San Andreas fault provides hazard clues.

Seismic reflection and refraction surveys of the deep crust show that the San Andreas (Calif.) fault goes straight through the crust and cuts through the Moho, the boundary between the crust and mantle of the Earth, instead of turning in the crust to connect with two other parallel faults in the area. The discovery, which helped discount recent suggestions that the faults linked at shallow depths, affects the predictions of the stress that builds up between the faults and the resultant assessments of earthquake hazards.

According to John Hole, assistant professor of geological sciences, Virginia Polytechnic Institute and State University, Blacksburg, and Alan Levander of the Department of Geology and Geophysics, Rice University, Houston, Tex., the San Andreas fault is the major boundary between the Pacific and North American geological plates of the Earth. Seismic surveys help determine the shape and composition of the structures in the deep crust.

Hole and his colleagues did their surveys in northernmost California at the Mendocino Triple Junction, where the Pacific, North American, and Gorda plates come together. The Triple Junction now is migrating northward parallel to the coast. As it goes, it lengthens the North American-Pacific plate boundary, the San Andreas fault. The rock boundaries such as the Moho are at different heights, or different rocks are juxtaposed on either side of the fault, not because one side has moved up or down, but because the rocks are moving northward so that those that are on one fault are not the same ones that formerly were on that side of the fault. The study done by Hole...