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Examines the vertical extent of the San Andreas fault at the Mendocino triple junction. Construction of three-dimensional seismic velocity model; Occurrence of lateral velocity contrast in the lower crust; Direction of growth of the San Andreas fault; Limit of the oceanic crust of the Pacific plate.

Seismic reflection and refraction images illuminate the San Andreas Fault to a depth of 1 kilometer. The prestack depth-migrated reflection image contains near-vertical reflections aligned with the active fault trace. The fault is vertical in the upper 0.5 kilometer, then dips about 70° to the southwest to at least 1 kilometer subsurface. This dip reconciles the difference between the computed locations of earthquakes and the surface fault trace. The seismic velocity cross section shows strong lateral variations. Relatively low velocity (10 to 30%), high electrical conductivity, and low density indicate a 1-kilometer-wide vertical wedge of porous sediment or fractured rock immediately southwest of the active fault trace. [ABSTRACT FROM AUTHOR]