What is the name of the fault that runs through most of California?

The fault that runs through most of California is the San Andreas Fault. It is a major right-lateral strike-slip fault forming part of the boundary between the Pacific Plate and the North American Plate.

The fault extends for roughly 750 miles through California, from the Salton Sea in the south to the Mendocino Triple Junction near the state’s northern coast. Its trace passes near or through areas including San Bernardino, Parkfield, Hollister, the Santa Cruz Mountains, and the San Francisco Peninsula.

“Right-lateral” means that, when viewed across the fault, the opposite side appears to move to the right. The Pacific Plate generally moves northwest relative to the North American Plate, producing the long-term motion associated with the fault. This movement averages roughly an inch per year in many parts of California, although it is not uniform.

A common misconception is that every California earthquake occurs on the San Andreas. California also contains important neighboring faults, including the Hayward, Garlock, San Jacinto, and Calaveras faults. The famous 1989 Loma Prieta and 1906 San Francisco earthquakes involved the San Andreas system, but the fault itself is only one part of California’s broader network of active faults.