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 north of the Indio Hills in the Coachella Valley can be interpreted in terms of waves propagating within narrow, low-velocity, high-attenuation, vertical zones. Evidence for our interpretation consists of: (1) emergent P arrivals prior to and opposite in polarity to the impulsive direct 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-zone wave energy; and (3) spatial selectivity of event-station pairs at which these data are observed, suggesting a long, narrow geologic structure. The observed waveforms are modeled using the analytical solution of Ben-Zion and Aki (1990) for a plane-parallel layered fault-zone structure. Synthetic waveform fits to the observed data indicate the presence of NS-trending vertical fault-zone layers characterized by a thickness of 50 to 100 m, a velocity decrease of 10 to 15% relative to the surrounding rock, and a P-wave quality factor in the range 25 to 50.
© 1994, by the Seismological Society of America. Manuscript received 18 August 1993. We gratefully acknowledge the cooperation and patience of the homeowners who allowed us to deploy instruments on their property. We thank E. Sembera, C. Mueller, G. Glassmoyer, L. Wennerberg, and S. Lydeen for their efforts in instrument deployment, maintenance, and data processing. We thank Jim Moil, Tom Heaton, and an anonymous reviewer for reviews of the manuscript, and M. Rymer for his review and helpful discussions.
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