Great Valley � Coast Range Boundary Thrust Faults

A series of thrust faults probably underlie the boundary between the eastern Coast Ranges and the Great Valley along the entire length of the Great Valley. In the San Francisco Bay area, these faults would be located in the subsurface in the vicinity of Vacaville, Fairfield, Pittsburg, and Tracy. These faults angle down and westward, and produce the uplift of the hills along this boundary. There is very little known about the seismic history or earthquake cycle of this fault system in the San Francisco Bay region. Slip rates for these faults estimated by the 1996 Working Group are low, approximately 1 to 2 mm/yr.

These faults are relatively short, and are thus thought capable of M6.4 to M6.7 sized earthquakes, but do pose substantial seismic hazards to the levee system in the Sacramento-San Joaquin Delta.