Bulletin of the Seismological Society of America, Vol. 74, No. 4, pp , August 1984 BY LUCILE M. JONES* ABSTRACT

Transcription

1 Bulletin f the Seismlgical Sciety f America, Vl. 74, N. 4, pp , August 1984 FORESHOCKS ( ) IN THE SAN ANDREAS SYSTEM, CALIFORNIA BY LUCILE M. JONES* ABSTRACT The spatial and tempral distributins f seismicity preceding mderate (M, _-> 5.) main shcks in the San Andreas fault system in Califrnia have been analyzed t recgnize and characterize the patterns f freshck ccurrence. Of 2 main shcks in the San Andreas system, 7, r 35 per cent, have been preceded by immediate freshck sequences that included events within 1 day and 5 km f the main shcks. A pssible crrelatin f the rate f freshck ccurrence with type f faulting was fund such that nne f the fur main shcks with reverse faulting had freshcks while 44 per cent f the strike-slip earthquakes had freshcks. Sme enhanced seismic activity was als bserved at relatively large distances frm the main shck (13 t 3 km) 1 t 5 days befre 4 per cent f the main shcks but this activity cannt be clearly distinguished frm the backgrund seismicity. Of the seven immediate freshck sequences, nly tw had the swarm-like appearance f the class II freshcks defined by Mgi. The ther freshck sequences appear t be single events (smetimes with their wn aftershcks) preceding the respective main shcks. Fur f these sequences are spatially crrelated with distinct physical discntinuities in their faults between the hypcenters f the freshck and main shck, and similar discntinuities may als be assciated with the ther sequences. The duratins f the freshck sequences were fund t decrease as the depths f the main shcks increase frm 3 t 11 km, which has been interpreted as a dependence n stress. T accunt fr this stress dependence f the duratin and the presence f discntinuities, a mdel fr freshcks ccurrence is presented. This mdel prpses that freshcks may represent a prcess f delayed multiple rupture and that the delay between ccurrence f freshck and main shck might represent the time needed fr static fatigue t break the strnger rck at the discntinuity in the fault. INTRODUCTION What freshcks are and why they ccur befre sme but nt all main shcks has lng been an unreslved questin in seismlgy (e.g., Richter, 1958). Mgi (1963) classified the types f seismicity bserved in Japan and included freshcks in his class II seismicity. This shwed Japanese freshcks as swarm-like events with several earthquakes f similar magnitude preceding the main shck. This has becme the standard mdel f freshcks s that when a swarm is bserved the chances f it being a freshck sequence are ften evaluated (e.g., Aki, 1981; Xu et al., 1982; Yamashina, 1981). Hwever, Utsu (197) shwed that many Japanese freshcks are single events, smetimes with their wn aftershcks that he called class Ib freshcks, rather than swarms. In Califrnia, while sme freshck sequences (like that befre the 1972 Bear Valley event) are swarm-like, many ther freshck sequences (like thse befre the 1968 Brreg Muntain and the 197 Lytle Creek earthquakes) appear t be single events r class Ib freshcks s that swarm-like activity may nt be the nly type f freshck activity in Califrnia. Althugh many individual freshck sequences in Califrnia have been recgnized * Present address: U.S. Gelgical Survey, Seismlgical Labratry, Califrnia Institute f Technlgy, Pasadena, Califrnia

2 1362 LUCILE M. JONES and studied (e.g., Ellswrth, 1975; Lindh et al., 1978; Bakun and McEvilly, 1979), the patterns f freshck ccurrence in Califrnia have nt yet been classified. Mrever, it is nt yet established hw ften freshcks ccur in Califrnia. Kagan and Knpff (1978), using statistical methds, fund that Califrnia may have anmalusly few freshck sequences but reprted that their data set was t small fr the results t be statistically valid. Therefre, ne gal f this study is t analyze using deterministic methds, the distributin f seismicity prir t main shcks in the San Andreas system in Califrnia t classify the type f freshcks and t determine hw ften they ccur. Mst f the theries fr freshck ccurrence assume that hetergeniety f the crust cntributes t freshck ccurrence. This was first prpsed by Mgi (1963) based in part n labratry experiments which shwed that a hmgeneus rck wuld fracture relatively unifrmly but a mre hetergeneus rck wuld exhibit mre micrcracking befre the fracture. Several mdels have been prpsed since then t explain in mre detail hw the hetergeneity leads t freshcks. One cmmn explanatin (e.g., Jnes and Mlnar, 1979; Kanamri, 1981) has been that the hetergeneity f the fault plane results in accelerating premnitry slip such as has been dcumented in the labratry (Dieterich, 1979). Das and Schlz (1982) suggested that freshcks may result frm lcalized decrease in rck strength within the initiatin zne f the main shck. They als suggested, as did Fuka and Furumt (1975), that freshcks culd represent a prcess f delayed multiple rupture. High-quality recrdings that are nw available fr earthquakes in Califrnia and recent wrk n the state f stress in the San Andreas have prvided a basis fr studying the details f freshck sequences. The data frm Califrnia are examined in this paper t determine which, if any, mdel is mst cmpatible with the bserved freshcks. DATA The main shck data set used in this study is the set f all earthquakes with ML -->_ 5. that ccurred in the San Andreas physigraphic prvince (as defined by Zback and Zback, 198) frm 1966 t 198. (This is all f Califrnia, except fr the Sierra Nevada. This regin has been excluded because the data frm there are nt f as high quality as the San Andreas data and because the extensinal tectnics f the regin might prduce different types f freshck sequences.) The 2 events in this categry are pltted in Figure 1 and their hypcentral parameters listed in Table 1. Since several seismic netwrks recrd earthquakes in Califrnia, three catalgs f earthquakes have been used t search fr pssible changes in seismicity befre these main shcks. Fr earthquakes in suthern Califrnia (suth f 36 N latitude), the catalg f the CIT/USGS netwrk in suthern Califrnia was used. The seismicity befre main shcks in nrthern Califrnia (nrth f 36 N latitude) after 1971 was studied using the recrds f the CALNET seismic netwrk perated by the USGS. Since CALNET was nt fully peratinal befre 1971, seismicity in nrthern Califrnia befre that time was taken frm the Califrnia Divisin f Mines and Gelgy Earthquake Catalgue f Califrnia (Real et al., 1978) which includes earthquakes reprted bth by CALNET and the seismic netwrk f the University f Califrnia, Berkeley. By using these three catalgs, the recrd befre each f the main shcks shuld be as cmplete as pssible. Table I shws which catalg was used fr each earthquake. All magnitudes in this paper are ML frm these catalgs. T search fr patterns f freshck ccurrence, the seismicity befre each main

