Edward F. Roth James F. Gibbs 1989 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. application/pdf 10.1193/1.1585538 en Seismic velocities and attenuation from borehole measurements near the Parkfield prediction zone, Central California article