O
mw W
CL 0.01
,1 1
0.01 0.I I.0 10.0 UNDAMPED NATURAL PERIOD (SEC)
FI GURE 361.17.4-8a HORIZONTAL SPECTRUM FOR M = 5.3, R =
3 km t
AttEllDt1EllT 21 OCTOBER 1900 361.17.4-29
\\
I O.0 mow m
Nh I
b 1.0 I
OBE o
o M =5.3
~
u R=3km w
P I
w a::
O.I c
3 W
m CL 0.01
,,,,,1
,,,,,,1 0.0 I o.
i.o I o.o UNDAMPED NATURAL PERIOD (SEC)
FIGURE 361.17.4-8b VERTICAL GPECTRUM FOR M = 5. 3, R =
3 km t
AliENDriEllT 21 OCTOBER 1930 361.17.4-30
IO.0 8
w m
N
[
SSE v
l.O
[
OBE 5
O Jw w
2 1--
Jwy Mu=5.3, o
0.1 7 R=10km oD w
m CL i
0.01
,,,,.I
,iil 0.0l O.I I.O I O.O UNDAMPED NATURAL PERIOD (SEC)
FIGURE 361.17.4-9a HORIZONTAL SPECTRUM FOR M =5.3, R = I O km t
AMEilDMEllT 21 OCTOBER 1980 361.17.4-31
Io.o mo W
m N
[
SSE v
1.0 f
oBE 8
/
/
W
>F M,=5.3, j
R=10km w
E o.1 oo w
m o.
0.01
,.,,,,r
,,,,,,i e i,-
0.01 0.1 1.0 10.o.
UNDAMPED NATURAL PERIOD (SEC) j FIGURE 361.17.4-9b VERTICAL SPECTRUM FOR M =5.3, R = 1 O km t
i AliEllDt1ErlT 21 OCTOBER 1930 361.17.4-32 1
l C.
C l
4 i
i i
i i iiii i
. i iiiii i
i i iiiiii r
7 I
7 F
1 r
1 b
w 4U in 8
SSE a
>E$
tj 2% DAMPING r
1 I
1 8
I 1
r 1
p I
- /
)
A C.
4 q
l l '..',/
.,
- j
~
F
~
v -.
I
- P QC
+,.....
....,., g
.o.
./
. \\s
=
Y
/
. 'e
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l r
./
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r 1
r
/
1 1
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/
T
/
I f_
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s /l
+/
w
- 9. /
s 3
- g..
A g.,.*
+/
s p-
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,1 i.,,,,
1 C. 2 y
2.
in.
,un
,n co.mouAc, rn re t
a w
>-<o O
U FIGURE 361.17.4-10 u
>. Y COMPARIS0N OF TARGET AND DESIGN SPECTRA Ou
=o4 s
At1EllDtiENT 21
~
OCTOBER 1900 a
361.17.4-33
361.18
- A graph of the common logarithm of cumulative number of events of magnitude M or greater can be plotted as a function of M.
Such a graphical representation is useful in describing the seismic history of a seismic zon.
Insofar as an assumption can be made that the magnitude distribution will continue as it has been, estimates of future seismicity can be made from the graph.
Although the latter assumption is not always tenable, such a graph is a useful resource in describing seismicity levels.
Therefore, produce a graphical representation of magnitudex cumulative frequency of occurrence of all events assumed to be associated with the reservoir impoundment. From this curve estimate the magnitude of the largest earthquake with a return period of one year, of ten years, and of 40-50 years (the life of the plant). (See, for example, Teledyne Geotech (1979c) No.
79-8, page 27, Figure 8). Compare these to the maximum potential event obtained in the previous question based on focal depth con-siderations. Discuss the limitations or uncertainties of those estimates.
RESPONSE
The magnitude-frequency distribution of earthquakes are conventionally represented by the Gutenberg and Richter (1944) relationship:
log N = a-b M (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 M ' mb, M L
s bLg). The fit to the equation of a plot of log N versus M is a straight or m line and yields a value for the slope, b.
The b-value varies with source AMENDMENT 21 OCTOBER, 1980 361.18-1
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 b-value may fluctuate between 0.3 and 1.5.
Gupta and Rastogi (1976) have noted the b-value of reservoir-induced earthquakes are usually greater than those of tectonic earths"akes in the same region.
To obtain the magnitude-frequency relation for Monticello, events recorded at JSC (a permanent seismographic station of the South Carolina network) were used. These events are catalogued according to their duration (D) in seconds, which is related to the local magnitude ML by the relation L
-1.83 + 2.04 log D (2)
M
=
The cumulative number of events with a duration greater than or equal to D, N (D), are plotted against D.
