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and seismic behavior of active fault segments in the northern Coachella Valley. The. 1986 North Palm Springs earthquake occurred between the Banning and ...
Bulletin of the Seismological Society of America, Vol. 86, No. 5, pp. 1331-1349, October 1996

Seismic Behavior of the Southern San Andreas Fault Zone in the Northern Coachella Valley, California: Comparison of the 1948 and 1986 Earthquake Sequences b y Craig N i c h o l s o n

Abstract Moderate earthquakes in 1948 and 1986 that occurred along the southern San Andreas fault zone are examined to help understand the subsurface geometry and seismic behavior of active fault segments in the northern Coachella Valley. The 1986 North Palm Springs earthquake occurred between the Banning and Mission Creek traces of the San Andreas fault zone. Based on data from both portable and permanent stations, aftershocks of the 1986 event define a nearly planar surface that strikes about N60 ° to 70°W, is about 15-km long, and dips northeast from near the Banning surface trace at about 45 ° to 50 °. Relocation of the mainshock using revised station corrections and a local velocity model inverted from the aftershock arrival times indicates a location at 34°N00.26 ', 116°W36.34 ', and a focal depth of 10.4 km. The first-motion focal mechanism for the mainshock indicates pure right-slip on a plane dipping northeast at 40 ° to 45 ° with a strike of N60°W. Previous momenttensor solutions (e.g., Hartzell, 1989) exhibit significant oblique-reverse slip on a plane with a more westerly strike and a seismic moment of 1.4 to 1.7 × 1018 N-m (Mw -- 6.1). Actual and synthetic Wood-Anderson recordings indicate a local magnitude of ML 6.0. The 1948 Desert Hot Springs earthquake produced waveforms similar to 1986, although amplitudes for 1948 are typically 20 to 30% larger. The 1948 mainshock is relocated at 33°N55.2 ', 116°W28.9 ', with a focal depth of 12 km, a revised magnitude of ML 6.3, and an estimated seismic moment of 2 to 3 × 1018 N-m. Based on arrival-time information from close ( < 2 0 km) portable stations deployed in 1948 to 1949, the 1948 aftershock distribution is about 15-krn long, 8- to 10-km wide, and abuts the 1986 aftershock sequence to the southeast along the Banning fault. The 1948 aftershock zone is consistent with a focal mechanism that exhibits predominantly right-slip motion (rake 169 °) on a plane that strikes N55°W and dips more steeply northeast at 60 ° to 70 °. In 1986, the horizontal extent of the rupture may have been controlled by secondary cross faults and possible changes in fault geometry, while the down-dip extent may have been controlled by the presence at depth of a regional detachment. In 1948, the rupture likely had a steeper dip than in 1986 and extended southeast as far as the northern Indio Hills, where the surface trace of the Banning fault undergoes an approximate 7 ° change in strike. These results indicate that the Banning fault is nonvertical, is likely segmented according to fault dip, as well as fault strike, and has been the primary locus of recent moderate-sized earthquake activity in the northern Coachella Valley.

Introduction The San Andreas fault zone in southern California, extending from San Gorgonio Pass to the Salton Sea (Fig. 1), has not experienced a major (M =>7.0) plate-rupturing earthquake in historical time. Although this segment is the pri-

mary locus of geodetic plate boundary shear strain (Savage et al., 1986) and exhibits fault creep (Louie et al., 1985), much of the fault zone appears to be nearly aseismic at the microearthquake level. This is similar in many respects to

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Figure 1. Regional geology of San Andreas fault zone in southern California (Rymer, 1996). White areas represent Quaternary alluvium; dark gray, Pliocene and Pleistocene stratified rock; light gray, pre-Cenozoic crystalline rock. BCF, Blue Cut fault; BF, Banning fault; DHS, Desert Hot Springs; GHF, Garnet Hill fault; MCF, Mission Creek fault; MVF, Morongo Valley fault; PMF, Pinto Mountain fault; SAF, San Andreas fault; SGP, San Gorgonio Pass; SJFZ, San Jacinto fault zone. Relocated epicenters (stars) are shown for major earthquakes in 1948, 1986, and 1992.

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(Rymer, 1996) other segments of the San Andreas fault zone responsible for large, plate-rnpturing earthquakes in 1857 and 1906 located farther north (e.g., Allen, 1981). Until 1986, much of the microearthquake activity in close proximity to the fault generally occurred as slip on secondary adjacent structures (Nicholson et al., 1986a; Williams et al., 1990), rather than right-slip along any of the major through-going fault strands. Nevertheless, the San Andreas fault is still believed to be one of the most likely places in southern California to generate a major damaging earthquake in the next few decades (Working Group on California Earthquake Probabilities, 1988). How, when, and where the southern San Andreas fault will fail will largely depend on a number of parameters, including the variation along strike of stress, strength, and fault geometry caused by bends, offsets, or other discontinuities that may affect rupture behavior and fault segmentation (Lindh and Boore, 1981; King and N~ibfilek, 1985). Thus, any information concerning the rupture behavior and geometry of the fault zone--particularly at seismogenic depths--has significant implications in terms of improved long-term probabilistic assessments of earthquake hazard and more realistic modeling of potential strong ground motion effects. In this article, I describe the seismic behavior of the southern San Andreas fault zone in the northern Coachella Valley based on an analysis of the only two known moderate earthquake ruptures along this fault segment during this cen-

