HISTORIC EARTHQUAKES (1918 TO 1923) AND AN ASSESSMENT OF SOURCE PARAMETERS ALONG THE SAN JACINTO FAULT SYSTEM BY DIANE I. DOSER

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1 Bulletin of the Seismological Society of America, Vol. 82, No. 4, pp , August 1992 HISTORIC EARTHQUAKES (1918 TO 1923) AND AN ASSESSMENT OF SOURCE PARAMETERS ALONG THE SAN JACINTO FAULT SYSTEM BY DIANE I. DOSER ABSTRACT Body waveform modeling is used to determine the source parameters of two earthquakes that occurred along the northern San Jacinto fault system in 1918 and The 1918 earthquake parameters are consistent with a 20- to 25-km rupture along the Claremont fault at a depth of km. Limited data for the 1923 event are more consistent with rupture on the San Jacinto fault than rupture along the San Andreas fault or a buried cross fault, although the greatest damage associated with the earthquake occurred closer to the San Andreas fault. Using the results of this study and previous studies of historic earthquakes along the southern San Jacinto fault, the relationships between rupture length and magnitude, focal depth and heat flow, and other parameters are examined in an effort to estimate the source properties of future events that are expected along currently quiescent portions of the fault zone. One important observation is that rupture lengths along the San Jacinto fault are shorter for an equivalent moment magnitude than those along the Imperial Valley-Mexicali fault system, The higher heat flow of the Imperial Valley region may be responsible for this difference. INTRODUCTION The San Jacinto fault system (Fig. 1) has been the most active fault zone in southern California in historic time. Since 1890, six to ten earthquakes of magnitude > 6.0 have occurred along the fault zone. The seismically active northern and southern portions of the fault zone are separated by a 20-km-long segment that is quiescent at depths < 12 km, termed the Anza gap (Sanders and Kanamori, 1984). (Unless otherwise noted, in this paper the term "Anza gap" refers to the gap defined by Sanders and Kanamori, rather than as defined by Thatcher et al. (1975).) The northern portion of the San Jacinto fault zone experienced two to four historical events between 1899 and Since 1930, magnitude > 6.0 earthquakes have been restricted to the southern San Jacinto fault. Waveform analysis of the historic events (1937 to 1954) along the southern segment has been discussed in Doser (1990). This study presents the results of waveform modeling of two earthquakes associated with the northern fault zone, the 21 April 1918 (San Jacinto) and 23 July 1923 (San Bernardino) earthquakes. Surface faulting was not reported following either event, making determination of the causative faults and fault rupture lengths difficult. The lack of reported surface faulting and the lower heat flow of the northern fault zone suggest that both earthquakes may have occurred at deeper depths (> 10 km) than events along the southern San Jacinto fault. Following a discussion of the northern San Jacinto events, an assessment of the source parameters of earthquakes is conducted to better quantify the faulting processes along the entire San Jacinto fault zone. Evaluation of the relationships of source parameters to one another and to factors such as heat 1786

2 HISTORIC EARTHQUAKES ALONG SAN JACINTO FAULT SYSTEM BF 34N, l16w 1937 O 17.5W \ i so km I B FIG. 1. Map of the San Jacinto fault system showing the location of large historic earthquakes from 1918 to 1987 (stars). Faults modified from Jennings (1975). BF = Banning fault, BRF = Buck Ridge fault, CCF = Coyote Creek fault, CF = Clark fault, EF = Elsinore fault, IF = Imperial fault, SAF = San Andreas fault, SHF = Superstition Hills fault, SJF = San Jacinto fault, SMF = Superstition Mountain fault. A box surrounds the Anza gap. The letters A, A', and B refer to seismicity versus depth cross sections shown in Figure 6. flow is conducted in order to estimate the source properties of future events that are expected along currently quiescent portions of the fault zone. PREVIOUS STUDIES Sanders and Kanamori (1984) have published an excellent summary of the history of large earthquakes along the northern and southern portions of the San Jacinto fault. The first well-documented earthquake that may be associated with the northern San Jacinto fault zone occurred on 22 July 1899, and has been assigned an intensity magnitude of 6.5 (Toppozada et al., 1981). The event was felt strongly in the Cajon Pass area near the intersection of the San Jacinto and San Andreas faults, but it is uncertain whether it occurred on the San Jacinto fault, San Andreas fault, or other nearby faults (Sanders and Kanamori, 1984). Two other earthquakes on 25 December 1899 and 21 April 1918 had greatest intensities near San Jacinto and Hemet, indicating locations on the San Jacinto fault system. Rasmussen (1981) has suggested that the December 1899 (M ~ 7.1) event produced significant surface rupture along the northeastward-dipping Casa Loma fault (Fig. 2). He believes the 1918 (M ~ 6.8) event may have ruptured

