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Fault-Zone Guided Waves from Explosions in the San Andreas Fault, Parkfield and Cienega Valley, California

Yong-Gang Li, Juan Madrid, Clifford H. Thurber, Peter E. Malin, & Keiiti Aki

Published February 1997, SCEC Contribution #286

Fault-zone guided waves were successfully excited by near-surface explosions in the San Andreas fault zone both at Parkfield and Cienega Valley, central California. The guided waves were observed on linear, three-component seismic arrays deployed across the fault trace. These waves were not excited by explosions located outside the fault zone. The amplitude spectra of guided waves show a maximum peak at 2 Hz at Parkfield and 3 Hz at Cienega Valley. The guided wave amplitude decays sharply with observation distance from the fault trace. The explosion- excited fault-zone guided waves are similar to those generated by earth- quakes at Parkfield but have lower frequencies and travel more slowly. These observations suggest that the fault-zone wave guide has lower seismic velocities as it approaches the surface at Parkfield. We have modeled the waveforms as S waves trapped in a low-velocity wave guide sandwiched between high-velocity wall rocks, resulting in Love-type fault-zone guided waves. While the results are nonunique, the Parkfield data are adequately fit by a shallow wave guide 170 m wide with an S velocity 0.85 km/sec and an apparent Q ~ 30 to 40. At Cienega Valley, the fault-zone wave guide appears to be about 120 m wide with an S velocity 0.7 km/sec and a Q ~ 30.

Li, Y., Madrid, J., Thurber, C. H., Malin, P. E., & Aki, K. (1997). Fault-Zone Guided Waves from Explosions in the San Andreas Fault, Parkfield and Cienega Valley, California. Bulletin of the Seismological Society of America, 87(1), 210-221.