tury. These are the Desert Hot Springs (DHS) earthquake of 4 December 1948 and the North Palm Springs (NPS) earthquake of 8 July 1986. In both cases, data from close (4) were restricted to depths of 11 to 12 km at each end of the rupture zone (to the northwest and southeast) and at relatively shallow depths (7 to 8 kin) directly updip from the mainshock hypocenter (squares, Fig. 4). Most of the smaller NPS aftershocks occurred between depths of 8 and 12 km and appear to outline an co-shaped pattern (Fig. 4). The co-shaped pattern is believed to outline the two areas of maximum dynamic slip during the mainshock (Nicholson et al., 1986c; Mendoza and Hartzell,

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1988). Nearly all the aftershocks that occurred on or close to the inferred slip plane of the 1986 mainshock exhibited focal mechanisms consistent with oblique-reverse slip (Jones et al., 1986; Seeber and Armbruster, 1995). Many of the shallow aftershock focal mechanisms exhibited slightly steeper nodal planes than the 40 ° to 45 ° dip of the 1986 mainshock, suggesting a possible listric or curvilinear shape to the geometry of the subsurface fault (Fig. 2). Separating aftershock hypocenters that occur to the east and west of the mainshock location (Fig. 4) (Nicholson et al., 1986c) or plotting the down-dip projection of the earthquakes (Seeber et al., 1987; Pacheco and N~b~lek, 1988) suggests that the nucleation point of the 1986 mainshock corresponded with a

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Figure 2. Map and vertical cross section (A-A') of aftershocks associated with the 1986 M 6.0 North Palm Springs earthquake (star). Triangles are locations of portable and permanent monitoring stations. Aftershocks define a nearly planar zone that dips northeast at about 50° from near the Banning fault. Additional aftershocks occur at shallow depths ( 1 0 km in depth) reflected a volume defor-

Seismic Behavior of the Southern San Andreas Fault Zone in the Northern Coachella Valley, California

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Figure 3. Left: lower-hemisphere equal-area projection of P-wave first motions for the 1986 NPS mainshock. Best-fitting nodal plane strikes N60°W, dips northeast at 40° to 45 °, and exhibits pure right-lateral strike-slip motion (rake 180°). Pluses are compressions; circles are dilatations; heavy symbols have take-off angles from nadir >90 ° (UP); light symbols have take-off angles 10 km) to the southwest of the mainshock (Fig. 2), that in many ways mimic elements of the previous seismic activity. Aftershocks east of the mainshock largely occurred along the northeast-trending Morongo Valley fault, and began 4 days after the mainshock. These events exhibited shallow focal depths ( 2) aftershocks (solid symbols, Fig. 2), many of which have focal mechanisms that exhibit a low-angle nodal plane consistent with slip on the previously inferred low-angle detachment. Figure 2 shows a number of aftershock focal mechanisms in map and cross section that exhibit nodal planes that dip at less than 20 °. These hypocenters are generally restricted to focal depths between 10 and 13 km and appear to define a planar surface dipping gently to the north and east. This surface tends to correlate with the lower extent of the seismic rupture and aftershock distribution of the NPS mainshock (Fig. 2). The change in focal depths of earthquakes across a presumed, northeast-dipping Banning fault (from deep seismicity on the west side to shallow seismicity on the east side, Fig. 2) may indicate that the Banning fault is a major tectonic strain boundary or that it, in turn, may vertically offset the detachment. Prior to 1986, the presence of this detachment was inferred based on both the observed change in character

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mechanism of 1948 can be determined. Figure 6 (top) shows the original location of the 1947 Morongo Valley and 1948 DHS earthquakes (Richter et al., 1958) relative to the 1986 NPS sequence (Table 1), as well as the locations of three sites occupied by portable seismographic equipment shortly after the 1948 mainshock. The systematic offset in the aftershock locations from the surface trace of the Mission Creek strand was originally used to infer a dip of 73 ° to the northeast of the Mission Creek fault (Richter et al., 1958; Allen, 1957). However, these original epicenters of the 1948 aftershocks were determined assuming a fixed focal depth of 16 kin, constant crustal and P, velocities, no station delays to account for lateral variations in velocity, and ignoring the S-wave arrivals ( S - P times) from the close portable stations. These factors suggest that the relative locations and accuracy of the 1948 hypocenters could be significantly improved, particularly if the information from the portable stations was included in the relocation procedure. Examples of some of the waveforms recorded in December 1948 at the portable sites Willis Palms (WPLM), Bennett Ranch (BNNT), and DHS are shown in Figure 7. Each site consisted of a self-contained portable trailer for photographic recording and an early version of the small Benioff seismometer. Although by 1948, Benioff had developed compact portable versions of both the vertical and horizontal seismometer, the sensors used for the Desert Hot Springs deployment arc believed to be single horizontal components typically oriented to record motion in a North-