3 1788 D. I. DOSER N, 116.9W % \ hemet 10 km 33.7N -t-- FIG. 2. Map of the San Jacinto valley region. HSF = Hot Springs fault, sj = town of San Jacinto. the southwestward-dipping Claremont fault, based on accounts of buckled concrete on a road north of San Jacinto. Sanders and Kanamori (1984), however, do not rule out the possibility that the events could have occurred on the Hot Springs fault east of San Jacinto or on the San Jacinto fault south of Hemet. The intensity for both earthquakes was less at Anza than at San Jacinto, so Sanders and Kanamori (1984) believe the events did not rupture into the Anza gap. The location of the 1923 (M = 6 i) 4 event is less certain. Sanders and Kanamori (1984) have studied the S-P times of seismograms of the mainshock and aftershocks recorded at Pasadena and believe the earthquake occurred either on the San Jacinto fault near Loma Linda or on the San Andreas fault northeast of the San Bernardino Valley (Fig. 3). Laughlin et al. (1923) found no

4 HISTORIC EARTHQUAKES ALONG SAN JACINTO FAULT SYSTEM 1789 evidence for surface rupture along the San Andreas fault, but the greatest damage from the earthquake occurred 2 to 5 km southwest near Highlands. The variation of aftershock S-P times suggests the fault rupture could have been as great as 20 km (Sanders and Kanamori, 1984). An increase in local felt seismicity was observed for 3 months preceding the earthquake. Hanks et al. (1975) determined the seismic moments of the 1918 and 1923 earthquakes using the AR method on seismograms recorded at Berkeley and the areal distribution of intensity VI isoseismals. The 1918 event was assigned a moment of dyne-cm and the 1923 event a moment of dyne-cm. DATA ANALYSIS Because the 1899 earthquakes occurred before most seismograph stations in the world were operating, the present study is restricted to analyzing the arrival-time, first-motion, and waveform information for the 1918 and 1923 r I, 10km~~ ~t~11 _~_ ~t~r~// FIG. 3. Map of San Bernardino valley region. CHC = Crafton Hills complex, hl = Highlands, 11 = Loma Linda, rl = Redlands, sb = San Bernardino.

5 1790 D.I. DOSER earthquakes. Relocations of these earthquakes were attempted using the bootstrap method of Petroy and Wiens (1989), however error ellipses for both relocations covered much of the northern San Jacinto fault. Station coverage for both earthquakes was too sparse for the determination of focal mechanisms from first-motion data, but available first-motion information was used to help select reasonable starting models for the waveform modeling studies. Seismograph stations and details of the instruments used in the waveform modeling studies are given in Table 1. Waveform modeling was conducted using the multi-step inversion technique of Baker and Doser (1988) to determine focal mechanism, source-time function shape, moment, and focal depth. A priori estimates of source parameters based on geological and first-motion data were used to speed the inversion process. Details of the methods used in generating synthetic seismograms are described in Baker and Doser (1988). In the technique of Baker and Doser (1988), the mean square error between the observed seismograms and synthetics is minimized using true amplitudes. Matching true amplitudes requires meaningful estimates of modeling uncertainties and signal-to-noise ratios. The maximum value of either the misfit of the best solution or of the initial covariance is used in estimating final covariances. This method gives a pessimistic estimate of uncertainty that is less likely to be biased by overly optimistic estimates of initial data or model quality, or by restrictive a priori information. Simple trapezoidal-shaped source-time functions were used as starting models. Unilateral and bilateral rupture lengths were estimated from the rupture duration using the method of Kanamori and Stewart (1976). The average fault displacement was estimated by dividing moment by fault area and shear modulus ( dyne/cm2). The uncertainties in the length, depth, and moment are reflected in the uncertainty bounds placed on the displacement. San Jacinto, 1918 RESULTS Since geologic and intensity information suggest that the 1918 earthquake occurred near San Jacinto on the Claremont or Casa Loma faults, the average strike, dip, and rake of these faults were used in forward modeling trials to determine the best starting model for the inversion. The best initial fit to the waveform data was obtained for a range of 120 to 160 in strike, 85 to 90 in TABLE 1 STATIONS USED IN WAVEFORM MODELING STUDIES Station Station Azimuth Distance Seismometer Abbreviation Name ( ) ( ) Period Magnification ebr Ebro, Spain esk Eskdalemuir, Scotland sas Saskatoon, Saskatchewan upp Uppsala, Sweden vic* Victoria, B.C vie Vienna, Austria zur Zurich, Switzerland *Only first motion used.

