Although many Bay Area residents remember the 1989 Loma Prieta as the "big one", it pales in comparison to these earlier fault ruptures. We know from paleoseismic investigations in Washington and Oregon and tsunami records in Japan, a magnitude 9 earthquake ruptured the Cascadia subduction zone in 1700. North of Cape Mendocino, the San Andreas fault merges with the plate boundary of the Cascadia subduction zone that lies offshore of northernmost California, Oregon, and Washington. The San Andreas fault southeast of San Bernardino has not experienced a major earthquake in the historical record, and paleoseismic investigations of this reach of the fault suggests it last ruptured in the late 17th Century. A 112-mile (180 km) long creeping section exists on the central portion of the San Andreas between the 18 ruptures. In addition to the 1906 rupture of the San Andreas fault in northern California, the San Andreas fault in south-central California also experienced a similar size earthquake in 1857, rupturing the San Andreas fault from Parkfield to just northwest of San Bernardino. These plates slide past each other when they move. It consists of the Pacific Plate and the North American Plate. The San Andreas fault is a long narrow valley where the plates that make up the fault meet. Plate Boundary Ruptures Along Western North America The San Andreas fault is an example of Transform tectonic plate boundary. Earthquakes in the Bay Area reflect the relative motion of the Pacific and North America plates. Notice how the location of earthquakes are concentrated along the plate boundaries of the Earth. Plate Boundary Ruptures Along Western North America. The two plates are moving alongside each other in opposite directions. The following file records earthquake activity around the world for the past week. The San Andreas fault is one of the boundaries between the North American tectonic plate and the Pacific plate. Of those, nearly 100,000 can be felt, and 100 cause damage. You may be surprised to learn that 500,000 detectable earthquakes occur globally each year. California is also one of the few places in the world where faults “creep”, that is, the fault slip is slow and continuous and occurs aseismically, which is to say that this slip does not occur as the result of an earthquake. Earthquakes are also associated with active volcanism and the flow of magma at depth. The word “earthquake” is used to describe both sudden slip on a fault and the resulting ground shaking. Most earthquakes occur in the upper 10-12 miles (15-20 km) of the Earth’s crust as the result of failure on faults caused by the strains induced by plate motions.