3 FORESHOCKS ( ) IN TIlE SAN ANDREAS SYSTEM, CA 1363 shck is examined ver a larger time and space windw than that expected t be ccupied by the pssible precursry seismicity s that a change in activity can be cmpared t sme backgrund level. This study is cncerned with immediate freshcks such as thse discussed by Mgi that ccur within hurs, days, r at mst weeks f the main shck. The ccurrence f lnger term changes in activity, 2O J/ Iti' 8, / / 15 IC FIG. 1. Map f the San Andreas fault system, Califrnia, shwing majr faults (frm gelgic map f Califrnia, 1972) and the lcatin f the main shcks listed in Table 1. Main shcks with freshcks are shwn by clsed circles, and main shcks withut freshcks are shwn by pen circles. smething that ccurs ver years, is nt cnsidered here. A year f seismicity befre each f the main shcks was examined, including all events within a 3 km radius (apprximately the maximum length f rupture fr main shcks f the size cnsidered here) f the epicenter f the main shck. Fr unifrmity in reprting befre all f the main shcks, nly events f ML --> 2. are included. This may be slightly lwer than the threshld f cmpleteness fr sme f the earlier main shcks, but any higher threshld wuld t greatly limit the magnitude differential available between main shcks and Dssib]e fr~,~halr~

4 TABLE 1 MODERATE (ML > ) MAIN SHOCKS IN THE SAN ANDREAS ( ) Main Shck Lcatin Magnitude Depth Catalg* Surce Parkfield ' Lindh and Bre, June ' Aki, 1979 WatsnviUe 37.6' Sept ' Brreg Muntain ' 9 Apr ' Allen and Nrdquist, 1972 Crbett and McNalley, 1981 Cyte Creek ' Apr ' Santa Rsa ' Oct ' Lytle Creek ' This study 12 Sept ' San Fernand ' Feb ' Superstitin ' Sept ' Bear Valley ' Ellswrth, Feb ' Pint Magu ' Feb ' Thanksgiving ' Nv ' Galway Lake ' This study 1 June ' Santa Barbara ' Aug ' New Year's ' Jan ' Hmestead ' Mar " 26.63' Huttn et al., 198 Stein and Liwski, 1983 Cyte Lake ' Oct ' Imperial Valley 32" 36.82' Oct ' Livermre 37" 49.63' Jan ' Scheimer and Ccheram, 1982, Blt et al., 1981 Anza ' Feb ' Eureka ' Nv ' * 1, Califrnia Divisin f Mines and Gelgy; 2, CALNET; 3, CIT/USGS.

5 FORESHOCKS ( ) IN THE SAN ANDREAS SYSTEM, CA 1365 Recgnitin f Freshcks ANALYSIS The seismicity preceding each f the main shcks is shwn in Figure 2. The time befre the main shck when the event ccurred is pltted against its distance frm the main shck fr each main shck separately. On the basis f these distributins f seismicity, the main shcks are divided int three categries, thse with freshcks immediately befre the main shck near the epicenter f the main shck (Figure 2a), thse with sme enhanced seismic activity at relatively large distances frm the main shck (Figure 2b), and thse withut any identifiable change in seismicity (Figure 2c). T cmpare the rate f freshck ccurrence with the rate f backgrund seismicity, the activity within 3 km radii circles centered at the epicenters f each f the 2 main shcks fr the perid f time 1 t 3 days befre each main shck is assumed t represent the backgrund rate f seismicity. The average rate f ccurrence (ML > 2.) in these circles is 4.1 x 1.5 events/day/kin 2. The rate f activity in different regins ranges frm 1.8 x 1.6 events/day/km 2 arund the Santa Rsa and Eureka main shcks t 1.2 x 1-4 events/day/km 2 arund the Imperial Valley and Galway Lake main shcks. The seven main shcks with immediate freshcks (Figure 2a) all have earthquakes within 1 day and 5 km f the ccurrence f the main shck. Based n the calculated backgrund rate, the chance f an event ccurring randmly within a 5 km radius circle n a given day is.32 per cent. Fr 2 main shcks, there is thus a 6 per cent chance that ne event wuld ccur randmly within 1 day and 5 km f ne f the main shcks. The 3 events recrded within 1 day and 5 km f 7 main shcks represent a rate f ccurrence that is 5 times larger than the backgrund rate. Nne f the ther 13 main shcks (Figure 2, b and c) are assciated with such a sudden jump in activity prir t their ccurrence. Althugh the seven main shcks in Figure 2b are preceded by sme enhanced seismicity, these earthquakes are nt clearly anmalus like the immediate freshcks discussed abve because a larger time and space windw is invlved. All f these main shcks were preceded by 1 t 5 days by anther event lcated 13 t 3 km away frm the main shck. The randm chance f having an earthquake in 5 days in a circle with a radius f 3 km (frm the abve backgrund rate) is 58 per cent. Thus in 2 such circles, ne wuld expect 12 events frm backgrund ccurrence; in fact, aside frm the immediate freshcks, 12 events were recrded in these 2 circles. Mrever, mst theries f freshcks (e.g., Jnes and Mlnar, 1979; Das and Schlz, 1981) assume that freshcks ccur n the same fault that slips during the main shck (which is reprted fr the seven immediate freshck sequences, as will be discussed belw). Mst f the distant events in Figure 2b did nt ccur n the same sectin f the fault that slipped during the main shck including thse preceding the Watsnville, Cyte Creek, Imperial Valley, Cyte Lake, and Santa Barbara earthquakes. Because f the different faults invlved and the higher backgrund level f activity in the 3 km radii, it is less likely that these distant events are related t the main shcks. N change in seismicity can be seen befre the six main shcks in Figure 2c. Thus, 7 f 2, r 35 per cent, f the main shcks f M _-> 5. in the San Andreas physigraphic prvince have been preceded by immediate freshck sequences. Of curse, this result is dependent upn the detectin level f the lcal seismic netwrk. Althugh the detectin capabilities in Califrnia have increased significantly since 1966, the rate f freshck ccurrence has nt (nte that the 1966 t 197 rate is