6 HISTORIC EARTHQUAKES ALONG SAN JACINTO FAULT SYSTEM 1791 dip, -160 to -180 in rake, and 5 and 12 km in focal depth. These dip and rake values suggest the earthquake occurred along the Claremont fault. The inversion starting model and results are shown in Figure 4. Synthetic seismograms obtained from the inversion results are shown in Figure 5. The starting model for the inversion was: strike 140 _+ 40, dip 85 _+30, rake , focal depth 8_+ 10 km, moment 9_ dyne-cm. The result was: strike , dip 87 _+ 6, rake -176 _+ 9, focal depth 7 _+ 5 km, moment dyne-cm. These final uncertainties allow for rupture along the Claremont or Casa Loma faults, although the geologic data (Rasmussen, 1981) support rupture along the Claremont fault. The moment is very comparable to that of Hanks et al. (1975) and gives a moment magnitude (Hanks and Kanamori, 1979) of 6.8. The strike of overlaps the observed strike of the Claremont fault (130 to 140 ) near San Jacinto (Fig. 2). Initial forward modeling indicated that the SH waveform at upp (Uppsala, Sweden) (Fig. 5) becomes nodal at 153. The.focal mech. uncertaint ies San Jac into,,, I. ~ source uncertaint ies FIG. 4. Focal mechanism and source-time function uncertainties for the 1918 San Jacinto ea~hquake. Moment rate has units of d~e-cm/sec 1025.

7 1792 D.I. DOSER vie sm v~e pr eb r eh ~ /"x..-. San /acinto 4/21/1B le.2 Cm 1 2B =ec FIG. 5. Observed (top) and synthetic (bottom) seismogram pairs for the 1918 earthquake. Station information is given in Table 1. pr = radial P wave, sh = transverse S wave. An asterisk indicates the waveform amplitudes have been reduced by a factor of 5 from scale shown at the bottom. fault strike obtained from the inversion would also match the strike of the southern end of the Hot Springs fault (Fig. 2), but the intensity reports (Sanders and Kanamori, 1984) would not be compatible with rupture so far to the south, and Sharp (1967) and Hill (1984) have found no evidence for Holocene movement along the Hot Springs fault. The rupture time for the earthquake gives a unilateral rupture length (Kanamori and Stewart, 1976) of km. Assuming the rupture occurred to a depth of km, an km rupture length would give an average fault slip of m. Since little or no surface faulting was associated with the earthquake, this slip value seems too great. If bilateral rupture is assumed, the rupture length would be km. This would give an average slip of m. The lower uncertainty bound of this slip estimate is more consistent with the observation that little to no surface faulting was associated with the earthquake. The Bouguer gravity anomaly map of the region (Oliver et al., 1980) indicates that the major gravity low of -100 mgals associated with the San Jacinto Valley is 20 to 30 km long, suggesting that the 1918 event could have ruptured the entire fault segment along the eastern side of the gravity low. The intersection of the Hot Springs and Claremont faults occurs near the middle of the

