DISCREPANCY BETWEEN GEOLOGICAL AND GEODETIC DEFORMATION RATES IN THE VENTURA BASIN

SPACE geodesy provides a new tool for measuring neotectonic deformation which, when combined with geological and geophysical observations and models, can be used to infer rates and styles of deformation. In the Ventura basin region of southern California, the geodetically measured convergence rates of approximately 7-10 mm yr-1 (ref. 1) are significantly lower than the 20-25 mm yr-1 estimated from geological data2,3 . Here we investigate the possible implications of this discrepancy. It might be, for example, that temporal variations cause the short- and long-term convergence rates to differ. But if this were the case we would expect to observe the long-term rate by extending the geodetic measurements over a broader region, yet only 11+/-3 mm yr-1 of regional convergence is observed4. Our preferred explanation of the discrepancy is that the geological models require revision, and to this end we present two deformation models that satisfy both the geological and geodetic constraints. Our modelling suggests that the faults bounding the basin are locked at the surface, but are slipping at depths below approximately 2-5 km.