Source mechanisms of three moderate California earthquakes of July 1986

Abstract

Abstract We studied the source processes of three moderate size earthquakes in California: the M s = 6.0 event of July 8, 1986, near Palm Springs, the M s = 5.8 shock of July 13, 1986, offshore of Oceanside, and the M s = 6.2 event of July 21, 1986, in the Chalfant Valley, on the east side of the Sierra Nevada. The centroidal source parameters are estimated by inversion of long-period P and SH waves recorded at teleseismic distances by seismographs of worldwide networks. A more detailed description of the source is then obtained by analyzing broadband seismograms. The North Palm Springs earthquake had a simple rupture process. Most of the seismic moment was released in the first 3 sec in a single step. The strike of the source fault plane follows the average trend of the Banning fault and its dip agrees with the dip of the fault plane delineated by the aftershocks. The faulting mechanism of the main shock is right-lateral strike-slip with a sizable reverse component that is consistent with uplift north of the Banning fault. The earthquake offshore of Oceanside had a slightly more complex rupture process. The far-field source-time function consists of two pulses that represent the release of most of the moment in the first 3 sec. The source mechanism indicates reverse faulting with the compressional axis trending northeast, almost perpendicular to the trend of mapped faults offshore. The event does not appear to be associated with major mapped faults in the area, but the strike of one of the nodal planes agrees with the trend of the major mapped faults offshore and its mechanism is consistent with the missing component of plate convergence not taken up by the onshore strike-slip fault systems, and the idea of a weak San Andreas fault system. The rupture process of the Chalfant Valley earthquake was yet more complicated. The source-time function indicates that in a 4 sec time window three asperities were broken. The source mechanism indicates right-lateral strike-slip faulting, with a small normal-faulting component on a fault that dips approximately 50° southwest. Earthquake activity suggests that presently the deformation in the northern part of the Owens Valley (which includes the Chalfant Valley) is dominated by conjugate strike-slip faults which strike obliquely to the main range bounding normal faults. The waveforms indicate the presence of a low-velocity zone at a depth of about 20 km in the crust in the Chalfant Valley. A synthesis of information from waveform modeling, first-motion fault-plane solutions, and aftershock distributions indicates that the nucleation and/or termination of rupture for the North Palm Springs and the Chalfant Valley events were controlled by preexisting fault geometry.