Seismic Slip Distribution Along the San Jacinto Fault Zone, Southern California, and Its Implications

Seismic Slip Distribution along the San Jacinto Fault Zone, Southern California, and Its Implications

WAYNE THATCHER National Center for Earthquake Research, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 JAMES A. HILEMAN Seismological Laboratory, California Institute of Technology, Pasadena, California 91109 THOMAS C. HANKS Seismological Laboratory and Earthquake Engineering Research Laboratory, California Institute of Technology, Pasadena, California 91109

ABSTRACT fault in central California. The reason for others (1972) obtained a similar rate for the this discrepancy in rates of relative motion past several thousand years from data ob- The amount and distribution of seismic is not known, although it is plain that rela- tained at a different locality along the fault. slip along 240 km of the San Jacinto fault tive motions of at least 3 cm/yr must be ac- The discrepancy between the geologic and zone between Cajon Pass and Superstition commodated across southern California. A seismic slip rates indicates that either more Mountain has been obtained from determi- significant proportion, although not all, of of the plate motion is now occurring along nations of seismic moment and estimates of this motion is currently being taken up by the fault zone or, more likely, that the seis- source dimension for each of the nine mod- seismic slippage along the San Jacinto fault mic strain release is episodic. erate earthquakes (6 < M < 7) which have zone. The remainder is accommodated In this, paper, we determine the detailed occurred there since 1890. either by aseismic slip or strain accumula- distribution of seismic slip along the San There are two significant gaps in seismic tion across the San Andreas or other Jacinto fault zone since 1890. This more slip, one between Cajon Pass and Riverside, northwest-trending faults of the region (Fig. detailed picture is important, because al- the other from Anza to Coyote Mountain. 1). though the M ^ 6 events are fairly evenly Each is about 40 km long and both are The seismic slip rate averaged along the distributed along the 240-km length of the characterized by complex fault zones and a entire length of the San Jacinto fault zone fault zone (Fig. 1), it is unclear whether all currently high level of minor seismicity (M was estimated by Brune (1968) to be 1.5 of the fault has ruptured as a result of these < 5). No aseismic fault creep has been cm/yr, using the seismicity recorded since earthquakes. Our major conclusion is that identified on either segment. These gaps 1912. Sharp (1967) estimated the average there are two conspicuous gaps in seismic may mark the sites of the next moderate rate during the past 2 m.y. to be 0.3 cm/yr slip which may mark the sites of future earthquakes (M = 6 —» 7) to occur along on the basis of geologic evidence; Clark and moderate earthquakes. These two segments the San Jacinto fault zone. The two remain- ing sections of the fault, Riverside and Anza, and Coyote Mountain to Supersti- tion Mountain, may have been ruptured along their entire lengths, in 1890—1923 and 1942—1968, respectively. Key words: seismology, earthquake moment, seismic slip, seismic gaps, earthquake prediction.

INTRODUCTION

The San Jacinto fault zone is the most seismically active strand of the San Andreas system in southern California. The succes- sion of moderate earthquakes (6 < M < 7) which have occurred since 1890 along this fault and its southern extensions to the head of the Gulf of California (Allen and others, 1965; Fig. 1) indicate that in this region, the San Jacinto fault zone currently is the most active element of the Pacific—North American plate boundary. The precise way in which the gross plate i lanc:^ \ motion is being accommodated across southern California is at present unclear. Magnetic lineations at the mouth of the Vj940 Gulf of California indicate that the relative plate motion there is nearly 6 cm/yr (Larson CALIFORNIA "MEXICO ' and others, 1968), but geologic (Dickinson and Grantz, 1968) and geodetic (Savage and Burford, 1973) observations suggest a Figure 1. Location map of the San Jacinto fault zone, showing epicenters of the largest earth- rate of ~3 cm/yr across the San Andreas quakes, 1890-1973.

Geological Society of America Bulletin, v. 86, p. 1140-1146, 7 figs., August 1975, Doc. no. 50814.

