Velocities of seismic P-waves are unusually low in the vicinity of a 600-meter deep well drilled into quartz diorite located 1.2 km from the San Andreas fault in central California. These low velocities are attributed to the presence of extensive fracturing of rocks along the fault. Nowhere in the well does the P-velocity exceed 70% of the velocity measured in samaples of core from the well. Analyses of electrical and gravity logs suggest that saturated macrocracks, while producing only 5--10% porosity, are responsible for the 30--50% reduction of in situ seismic wave velocities. These macrocracks fail to close in response to confining stress. We attribute this to dilatancy indced by deviatoric stresses associated with the San Andreas fault, although the possibilities of high pore fluid pressures in the rocks or of an increase in fracturing and gouge formation with depth cannot be ruled out. Serious questions are raised concerning the use of laboratory data to model fault zones. In situ measurements of stress, seismic velocity and fluid pressures are necessary before we can hope to completely understand the distribution and mechanism of shallow focus earthquakes.