8 HISTORIC EARTHQUAKES ALONG SAN JACINTO FAULT SYSTEM 1793 gravity low and may have served as the nucleation point for the 1918 event. As suggested previously by Sanders and Kanamori (1984) and Rasmussen (1981), the 1899 event probably represented rupture along the western edge of the San Jacinto Valley on the Casa Loma fault. San Bernardino, 1923 Because of its small magnitude, only one station, Saskatoon, Saskatchewan, was available for the analysis of this earthquake. Waveform inversion was not attempted, although uncertainty estimates on source parameters were made in a manner similar to those used in the inversion routine. Forward modeling was used to compare the observed waveforms to synthetic seismograms generated for a right-lateral strike-slip fault striking 280 to 340 (the range of strikes for active right-lateral faults within the region) with focal depths of 2 to 20 kin. Synthetic seismograms were also generated for left-lateral strike-slip faults with strikes of 230 to 270, since Nicholson et al. (1986) have indicated that left-lateral faults striking northeast to east-northeast within this region are also seismically active. One and two source models were tried for each range of strikes and focal depths. The best match to the waveforms was obtained for strike = , dip = , rake = , and focal depth of km (Fig. 7), values similar to the San Jacinto fault. A one-source model appears to fit the SH waveform well, but a two-source model gives a better match to the PR waveform. The source models have moments of 2.1 and dyne-cm (moment magnitudes of 6.2 to 6.3). Laughlin et al. (1923) noted that many people felt two shocks during the earthquake, however the observers may have been feeling the P and S waves from a single shock. Waveforms generated with the same source-time functions, but a strike of 300, the strike of the San Andreas fault near Highlands, had too small of PR amplitude and too large of SH amplitude compared to the observed data (Fig. 7). The best-fit mechanism for cross faults similar to those observed by Nicholson et al. (1986) had a strike of 235, dip of 70, and rake of -20. This mechanism does not match the observed P-wave polarity at either Saskatoon or Victoria (Fig. 7). The unilateral rupture length for this event is 15 ± 4 kin. This is comparable to the 20-kin length based on S-P times of aftershocks (Sanders and Kanamori, 1984). It is also possible that some of the aftershocks occurred on cross faults. Rupture for 15 km along the fault starting at a point near San Bernardino, where there appears to be a bend in the fault (Fig. 3), would place the end of the rupture at the point where the Crafton Hills complex intersects the fault. An epicentral location near San Bernardino would also place rupture nucleation in 1923 immediately to the south of a zone of shallow seismicity along the fault (Fig. 6) in a region where the upper portion of the fault may currently be locked (Sanders, 1990). DISCUSSION Previous studies (Doser and Kanamori, 1986; Sanders, 1990) have found that background seismicity along the San Jacinto fault system deepens as heat flow decreases (Fig. 6). Sanders further suggests that the band of seismicity with an upper depth limit of 11 km located north of the 1937 rupture is due to seismic

9 1794 D.I. DOSER E O' -10" A A' u, " ' l 1899."~ =,.L, J AG n [~ = 1890?' 'I" I Dec ' J~'~ogl"-e--~, J~a,L J_ g I I p r, e- ~ -20 [~:P~ ~ =:~'~ "= ~ ~,. ~ 120 ~E 1oo Distance, Magnitudes for A-A' = [] >4.0 km along San Jacinto fault -1 0 ~ m ~ -2 0 A' 1987~ B Distance, km E E o ~ FIG. 6. Seismicity versus depth along the San Jacinto fault system. Points A, A', and B are shown on Figure 1. Heat flow variation along the fault from Lachenbruch et al. (1985) is shown at bottom of figure. Earthquakes from A to A' taken from Sanders (1990). These earthquakes are A quality events from 1980 to 1986 with M > 2. Earthquakes from A' to B taken from Doser and Kanamori (1986). These are relocated earthquakes occurring between 1977 and Rupture lengths and focal depths for events from 1918 to 1987 (solid dots) taken from Table 2. Location and rupture lengths for pre-1918 events (open dots) are discussed in the text. Focal depths for pre-1918 events are arbitrarily set at 10 km. Vertical dashed lines define limit of the Anza gap (AG). -1- and aseismic slip at this level. Above 11 km he suggests that the fault is locked. A zone of shallow seismicity is seen near the location of the 1923 earthquake, and Sanders believes that this may reflect seismicity on cross faults near the San Jacinto fault or local interactions of the San Jacinto fault with these cross faults. The estimated focal depths for the 1918 and 1923 earthquakes place them at or just above the depth of background seismicity along the northern San Jacinto fault, in agreement with Sanders' suggestion that larger earthquakes will nucleate near the base of the locked zone and rupture toward the surface. South of the Anza gap the depths of large earthquakes generally decrease with increasing heat flow. The one exception is the 1937 event that occurred in a region of lower heat flow (~ 70 to 80 mw/m 2) at a depth of kin. The shallow depth may be a local effect related to strain adjustments at the edge of the Anza gap.