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7 FORESHOCKS ( ) IN THE SAN ANDREAS SYSTEM, CA per cent); hence, this is nt cnsidered a significant prblem. It shuld als be nted that there is a reginal variatin in the rate f freshck ccurrence. Nne f the earthquakes n the reverse faults in the Transverse Ranges (Sylmar, Pint Magu, Santa Barbara, and New Year's Day) have been preceded by freshcks, while 44 per cent f the main shcks n strike-slip faults f the San Andreas have had freshcks. The enhanced seismicity shwn in Figure 2b is seen befre bth reverse and strike-slip events. Characteristics f Freshcks The seven sequences f immediate freshcks (Figure 2a) all include an event within 5 km and 1 day f the main shck which culd be used as a criteria fr 1966 JUNE 28 PARKFIELD " "-,, 12(Y'3' 36~ ' 5.2 ~.~.,,, Mainshck,,,5 a 24 ~2 IT,, ]I 6/6128 2: 4: 1: 3= ' %% %'. FIG. 3. The spatial distributin f the freshcks (circles) and main shck (star) f the Parkfield freshck-main shck sequence. The details f the San Andreas fault are als shwn. The dtted line shws the prjectin f the strike f the nrthern sectin f the fault nt the sectin suth f the bend. The difference in strike is 5 (frm Lindh and Bre, 1981). The inset shws a plt f time versus magnitude fr the freshcks and main shck. defining freshcks in the San Andreas system. The tempral and spatial distributin f each f these freshck sequences is described belw. In sme cases, relative relcatins f the freshck-main shck sequences have been carried ut [see Jnes et al. (1982) fr a discussin f this technique]. Parkfield, 28 June This earthquake ccurred n the central sectin f the San Andreas fault just at the suthern end f the creeping sectin. Lindh and Bre (1981) have nted that there is a change in strike f the San Andreas fault f abut 5 just nrth f the epicenter f the main shck (Figure 3). A cluster f immediate freshcks was lcated just nrth f this change in strike starting 3.5 hr befre the main shck. The largest f these freshcks was a ML = 5.1 event that ccurred 17 min befre the main shck; the rest f the events were M < 3.1. Three events f M - 2 recrded in May and early June nrth f these freshcks and are nt cnsidered

8 1368 LUCILE M. JONES part f the freshck sequence since they ccurred in the seismically active creeping sectin f the San Andreas fault. Brreg Muntain, 9 April The Brreg Muntain earthquake was lcated n the San Jacint fault just nrth f a large en-echeln step in the fault, the Octill Badlands (Allen and Nrdquist, 1972). The hypcenter is east f the surface fault strand that was ffset in the event (Figure 4). Surface rupture and aftershcks were bserved bth nrth and suth f the Octill Badlands s it appears that rupture prpagated thrugh the ffset. One freshck f M = 3.7 ccurred 8 sec befre the I,%, I. / ".:..i I '~',""':~'"'". ;'/ "":":" 5"ALTON ~:- ", "X~"" BORREGO M ~ N "', ',, ~F" "~ ~ ;_~ i~ SEA.z.~.,:,... ~, "--Q V F / /,. ~ "'Q,//i:: 5 ~ I 5 IOkm I I I I ' 116 ' 1155'w FIG. 4. The spatial distributin f the freshck (circle) and main shck (star) f the Brreg Muntain freshck-main shck sequence. The details f the San Jacint fault are als shwn (frm Wyss and Hanks, 1972). The lcatin f the main shck is frm Crbett (1981), the freshck was relcated relative t the master (this study). The inset shws a plt f time versus magnitude fr the freshck and main shck. main shck. Relative relcatin f the event with respect t the main shck (this study) shws that the freshck ccurred significantly t the suth f the main shck acrss the en-echeln step. Lytle Creek, 12 September 197. The epicenter f the Lytle Creek main shck is nt clearly assciated with any fault, but lies between the San Andreas and the San Jacint faults near the intersectin f these tw faults. N detailed study f this earthquake has been published, but relative relcatin f the main shck with respect t the Lytle Creek earthquake f 19 Octber 1979 has prvided a reasnably accurate hypcentral lcatin (this study; Table 1). Lcatins f the aftershcks by Hanks (unpublished data) d nt define a simple planar feature, althugh they are cmpatible with ccurrence n a fault parallel t the San Andreas and San Jacint

