The San Andreas fault is a major continental right lateral strike slip transform fault that marks the boundary between the Pacific Plate and the North American Plate, and it extends roughly 800 miles through California into Baja California and Sonora, Mexico.
The fault forms a complex zone of crushed and broken rock from a few hundred feet to a mile wide, and it reaches depths of at least 10 miles, producing linear troughs, straight escarpments, small ridges and sag ponds along much of its length.
Blocks on opposite sides of the fault move horizontally, with the block west of the trace moving northwest relative to the block to the east, a motion geologists describe as right lateral strike slip, and total long term displacement along the system has reached at least several hundred miles since it formed millions of years ago.
The fault is traditionally divided into northern, central and southern segments, and it has produced major historical earthquakes including events in 1812, 1838, 1857 and the 1906 San Francisco earthquake, which the U.S. Geological Survey materials estimate at about magnitude 7.7 on the moment magnitude scale and at about 8.3 on the Richter scale in earlier accounts, with reported human losses differing between sources.
Some stretches of the fault creep steadily and do not produce great shocks, while other stretches lock for a century or more, then release strain in large ruptures, and surveying has recorded modern drift rates reported as high as 2 inches per year in one U.S. Geological Survey account and as 0.79 to 1.38 inches per year in another summary of the fault.
Risks Research And Preparedness
Scientists and agencies have focused on forecasting and on building resilience, and the U.S. Geological Survey and university teams have highlighted seismic gaps, especially on the southern San Andreas where large earthquakes have not occurred for centuries and where stress accumulation may be large.
Parkfield in central California has produced moderate earthquakes at roughly 20 to 22 year intervals, and the U.S. Geological Survey has installed dense instruments there and pursued the San Andreas Fault Observatory at Depth, SAFOD, to drill and observe fault behavior at depth during the 2004 to 2007 campaign.
Research papers have flagged elevated risk on the southern section, notably a 2006 study by Yuri Fialko reporting sufficient interseismic strain accumulation for magnitude greater than 7.0 events, and a 2023 study linking high water levels in ancient Lake Cahuilla to increased stress and possible triggering of southern San Andreas ruptures.
Long term forecasts and scenarios by the U.S. Geological Survey include the UCERF3 assessment and regional hazard scenarios such as the HayWired and Southern California rupture studies, which model casualty and economic impacts and underline building code upgrades, land use choices and monitoring as primary mitigation strategies.