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of the fault are each examined in more de- TABLE 1. SOURCE PARAMETERS, SAN JACINTO FAULT Average seismic slip ZONE EARTHQUAKES 1890-1969* tail, and suggestions are made for experi- Seismic Moment ments which would increase our under- Mo (1025 dyne-cm) (km) (cm) standing of them. M0= fiiiA 1923 6 1/4 0.8 -4.+ 62 (fL - shear modulus) SOURCE PARAMETERS OF THE 1890, 1899 -7 -15. 1918 6.8 15. -11 + 127 MAJOR EARTHQUAKES 1937 6.0 0.2 -2. 53 1969 5.8 0.5 3. 59 Fault area A 1954 6.2 4.4 11.9 33 The distribution of seismic slip along the 1968 6.4 8.3 16. 34 fault has been obtained from a determina- 1942 6.5 9.4 10.8 85 tion of the seismic moment (Mo); an esti- * North-to-south along the fault, t Estimated approximately by using Mi = log M0 - mate of the area of rupture (A) for each of 3/2 log r - 17.8 (Thatcher and Hanks, 1973). the moderate earthquakes is listed in Table 1. The average slip (u) over the fault area ters, it appears that there is a significant may then be computed from deficiency of seismic slip in at least part of the region between the town of San Jacinto z*u (1918 event) and Coyote Mountain (1969 4 M0 = flu A (1) shock). The more refined source parameter lM0= M M'®'+M' where /x is the shear modulus, taken here as estimates listed in Table 1 bear out this 3 X 10 11 dynes/cm2. The method is shown supposition. Figure 2. (Top) Schematic illustration of the definition of seismic moment. (Bottom) Drawing There appear to be several similarities be- schematically in Figure 2 and is discussed showing how the average seismic slip over a rec- further below. M0 can be calculated from tween the 1937 and 1969 earthquake se- tangular segment of a vertical strike-slip fault is the amplitude spectra of seismic surface quences. Both had relatively large mag- determined from the moments of individual waves or body waves (obtained from seis- nitudes for their moments, an indication of earthquakes. The method is that first proposed mograms). The area, A, may be estimated relatively small source size and large stress by Brune (1968). from the areal extent of an aftershock zone drop (Thatcher and Hanks, 1973). In addi- or from the spectra of body waves. When tion, relative locations of aftershocks as in- timate the seismic slip along lithospheric surface faulting occurs, fault length and dicated by small variations in S-P times at plate boundaries using the record of his- surface offsets may be used to estimate A the California Institute of Technology re- toric seismicity. and u, thus yielding M0 directly. (Depth of gional seismic stations support the conclu- Slip from the 1890, 1899 (Dec. 25), faulting must either be assumed or obtained sion that the source sizes are small (< 10 1918, and 1923 earthquakes has been dis- from other data.) These source parameters km). The 1969 sequence has been studied in tributed uniformly over a fault area 75 km for the nine moderate San Jacinto fault some detail by Thatcher and Hamilton long by 15 km deep extending from just zone earthquakes used in this study are (1973), who found that most aftershock north of Riverside to slightly north of Anza. listed in Table 1. They have been taken hypocenters were confined to depths of be- This section was chosen partly to corres- from a companion paper by Hanks and tween 10 and 13 km, that the source size others (1975, this issue) on the moments of was less than about 6 km, and that after- moderate and major southern California shock activity was relatively low. All of earthquakes since 1890. Note that the these observations indicate that the con- 1923 Riverside cumulative seismic slip produced by all of tribution of these two earthquakes to the M0 = the smaller earthquakes in the 1890—1973 seismic slip between Anza and Coyote interval is at most about equal to that pro- Mountain is insignificant except in a very duced by the largest events (Brune, 1968). small region (~ 5 km) around their Figure 3 shows seismograms for the four hypocenters. moderate San Jacinto fault zone earth- SEISMIC SLIP DISTRIBUTION quakes which occurred between 1918 and 1918 San Jacinto 25 1942, arranged north-to-south in the figure. M0 - 15.x IO The records were written at Berkeley (A = The principal findings of this paper are 750 km) on well-calibrated, low-gain (V = summarized in Figure 4, a plot showing 40 to 80), horizontal component, long- seismic slip as a function of position along period (T0 = 10 to 16 sec) Bosch-Omori the San Jacinto fault zone, measured south- seismograph systems which have been op- east from Cajon Pass to Superstition erated by the University of California at Mountain.