The data can be defined by the relation log N (D) = a*-b* log D (3) 2.04b where b*
=
0.5b*
or b
=
The b-value is thus about h b*-value obtained by plotting log N (D) versus D.
RESULTS Reservoir-induced seismicity at Monticello Reservoir started in the last week of December 1977. Consequently data recorded in a two-year period December 1977-December 1979 were analyzed in two ways.
The data were divided into smaller time periods, and the b*-value was determined. A least square line was fit to the data, after ignoring the end points.
The results are given in Table 361.18-1. These data are compared with the monthly number of larger events (ML 2 2.0) (Figure 361.18-1).
AMENDMENT 21 OCTOBER, 1980 361.18-2
We note that' the b-value is around 1 and appears to decrease during'the swarms in. February '78 and October '79.
.The frequency (N(D)} versus D data for the period December '77-December '79 are shown in' Figure 361.18-2.
(The corresponding magnitude scale is also shown.)
Ignoring data for small durations, a b*-value 'S 2.67 (or a b-value of 1.34)
J is obtained.
{Y = 6.82 -2.67X, where X = log D and y = log N(D)}.
Alternatively, the b-slope can be computed by Utsu's formula (Utsu,1965) in a form given by Aki (1965):
b = log (e)/ (II - Mnin)
(4) where log (e) = 0.4343, Fi is the average magnitude, and Mmin is the minimum nagnitude in a given sample data. Aki (1965) showed that Eq (4) was the maximum likelihood estimate of the b-slope.
Using the same data, and Utsu's method, a b-value of 1.67 was obtained.
In trying to use the b-values to obtain recurrence rates, it is important to realize the assumptions involved. The fundamental assumption is that the frequency distribution is a Poisson process and that the same tectonic forces a
will continue. The nature of induced seismicity is episodic, and thus such extrapolations have to be carefully interpreted.
One such method was to compare the seismicity at Monticello Reservoir with that at Lake Jocassee, where a longer sample is available. Both Lake Jocassee and the Monticello Reservoir are in the Piedmont; thus, it is appropriate to
. compare their seismicities. The cumulative seismicity at Lake Jocassee for the period December 1975-December 1979 is_ also shown on Figure 361.18-2. The b-values obtained by f'tting a least square line and by Utsu's formula are AMENDMENT.1 361.18-3 OCTOBER,1980
1.27 and 1.21, respectively. Both the Lake Jocassee data and the Monticello Reservoir data were then used to obtain recurrence rates. The object of using the Lake Jocassee data was to see if the calculations yielded meaningful results which could be compared with the observed seismicity there.
RECURRENCE RATES If x is the mean return period of an earthquake exceeding a magnitude M, then L
for two years, N, the number of earthquakes per one year exceeding that magnitude T = 2/N. For Lake Jocassee, where 4 years data have been used, t = 4/N. For Monticello Reservoir, the return periods of 1,10, 40 and 50 years correspond to N = 2, 0.2, 0.05 and 0.04, respectively.
In the calcula-tions a b*-slope value of 2.67 (b-value of 1.34) is used.
(It yields more conservative results than by using that obtained by Utsu's formula.) The magnitudes corresponding to these return periods are given in Table 361.18-2 where they are compared with results from Lake Jocassee.
The largest earthquake recorded at Lake Jocassee (in about 7 years since its impoundment) is of the same order as predicted for a 40-50 year return period.
Thus, it appears that the estimates obtained by using b-value extrapolations yield usable order of magnitude values.
For Monticello Reservoir, this method suggests that the largest earthquake with a return period of 10-50 years is ML = 4.0 - 4.5.
A note of caution is in order.
In about 3 years of continuous monitoring, no event with ML > 3.0 has been observed.
It is entirely likely that the maximum size of the earthquake is more closely related to the local geology, fractures and stress conditions rather tnan to the conclusions of a statist.ical exercise.
l 361.18-4 AMENDMENT 21 OCTOBER, 1980
AN EMPIRICAL APPROACH Severy et al (1975) examined the seismicity at 59 reservoirs which were constructed in the Piedmont Province between 1891 and 1974. Of these,12 were noted to be associated with seismic activity. This list has been updated to inc19 Lake Jocassee and Monticello Reservoir (Table 361.18-3 and Figure 361.18-3) Although the depths of the reservoirs and their capacity varied by as much as a factor of 10, no_ reservoir in the Piedmont was associated with an earthquake of PE intensity greater than VI. This record is complete and, although the years elapsed between the impoundment and the observation of seismicity vary widely, the largest reported intensity is Modified Mercalli VI. This cbservation suggests that MM VI is the largest estimate of an induced earthquake in the Piedmont region.