9 FORESHOCKS ( ) IN THE SAN ANDREAS SYSTEM, CA 1369 faults (Figure 5). The main shck was preceded by tw freshcks 2 min befre it, a M = 4.2 event with its wn aftershck fm = 2.4 (Figure 5). The largest freshck is 2 km away frm the main shck, perpendicular t the trend f the aftershcks. Hwever, it is nt clear if this is a real feature f the rupture zne. Bear Valley, 24 February Ellswrth (1975) shwed that the main shck ccurred n the central creeping sectin f the San Andreas fault and that the aftershcks which preceded the main shck fr 2 days were lcated n a separate, small fault cnjugate t the San Andreas. Althugh this area f the fault is seismically very active, there is a clear start t the freshck sequence by a M = SEPTEMBER 12 LYTLE ' 1175 ' CREEK 2 i i i i ~ ~~ km 4 I 34 2' "~".~, freshck. Oa~ ~, '~''- "~.. ~ O ~ ~" "~ ~ -~. ~. -~ ' ~" ~5 ~2, SAN JACINTO FAULT ZONE, i", 14: 14:3 FIG. 5. The lcatins f the main shck (star), aftershcks (pen circle), and largest freshck (clsed circle) f the Lytle Creek sequence. The San Andreas fault and strands f the San Jacint fault zne are shwn. The lcatins f the main shck and aftershcks are frm Hanks (unpublished study) and that f the freshck is a lcatin relative t the main shck (this study). The inset shws a plt f time versus magnitude fr the freshcks and main shck. event (Figure 6). Only tw M = 3 freshcks ccurred almst at the epicenter f the main shck in the minute befre it ccurred. Galway Lake, 1 June The Galway Lake earthquake caused surface rupture n a small nrth-suth trending fault in the Mjave desert which was mapped by Hill and Deeby (1977) wh fund a change in strike f the fault f a few degrees {Figure 7). The hypcenter f the main shck was exceptinally shallw (2.8 km) as were thse f all f its freshcks and aftershcks. The main shck was preceded by several weeks f freshck activity all within a few kilmeters f the main shck. The first M => 2. freshck was recrded 4 days befre the main shck, but a M = 1.9 event als ccurred almst 12 weeks befre the main shck n 9 March 1975 in the same regin which might be the start f the freshck sequence (Figure 7). The tw largest freshcks preceded the main shck by less than 1 hr. Abslute

10 137 LUCILE M. JONES lcatins f them and the main shck place them nrth and suth, respectively, f the change in strike f the fault mentined abve. In additin, Lindh et al. (1978), shwed that there was a change in the fault plane slutin between the freshcks and main shck cnsistent with this change in strike f the fault. Hmestead, 15 March This earthquake sequence appears t have activated tw almst perpendicular faults (nrth-suth and east-west trending) in the Mjave desert (Huttn et al., 198). The whle sequence is smetimes cnsidered a swarm because there were fur earthquakes f M -~ 5 in the sequence, but in this study, 1972 FEBRUARY 24 BEAR VALLEY 3~ 35 km 5 I I I I I I " 5 I.d :) ~4 z ~2 t l[ r [llll tl 2/22 2/23 2/24 4 h ',,-.-4 ~.~ 1211 ' ' FI. 6. The spatial distributin f the freshcks and main shck f the Bear Valley freshck-main shck sequence. The first freshck and the last tw freshcks are shwn by circles and the main shck by a star. The rest f the freshcks were lcated within regin A. Regin B is the area f the aftershcks. The details f the San Andreas fault are als shwn (frm Ellswrth, 1975). The inset shws a plt f time versus magnitude fr the freshcks and main shck. the largest earthquake is taken as the main shck. It was preceded by 2 min by a large freshck (M = 4.9) which had its wn aftershck sequence. The freshcks and main shck ccurred n the same fault but the secnd fault intersected the main fault between the hypcenters f the largest freshck and main shck (Figure 8). Fault plane slutins f the largest freshck and main shck (Huttn et al., 198) als shw a change in strike f the main fault plane f 7 between the tw events. Livermre, 24 January 198. This earthquake sequence ccurred n a small fault subparallel t the San Andreas in the Livermre Valley. One freshck preceded the main shck by 9 sec ccurring almst directly abve the main shck within a few hundred meters (Scheimer and Ccheram, 1981). There were tw grups f

11 FORESHOCKS ( ) IN THE SAN ANDREAS SYSTEM, CA JUNE 4 GALWAY LAKE I I I 2km 5/2 2.2 ~ ke /I ' w /4 } 1 5/2 2.1 (. 5/I ~ 3.9-5~' 34 I 3/74/I 5/I 6/I I I I 31' 116 :3 ' 29' Fr. 7. The lcat}ns f the main shck (star), and freshcks (clsed circles) f the Galway Lake sequence. The Galway Lake fault as mapped frm surface rupture during this sequence by Hill and Deeby (1977) is als shwn. The lcatins f the main shck and freshcks are abslute lcatins (this study). The inset shws a plt f time versus magnitude fr the freshcks and main shck. 2:2 2:4 21: 2' '::~}ii;~i}:}?::?:}::::?;?.:.i:::,:::... "".. k,m 2 ] 116 2,.5' 1979 MARCH 15 HOMESTEAD VALLEY FIG. 8. The spatial distributin f the Hmestead Valley sequence. The stippled regin is the area f aftershcks, the circles are freshcks, and the star is the main shck. The heavy line is the surface rupture during the sequence (frm Huttn et al., 1979). The inset shws a plt f time versus magnitude fr the freshcks and main shck. I