10 HISTORIC EARTHQUAKES ALONG SAN JACINTO FAULT SYSTEM 1795 San Jacinto I event pr 5an lqndreas cross?auit 2 events IO.2 cm 1 2~ sec FIG. 7. Observed (top) and synthetic (bottom) seismogram pairs for the 1923 event recorded at Saskatoon. Synthetic seismograms were generated for models that placed the earthquake on the San Andreas fault (left), a cross fault (middle), San Jacinto fault-1 event (top right), and San Jacinto fault-2 events (bottom right) as discussed in text. The solid focal mechanism is for the San Jacinto fault, the dashed focal mechanism for the San Andreas fault, and the dotted mechanism for the cross fault. The open circle denotes the dilatational first motion observed at Victoria; the open square, the dilatational first motion at Saskatoon. The estimated rupture area for each large historic earthquake is also shown in Figure 6. If we assume that the 1923 earthquake ruptured km to the southeast from a point near San Bernardino, there would be a 15- to 20-km gap between the 1918 and 1923 ruptures and a 30-km gap between the 1923 rupture and the intersection of the San Jacinto and San Andreas faults that have not slipped since Thatcher et al. (1975) have suggested that the July 1899 event occurred on the northernmost San Jacinto fault near Cajon Pass. The estimated moment of the event is 4 x 1025 dyne-cm (Hanks et al., 1975), suggesting that the event is similar in size to the 1923 or 1954 events and might be expected to have a similar rupture length of 15 to 20 km. An event of this size on the northernmost San Jacinto fault would not completely fill the slip gap to the 1923 rupture, but it would completely rupture the northern portion of the fault that is not associated with shallow ( < 8 km depth) earthquakes (Fig. 6). It is important to note that intensity data cannot resolve the location of the July 1899 event

11 1796 D.I. DOSER (Sanders and Kanamori, 1984), and it is quite possible that the San Andreas fault or a cross fault ruptured, leaving a significant slip gap on the northernmost San Jacinto fault. Thatcher et al. (1975) place an earthquake in 1890 south of the 1923 event (Fig. 6), with rupture between Riverside and San Jacinto. The 1890 event has been assigned a moment of ~ dyne-cm (Hanks et al., 1975), comparable to the moment of the 1918 event. This would imply an event with a rupture length of at least 22 km, a length more than adequate to fill the slip gap between the 1923 and 1918 events (Fig. 6). Sanders and Kanamori (1984), however, assign an intensity magnitude of 6.3 for the 1890 event and feel that intensity reports place the earthquake southeast of the Anza gap in the vicinity of the 1937 and 1954 events. Another possibility is that the December 1899 event, believed to be larger in magnitude than the 1918 event (Sanders and Kanamori, 1984), may have ruptured further to the northeast than the 1918 event, filling most of the observed slip gap between the 1918 and 1923 events. Another slip gap occurs between the end of the 1918 rupture and the southern end of the Anza gap. This gap, about 40 km in length, is the "Anza Gap" of Thatcher et al. (1975). The slip gap may be 5 to 10 km shorter, depending on how far to the south the 1918 or December 1899 ruptures extended, but there is no evidence that either event ruptured through the Anza gap. South of the Anza gap the San Jacinto fault branches out into the Buck Ridge (north), Clark (middle), and Coyote Creek (south) segments (Fig. 1). The 1937 Buck Ridge earthquake occurred 10 km southeast of the Anza gap with a source mechanism suggesting rupture along the Clark fault (Doser, 1990). Aftershocks of the earthquake fall between the Buck Ridge and Clark faults (Sanders et al., 1986). The earthquake occurred at a shallow depth (~ 3 kin), rupturing a portion of the fault that is currently quiescent at depths < 5 km (Fig. 6). If the 1937 earthquake occurred along the Clark fault, a slip gap of 25 km exists along the Clark fault between the 1937 and 1954 earthquakes (Fig. 6). This is the region where Sanders and Kanamori (1984) suggest that the 1890 and 1892 earthquakes may have occurred, each with estimated magnitudes of ~ 6.0. Earthquakes of this magnitude would produce rupture lengths of 10 to 15 km, so the two events could have filled the slip gap on the Clark fault. The earthquakes also could have occurred on the Buck Ridge fault, rupturing the entire fault segment from the Anza gap southward, or along the northernmost 20 km of the Coyote Creek fault, a segment that has not ruptured in a large earthquake this century. The 1969 (M n = 5.8), Coyote Mountain earthquake appears to have ruptured a cross fault to the Coyote Creek fault (Petersen et al., 1991). The 1969 event occurred near the base of the seismogenic zone (12 1 km), and aftershock locations suggest that the rupture was confined to depths of 10 to 13 km (Thatcher and Hamilton, 1973). Therefore, the entire portion of the Coyote Creek fault from its northern end to the surface rupture associated with the 1968 Borrego Mountain event may represent a 30-km-long slip gap for shallow rupture. About 32 km of surface faulting occurred during the 1968 Borrego Mountain earthquake (Clark, 1972), with rupture extending to the mapped southern end of the Coyote Creek fault. The mainshock location for the 1942 Borrego Valley earthquake (Sanders et al., 1984) is just to the west of the southern end of the 1968 rupture (Fig. 1). Some aftershocks of the 1942 event align themselves in a

