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.

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