Shear-wave anisotropy in the Parkfield Varian well VSP

A vertical seismic profile (VSP) survey was run to 1334 m depth in the instrumented Varian well, 1.4 km from the San Andreas fault trace at Parkfield, California, to test the sensor string shortly after its permanent installation. The cable subsequently failed near the 1000 m level, so the test survey represents the deepest data acquired in the study. A shear-wave vibrator source was used at three ofsets and two orthogonal orientations, and the data have been processed for P- and S-wave velocities and for S-wave velocity anisotropy. Velocities are well-determined (3.3 and 1.9 km/sec, respectively, at the deeper levels), and the S waves are seen clearly to be split by anisotropy below about 400 m. Some 8 per cent velocity difference is apparent between polarizations parallel to and perpendicular to the San Andreas fault (faster and slower, respectively), and the difference seems to decrease with distance from the fault, suggesting that the cause may be the fabric of the fault zone. Repeated surveys at the 1000 m depth are being conducted as part of the Parkfield monitoring program.