South direction (C. Allen, personal comm., 1986). In many cases, S - P times at Bennett Ranch and Desert Hot Springs were on the order of 1.0 sec or less. Incorporating the P- and S-wave arrival times from the portable stations into the earthquake location procedure moves the 1948 aftershock epicenters between the Banning and Mission Creek faults, where they abut the 1986 aftershock zone to the southeast (Fig. 6). Inclusion of the arrival-time delays observed at regional stations for the 1986 NPS event (Fig. 5) in the relocation of the 1948 DHS mainshock also moves the mainshock epicenter between the traces of the Banning and Mission Creek faults (Table 1, Fig. 6). Both the 1948 and 1986 aftershock zones are of comparable size and define zones that dip to the northeast consistent with slip on a presumed NE-dipping Banning fault. Richter (1949, 1958) and Richter et al. (1958) repeatedly commented that the original location of the 1948 sequence (mainshock and aftershocks) east of the Mission Creek fault was incompatible with the short S - P times recorded at the close portable stations--a problem that does not exist with the new relocated hypocenters. The focal mechanism of the 1948 DHS earthquake derived from regional and teleseismic first motions (Fig. 8) is nearly the same as the teleseismic and long-period pattern found for the 1986 NPS event. It shows predominantly rightlateral strike-slip motion (with possibly a small reverse component) on a plane dipping northeast at about 65 ° to 70 °. The first-motion focal mechanism is reasonably consistent with

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Figure 6. Revised 1948 Desert Hot Springs mainshock (filled star) and aftershock (circles) locations relative to the 1986 NPS sequence (shaded area with star) using S P times from local portable stations (triangles), regional arrival times, and station delays derived from the 1986 event. Shaded diamond is the relocated 1948 hypocenter derived without the 1986 station delays (Table 1). The 1948 DHS aftershocks abut the 1986 NPS earthquakes to the southeast and are consistent with slip on a fault that dips northeast. Dashed lines that dip at 65 ° from the Banning (BF) and Mission Creek faults (MCF) are provided for reference. Original aftershock areas of the 1948 and 1947 earthquakes (open stars) (Richter et al., 1958) are shown as oblong regions northeast of Mission Creek fault. Original 1948 aftershock area was inconsistent with arrival times at portable stations (DHS, Desert Hot Springs; BNNT, Bennett Ranch; WPLM, Willis Palms) deployed within 8 h of the 1948 mainshock. Sites A through E refer to locations of the 1948 intensity effects referred to in text.

Seismic Behavior of the Southern San Andreas Fault Zone in the Northern Coachella Valley, California

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Figure 7. Examples of waveforms recorded at portable stations deployed following the 1948 Desert Hot Springs (DHS) mainshock to monitor aftershock activity. See Figure 6 for station locations. Notice extremely short S-P times for earthquakes recorded at Bennett Ranch and Desert Hot Springs. the spatial distribution of the relocated aftershock hypocenters, and with the similarity in waveforms recorded at DeBilt (DBN, A ~- 81 °) and Mt. Hamilton (MHC, A ~ 6 °) for both the 1948 and 1986 earthquakes (Fig. 9). Long-period surface waves recorded at DBN for the 1948 earthquake are only 20 to 30% larger than the 1986 NPS event (Fig. 9, top). Peak amplitudes at MHC are also larger by 30% for the 1948 event, although amplitudes of 3- to 6-sec surface waves are systematically smaller (Fig. 9, bottom). This is consistent with the inferred change in focal mechanism (Harkrider, 1964) between the 1948 and 1986 ruptures, with the 1986 earthquake exhibiting a larger component of dip-slip motion and a less steeply dipping inferred fault plane. A survey of peak amplitudes for the 1948 DHS and 1947 Morongo Valley earthquakes recorded at regional and teleseismic stations is given in Tables 3 and 4, respectively. Because stations in the San Francisco Bay area are known to systematically overestimate ML values for earthquakes in southern California (Hutton and Boore, 1987), equivalent magnitudes for the 1948 and 1947 events were recalculated

from these stations, assuming that the amplitudes in 1986 were produced by a magnitude ML 6.0 event. These "relative-magnitude" values are identified in Tables 3 and 4 with an asterisk (*). The data indicate that more appropriate ML values for the 1948 DHS and 1947 Morongo Valley earthquakes are 6.3 and 5.3, respectively, rather than previous catalog values of 6.5 and 5.5. In summary, based on arrival-time information from close (