12 HISTORIC EARTHQUAKES ALONG SAN JACINTO FAULT SYSTEM 1797 direction parallel to but 5 km west of the Coyote Creek fault, while others align with the Split Mountain fault, a cross fault between the Elsinore and San Jacinto fault zones (Rockwell et al., 1990), suggesting that rupture during 1942 did not occur along the same portion of the Coyote Creek fault that ruptured in The focal mechanism of the event is consistent with rupture along either trend of aftershocks (Doser, 1990). South of the Coyote Creek fault, the San Jacinto system continues along the Superstition Hills and Superstition Mountain faults. Large earthquakes have not occurred in historic time along the 27-km-long Superstition Mountain fault, but 25 km of surface rupture occurred along the Superstition Hills fault in 1987 (Sharp et al., 1989). Tl~us, most of the southern end of the San Jacinto fault system has experienced rupture within the past 50 years. Seismicity south of the Superstition Hills and Superstition Mountain faults is diffuse, making it difficult to determine whether the San Jacinto system merges with the Imperial fault or continues southward into Mexico. Moment magnitude versus rupture length for large historical earthquakes of the San Jacinto fault system is shown in Figure 8a. The figure indicates that slip gaps of 10 to 15 km in length along the San Jacinto system should produce earthquakes of moment magnitude 6.0 to 6.5, while gaps of > 20 km should produce earthquakes exceeding moment magnitudes of 6.5. The solid line in Figure 8a is the magnitude-rupture length relationship given by Anderson and Luco (1983) assuming a rupture width of 10 km, a shear modulus of dyne-cm 2, and a ratio of average slip to rupture length of This relationship is only valid for rupture lengths greater than the width of the seismogenic zone, which should only exclude the 1937 and 1969 events (Table 2). The relationship fits data collected by Anderson and Bodin (1987) for the Imperial-Mexicali Valley fault system well, but it overestimates rupture lengths for the San Jacinto events. Heat flow versus rupture length for the San Jacinto system is shown in Figure 8b. Note that the longest ruptures tend to be in higher heat flow regions. This is consistent with the observation that rupture lengths of Imperial Valley- Mexicali events are also greater than most San Jacinto events. Earthquakes in higher heat flow regions nucleate at shallower depths, so ruptures must be greater to give the same fault area (and hence same moment). The shear modulus is also usually lower in regions of high heat flow, and again rupture length must be longer in high heat flow regions to give moments equivalent to events in lower heat flow areas. Anderson and Bodin (1987) have noted a lack of earthquakes with moment magnitudes between 5.9 and 6.3 and 6.7 and 7.0 in the Imperial Valley-Mexicali region, with a cluster of events with moment magnitudes of 6.3 to 6.7. This has led them to suggest that the magnitude distribution in the region does not fit a Gutenberg-Richter type model. Along the San Jacinto fault system, there seems to be a lack of events with moment magnitudes of 5.8 to 6.1 and 6.4 to 6.7, again suggesting deviation from a Gutenberg-Richter type magnitude distribution. Thatcher et al. (1975) estimated a slip rate of 0.8 cm/yr from the seismic moments of large earthquakes between 1890 and 1973 along the entire San Jacinto system ( km). Results from Table 2 suggest a slip rate of 0.6 cm//yr from 1918 to 1991 along a fault system with dimensions of km. Geologic slip rates of 0.8 to 1.7 cm//yr have been estimated for the Clark fault (Sharp, 1981; Merifield et al., 1987), a rate of 0.54 cm//yr for the Claremont

13 1798 D.I. DOSER 7.0 a) "ID. I e.- E I 6.0- E O E 5.0 i i a i l I b) rupture length, km O E E o N,,- t~ (1) i W ii Hi ' 'IF I I rupture length, km FIG. 8. (a) Moment magnitude versus rupture length for the San Jacinto fault system (data from Table 2). Solid line is the magnitude-rupture length relationship that fits data for the Imperial Valley-Mexicali region (Anderson and Bodin, 1987). (b) Heat flow versus rupture length. Open circles denote the 1987 Superstition Hills earthquake that may have a rupture length uncertainty of as much as 20 km.