In the eastern U.S., which is characterized by low attenuation, an MM Intensity VI would be equivalent to aML 4.0 earthquake. Thus, the largest induced earthquake based on empirical data covering about 90 years is ML s4.0.
All the observed induced seismicity at Monticello Reservoir in the first 2 years has been < 2 km deep and none of the events has exceeded a local magnitude of 3.0.
Thus, it appears unlikely that an induced earthquake with ML s4.0 would occur at a much shallower depth.
A TECTONIC EARTHQUAKE The largest tectonic earthquake in the Piedmont is the RF* Intensity VIII event (=MM Intensity VII) that occurred on January 1,1913 at Union County.
The exact cause of its occurrence is not known, although it has been suggested (Talwani,1980, Personal Cotnm.) that it may be associated with the King's Mountain belt-Charlotte belt contact.
If it is assumed, however, that the Union County event was a random event, and could occur anywhere in the Piedmont (as has been done in Section 2.5.2.9 of the FSAR), then the largest
- Rossi-Forel Scale 361.18-5 AftENDMENT 21 0CTOBER, 1980
4a A
y a
se.m p
.-4 m
-- 7m
~
event that could' occur near the Monticello Reservoir ir
- Intensity VII.
. From the nature and extent.of-the isoseismals of the Union County event, its depth has to be at least s5 km.
^
_SUMARY From the empirical analysis which is based on all existiig data, the largest
. induced earthquake is estimated to be sML 4.0 with a depth of at least 2 km.
The largest tectonic earthquake is estimated at MM Intansity VII (or s M 5.3 for eastern U.S.) with a' depth of at least 5 km.
REFERENCE
- Aki, K., (1965) Maximum likelihood estimate of b in the formula log N =
a + b m and its confidence limits. Bull. Earthq. Res. Inst. Vol. 43_,
- p. 237-239.
4 1
. Gupta, H. K., and B.' K. Rastogi, (1976), Dams and Earthquakes, Elsevier, Amsterdam, 229 pp.
Gutenberg, B. and C. F. Richter (1944), Frequer.cy of Earthquakes in i
California, Bull. Seis. Soc. Am., 34, 185-188.
- Severy, N.
I., G. A. Bollinger, and H. W. Bohannon, Jr., (1975), A Seismic Comparison of Lake Anna and_other Piedmont Reservoirs in the Eastern i
U.S.A., 1st Internat. Symposium on Induced Seismicity, Banff, Alberta, Canada, Sept. 15-19, 1975.
4 Utsu, T., (1965) A method for determining the value of b in a formula l
log N = a - b M showing the magnituda-frequency relation for earthquakes. Geophys. Bull. Hokkaido Univ. 13_,99-103 (In Japanese
{
with English Summary).
l I
4 4
l 4
AMENDMENT 21 361.18-6 0CTOBER,1980
}"
.. 7.-
4 TABLE 361.18 Temporal Changes in b-Valuas Total No. of Period Events b*-Value 12/77-1/78 700 2.18 2/78 1341 1.81 3/78 523 2.94 4/78 217 1.90 5/78 146 1.83 i
6-8/78 218 2.18 9/78 221 1.59 10/78 197 1.83 11/78 225 1.77 12/78 167 2.08 1-5/79 193 1.58 6-9/79 168 2.35 10/79 656 2.00 11-12/79 212 2.21 1-4/80 221 2.11 12/77-12/7f 5204 2.67 4
AMENDMENT 21 OCTOBER,.1980 361.18-7
- g Table 361.18-2 ESTIMATE OF LARGEST EARTHQUAKE (M )
L Return Period Monticello Lake (Years )
Reservoir Jocassee 1
3.15 2.30 10 3.92 3.10 i
40 4.37 3.59 53 4.45 3.67 l'
1 A
i At.tENDMENT 21 OCTOBER,1980 i -
p 361.18-8
i TABLE 361.18-3 PIEDM0NT PROVINCE RESERVOIRS WITH SEISMICITY RESERV0IR NAME/
DAM CAPACITY SEISMICITY YEARS 6
DATE LOCATION HEIGHT (H.)
(M.3x10 )
DATE/M.M.
ELAPSED REMARKS' 1910 LLOYD SH0ALS 30 132.0 3-5-14/VI 4
(INDUCED ?)
