SEISMIC TOMOGRAPHY AND DEFORMATION MODELING OF THE JUNCTION OF THE SAN-ANDREAS AND CALAVERAS FAULTS

Local earthquake P traveltime data is inverted to obtain a three-dimen sional tomographic image bf the region centered on the junction of the San Andreas and Calaveras faults. The resulting velocity model is the n used to relocate more than 17,000 earthquakes and to produce 8 model of fault structure in the region. These faults serve as the basis for modeling the topography using elastic dislocation methods. The region is of interest because active faults join, it marks the transition zo ne from creeping to locked fault behavior on the San Andreas fault, it exhibits young topography, and it has a good spatial distribution of Seismicity. The tomographic data set is extensive, consisting of 1445 events, 96 stations, and nearly 95,000 travel time readings. Tomograph ic images are resolvable to depths of 12 km and show significant veloc ity contrasts across the San Andreas and Calaveras faults, a low-veloc ity zone associated with the creeping section of the San Andreas fault , and shallow low-velocity sediments in the southern Santa Clara valle y and northern Salinas valley. Relocated earthquakes only occur where v(p)>5 km/s and indicate that portions of the San Andreas and Calavera s faults are non vertical, although we cannot completely exclude the p ossibility that all or part of this results from ray tracing problems. The new dips are more consistent with geological observations that di pping faults intersect the surface where surface traces have been mapp ed. The topographic modeling predicts extensive subsidence in regions characterized by shallow low-velocity material, presumably the result of recent sedimentation. Some details of the topography at the junctio n of the San Andreas and Calaveras faults are not consistent with the modeling results, suggesting that the current position of this ''tripl e junction'' has changed with time. The model also predicts those part s of the fault subject to contraction or extension perpendicular to th e fault strike and hence the sense of any dip-sip component. In each l ocality the relative vertical motion accross the fault is consistent w ith the fault dips found with the new hypocentral locations.