14 HISTORIC EARTHQUAKES ALONG SAN JACINTO FAULT SYSTEM 1799 TABLE 2 SOURCE PARAMETERS OF LARGE EARTHQUAKES ALONG THE SAN JACINTO FAULT SYSTEM Focal Rupture Date Depth Moment Length Event (m/d/y) M L M (km) ( dyne-cm) (km) Reference San Jacinto 4/21/ ± 5 14 ± 5 22 ± 4 This study San Bernard±no 7/23/ i i i 4 This study Buck Ridge 3/25/ ± ± ± 2 Doser (1990) Borrego V. 10/21/ ± ± i 2 Doser (1990) SaladaWash 3/19/ _ ± 2 Doser(1990) Borrego Mtn. 4/9/ ± i 4 Petersen et al. (1991) Coyote Mtn. 4/28/ ± ± ± 2 Petersen etal. (1991) SuperstitionH. 11/24/ i i 20 Bent et al. (1989) fault near San Bernard±no (Wesnousky et al., 1987), and rates of 0.2 to 0.4 cm/yr for the Coyote Creek fault (Sharp, 1981). CONCLUSIONS Results of waveform modeling of the 1918 San Jacinto earthquake are consistent with rupture along the Claremont fault for a distance of 22 ± 4 km. This rupture length is comparable to the length of the i00 mgal gravity low associated with the San Jacinto valley. The intersection of the Claremont and Hot Springs faults may have served as the rupture nucleation point. The 1923 San Bernard±no event occurred on the San Jacinto fault and had a rupture length of 15 _+ 4 km. The event may have ruptured between cross structures seen in gravity and seismicity data just north of San Bernard±no to the Crafton Hills complex. Figure 6 indicates that slip gaps of I0 to 15 km and > 30 km occur in several locations along the fault. These lengths correspond to rupture lengths associated with moment magnitude 6 to 6.4 and > 6.7 earthquakes, respectively. Rupture lengths along the San Jacinto fault system are shorter for an equivalent moment magnitude than those along the Imperial Valley-Mexicali system. The higher heat flow of the Imperial Valley may be responsible for this observed difference. ACKNOWLEDGMENTS G. Kaip, J. Rodriguez, and C. Montana helped in digitizing maps and seismograms for this study. C. Sanders provided reprints of some obscure publications and gave a thorough review of the paper. An anonymous reviewer's comments are also appreciated. I would like to thank seismic observatories from around the world for sending seismograms and instrument response information. The contents of this publication were developed under grant G1954 from the USGS, however these contents do not necessarily represent the policy of this agency and should not be assumed to be an endorsement by the Federal government. REFERENCES Anderson, J. G. and P. Bodin (1987). Earthquake recurrence models and historical seismicity in the Mexicali-Imperial Valley, Bull. Seism. Soc. Am. '/7, Anderson, J. G. and J. E. Luco (1983). Consequences of slip rate constraints on earthquake occurrence relations, Bull. Seism. Soc. Am. 73, Baker, M. R. and D. 1. Doser (1988). Joint inversion of regional and teleseismic earthquake waveforms, J. Geophys. Res. 93,