JACKSON, GA.
1919 BRIDGEWATER 50 370.0 1919/VI 0
(INDUCED ?)
MARION, N.C.
1925 RH0DHISS 21.9 83.4 7-8-26/VI 1
(INDUCED ?)
GRANITE FALLS, N.C.
c, S
1928 0XFORD 35.4 151.0 9-9-70/V 42 HICK 0RY, N.C.
go e
1930 SALUDA 63 2,600.0 7-26-45/VI 15 (FAULT PRESENT)
IRM0, C.S.
1940 BUZZARD'S ROOST 25 334.0 12-73/ FELT 34 GREENWOOD, S.C.
REPORTS IN C0.
1952 CLARK HILL 67 3,096.0 1960/ FELT 8
(FAULT PRESENT)
AUGUSTA, GA.-S.C.
REPORTS 4
8-2-74/VI 22 1953 SINCLAIR 32 407.0 3-12-64/V 11 MILLEDGEVILLE, GA.
1961 HARTWELL 73 3,145.0 10-20-68/V 7
HARTWELL, GA.-S.C.
AMENDMENT 21 OCTOBER, 1980
TABLE 361.18-3 (CONTINUED)
RESERVOIR NAME/
DAM CAPACITY SEISMICITY YEARS 6
DATE LOCATION HEIGHT (M.)
(M.3x10 )
DATE/M.M.
ELAPSED REMARKS 1963 SMITH MOUNTAIN 69 1,357.0
--/V ALTAVISTA, VA.
1969 KE0 WEE 53 1,179.0 7-31-71/V 2
INDUCED SENECA, S.C.
1973-74 J0CASSEE, S.C.
133 1,490.0 11/75-PRESENT 2
INDUCED 8/25/79/VI 1977 MONTICELLO RESERVOIR, S.C.
35 493.6 12/77-PRESENT 0
INDUCED ML < 3.0 b
b AMENDMENT 21 OCTOBER, 1980
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I N
S U
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E
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AMENDMENT 21 5
0CTOBER 1980 i
!2; 361.10-11
4 10 o
4 >
a 10 -
u, 5
MONTICELLO a
5>
/.67 2
2 10 _
e 0
e JOCASSEE 5
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.05 s
.04 o
DURATION ISEC) 10 100 1000 10,000 i
i di i
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l l
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AMENDMENT 21 OCTOBER 1980 s
5 361.18-12
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S EE E.C:PS V :TH FIGURE 361.10-3 q EE EVtTY ~;; a AMENDMENT 21 NOTE: ADAPTED FROM SEVERY,1975 OCT0 DER 1980 4 5 361.18-13
9 361.19: Using historical earthquake catalogs for the southeastern U.S., and excluding the microearthquakes induced by Monticello Reservoir, obtain recurrence intervals for Modified Mercalli (M.M.) epicentral intensities, I, for (a) I = VIII within 100 km of the V. C. o n Summer Site and (b) I = IX within 225 km of the site. n
RESPONSE
This question was addressed during the July 30, 1980 meeting with NRC representatives. At that meeting it was agreed that a more appropriate topic of investigation would be to determine the probabilities of exceedance, or return periods, associated with various levels of Modified Mercalli inten-sity at the plant site. Several mathematical ~ models must be assumed before a seismic hazard analysis at a site can be conducted. Sources of future seismicity (seismogenic zones) must be delineated; activity rates, b-values, and maximum earthquake sizes must be selected for these zones; and an attenuation function estimating ground motion as a function of earthquake size and distance from the site must be selected. Once these models have been defined, calculating proba-bilities of exceedance follows proc ~ dures which are well established in the e scientific literature. For this study of the Virgil C. Summer Nuclear Station, the basic data base used was t' e earthquake catalog of Bollinger for the southeastern U.S. This n is the most accurate source of earthquake data (dates, M.M. intensities, and epicentral locations) available. The seismogenic zones selected for the basic analysis are those presented in the FSAR for the V. C. Summer Nuclear Station. They consist of the Coastal 361.19-1 AMENDMENT 21 OCTOBER, 1980
9 Plain, Piedmont, Blue Ridge, Valley and Ridge, and Appalachian Plateau zones. For each zone, the historical occurrence of earthquakes was determined using the Bollinger catalog. Rates of occurrence of earthquakes with M.M. inten-sity greater than V were determined by correcting each intensity level for incomplete reporting of events. A Richter b-value of 0.5 was assumed because this has been widely reported in the literature for eastern North America; the data used in this study indicate a b-value of approximately 0.5, although some statistic.al variations from zone to zone occur. Maximum possible inten-sities for each zone were selected to be equal to the largest historical earth-quake plus one unit; sencitivity to this selection is discussed below. / The M.M. intensity at a site $ was estimated from the following equation: 3 Is = 3.08 + Ie 1.34 a a>10 km Is
- I a 510 km e
where l is epicentral M.M. intensity and a is epicentral distance to the site. e Uncertainty in this estimate was modeled by a Gaussian probability distribution with a standard deviation of 1.19 intensity units, except that the distribution was truncated so that site intensities could not exceed epicentral intensities. The above equation is very close to that reported by Bollinger for the 1886 Charleston earthquake; the Gaussian distribution is a typical one used to represent uncertainty in ground motion estimates, i The first two rows of Table 361.19-1 indicate annual probabilities of exceedance (and corresponding return periods) for various intensity levels, using the models just described. The annual probability of exceedance associated with inten-sity VII is.22x10-3, implying a return period of 4500 years. If the assumption on maximum earthquake size is modified to say that the maximum possible event 361.19-2 AMENDMENT 21 OCTOBER, 1980