Berkeley (UCB) since 1910. The important The method which has been used to de- 1937 point to note in Figure 3 is the small long- termine the average seismic slip over vari- Terwilliger Valley J

period amplitudes of the 1937 Terwilliger ous fault zone segments is shown schemati- M0- 0.2 xlO - Valley event (the long-period amplitudes cally in Figure 2. The upper part sketches are directly proportional to seismic mo- the slip surface from an individual earth- 1942 Lower Borrego Valley ment). Figure 1 shows that the 1937 shock quake, with the definition of the seismic Mn= 9.4 xlO25 EW is the sole moderate earthquake between moment for that event. The lower portion Anza and Coyote Mountain (1969 shock) shows,- for a cross section parallel to the from 1890 to 1973. Other seismograms strike of a vertical strike-slip fault, the slip Figure 3. Seismograms of four moderate San show that the 1969 earthquake, which is surfaces of four earthquakes (left), and, on Jacinto earthquakes, 1918-1942, arranged north-to-south. Recordings were made on long- near Coyote Mountain, has a relatively the right, that slip averaged over a single period Bosch-Omori seismographs operating at small moment, similar to that of the 1937 rectangular fault segment. This technique is Berkeley, about 750 km from the epicenters. event. Then, purely on the basis of these essentially that proposed by Brune (1968) Note the small long-period amplitudes of the seismograms and the earthquake epicen- and used by Davies and Brune (1971) to es- 1937 event.

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estimate source dimension. [Seismic slip (u) 1968 is then computed from (1) assuming a circu- lar rupture area.] It appears that the entire •JACINTO^ 0 ZONE 75-km-long segment between about Coyote 1942 Mountain and Superstition Mountain has been ruptured by the 1954,1968, and 1942 earthquakes. If, as was done for the strip of fault from Riverside to Anza, the seismic slip is distributed evenlv over the 75 X 15 km2 southern zone, ana average slip would be 80 cm, quite comparable to the 135 cm NW Coyote SE obtained for the northern strip. This aver- Son Ama Mtn Superstition Cajón Pass Riverside Mtn age for the southern zone is also shown in Jacint• o _LJ t Figure 4. GAP GAP If we ignore the two 40- and 45-km-long E 1954 o 30 • I • slip gaps and sum the moments of all nine 1968 major events and distribute the slip uni- 2 a. 60 1937 formly over a 240 x 15 km area, the av- 1969 55 1942-1968 erage slip would be 65 cm or 0.80 cm/yr I 90 • during the interval 1890-1973. Using a u 1942 moment-magnitude relation, and mag- I i» 120 nitudes for the interval 1912-1963, Brune 5 1890- 1923 V) (1968) obtained 1.5 cm/yr by a similar cal- culation. Figure 4. Seismic slip along the San Jacinto fault, 1890-1973, plotted as a function of distance along the fault southeast from Cajon Pass. Upper half of figure, at the same scale, shows the fault and These short-term, time-averaged seismic the epicenters of the largest earthquakes. slip rates for the San Jacinto fault may not be simply related to the long-term average slip for the zone. Sharp (1967) observed 5.1 km of offset in Pleistocene gravels north of pond to the gap in seismicity shown in Fig- the northern one crosses a densely popu- Anza; from them, he estimated an average ure 5 and partly to accord with the epicen- lated urban area, and in neither segment is displacement rate of about 0.25 cm/yr over ters of the four earthquakes and some in- there positive evidence for fault creep. the past ~ 2 m.y. Clark and others (1972) spired guesswork as to their rupture lengths However, there is a suggestion of progres- examined offsets in datable sediments of (see Table 1). These sources were grouped sive offset on a freeway interchange in San Holocene Lake Cahuilla, which lie astride together because of our generally poorer Bernardino (C. R. Allen, 1972, personal the San Jacinto fault zone along the 1968 knowledge of the source parameters of the commun.), and Clark (1972) reported evi- Borrego Mountain rupture. Their data sug- early events: only the moments of the 1918 dence of possible pre-1968 creep on the gest recurrence intervals at the same site of and 1923 events have been instrumentally southern part of the Borrego Mountain ~ 200 yr for 1968-size earthquakes in this determined; since intensity reports do sug- break. Along the entire San Jacinto fault region and an average slip rate for the past gest the 1890 and 1899 events are both zone, aseismic slip has been definitely several thousand years which is similar to comparable to the 1918 earthquake, identi- confirmed at only two localities. Clark Sharp's (1967) estimate. Ambraseys (1971) cal moments have been assigned to these (1972) observed creep at the southern end and Wilson (1972) examined the long-term three events (Hanks and others, 1975). of the line of surface faulting associated (~ 2,000 yr) historic seismic records of the Inferred slip gaps to the north and south with the 1968 Borrego Mountain earth- Middle East and China, respectively, and of the 75-km-long uniform slip zone are quake, with much of the motion attributa- suggest that in these regions short periods shown in Figure 4. Although we do not feel ble to after-slip following the ML 6.4 main- of intense activity are interspersed by long it is likely, we cannot wholly exclude the shock (Burford, 1972). Several millimeters intervals of relative quiescence. An analo- possibility that portions of these gaps may of right-lateral slip, apparently unrelated to gous pattern may exist for the San Jacinto have been filled by displacements which oc- earthquake activity, has also been reported fault zone. curred at the times of the four earliest (Hileman and others, 1971) near the ex- It is also worth noting that there has been earthquakes. However, in order to treme southeastern end of the San Jacinto substantial seismic slip and surface faulting significantly decrease these gaps, the epicen- fault zone at Dixieland (about 20 km since 1900 on the Imperial and related ters of the 1890, 1899, and 1918 earth- southeast of the 1942 epicenter shown in faults which extend southeast from near the quakes would have to be in error by at least Fig. 1). United States—Mexico border to the head of 40 km and (or) their rupture lengths would Since the source parameters of the the Gulf of California. Using moment esti- each have to be 50 km or greater. Finally, 1937-1969 earthquakes in the southern mates from Hanks and others (1975) of the we note that historical evidence accumu- San Jacinto fault zone are well determined, five largest earthquakes in this 200 X 15 lated by Townley and Allen (1939) shows their slips have been plotted individually in km2 area, we obtain an average rate of 2.0 that no 1918-size earthquakes had been re- Figure 4. Moments and source dimensions cm/yr over the interval 1903-1973. The ported for the San Jacinto fault zone prior (Table 1) have been obtained from body- similarity of this rate with those found for to 1890. wave spectrum measurements (1942 and the San Jacinto fault zone, as well as the Again, although available evidence sug- 1954 from Thatcher and Hanks, 1973; similar trends of the two fault systems, sug- gests it is unlikely, we cannot entirely ex- 1968 from Hanks and Wyss, 1972). For the gests that in this region the major part of clude the possibility that all or part of the 1937 earthquake, moment is determined the relative motion between the Pacific and deficiency in seismic slip has been taken up from surface-wave amplitudes, and the the North American plates is currently by aseismic fault creep. Although few cul- theoretical Mi-source parameter relation of being accommodated along these fault tural features cross the southern slip gap, Thatcher and Hanks (1973) is then used to lines.

