THE PARADOX OF THE LOMA-PRIETA EARTHQUAKE - WHY DID RUPTURE TERMINATEAT DEPTH

The 1989 Loma Prieta earthquake was an oblique thrusting event that ru ptured a moderately dipping fault that is subparallel to the San Andre as fault. Rupture initiated at 18 km and propagated both updip and bil aterally. Seismic and geodetic studies indicate that rupture terminate d between 8 and 5 km depth. We examine the role of the elastic propert ies of the crust as a mechanism for reducing stress in the upper crust and terminating rupture. We use linear elastic fracture mechanics and the finite element method to model the Loma Prieta fault rupture as a mode II and mode III shear crack in a layered elastic crust. Our anal ysis shows that a low-stress condition, generated in our models by a c hange in material properties in the upper crust, is capable of stoppin g rupture. We also examine the stress condition at the intersection of the San Andreas and Loma Prieta faults to determine the present state of stress and seismic hazard in the region. We find that the Loma Pri eta fault in the upper crust is still closer to the point of failure t han the San Andreas fault over the same depth. Rupture of the Loma Pri eta fault did not increase the seismic hazard on the San Andreas fault within the crustal volume above the Loma Prieta rupture.