Velocities and driving pressures of clay-rich sediments injected into clastic dykes during earthquakes

Abstract

SUMMARY We studied the velocities and driving pressures associated with clastic-dyke formation in the Ami’az plain, where hundreds of clastic dykes cross-cut the soft rock of the late Pleistocene lacustrine Lisan Formation, within the seismically active Dead Sea basin. Flow of clastic material into fractures and opening of the fractures are two mechanisms that occur during earthquake-induced clastic dyke emplacement. Two analytic models were established, based on field observations and experimental viscosity tests, to estimate the velocities and driving pressures that were associated with dyke emplacement: (a) a channel flow for upward injection of a clay–water mixture and (b) a profile of fracture dilation based on the elastic theory analysis. The two models predict that pressures between 1 and 10 MPa are generated in the source layer and dykes in the last stage of the injection process. In addition, the channel flow model predicts that the injection velocity reaches metres to tens of metres per second and the emplacement time of the clastic dykes is on a scale of seconds. It is suggested that the high pressure values represent the static stress drop during earthquake events or represent dynamic stresses resulting from the seismic waves which passed through the soft lacustrine rocks. In both cases, the predicted high pressure values indicate that the clastic dyke was emplaced in close proximity of an active segment of the Dead Sea Fault during the late Pleistocene-Holocene.