15 1800 D.I. DOSER Bent, A. L., D. V. Helmberger, R. J. Stead, and P. Ho-Liu (1989). Waveform modeling of the November 1987 Superstition Hills earthquakes, Bull. Seism. Soc. Am. 79, Clark, M. M. (1972). Surface rupture along the Coyote Creek fault, U.S. Geol. Surv. Profess. Paper 787, Doser, D. I. (1990). Source characteristics of earthquakes along the southern San Jacinto and Imperial fault zones (1937 to 1954), Bull. Seism. Soc. Am. 80, Doser, D. I. and H. Kanamori (1986). Depth of seismicity in the Imperial Valley region ( ) and its relationship to heat flow, crustal structure, and the October 15, 1979, earthquake, J. Geophys. Res. 91, Hanks, T. C., J. A. Hileman, and W. Thatcher (1975). Seismic moments of the larger earthquakes of the southern California region, Geol. Soc. Am. Bull. 86, Hanks, T. C. and H. Kanamori (1979). A moment magnitude scale, J. Geophys. Res. 84, Hill, R. I. (1984). Petrology and petrogenesis of batholithic rocks, San Jacinto Mountains, southern California, Ph.D. Thesis, California Institute of Technology, Pasadena, 800 pp. Jennings, C. W. (1975). Fault map of California, Calif. Div. Mines Geol., Geol. Data Map No. 1. Kanamori, H. and G. S. Stewart (1976). Mode of strain release along the Gibbs fracture zone, Mid-Atlantic ridge, Phys. Earth Planet. Interiors 11, Lachenbruch, A. H., J. H. Sass, and S. P. Galanis, Jr. (1985). Heat flow in southernmost California and the origin of the Salton Trough, J. Geophys. Res. 90, Laughlin, H., R. Arnold, and W. S. W. Kew (1923). Southern California earthquake of July 22, 1923, Bull. Seism. Soc. Am. 13, Merifield, P. M., T. I. Rockwell, and C. C. Loughman (1987). Slip rate of the San Jacinto fault zone in the Anza seismic gap, southern California (abstract), Geol. Soc. Am. Abs. with Progs. 19, no. 6, Nicholson, C., L. Seeber, P. Williams, and L. R. Sykes (1986). Seismicity and fault kinematics through the eastern transverse ranges, California: block rotations, strike-slip faulting and low-angle thrusts, J. Geophys. Res. 91, Oliver, H. W., R. H. Chapman, S. Biehler, S. L. Robbins, W. R. Hanna, A. Griscom, L. A. Beyer, and E. A. Silver (1980). Gravity map of California and its continental margin, Calif. Div. Mines and Geol., Geol. Data Map No. 3. Petersen, M. D., L. Seeber, L. Sykes, J. N~bSlek, J. Armbruster, and K. Hudnut (1991). The interaction between secondary and master faults within the southern San Jacinto fault zone, southern California, Tectonics 10, Petroy, D. E. and D. A. Wiens (1989). Historical seismicity and implications for a diffuse plate convergence in the northeast Indian Ocean, J. Geophys. Res. 94, 12,301-12,319. Rasmussen, G. S. (1981). Nature of surface rupture and recurrence interval, Casa Loma fault, in Geology of the San Jacinto Mountains, A. R. Brown and R. W. Ruff (Editors), South Coast Geological Society Guidebook No. 9, Rockwell, T., R. Blom, R. Crippen, R. Klinger, A. Stinson, and N. Thomas (1990). Recognition, extension, and significance of northeast trending faults between the Elsinore and San Jacinto fault zone using combined SPOT and LANDSAT imagery, Friends of the Pleistocene Field Trip Guidebook--Western Salton Trough Soils and Neotectonics, Sanders, C. O. (1989). Fault segmentation and earthquake occurrence in the strike-slip San Jacinto fault zone, California, U.S. Geol. Surv. Open-File Rept , Sanders, C. O. (1990). Earthquake depths and the relation to strain accumulation and stress near strike-slip faults in southern California, J. Geophys. Res. 95, Sanders, C. O. and H. Kanamori (1984). A seismotectonic analysis of the Anza seismic gap, San Jacinto fault zone, southern California, J. Geophys. Res. 89, Sanders, C. O., H. Magistrale, and H. Kanamori (1986). Rupture patterns and preshocks of large earthquakes in the southern San Jacinto fault zone, Bull. Seism. Soc. Am. 76, Sharp, R. V. (1967). San Jacinto fault zone in the Peninsular Ranges of southern California, Geol. Soc. Am. Bull. 78, Sharp, R. V. (1981). Variable rates of Late Quaternary strike-slip on the San Jacinto fault zone, southern California, J. Geophys. Res. 86, Sharp, R. V., K. E. Budding, J. Boatwright, M. J. Ader, M. G. Bonilla, M. M. Clark, T. E. Fumal, K. K. Harms, J. J. Lienkaemper, D. M. Morton, B. J. O'Neill, C. L. Ostergren, D. J. Ponti, M. J. Rymer, J. L. Saxton, and J. D. Sims (1989). Surface faulting along the Superstition Hills fault zone and nearby faults associated with the earthquakes of 24 November 1987, Bull. Seism. Soc. Am. 79,

16 HISTORIC EARTHQUAKES ALONG SAN JACINTO FAULT SYSTEM 1801 Thatcher, W. J. and R. M. Hamilton (1973). Aftershocks and source characteristics of the 1969 Coyote Mountain earthquake, San Jacinto fault zone, California, Bull. Seism. Soc. Am. 63, Thatcher, W., J. A. Hileman, and T. C. Hanks (1975). Seismic slip distribution along the San Jacinto fault zone, southern California, and its implications, Geol. Soc. Am. Bull. 86, Toppozada, T. R., C. B. Real, and D. L. Parke (1981). Freparation ofisoseismal maps and summaries of reported effects for pre-1900 California earthquakes, Open-File Rept SAC, Calif. Div. Mines and Geol., Sacramento. Wesnousky, S. G., C. S. Prentice, and K. E. Sieh (1987). Fault slip-rate determination on the northern segment of the San Jacinto fault, San Bernardino, California (abstract), EOS 68, DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF TEXAS AT EL PASO EL PASO, TEXAS Manuscript received 7 November 1991