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Bulletin of the Seismological Society of America; April 1979; v. 69; no. 2; p. 397-415
© 1979 Seismological Society of America
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Pressure-induced velocity gradient: An alternative to a Pg refractor in the Gabilan Range, central California

DONALD J. STIERMAN*, JOHN H. HEALY and ROBERT L. KOVACH

DEPARTMENT OF GEOPHYSICS STANFORD UNIVERSITY, STANFORD, CALIFORNIA 94305
U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025

Abstract

Seismic refraction experiments carried out in central California show that a wedge of fractured rock characterized by unusually low P-wave velocities extends at least 1 km into the Gabilan Range south and west of the San Andreas fault. Lateral refraction of seismic waves by this low-velocity wedge precludes standard inversion of travel-time data obtained from sources or stations located near the San Andreas fault. Granitic rocks in the Gabilan Range exhibit near-surface P-wave velocities higher than velocities measured near the fault, but which are still significantly lower than P-wave velocities measured in saturated laboratory specimens of crystalline rock. These low velocities are also attributed to the presence of large in situ fractures. Travel-time data are fit with a velocity-depth function of the same form used to fit pressure-velocity behavior of laboratory rocks. This relation suggests that a velocity gradient is a realistic alternative to the Pg refracting interface for explaining travel-time data in the Gabilan Range. This velocity gradient is probably caused by the closing of macrocracks at shallow depths. If these low velocities are due to large fractures, the P-wave velocity sensitivity of these rocks to stress changes in the upper 4 km of the crust should be higher than the sensitivity of saturated laboratory specimens.

Footnotes

* Present address: Institute of Geophysics and Planetary Physics, University of California (Riverside), Riverside, California 92521.




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