Earthquake source parameters and fault kinematics in the Eastern California Shear Zone

摘要

Based on waveform data from a profile of aftershocks following thenorth-south trace of the June 28, 1992 Landers rupture across the Mojavedesert, we construct a new velocity model for the Mojave region which featuresa thin, slow crust. Using this model, we obtain source parameters, includingdepth and duration, for each of the aftershocks in the profile, and inaddition, any significant (M>3.7) Joshua Tree--Landers aftershock betweenApril, 1992 and October, 1994 for which coherent TERRAscope data wereavailable. In all, we determine source parameters and stress-drops for 45significant (M_w > 4) earthquakes associated with the Joshua Tree and Landerssequences, using a waveform grid-search algorithm. Stress drops for theseearthquakes appear to vary systematically with location, with respect toprevious seismic activity, proximity to previous rupture (i.e., with respect tothe Landers rupture), and with tectonic province. In general, for areas northof the Pinto Mountain fault, stress-drops of aftershocks located off the faultsinvolved with the Landers rupture are higher than those located on the fault,with the exception of aftershocks on the newly recognized Kickapoo (Landers)fault. Stress drops are moderate south of the Pinto Mountain fault, where thereis a history of seismic swarms but no single through-going fault. In contrastto aftershocks in the eastern Transverse ranges, and related to the 1992 BigBear, California, sequence, Landers events show no clear relationship betweenstress-drop and depth. Instead, higher stress-drop aftershocks appear tocorrelate with activity on nascent faults, or those which experiencedrelatively small slip during mainshock rupture.