We have examined S-wave arrivals from local earthquakes at a three-station seismograph array in the Franciscan terrane of the Diablo Range, California. A single crustal S-wave phase is observed with a velocity of 3.30 km/sec. Poisson's ratio calculated for the crust from a composite Wadati diagram is 0.27. Beyond epicentral distances of 90 km we have tentatively identified an Sn phase with a velocity of 4.35 km/sec. Other investigators have reported a Pn velocity of 8.0 km/sec corresponding to an upper mantle Poisson's ratio of 0.29.
The 3.30 km/sec crustal S-wave velocity is intermediate in value between crustal S-wave velocities measured in similar terranes 75 km north at Berkeley and 90 km south at Bear Valley, suggesting a NW-SE crustal S-wave velocity gradient east of the San Andreas fault in the Franciscan terrane. This may be indicative of an increase in crustal rigidity from southeast to northwest, possibly associated with the differing levels of seismic activity observed along portions of the San Andreas fault.