is equal to the largest historical event, the annual probabilities are reduced, as shown in the second pair of rows in Table 361.19-1. In this case the return period associated with M.M. Intensity VII is about 10,000 years. A final analysis was conducted using the seismogenic zones proposed by Algermissen and Perkins in 1976. For the Virgil C. Summer Nuclear Station, this implies that the 1886 Charleston earthquake, M.M.-intensity X, can occur in the vicinity of the site. However, the probability of occurrence of such an event is low, even assuming that these seismogenic zones are appropriate. The annual probabilities are shown in the last two rows of Table 361.19-2, and indicate marginally larger probabilities compared to the results of previous analyses shown in the table. It is concluded that the seismic hazard at the V. C. Summer fluclear Station is relatively low: M.M. Intensity VII corresponds to a return period on the order of 3,000 to 10,000 years. By comparison, analyses conducted by the NRC suggest that the SSE for existing facilities should be compared to~ ground motions with return periods on the order of 1,000 years. Additional Analyses Several additional analyses were conducted for comparison purposes and in anticipation of future questions regarding comparative risks for natural and induced earthquakes. First, the seismic hanrd analysis for natural (tectonic) earthquakes was carried out using, hs a description of earthquake size, body-wave magnitude M. These were estimated from epicentral inten-b sities using the Nuttli relation: Mb = 0.5 (Ie + 3.5) Conversion of intensities to Mb was necessary because the data available in 361.19-3 AMENDMENT 21 OCTOBER, 1980
the Bollinger catalog is primarily in the form of intensities rather than mag-nitudes. The maximum value of Mb was estimated by determining the largest historical earthquake in each zone in terms of Ie, converting this to Mb by the above equation, and raising this value of Mb by 0.25 magnitude units. Thus, for instance, a largest historical intensity of VII would imply a largest possible magnitude Mb of 5.5, which in turn corresponds to intensity VII-VIII. A depth of 5 km was used to represent the source of energy during these events. For this analysis, ground motion was characterized by peak horizontal accelera-tion; this was estimated using equations derived by Nutt11 (1979) for the cen-tral U.S., which are thought to be appropriate for the southeast U.S. The theory of Nutt11 was modified to estimate the peak acceleration from a sus-tained acceleration, and to estimate the average of the two horizontal motions rather than the larger. A log normal probability distribution was used to represent uncertainty in the estimate of peak acceleration. For the FSAR sources, with parameter values and-attenuation model as discussed above, Table 361.19-2 gives annual probabilities and return periods associated with various levels of acceleration. A peak acceleration of 0.15 g has an esti-mated return period of approximately 3,400 years, by this analysis. For purposes of comparison, we have also estimated annual probabilities of exceedance from induced earthquakes. These were assumed to occur at 1 km depth, and to have magnitudes (M ) in the range 1.6 to 3.6, which corresponds b to local magnitudes. in the range 2.0 to 4.0 using the Nuttli relation: ML = 1.023 Mb + 0.3 The rate of occurrence of induced seismicity in this magnitude range was esti-mated to.be 35 events per year (Talwani, personal comunication,1980), and 361.19-4 AMENDMENT 21 OCTOBER, 1980
e the..'c.hter b-value was assumed to be 1.34 based on Talwani's analysis of ob-served events. The Nuttli attenuation equation was used to estimate accelerations for these events, as it was for tectonic earthquakes. By comparison with the recorded earthquake of August 27, 1978 (ML = 2.7, R + 0.8 km), Nuttli's theory esti-mates 0.12 g for this event, which is in excellent agreement with the (digitized) peak horizontal acceleration values of 0.11 g and 0.13 g. Table 361.19-2 indicates the calculated annual probabilities associated with induced seismicity. These probabilities are approximately 100 timer, as large as those for tectonic events, primarily due to the large number of them (35 per year), their shallow depth, and their proximity to the facility. It is also a property of small magnitude events close to the site that they can generate large peak accelerations, but may produce no damage because of short duration and lack of energy at lower frequencies. 351.19-5 AMENDMENT 21 0CTOBER, 1980
TABLE 361.19-1 ANNUAL PROBABILITIES OF EXCEEDANCE AND RETURN PERIODS ASSOCIATED WITH VARIOUS M.M. INTENSITIES M.M. Intensity Seismogenic Zones V VI VII VIII FSAR zones with Annual
