The U.S. Geological Survey's telemetered seismic network in central California was supplemented over a 7-week period with 13 portable seismograph stations in the area of the Calaveras Fault southeast of San Jose. Travel-time data from small quarry explosions indicate a vertical low-velocity zone apparently associated with the fault and extending to a depth of at least 6 km.

When this zone is taken into account, the hypocenters of 55 earthquakes recorded to study this effect show an arrangement of a near-vertical dipping plane that coincides remarkably well with the mapped trace of the fault. The previously determined distribution of foci was off the fault by several kilometers to the east.

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