Seismic behavior of the southern San Andreas fault zone in the northern Coachella valley, California: Comparison of the 1948 and 1986 earthquake sequences

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 1985 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 degrees to 70 degrees W, is about 15-km long, and dips northeast from near the Banning surface trace at about 45 degrees to 50 degrees. Relocation of the mainshock using revised station corrections and a local velocity model inverted from the aftershock arrival times indicates a location at 34 degrees N00.26', 116 degrees 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 degrees to 45 degrees with a strike of N60 degrees W. Previous moment-tensor 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 X 10(18) N-m (M(w) = 6.1). Actual and synthetic Wood-Anderson recordings indicate a local magnitude of M(L) 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 degrees N55.2', 116 degrees W28.9' with a focal depth of 12 km, a revised magnitude of M(L) 6.3, and an estimated seismic moment of 2 to 3 X 10(18) N-m. Based on arrival-time information from close (<20 km) portable stations deployed in 1948 to 1949, the 1948 aftershock distribution is about 15-km long, 8- to IO-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 degrees) on a plane that strikes N55 degrees W and dips more steeply northeast at 60 degrees to 70 degrees. 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 degrees 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.