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 re- gion, 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. 1140 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/8/1140/3444064/i0016-7606-86-8-1140.pdf by guest on 02 October 2021 1 143 SEISMIC SLIP, SAN JACINTO FAULT ZONE, SOUTHERN CALIFORNIA 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.