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MINOR SEISMICITY could be demonstrated that minor seismic- information is currently available on this ity preceded the occurrence of nine moder- section of the San Jacinto fault zone, several Figure 5 shows the ML 5= 4 seismicity in ate and large California earthquakes. new experiments are planned. No creep- and near the San Jacinto fault zone for the meters or alignment arrays are sited here, interval 1932-1972 (Hileman and others, SEISMIC SLIP GAP: and, except for Highway 71 (Fig. 6), no cul- 1973). In Figure 5A, all events within the ANZA TO COYOTE MOUNTAIN tural features cross the fault. No offsets or dashed rectangular box are included, while cracks have been observed in this asphalt Figure 5B excludes shocks obviously off the This region is shown in Figure 6, which road where it crosses the fault trace. The San Jacinto trend and aftershocks occurring includes mapped faults (Geol. Map of closest permanent seismographic station, at within one year of the 6 earthquakes. California, Santa Ana sheet, 1965), cultural Palomar Mountain, is about 40 km away. The segment of the San Jacinto fault zone features, and geophysical field experiments One strong-motion seismograph installa- between about Riverside and Anza is mar- either planned or being currently carried tion is located at Anza, although the Earth- kedly aseismic at the ML > 4 level in this out in the area. The city of Palm Springs lies quake Engineering Research Laboratory of time interval, with only two events shown off the map about 25 km north of Anza. the California Institute of Technology is on Figure 5 during this 41-yr time interval. The San Jacinto fault zone is quite com- currently installing 40 stations with WWVB No events larger than magnitude 6 have oc- plex in this region, being composed of three timing along the San Jacinto fault zone, and curred in this region since 1932, but this distinct strands, all of which show geomor- those located in the Anza-Coyote Moun- segment was the site of four such events be- phic evidence of recent displacement (R. V. tain area are shown in Figure 6. tween 1890 and 1923. The occurrence of Sharp, 1973, personal commun.). If a mod- The California Division of Mines and six Ml S 6 earthquakes along the southern erate earthquake is anticipated on this sec- Geology has four geodimeter lines which San Jacinto fault zone from Anza to the In- tion of the fault, all three strands appear cross the fault (Fig. 6), but they were first ternational Border (1937-1969) has been equally likely sites on the basis of presently surveyed in 1970; to date, only the north- associated with essentially continuous ac- available evidence. ernmost line has been remeasured. It tivity at the Ml 2= 4 level since 1932. The minor seismicity shows a broad and showed no observationally significant Both of the seismic slip gaps defined ear- diffuse pattern (Fig. 4) which is not change in length during a 2-yr period. lier have been quite active at the ML = 4 —> significantly sharpened by more precise epi- However, generally, measurements over at 5 level (Fig. 5B), demonstrating that these center locations (Arabasz and others, least a 5-yr interval are required to unam- segments of the fault are currently being 1970). Both the seismicity and the distribu- biguously resolve changes in lengths of subjected to shear stresses large enough to tion of active faults thus suggest that a geodimeter lines (Savage and Prescott, produce at least localized failure. Wesson complex zone ~ 20 km wide is being 1973). Also, a laser strainmeter has been and Ellsworth (1973) have argued that such sheared right laterally, with displacements operated since early 1971 by the University a situation is a logical prelude to the oc- not confined to any single, simple through- of California at San Diego at Pinon Flat, currence of larger earthquakes; they found going fault line. about 12 km east of the fault (Fig. 5). Cur- that where sufficient data were available, it Although very little detailed geophysical rently, there are three horizontal compo-