- 51x10-2
.12x10-2 .22x10-3 .31x10~4 max. intensity = arob. largest hist, l.P., 200 830 4500 32,000 intensity + 1 years FSAR zones with Annual .37x10-2 .70x10-3 .99x10-4 .15x10-4 max. intensity = 3 rob. largest hist. .R.P., 270 1400 10,000 67,000 intensity years Algermissen-Perkins Annual .55x10-2 .14x10-2 .32x10-3 .66x10-4 zones with max. Prob. intensity = largest R.P., 180 714 3100 15,000 hist. int. years l i l 361.19-6 AMENDMENT 21 0CTOBER, 1980
I A!:Lc 357.19-2 ANNUAL FROBABILITIES OF EXCEEDANCE AND RETURN PERIODS ASSOCIATED WITH VARIOUS ACCELERATION LEVELS Peak Horizontal Acceleration (g) Seismogenic Zones (s) '0.07 0.10 0.15 0.20 ^" .16x10-2 .76x10-3 .29x10-3 ,14x10-3 FSAR zones. )ro (tectonic events) 1.P., 630 1300 3400 7100 i years 0.47 0.21 .61x10-1 .28x10-1 Annual Reservoir zones arob. 1 (induced events) .l.P., 2.1 4.6 15 36 years 361.19 AMENDMENT 21 OCTOBER, 1980
v 0 361.20 Re: Reservoir-induced seismicity occurring on pre-existing joint of fracture planes. (1) In Talwani, et al.,1980, the authors stress the observation that the nodal planes of the focal mechanisms for earthquakes under the Monticello Reservoir are oriented parallel to existing fracture planes. What significance does this observations have, particularly with respect to the maxmimisnearthquake potential? Explain the significance. (2) What would be the consequences of a Reservoir-induced earthquake on one joint or fracture plane nucleating a series of events (a multiple event) on nearby joint or fracture planes? Estimate the possibility of such an event and describe the conditions that would mitigate against such an event.
RESPONSE
(1) From an analysis of fault plane solutions of events recorded on the Monticello network, Talwani et al (1980) suggests that the earthquakes are oriented parallel to existing fracture planes. A similar conclusion was also obtained by Zoback (pers. com.) based on an analysis of the density and orientation of joints encountered in the two deep USGS holes. It was also noted that the joint patterns observed in the two holes were dissimilar. These observations suggest that, although the joint directions etc. inferred from fault plane solutions in different regions may agree qualitatively, they do not indicate the presence of through-going buried faults. The diffuse nature of the seismicity together with an absence of any long (s kms) surface AMENDMENT 21 l 361.20-1 OCTOBER, 1980
faults implies that they are absent at depth. Thus, the observation that the focal mechanisms for earthquakes under Monticello Reservoir are oriented parallel to existing fracture planes puts an upper bound on the size of the fault (joint) surface available for an earthquake. Field data suggest that such features rarely exceed lengths of s 100 m. Assumed stress drops of s 100 bars (an upper bound) suggest ML 2.9 earthquakes. (The assumptions etc. involved in arriving at this number are discussed in response to Question 361.21). In trying to estimate the largest earthquake, it is necessary to have a valid estimate of the fault length (or source radius). In any given spurt of seismicity observed at Monticello Reservoir, the largest dimension observed is 1 3 km. Thus, if this whole length was to rupture at once, the maximum plausible source dimension is s 3 km. If a stress drop of s100 bars is assumed (an upper bound estimate) the largest associated earthquake is ML $5.3. A note of caution is necessary here. For the following reasons, this estimate is unrealistically large: (a) The seismicity in any recorded sequence has occurred in clusters and has not defined any buried fault plane. (b) From fault plane solutions, the seismicity appears to be associated with existing fractures. (c) The fractures are oriented in different directions. (d) In over 2 years of observations, no noticeable increase in the size of the largest earthquake magnitude has.been noted. This argues against small fractures coalesing into larger ones to form through-going faults. (e) No surface faulting has been observed. AMENDMENT 21 361.20-2 OCTOBER, 1980
(f) In over two years of observations, no increase in hypocentral depth beyond s2 km has been observed based on depth compe:tations using magnetic tape data. As described in response to Question 361.17.4, a M s5.3 earth-L quake would require at least a 5 km depth. (2) Observations at Lake Keowee (Talwani et al,1979) suggest the migration of seismicity from one joint system to another. Multiple events have been observed at Lake Jocassee (4/21/761 However, the magnitude of no event has exceeded (M ) 2.1. Subsurface data with detailed descriptions of joint L patterns are not available and thus it is not possible to " estimate the possibility of such an event". REFERENCES Talwani, Pradeep, D. Stevenson, D. Amick, and J. Chiang, (1979) An Earthquake Swarm at Lake Keowee, South Carolina, Bull. of the Seism. Soc. of Am., Vol. 69, No. 3, pp. 825-841.
