FAULT-ZONE GUIDED-WAVES FROM EXPLOSIONS IN THE SAN-ANDREAS FAULT AT PARKFIELD AND CIENEGA VALLEY, CALIFORNIA

Fault-zone guided waves were successfully excited by near-surface expl osions in the San Andreas fault zone both at Parkfield and Cienega Val ley, central California. The guided waves were observed on linear, thr ee-component seismic arrays deployed across the fault trace. These wav es were not excited by explosions located outside the fault zone. The amplitude spectra of guided waves show a maximum peak at 2 Hz at Parkf ield and 3 Hz at Cienega Valley. The guided wave amplitude decays shar ply with observation distance from the fault trace. The explosion-exci ted fault-zone guided waves are similar to those generated by earthqua kes at Parfield but have lower frequencies and travel more slowly. The se observations suggest that the fault-zone wave guide has lower seism ic velocities as it approaches the surface at Parfield. We have modele d the waveforms as S waves trapped in a low-velocity wave guide sandwi ched between high-velocity wall rocks, resulting in Love-type fault-zo ne guided waves. While the results are nonunique, the Parkfield data a re adequately fit by a shallow wave guide 170 m wide with an S velocit y 0.85 km/sec and an apparent Q similar to 30 to 40. At Cienega Valley , the fault-zone wave guide appears to be about 120 m wide with an S v elocity 0.7 km/sec and a Q similar to 30.