SAN JACINTO FAULT ZONE ML 1932-1972 ML>4.0 (DELETED SAMPLE) X 4.0 - 4.9 x 5.0 - 5.9 > 6.0

Figure 5. Epicenters of earthquakes with ML 3= 4 located in or near the San Jacinto fault zone for the interval 1932-1972 (by Calif. Inst. Technology network). A. (Left) All epicenters within the dashed rectangular box are included. B. (Right) Epicenters obviously not associated with the San Jacinto have been excluded, as are aftershocks of the 1937,1942, 1968, and 1969 events. Abbreviations: C.P. = Cajon Pass, RVR = Riverside, S.J. = San Jacinto, A = Anza, C.M. = Coyote Mountain, S.M. = Superstition Mountain.

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nents, each 800 m long, operating contin- SEISMIC SLIP GAP: fault. Currently, two seismograph stations uously, and secular strains are less than CAJON PASS TO RIVERSIDE operate in this region. However, by early 10"V (Berger and Wyatt, 1973). 1974 a dense array of 32 stations, run We believe that valuable information Figure 7 shows a detailed map of the San jointly by the California Institute of Tech- could be gained by several new experiments Jacinto fault zone between Cajon Pass and nology and the U.S. Geological Survey, and some modifications to existing studies. Riverside. In this region, where the will begin operation in the northern half of A 15- to 20-station microearthquake array northwest-striking San Andreas zone and the map area of Figure 7. would, within 1 to 2 yr, provide enough ac- east-trending Transverse Ranges structures Planned locations for EERL accelero- curate hypocenter locations and fault-plane meet, there exists a complex pattern of re- graph stations equipped with WWVB time solutions to significantly improve our un- cent faults of diverse type and orientation. code receivers are shown in Figure 7. Six ex- derstanding of the contemporary tectonics Here, deformation may be broadly distrib- isting stations are shown as well. of this segment of the fault zone. Alignment uted across many faults comprising a zone Geodimeter lines measured by the DMG arrays and creepmeters could be im- roughly bounded by the San Andreas and which lie between Cajon Pass on the north mediately located on the basis of mapped San Jacinto trends. This segment of the San and Riverside on the south are shown in geologic features, and new microearth- Jacinto fault does, however, show abun- Figure 7. For those lines with a sufficient quake data would be useful in more criti- dant evidence of recent displacement number of measurements to define a trend, cally siting these installations. A new (Sharp, 1972) and is the dominant Savage and others (1973) have summarized geodimeter network has already been de- northwest-trending fault of the region. results obtained to date. We include lines signed for this region by the U.S. Geological Seismicity once more shows a rather dif- 67, 70, 71, 74, 75, and 76, all resurveyed Survey and will be surveyed in the near fu- fuse pattern, most of it in a band 10 to 15 intermittently over a 14-yr period. Of these ture (J. C. Savage, 1973, personal com- km wide located to the west of and roughly 7 lines, only line 67 actually crosses the San mun.). bounded on the east by the San Jacinto Jacinto fault zone, and it spans the San An-

roearthquake arrays on these portions of the fault would provide valuable new information in this respect.

