4. Seismic Velocities and Attenuation from Borehole Measurements near the Parkfield Prediction Zone, Central California

Abstract

Shear (S)‐ and compressional (P)‐ wave velocities were measured to a depth of 195 m in a borehole near the San Andreas fault where a recurrence of a moderate Parkfield earthquake is predicted. S‐wave velocities determined from orthogonal directions of the S‐wave source show velocity differences of approximately 20 percent. An average shear‐wave Q of 4 was determined in relatively unconsolidated sands and gravels of the Paso Robles Formation in the depth interval 57.5‐102.5 m.