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.
Automatic identification of fault zone head waves and directPwaves and its application in the Parkfield section of the San Andreas Fault, California