12 1372 LUCILE M. JONES aftershcks, ne lcated directly abve the hypcenter f the main shck and ne extending suth frm the main shck alng the fault, cnfined t apprximately the same depth as the main shck, 9 t 12 km (Figure 9). Thus, there is a suggestin that bth the main shck and the freshck culd have ccurred at the intersectin f tw faults. Summary. Five f the seven freshck sequences begin in the last day befre the main shck. All f these sequences have either nly ne freshck (Brreg Muntain and Livermre) r ne large freshck with its wn aftershcks (Parkfield, Lytle Creek, and Hmestead). (The Hmestead sequence verall appeared mre like a swarm, but the events befre the main shck were ne large event with aftershcks.) The Bear Valley freshck sequence preceded the main shck by 58 hr and appeared mre swarm-like with 14 events f 2. -< M =< 3.5 in the sequence. The Galway Lake freshck sequence als had a swarm-like character with a gradual increase in number and magnitude f the events. Thus, f seven sequences, DISTANCE ALONG N$O W, KM 5 I I I I I I I I -- O ; /F " h ck -5 w5 D 4 I-- Z ~5 < O / ~f~ O - ~ ~ MAG, (:9-1 m "1- III JANUARY24 LIVERMORE FIG. 9. The depth distributin f the Livermre Valley sequence. The prjectins f the main shck (star), aftershcks (pen circle), and freshck (clsed circle) alng the strke f the aftershck zne are shwn (frm Scheimer and Ccheram, 1981). The inset shws a plt f time versus magnitude fr the freshcks and main shck. nly tw, r 1 per cent f all the main shcks, had freshcks similar t thse described by Mgi. One sequence, Hmestead, culd be a class IIIa sequence [as mdified by Utsu (197)] r multiple main shcks. The remaining fur freshck sequences are single events, smetimes with their wn aftershcks, r the class Ib freshcks f Utsu (197). Fur f the freshck sequences had discntinuities in the fault surface between the freshcks and main shcks. Parkfield had a change in strike, Brreg Muntain had an en-echeln step, and Bear Valley and Hmestead had intersecting faults. The ther three sequences als had pssible discntinuities in their faults. This suggests that the physical discntinuities in the faults may be related t the ccurrence f the freshcks and is in agreement with previus findings that the

13 FORESHOCKS ( ) IN THE SAN ANDREAS SYSTEM, CA 1373 gemetry f fault znes can play an imprtant rle in determining seismicity patterns (Bakun et al., 198; Bakun, 198). Magnitudes The magnitudes f the largest freshcks are pltted against the magnitudes f the main shcks in Figure 1a. As was fund in a wrldwide study f freshcks (Jnes and Mlnar, 1979), the magnitudes f the freshcks d nt seem t be related t thse f the main shcks. In additin, the magnitudes f the freshcks d nt seem t be related t the time between the freshcks and main shcks (Figure 1b). Main Shck Depth and Duratin f Sequence The duratins f the freshck sequences vary frm 8 sec at Brreg Muntain t perhaps as much as 12 weeks at Galway Lake. Althugh the duratin f a sequence des nt depend n the magnitude f the freshck (Figure 1), there des seem t be a relatinship between the depth f the main shck and the time between the earliest freshck and the main shck. In Figure 11, the depth f the main shck is pltted against the lgarithm f the duratin f the freshck sequence. With the exceptin f the Hmestead sequence, the lgarithm f the duratin f the freshck sequence decreases linearly as the depth f the main shck increases. Of the parameters that vary with depth, the stress state is ne f the mst likely t affect the ccurrence f earthquakes. McGarr et al. (1982) have estimated the variatin in minimum cmpressive stress with depth near the San Andreas frm measurements in the tp kilmeter f the crust as ~1 = (19.28 _ 1.52)*z (1) where ~1 is in MPa and z is in kilmeters. The stress calculated frm this relatinship is shwn with the depths in Figure 11. Hence, the duratins f the freshck sequences are inferred t decrease with increasing stress. INTERPRETATION T understand hw the depth f the main shck culd be affecting the duratin f the freshck sequences, the relatinships between the stresses clse t failure in the fault zne need t be examined. Failure in brittle materials, either fracture f the material r frictinal sliding n a preexisting surface, beys Culmb's Law (e.g., Jaeger and Ck, page 95, 1976); as=c+q.al (2) where as and ~1 are the maximum and minimum principal effective stresses, respectively, C is the chesin, and q is the cefficient f frictin. In additin, failure can ccur at lwer stresses than thse given abve fr instantaneus failure, by the prcess f static fatigue (e.g., Schlz, 1972). The rck will nt fail instantaneusly after the applicatin f these lwer stresses but will after a perid f time that depends n the rati f the applied stress t the instantaneus strength, r [ t= T exp -Ka~ (3) ~c/

14 1374 LUCILE M. JONES Cl N" C) T ~ m n- O LL 5. F- SO W ry 3 b I:D W ' 5: ' 6: ' MAGNITUDE OF MAINSHOCK 7. b C~ i W h I-- Ld 4. LL W rm z.5. :1 1: 16. l5. ld. DURATION OF FORESHOCK SEQUENCE, hr Fr. 1. (a) The magnitude f the main shck pltted against the magnitude f the largest freshck fr the seven freshck-main shck sequences in the San Andreas. (b) The magnitude f the largest freshck pltted against the lgarithm f the duratin f the freshck sequence fr the seven sequences in the San Andreas. where T and K are cnstants, t is the time t failure, ~ is the applied maximum principal stress, and ~c is the maximum principal stress needed fr instantaneus failure [which is a functin f pressure by equatin (2). There is als a slight additinal effect f cnfining pressure which increases the time needed as pressure

