Los Angeles is slowly sliding north along the San Andreas Fault toward San Francisco
West of the San Andreas Fault, California rides the Pacific plate northwest by a few centimetres each year. Keep that up for perhaps twenty million years and Los Angeles will glide past San Francisco and carry on toward the Aleutian Trench. The fault is a seam between two plates, grinding sideways.
The fault runs about 1,210 km through California, from the Mendocino triple junction offshore near Eureka, where three plates meet, down to Bombay Beach on the Salton Sea. There the sideways motion gives way to pulling apart, and the Salton Trough is slowly becoming an extension of the Gulf of California, kept from flooding only by Colorado River sediment. Across the state, slip averages roughly 33 to 37 mm a year, and compression where the fault bends has pushed up the Transverse Ranges and the Coast Ranges.
Andrew Lawson of UC Berkeley identified the fault in 1895 and named it for the San Andreas Valley, not, as often assumed, San Andreas Lake. After the 1906 San Francisco earthquake he mapped fences and roads sliced apart by the rupture and found they traced a near perfect line along his fault, which he then realised continued into Southern California. In 1953 Mason Hill and Thomas Dibblee proposed that the two sides had shifted hundreds of miles, an idea considered radical until plate tectonics confirmed it.
Different stretches behave differently. The central section mostly creeps, slipping steadily without earthquakes. Near Parkfield, a magnitude 6.0 quake arrived roughly every 22 years, in 1857, 1881, 1901, 1922, 1934 and 1966, so scientists forecast the next for 1993; it came in 2004. That regularity made Parkfield a world centre for earthquake research, and from 2004 a project drilled right through the fault there to sample it at depth. The southern segment, passing about 35 miles from Los Angeles, could produce a magnitude 8.1 event.
Some puzzles remain. The San Andreas and its branches account for only about 75 percent of the plates' relative motion; the rest is taken up east of the Sierra Nevada. A 2023 study linked past big quakes in the south to high water in ancient Lake Cahuilla, whose weight could more than double stress on the fault.
Source: San Andreas Fault
