SEISMIC REFLECTIONS FROM THE NEAR-VERTICAL SAN-ANDREAS FAULT

Major near-vertical strike-slip faults should reflect seismic energy d ue to velocity contrasts within and across the fault zone, analogous t o sub-horizontal faults commonly imaged by both multichannel and wide- angle reflection surveys. An increase of velocity with depth in the up per crust allows such reflections to be detected in surface seismic su rveys. We have observed fault reflection events from both sides of the San Andreas Fault in long-offset common-receiver gathers acquired nea r San Francisco. The events can be traced to the known surface positio n of the fault and provide constraints on the subsurface position to 3 -7 km depth. Such reflections can be used to find the geometry of majo r near-vertical faults, even in areas without active seismicity, provi ding information crucial to the understanding of the tectonics and sei smic hazards of strike-slip faulting.