15 FORESHOCKS ( ) IN THE SAN ANDREAS SYSTEM, CA 1375 increases but this effect is nt very strng when the rck is saturated with water (Kranz, 198).] The equatin abve is valid fr fracture f brittle materials but a similar failure mde has been recgnized in frictinal defrmatin (Dieterich, 1981). The prpsed mdel fr freshck ccurrence assumes that the strength f the future rupture surface f the main shck is variable [i.e., that C and q in equatin (2) are functins f psitin alng the fault] such that the discntinuities in the faults (seen fr the freshck-main shck sequences, Figures 3 t 1) wuld be the strngest parts f the faults and the fci f the main shcks. The strength f the I I I I I C~I D ~i_ ~i (~B r reg Mtn. ~" ytle Cree arkf/eld hi T" ~.-I <C OA ear Valley ~ ~_) " :2 meeted Galway Lake ~ ~" %2. 21 ' '. 3'. 6. I O 2: O 4.~ LOG (DURATION hrs) FIG. 11. The lgarithm f the duratin f the freshck sequences in hurs versus the depth f the main shck. Als shwn is the minimum principal stress calculated fr thse depths frm the relatin f McGarr et al. (1982). Fr the Galway Lake sequence, the duratin shwn is frm the first M freshck. fault at the main shck fci wuld be given by ~TSm = Cm -Jr- qm.as. (4) Suppse a nearby sectin is smewhat weaker with a strength f asf = Cf + q[. " 2 (5) where C~ < Cm and q/_-< qm. As the shear stress n the fault increases, the failure strength f the weaker patch will be reached first. If the failure f this weaker sectin is unstable, an earthquake (freshck) will ccur, lading the nearby discntinuity with a stress at the failure strength f the weaker sectin [given by equatin (5)]. If the differences in strength between the tw regins is sufficiently large, failure will nt ccur instantaneusly but the prcess f static fatigue will be accelerated. The time t failure (duratin f the freshck sequence) by equatin (3) will depend n the rati f failure strengths f the surce regins f the freshck and main shck lci.

16 1376 LUCILE M. JONES Cmbining equatins (1), (4), and (5), the rati f strengths is as/ Sf + q/. ( z) as,~ Sm+ qm.( z) " (6) Thus the rati (and hence the duratin f the freshck sequence) depends n the chesins and cefficients f frictin f bth the strnger and weaker patches and the depth. The depth dependence is quite strng because f the large increase in minimum cmpressive stress with depth. ;1 I I I I I U3 ~ wc~" U3 tf) r~ 3 (3 I-4 p-o r~ 6" i O '1 O' 2' 3' LOG (DURATION hrs) FIG. 12. The rati f maximum principal effective stresses needed fr failure f a weak patch f the fault divided by that needed fr failure f a strng patch versus duratin f the freshck sequence. T examine the depth dependence, a test case has been calculated where the weaker patch (the freshck fcus) has a strength f ~3 = ~1 (Byerlee's Law) while the strnger patch has sme chesin fr a strength f ~3 = 85 MPa This rati f strengths has been calculated fr the depths f each f the seven freshck-main shck sequences (Table 1) and pltted against the lgarithm f the duratins f the freshck sequences in Figure 12. This relatinship between rati and duratin is in general agreement with labratry results. Fr instance, fr dry Barre granite, Kranz (198) fund that the time t failure wuld be 1 min t 1 hr fr applied stresses that were 9 per cent f the instantaneus fracture strength and n the rder f mnths fr 7 per cent f the instantaneus fracture strength.

17 FORESHOCKS ( ) IN THE SAN ANDREAS SYSTEM, CA 1377 Thus, static fatigue f rck with variable strength similar t that fund in the labratry culd prduce the duratins f freshck sequences seen in Califrnia. DISCUSSION This mdel culd explain why nne f the thrust faults in the Transverse Ranges have experienced freshcks. The stress versus depth relatin f McGarr et al. (1982) is valid nly fr the strike-slip faults f the San Andreas system, where the vertical stress is the intermediate principal stress. Fr thrust faulting, the vertical stress is the least principal stress leading t principal stresses much higher than in a strike-slip regime. Thus, the thrust faults culd be discntinuus [such as the prpsed dwn-dip bend in the fault the 1971 Sylmar earthquake, e.g., Heatn (1982)] but the stress n the fault wuld be t high fr the "freshck" and "main shck" t be separated in time. This culd als explain why the Hmestead sequence des nt fit in the apparent relatinship (Figure 11). The difference in magnitude between freshck and main shck is much smaller fr the Hmestead sequence than fr the ther freshck sequences. Such a large freshck culd cncentrate stress at the hypcenter f the main shck. This stress cncentratin factr is the ttal area f the fault that slips in the sequence divided by the area that did nt slip in the freshck (Jnes and Mlnar, 1979). If the freshck is small cmpared t the main shck, this factr is clse t 1; fr all f the sequences except Hmestead it is less than 1.5. Assuming the areas f the earthquakes frm the relatinship between area and magnitude f Wyss and Brune (1968), the factr fr Hmestead is 1.6. Several ther mdels fr freshck ccurrence have been prpsed but at least tw f these mdels seem t be incmpatible with the data presented here. One has suggested that freshcks culd result frm accelerating premnitry creep n a frictinal surface (e.g., Jnes and Mlnar, 1979). Hwever, labratry studies f this prcess (Dieterich, 1978) have shwn that the time frm nset f creep t failure increase with increasing stress. While there are large errrs in the values f stress assumed here, it is clear that the duratin f the sequence des decrease with increasing stress, nt increase as in the labratry experiments, s that just creep des nt seem a likely mechanism fr prducing these freshcks. Other mdels (e.g., Jnes et al., 1982) have suggested that increased pre pressure caused by the freshcks culd be triggering the main shck and thus cntrlling the duratin f the freshck sequences. If this were true, the duratins f the sequences shuld decrease as the magnitudes f the freshcks increase. N such dependence is seen in these data (Figure 1), suggesting that this mdel is nt applicable t Califrnia freshcks. While the data presented here are cmpatible with a freshck mdel f delayed multiple rupture, this des nt require that all freshcks everywhere result frm the same prcess. Freshcks prduced by delayed multiple rupture seem mre likely when the magnitudes f the freshcks are relatively large. As nted befre, tw types f freshck sequences, swarms and single events, are seen in bth the San Andreas and Japan. Das and Schlz (1981) suggested that tw prcesses, delayed multiple rupture and initiatin cracking due t lcalized decreases in strength, culd prduce freshcks. If the delayed multiple rupture prduces the single freshcks, perhaps initiatin cracking causes swarm-like freshcks. Hwever, the tw Califrnian sequences with the appearance f swarms, Bear Valley

