Seismic reflections from the near-vertical San Andreas fault

Major near-vertical strike-slip faults should reflect seismic energy due to velocity contrasts within and across the fault zone, analogous to sub-horizontal faults commonly imaged by both multichannel and wide-angle reflection surveys. An increase of velocity with depth in the upper crust allows such reflections to be detected in surface seismic surveys. We have observed fault reflection events from both sides of the San Andreas Fault in long-offset common-receiver gathers acquired near San Francisco. The events can be traced to the known surface position 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 major near-vertical faults, even in areas without active seismicity, providing information crucial to the understanding of the tectonics and seismic hazards of strike-slip faulting.