Abstract
For more than 3 years the propagation characteristics of shear waves have been monitored for paths near the 1966 hypocenter at Parkfield, the presumed nucleation site for the expected next M6 earthquake there. Data have been collected repeatedly (33 sets as of April 1991) from eight S-wave Vibroseis source positions into the 10 borehole-installed three-component seismometers of the local high-sensitivity digital network. Twenty-second correlated records from a 6- to 24-Hz sweep are acquired, and the entire seismogram is viewed for analysis as the elastic response of the local crustal structure, which includes the San Andreas fault zone. Amplitudes, travel times, spectra, and particle motions of the P and S waves are monitored for indications of any changes in these properties that may be attributed to processes associated with nucleation. The horizontal vibrator at each source point is positioned at three surface-orientations to study anisotropy. Unorthodox methods have been developed to display the waveform properties in time in order to visualize the resulting massive data sets. The first-order variations seen in some of the parameters are attributed to changes from dry to wet conditions in the shallow subsurface due to the seasonal rainfall, which affects the source function of the vibrator. Corrections have been devised for these source-specific variations. Secular variations not obviously coupled to seasonal near-surface changes are also seen in some localized time intervals within the 20-sec records. The most striking of such changes is a progressive travel-time decrease at rates of 3 to 7 msec / year seen for late arrivals (7 to 11 sec travel time) on at least five paths into station VCA, which sample the region southeast of the anticipated epicenter at Middle Mountain. This anomaly appears to be genuine and is now the subject of intensified study. In the same general area, along the fault in the southwestern block, the direct S wave is clearly split, with the faster of the split phases polarized parallel to the fault zone, a result in agreement with that from the VSP survey in the Varian well on the northeast side of the fault.