ACKNOWLEDGMENTS

The staff of the Seismographic Station of the University of California at Berkeley, particularly T. V. McEVilly, are thanked for their assistance in providing access to the seismographic records from the Berkeley station. R. V. Sharp, D. P. Hill, and J. C. Savage provided helpful discussion. M. M. Clark's careful review considerably im- proved the manuscript.

REFERENCES CITED

Allen, C. R., P. St.-Amand, P., Richter, C. F., and Nordquist, J. M., 1965, Relationship between seismicity and geologic structure in the southern California region: Seismol. Soc. America Bull., v. 55, p. 753-797. Ambraseys, N., 1971, Value of historical records of earthquakes: Nature, v. 232, p. 375-379. Figure 7. Detailed map of the seismic slip gap between Cajon Pass and Riverside. Arabasz, W. J., Brune, J. N., and Engen, G. R., 1970, Locations of small earthquakes near the trifurcation of the San Jacinto fault dreas as well. Savage and others suggest its aftershocks ranged in depth from 5 to 12 southeast of Anza, California: Seismol. Soc. that the observations may be consistent km (Hanks and others, 1971). America Bull., v. 60, p. 617-627. with right-lateral motion of about 7 mm/yr Suggestions for new research in this re- Berger, J., and Wyatt, F., 1973, Measurements of across the San Andreas—San Jacinto system gion parallel those made earlier for the slip strain accumulation between the San An- between Palmdale and Riverside. (Palmdale gap near Anza. New information to be pro- dreas and San Jacinto faults, in Kovach, R. is 70 km northwest of Cajon Pass near the L., and Nur, A., eds., Proceedings of Con- vided by the CIT-USGS microearthquake ference on Tectonic Problems of the San San Andreas fault.) However, these line- array around San Bernardino should prove Andreas Fault Zone: Stanford Univ., School length data do exhibit considerable scatter, useful in planning these and other experi- of Earth Sci., p. 80-85. and consistent trends do not exist for all ments in this slip gap. Brune, J. N., 1968, Seismic moment, seismicity lines. and rate of slip along major fault zones: Two early earthquakes (July 22, 1899, CONCLUSIONS Jour. Geophys. Research, v. 73, p. and 1907), probably in the M = 5 —» 6 777-784. range, may have occurred on this segment Determination of the seismic slip history Burford, R. O., 1972, Continued slip on the of the San Jacinto fault zone (Fig. 1). Hanks on the San Jacinto fault during the past 85 Coyote Creek fault after the Borrego and others (1975) estimated the seismic yr and examination of patterns in the recent Mountain earthquake: U.S. Geol. Survey Prof. Paper 787, p. 105-111. moments of these two shocks and found M 0 minor seismicity have delineated two ~ 40- Clark, M. M., 1972, Surface rupture along the < 1025 dyne-cm for the 1907 event but M„ km-long segments of the fault which 25 Coyote Creek fault: U.S. Geol. Survey Prof. ~ 4 x 10 dyne-cm for the 1899 earth- have not slipped significantly since at least Paper 787, p. 5-86. quake. The location of this 1899 event is 1890. During this time, most, if not all, of Clark, M. M., Grantz, A., and Rubin, M., 1972, not well determined, but it could well be on the adjacent portions of the fault have sus- Holocene activity of the Coyote Creek fault the San Jacinto fault northwest of San Ber- tained displacements of 30 cm or more dur- as recorded in sediments of Lake Cahuilla: nardino (Townley and Allen, 1939). Locat- ing moderate earthquakes. U.S. Geol. Survey Prof. Paper 787, p. ing this event on the fault zone and dis- Potential slip may thus be accumulating 112-130. tributing its ~ 20 cm of slip uniformly over in these two seismic slip gaps, both of Davies, G. F., and Brune, J. N., 1971, Regional 2 and global slip rates from seismicity: Na- a 40 X 15 km area would substantially de- which deserve special attention in experi- crease the slip deficit of the region between ture Phys. Sci., v. 229, p. 101. ments designed to detect premonitory ef- Dickinson, W. R., and Grantz, A., eds., 1968, Cajon Pass and Riverside. Given the epicen- fects suspected to occur prior to moderate Proceedings of Conference on Geologic tral uncertainty of the early earthquake, we earthquakes. Small-scale trilateration Problems of the San Andreas Fault System, must accept the possibility of this interpre- figures, creepmeter installations, and Vol. 11: Stanford Univ. Pubs. Geol. Sci., tation. Accordingly, this slip gap must be alignment arrays would be particularly use- 374 p. considered somewhat less likely to sustain a ful in detecting any possible strain build-up Hamilton, R. M., 1972, Aftershocks of the Bor- moderate earthquake than the fault seg- and in discriminating between accumula- rego Mountain earthquake from April 12 to ment between Anza and Coyote Mountain. tion and aseismic slip. June 12, 1968: U.S. Geol. Survey Prof. In addition, the 1970 Lytle Creek earth- However, planning of experiments here Paper 787, p. 31-54. Hanks, T. C., Brune, J. N., Thatcher, W., and quake, M = 5.4, was located 25 km south- L is made difficult by the complexity of recent Tucker, B. E., 1971, Report on the Lytle east of Cajon Pass near the San Jacinto faulting and diffuse character of minor Creek aftershock sequence: Seismol. Soc. fault. This event had a source dimension of 5 seismicity. Further detailed mapping and America, Abs. with Programs (Cordilleran 24 km or less, a moment of 10 dyne-cm, and installation of two densely spaced mic- Sect. Mtg.), v. 3, p. 131.'

