Abstract

High-gain three-component seismometers from 0- to 1-km deep along the Varian A-1 well at Parkfield, California, were used to record the waveforms of nearby microearthquakes. Despite being in the thick Tertiary sediments of the Parkfield Syncline, the S-wave amplification at this site is only about a factor of 3. The spectral content and spectral ratios of S waves along the well show that the average Qs in the top 1 km at this site is 37, with the Qs in different subintervals varying between 8 and 65. Based on initial S-wave polarizations, a complex S-wave velocity structure must exist at and below the Varian site. This structure appears to include position-dependent anisotropy as well as steep lateral velocity gradients. At a depth of 1 km, S-wave splitting parallel and normal to the San Andreas fault zone is consistently observed. This splitting scales at roughly 0.01 sec/km. Subsequent to the split S waves, the particle motion seems to be controlled by event focal mechanism. Above 1 km, the upgoing S waves attenuate and change directions of polarization, with a new splitting rate of 0.1 sec/km. Uniquely, for some events on the San Andreas fault immediately below the Varian site, large, post-S-wave signals with normal dispersion are present. We propose that these phases are fault-zone guided waves channeled from the San Andreas fault to the Varian site along the Gold Hill fault.

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