Dilatancy and crustal uplift

Abstract

Opening of cracks before an earthquake (dilatancy) causes inelastic crustal uplift. Raising the pore pressure in these cracks as pore fluid diffuses into the dilatant zone produces further elastic uplift. Since the time between resaturation of the dilatant zone and the earthquake is shorter than the life of the v p /v s anomaly, the elastic uplift is less than the inelastic. The magnitude of the V p /v s anomaly gives a lower bound on the compressibility of the dilatant zone. Velocity and uplift measurements lead to the conclusion that if dilatancy occurred before the 1971 San Fernando earthquake, the pore pressure in the hypocentral region at the time of the event must have been at least an order of magnitude below hydrostatic.