EVIDENCE FOR ASEISMIC DEFORMATION IN THE WESTERN TRANSVERSE RANGES, SOUTHERN CALIFORNIA - IMPLICATIONS FOR SEISMIC RISK ASSESSMENT

Recent studies in southern California suggest that long-term deformati on rates are far in excess of that which can be accounted for by histo rical seismicity, and thus, a deficit of moderate and/or large earthqu akes exists in southern California. Although possible, this conclusion is not unique because aseismic deformation may have contributed to bu lk regional strain. We examined Cretaceous to Pleistocene sedimentary rocks exposed in the Ventura basin along four cross-strike traverses t o evaluate the possibility that aseismic deformation contributed to re gional shortening. Our field and microstructural investigations sugges t that aseismic deformational mechanisms, particularly pressure soluti on, contributed significantly to permanent shortening strain during th e late Neogene and that the proposed seismic deficiency may be overest imated.