- Talwani, P., B.K. Rastogi and D. Stevenson, (1980) Induced Seismicity and Earthquake Prediction Studies in South Carolina, Tenth Technical Report, USGS Contract No. 14-08-0001-17670, 212 pp, i
1 AMENDMENT 21 OCTOBER,1980 361.20-3
m 361.21 The USGS measured in situ stresses at greater depths than those reported in the FSAR (Appendix 2D). Summarize those measurements. Do those data support the hypothesis t,;t in situ normal and shear stresses are sufficiently close to shear movement along pre-existing planes of weakness that raising pore pressure to hydrostatic levels gover.ned by Lake Monticello has caused the observed seismicity in the area of the stress measurements? What is the stress drop associated with the maximum potential earthquake that might occur from the greatest observed (Sj-S ) 3 stress condicions? Could the response spectrum of an earthquake with this stress drop exceed the design response spectrum? J
RESPONSE
Figure 361.21 -1 shows the locations of the two USGS deep wells. Both were drilled to depths exceeding 1 km and in the epicentral areas of greatest activity. The results presented below are summarized from information obtained from Zoback (pers. comm.). In situ stress was measured at depth using the hydraulic fracturing technique (Zoback et al,1977). The results of these measurements are shown in Figure 361.21-2 (a) and (b). The error bars represent the least and greatest horizontal principal stresses. The vertical principal stress is represented j 3 by the lithostatic gradient (solid line) for a density of 2.7 gm/cm. In Monticello 1, the difference in magnitude between the two horizontal principal stresses is relatively small and there is only a minor increase in stress with depth. However, at shallow depth the greatest horizontal principal stress is AMENDMENT 21 OCTOBER, 1980 361.21-1 e v
- 9 substantially greater than the vertical stress. This stress configuration is appropriate for thrust faulting.
The data in Figure 361.21-2(a) indicate that the tendency for this thrust-type faulting is limited to the upper 300 m or so, as the measurement at 486 m shows that the vertical stress is no longer the least principal stress (a requirement for thrust faulting). Figure 361.21-2(b) shows that the magnitude of in situ stresses at Monticello 2 are markedly different than in Monticello 1, as both horizontal stresses are approximately equal to the vertical stress. Stress measurements were not made above 200 m because of dense natural fracturing. At depths greater than 400 m attempted stress measurements were unsuccessful because the rock could not be hydraulically fractured due to its high tensile strength and the apparently low difference in horizontal stresses. According to Zoback, the available data indicate that if there are high horizontal stresses relative to the vertical stress, as in Monticello 1, it is probably only in the upper 150 meters, or so. From the data presented above, it is only at depths up to s150 m that data support the hypothesis that in sito normal and shear stresses are sufficiently close to shear movement along pre-existing planes of weakness such that increasing pore pressure to hydrostatic levels as governed by Monticello Reservoir has induced the observed seismicity. I However, due to the spatial and temporal association of seismicity with filling, the thrust fault nature of the seismicity suggest that the stress measurements may not represent the actual stress conditions at the site of the seismicity itself. hbhbhfb,1980 361.21-2
