Abstract
ault-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 earthquakes
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.
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