The San Jacinto Fault Zone runs about 210 kilometres through Southern California, from near San Bernardino southeast toward the Imperial Valley, running roughly parallel to and just west of the southern San Andreas. It is a right-lateral strike-slip fault that helps carry the horizontal motion between the Pacific and North American plates, and it is often described as the most seismically active fault in the region.
Rather than a single clean break, the zone is a braid of overlapping strands with names like Clark, Coyote Creek, Anza and Hot Springs. This complex geometry produces frequent moderate earthquakes and makes the fault a favourite target for seismologists studying how strain is shared between neighbouring faults.
The San Jacinto has generated numerous damaging shocks, including magnitude 6-plus events in 1899, 1918 and 1968 (the Borrego Mountain earthquake). Because it threads through fast-growing inland communities, researchers watch its quieter 'Anza gap' segment closely as a candidate for a future major rupture.