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Hanks, T. C., Thatcher, W., and Hileman, J. A., search, v. 78, p. 832-845. parameters of southern California earth- 1975, Seismic moments of the larger earth- Savage, J. C., Prescott, W. H., and Kinoshita, W., quakes: Jour. Geophys. Research, v. 78, p. quakes of the Southern California region, 1973, Geodimeter measurements along the 8547-8576. 1890-1973: Geol. Soc. America Bull., v. 86, San Andreas fault, in Kovach, R. L., and Townley, S. D., and Allen, M. W., 1939, De- p. 1131-1139. Nur, A., eds., Proceedings of Conference on scriptive catalog of earthquakes of the Hileman, J. A., Allen, C. R., Brune, J. N., and Tectonic Problems of the San Andreas Fault Pacific coast of the United States, 1769 to Wyss, M., 1971, Measurements of active Zone: Stanford Univ. School Earth Sei., p. 1928: Seismol. Soc. America Bull., v. 29, p. slip on faults in the Imperial Valley region, 44-53. 1-297. California: Geol. Soc. America Abs. with Sharp, R. V., 1967, San Jacinto fault zone in the Wesson, R. L., and Ellsworth, W. L., 1973, Seis- Programs, v. 3, no. 2, p. 136. Peninsular Ranges of southern California: micity preceding moderate earthquakes in Hileman, J. A., Allen, C. R., and Nordquist, J. Geol. Soc. America Bull., v. 78, p. 705-730. California: Jour. Geophys. Research, v. 78, M., 1973, Seismicity of the southern 1972, Map showing recently active breaks p. 8527-8546. California region 1 January 1932 to 31 De- along the San Jacinto fault zone between Wilson, J. T., 1972, Seismicity of China [abs.]: cember 1972: Pasadena, Calif., Calif. Inst. the San Bernardino area and Borrego Val- EOS (Am. Geophys. Union Trans.), v. 53, Technology Seismol. Lab. ley, California: U.S. Geol. Survey Misc. p. 519-520. Larson, R. L., Menard, H. W., and Smith, S. M., Geol. Inv. Map 1-675. 1968, The Gulf of California: The result of Thatcher, W., and Hamilton, R. M., 1973, After- ocean-floor spreading and transform fault- shocks and source characteristics of the MANUSCRIPT RECEIVED BY THE SOCIETY JULY 4, ing: Science, v. 161, p. 781-784. 1969 Coyote Mountain earthquake, San 1974 Savage, J. C., and Burford, R. O., 1973, Geodetic Jacinto fault zone, California: Seismol. Soc. REVISED MANUSCRIPT RECEIVED FEBRUARY 18, determination of relative plate motion in America Bull., v. 63, p. 647-661. 1975 central California: jour. Geophys. Re- Thatcher, W., and Hanks, T. C., 1973, Source MANUSCRIPT ACCEPTED MARCH 7, 1975

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