9 From Figure 361.21-2(a), the stress values at s1 km depth are ol(Horiz) " 290 bars, 2(Vert) = 270 bars, o3(Horiz) = 190 bars. If we assume that in an earthquake the maximum stress drop occurs (i.e. oj-o3), we obtain ao=100 bars. It should be noted that the assumption of 100 bars stress drop (as suggested by Question 361.21) is untenable. It assumes 100% seismic efficiency, against an ambient stress field of s200 bars. There are no known examples or data which suggest that such large seismic efficiencies are observed. Estimates of s20% for seismic efficiency are more valid, and would reduce the estimated magnitudes. Also, it has been noted that large stress drops are localized and associated with very small fault dimensions. Thus, estimates made of maximum earthquakes based on large stress drops (s100 bars) and large source dimensions are unrealistic and apt to greatly overestimate the largest magnitudes. Further, the calculations are based on the application of Brune's model (1970-71) and empirical scaling laws derived for California (Thatcher and Hanks, 1973). Thus, the results presented below should be used - with utmost caution. The earthquake stress drop a is related to the seismic moment M and source o o radius r in the Brune model (1970,1971) by d =7M o o 16 3 (1) r The local magnitude is estimated from the relationship of Thatcher and Hanks (1973) log Mo = 1. 5 ML + 16 (2) L is local magnitude. where M 1 l AMENDMENT 21 OCTOBER,1980 361.21-3 - l
y The basic assumptions in estimating the magnitude is the applicability of the Biene model and the Thatcherand Hanks relationship to smaller magnitudes. If all, these assumptions are satisfied, ML can be estimated from equation (1) and (2) for assumed values of r, the source radius. For source radii of 10 and 100 m, the calculated potential magnitudes are 0.9 and 2.9, respectively. A maximum stress drop of 100 bars was assumed in the response to Question 361.17.4. REVERENCES Brune, J.N. (1970) Tectonic stress and the spectra of seismic shear waves from earthquakes, Journal of Geophysical Research, ));, 1997-2009. Brune, J.N. (1971) Correction, JGR, 76, 5002 Thatcher, W., and T. C. Hanks (1973) Source Parameters of Southern California earthquakes, JGR, ))[, 8547-8576. Zoback, M.D., et al., (1977) Preliminary stress measurements in central California usTng hydraulic fracturing technique, PAGE0PH,115,135-152. AMENDMENT 21 1 361.21-4 0CTOBER, 1980
MONTICELLO EARTHOURKES DEC 77 - DEC 79 MAG GE 2.0 al'p2. pi. pc. p. ps. p. ps. l,,, _n. o I y ._22.s g, - 22.o a: a _2i.c Jl.0 og m MONTICELLO 41 _2o.5 L 0 0 -20.c BA 0 g C a 2.s a 3.c g gg au GeG aso g 6 0 w p g a 2.5 C MONTICELLO #2 1 _ 2 7.c 0 au a s.o W D 5 IKM1 g i
- - i t
i i 34.00 1 i ) I i I i ~ E 0 = c e O E] e o a + x o 5 E
- UU l.
O I. 2 3. 4. 3. t. 2. 3. 5 4 MCGNITUDE DEPTH (KM1 3 FI GURE 361.21-1 y AMENDMENT 21 Z OCTOBER 1980 a J 361.21-5
9 e W 6-4 hh O \\t* ~ sf g = ts.1 ~ ~ a m b-gt e g 2$ w 2 O o e 2 o o I g f f f f 1 I f f f I 8 f ~ w ao o o o o o o o $8313n0111
- Hid30 E-I de EE ST w
\\%* o w n$ \\$ d m o - e- .t 2 C 7 (( z
Osgdy\\ I w w2 0 T -= z = o m x g .= w E = o e () t t t t f I f f f I i m ~ a. o o o g o o o o w o 58313n0lli
- Hid30
>Z w eo l = 3 FI GURE 361.21-2 l g At4ENDt4ENT 21 OCTOBER 1980 i a J 361.21-6
1 361.22 'On Page 2.5-14, paragraph 1, it is reported that McKenzie postulated northwest-trending faults with 1500 feet of displacement. What evidence has been observed which supports the conclusion that these " faults" are an unsupported hypothesis by McKenzie.
RESPONSE
The subject fault was discussed by McKenzie in his masters thesis in 1962. Since that time the area has been visited by several geologists, including the applicant's consultants and independent geologists, who could not cor-roborate the existance of the fault. The area was mapped by Thomas L. Kessler in 1972 who showed the apparent displacement to be a syncline, and not a fault. l l I 'T AMENDMENT 21 361.22-1 OCTOBER, 1980 ..}}