18 1378 LUCILE M. JONES and Galway Lake, fit the depth-time relatin in Figure 11, s mre freshck sequences need t be studied t answer this questin. The generally small tempral spacing between freshcks and main shcks in the San Andreas system may limit the usefulness f freshcks in earthquake predictin in that area. Hwever, since the duratin f the freshck sequence is a functin f the gemetry f the fault and the material prperties f the rcks, the duratin f the sequence shuld be the same fr different sequences at the same lcatin. Indeed, the tempral distributins f several Parkfield freshck-main shck sequences have been very similar. Thus, a repeat f the 1857 Frt Tejn earthquake culd have the same freshck distributin as the 1857 event (Sieh, 1978). If these freshcks culd be recgnized in time, this might allw a few hurs fr preparatin. CONCLUSIONS Immediate freshcks, as a phenmena distinct frm backgrund seismicity, have ccurred befre 35 per cent f the 2 mderate and larger main shcks in the San Andreas system f Califrnia. These freshck sequences can be defined as having an event within I day and 5 km f the main shck. Nne f the fur reverse-faulting earthquakes f the Transverse Ranges had freshcks while 44 per cent f the strike-slip earthquakes did have. Enhanced seismic activity at relatively large distances frm the main shck (13 t 3 km) has als preceded 4 per cent f the main shcks by 1 t 5 days but this activity cannt be clearly distinguished frm the backgrund seismicity. Of the seven immediate freshck sequences, nly tw had the swarm-like appearance f the class II freshcks defined by Mgi. The ther freshck sequences appear t be single events (smetimes with their wn aftershcks) preceding the main shcks. Fur f these sequences were crrelated with physical discntinuities in their faults between the hypcenters f the freshck and main shck, and this is als pssible fr the ther fur sequences. The duratins f the freshck sequences are fund t decrease as the depths f the main shcks increase frm 3 t 11 km, which is interpreted as a dependence n stress. This suggests that freshcks may represent a prcess f delayed multiple rupture and that the delay between ccurrence f freshck and main shck might represent the time needed fr static fatigue t break the strnger rck at the discntinuity in the fault. ACKNOWLEDGMENTS The authr thanks all f thse wh supplied data frm Califrnia, including Allan Lindh, Rbert Ccheram, Jerry Eatn, Kate Huttn, Carl Jhnsn, and Art Frankel. This wrk was supprted by NSF Grant EAR and by the authr's Lamnt Pstdctral Fellwship. Terry Engelder and Egill Haukssn critically reviewed the manuscript. REFERENCES Aki, K. (1979). Characterizatin f barriers n an earthquake fault, J. Gephys. Res. 84, Aki, K. (1981). A prbabilistic synthesis f precursry phenmena, in Earthquake Predictin: An Internatinal Review, Maurice Ewing Series, 4, D. Simpsn and P. Richards, Editrs, Am. Gephys. Unin, Publ., Washingtn, D.C., Allen, C. R. and J. M. Nrdquist (1972). Brreg Muntain earthquake: freshck, mainshck and larger aftershcks, U.S. Gel. Surv. Prfess. Paper 787, Bakun, W. H. (198). Seismic activity n the suthern Calaveras fault, Bull. Seism. Sc. Am. 7, Bakun, W. H. and T. V. McEvilly (1979). Are freshcks distinctive? Evidence frm the 1966 Parkfield and 1975 Orville, Califrnia sequences, Bull. Seism. Sc. Am. 69,

