2-DIMENSIONAL SEISMIC IMAGE OF THE SAN-ANDREAS FAULT IN THE NORTHERN GABILAN RANGE, CENTRAL CALIFORNIA - EVIDENCE FOR FLUIDS IN THE FAULT ZONE

A joint inversion for two-dimensional P-wave velocity (Vp), P-to-S vel ocity ratio (Vp/Vs), and earthquake locations along the San Andreas fa ult (SAF) in central California reveals a complex relationship among s eismicity, fault zone structure, and the surface fault trace. A zone o f low Vp and high Vp/Vs lies beneath the SAF surface trace (SAFST), ex tending to a depth of about 6 km. Most of the seismic activity along t he SAF occurs at depths of 3 to 7 km in a southwest-dipping zone that roughly intersects the SAFST, and lies near the southwest edge of the low Vp and high Vp/Vs zones. Tests indicate that models in which this seismic zone is significantly closer to vertical can be confidently re jected. A second high Vp/Vs zone extends to the northeast, apparently dipping beneath the Diablo Ranger Another zone of seismicity underlies the northeast portion of this Vp/Vs high. The high Vp/Vs zones cut ac ross areas of very different Vp values, indicating that the high Vp/Vs values are due to the presence of fluids, not just lithology. The clo se association between the zones of high Vp/Vs and seismicity suggests a direct involvement of fluids in the faulting process.