FAULT-ZONE WAVES OBSERVED AT THE SOUTHERN JOSHUA-TREE EARTHQUAKE RUPTURE ZONE

Waveform and spectral characteristics of several aftershocks of the M 6.1 22 April 1992 Joshua Tree earthquake recorded at stations just nor th of the Indio Hills in the Coachella Valley can be interpreted in te rms of waves propagating within narrow, low-velocity, high-attenuation , vertical zones. Evidence for our interpretation consists of: (1) eme rgent P arrivals prior to and opposite in polarity to the impulsive di rect phase; these arrivals can be modeled as headwaves indicative of a transfault velocity contrast; (2) spectral peaks in the S wave train that can be interpreted as internally reflected, low-velocity fault-zo ne wave energy; and (3) spatial selectivity of event-station pairs at which these data are observed, suggesting a long, narrow geologic stru cture. The observed waveforms are modeled using the analytical solutio n of Ben-Zion and Aki (1990) for a plane-parallel layered fault-zone s tructure. Synthetic waveform fits to the observed data indicate the pr esence of NS-trending vertical fault-zone layers characterized by a th ickness of 50 to 100 m, a velocity decrease of 10 to 15% relative to t he surrounding rock, and a P-wave quality factor in the range 25 to 50 .