19 FORESHOCKS ( ) IN THE SAN ANDREAS SYSTEM, CA 1379 Bakun, W. H., R. M. Stewart, C. G. Bufe, and S. M. Marks (198). Implicatin f seismicity fr a failure f a sectin f the San Andreas fault, Bull. Seism. Sc. Am. 7, Blt, B. A., T. V. McEvilly, and R. A. Urhammer (1981). The Livermre, Califrnia, sequence f January 198, Bull. Seism. Sc. Am. 71, Crbett, E. J. and K. C. McNally (198). Seismicity f the Brreg Muntain regin (abstract), Earthquake Ntes 5, 4. Das, S. and C. H. Schlz (1981). Thery f time-dependent rupture in the earth, J. Gephys. Res. 86, Dieterich, J. H. (1978). Time-dependent frictin and the mechanics f strike-slip, Pure Appl. Gephys. 116, Dieterich, J. H. (1979). Mdeling f rck frictin, 1, Experimental results and cnstitutive equatins, J. Gephys. Res. 84, Dieterich, J. H. (1981). Cnstitutive prperties f faults with simulated guge, in Mechanical Behavir f Crustal Rcks, Gephysical Mngraph 24, Am. Gephys. Unin, Publ., Washingtn, D.C Ellswrth, W. L. (1975). Bear Valley earthquake sequence f February-March, 1972, Bull. Seism. Sc. Am. 65, Fuka, Y. and M. Furumt (1975). Freshcks and multiple shcks f large earthquakes, Phys. Earth Planet. Interirs 1, Heatn, T. H. (1982). The 1971 San Fernand earthquake: a duble event? Bull. Seism. Sc. Am. 72, Hill, R. J. and D. J. Deeby (1977). Surface faulting assciated with the 5.2 magnitude Galway Lake earthquake f May 31, 1975: Mjave Desert, San Bernardin Cunty, Califrnia, Gel. Sc. Am. Bull. 88, Huttn, L. K., C. E. Jhnsn, J. C. Pechman, J. E. Ebel, J. W. Given, D. M. Cle, and P. T. German (198). Epicentral lcatins fr the Hmestead Valley earthquake sequence, March 15, 1979, Calif. Gel. 33, Jaeger, J. C. and N. G. W. Ck (1976). Fundamentals f Rck Mechanics, Halstead Press, New Yrk, 45 pp. Jnes, L. M. and P. Mlnar (1979). Sme characteristics f freshcks and their pssible relatin t earthquake predictin and premnitry slip n faults, J. Gephys. Res. 84, Jnes, L. M., B. Q. Wang, S. X. Xu, and T. J. Fitch (1982). The freshck sequence f the February 4, 1975 Haicheng, China earthquake, J. Gephys. Res. 87, Kagan,. and L. Knpff {1978). Statistical study f the ccurrence f shallw earthquakes, Gephys. J. Ry. Astr. Sc. 55, Kanamri, H. (1981). The nature f seismicity patterns befre large earthquakes, in Earthquake Predictin: An Internatinal Review, Maurice Ewing Series, 4, D. Simpsn and P. Richards, Editrs, Am. Gephys. Unin, Publ., Washingtn, D.C., Kranz, R. L. (198). The effects f cnfining pressure and stress difference n static fatigue f granite, J. Gephys. Res. 85, Lindh, A., G. Fuis, and C. Mantis (1978). Freshck amplitudes and fault plane changes: a new earthquake precursr? Science 21, Lindh, A. G. and D. M. Bre (1981). Cntrl f rupture by fault gemetry during the 1966 Parkfield earthquake, Bull. Seism. Sc. Am. 71, McGarr, A., M. D. Zback, and T. C. Hanks (1982). Implicatins f an elastic analysis f in situ stress measurements near the San Andreas fault, J. Gephys. Res. 87, Mgi, K. (1963). Sme discussins n aftershcks, freshcks, and earthquake swarms: the fracture f a semi-infinite bdy caused by an inner stress rigin and its relatin t earthquake phenmena (third paper), Bull. Earthquake Res. Inst., Tky Univ. 41, Real, C. R., T. R. Tppzada, and D. L. Parke (1978). Earthquake Catalgue f Califrnia, January 1, 19-December 31, 1974, Califrnia Divisin f Mines and Gelgy, Sacrament, Califrnia. Richter, C. F. (1958). Elementary Seismlgy, W. H. Freeman and C., San Francisc, Califrnia, 768 pp. Scheimer, J. F. and R. S. Ccheram (1982). Livermre Valley earthquake sequence January 24-February 29, 198 (abstract), Trans. Am. Gephys. Unin 63,374. Schlz, C. H. (1972). Static fatigue f quartz, J. Gephys. Res. 77, Sieh, K. E. (1978). Central Califrnia freshcks f the great 1857 earthquake, Bull. Seism. Sc. Am. 68, Stein, R. and M. Liwski {1983). The 1979 Hmestead Valley earthquake sequence, Califrnia: cntrl

20 138 LUCILE M. JONES f aftershcks and pstseismic defrmatin, J. Gephys. Res. 88, Utsu, T. (197). Aftershcks and earthquake statistics (II)--Further investigatin f aftershcks and ther earthquake sequences based n a new classificatin f earthquake sequences, J. Faculty Sci., Hikkaid Univ. 3, Wyss, M. and J. Brune (1968). Seismic mment, stress and surce dimensins fr earthquakes in the Califrnia-Nevada regin, J. Gephys. Res. 73, Wyss, M. and T. Hanks (1972). Surce parameters f the Brreg Muntain earthquake, U.S. Gel. Surv. Pr/ess. Paper 787, Xu, S. X., B. Q. Wang, L. M. Jnes, X. F. Ma, and P. W. Shen (1982). The Haicheng earthquake sequence and earthquake swarms--the use f freshck sequences in earthquake predictin, Tectnphysics 85, Yamashina, K. (1981). Sme empirical rules n freshcks and earthquake predictin, in Earthquake Predictin: An Internatinal Review, Maurice Ewing Series, 4, D. Simpsn and P. Richards, Editrs, Am. Gephys. Unin, Publ., Washingtn, D.C., Zback, M. L. and M. D. Zback (198). State f stress in the cnterminus United States, J. Gephys. Res. 85, LAMONT-DOHERTY GEOLOGICAL OBSERVATORY OF COLUMBIA UNIVERSITY PALISADES, NEW YORK 1964 Manuscript received 28 Nvember 1983