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The San Andreas Fault
California's 1,200-kilometer seam between the Pacific and North American plates
From the air over the Carrizo Plain, the San Andreas Fault looks like a crease pressed into the earth: a straight line of offset streams, sag ponds, and pressure ridges running toward the horizon. It is the boundary where the Pacific Plate grinds northwest past the North American Plate, carrying Los Angeles slowly toward San Francisco. The fault produced the great earthquake that destroyed much of San Francisco in 1906, and its southern half has been quiet for more than three centuries in places. That silence is why seismologists treat it as the single most closely watched fault in the United States and why California builds, insures, and drills around it.
The fault runs about 1,200 kilometers from the Salton Sea in the southern California desert to Cape Mendocino on the north coast, where it meets the Mendocino triple junction and heads out to sea. It is a right-lateral strike-slip fault: stand on either side and the far side moves to your right. Long-term slip on the main trace is about 20 to 35 millimeters a year, part of roughly 50 millimeters of total plate motion shared with the Hayward, San Jacinto, and other faults. Between San Juan Bautista and Parkfield a creeping section slides steadily without large quakes, while the locked sections north and south store strain for decades. The trace passes within about 60 kilometers of downtown Los Angeles and runs offshore just west of San Francisco.
Geologist Andrew Lawson named the fault in 1895 after San Andreas Lake on the San Francisco Peninsula, and the state commission report on the 1906 earthquake that he led included Harry Fielding Reid's elastic rebound theory of how faults store and release strain. The 1857 Fort Tejon earthquake, about magnitude 7.9, ruptured some 350 kilometers of the southern section. The USGS UCERF3 forecast, still the official long-term model for the state, gives California a greater than 99 percent chance of a magnitude 6.7 or larger quake in 30 years and puts the southern San Andreas near Mojave at 19 percent. For the Bay Area, the USGS gives a 72 percent chance of a magnitude 6.7 or larger quake by 2043 on some fault, 22 percent on the San Andreas itself.