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Quick Facts about the San Andreas Fault


David K. Lynch


The SAF is entirely within California.

From south to north, the counties it passes through are Imperial, Riverside, San Bernardino, Los Angeles, Kern, San Luis Obispo, Monterey, San Benito, Sants Cruz, Santa Clara, San Mateo, Marin, Sonoma, Mendocino and arguably Humbolt.

It is about 700 miles long and reaches from the Salton Sea in Imperial County to Cape Mendocino in Humboldt County. Neither its northern or southern end is well defined.

Its highest point is at the Big Pines Ranger Station in Los Angeles County (6874 ft) and its lowest point is just east of the Salton Sea in Imperial County (-235 ft). One could say that its lowest point is in the Pacific Ocean west of Point Arena, CA (Mendocino County).

The SAF is part of the boundary between the North American and Pacific Plates. The plate boundary extends far beyond California.

The SAF is a "transform fault." This means that the two plates are moving horizontally past each other, like two pieces of pizza on a table.

The Pacific Plate is moving northwest relative to the North American Plate at a rate of about 2.5 inches per year, about the same rate that your fingernails grow.

Motion along the fault is not steady. The plates don't move for many years while building up stress, then suddenly part of a contact region between the two plates shifts, causing an earthquake.

San Diego, Los Angeles, Santa Barbara, Lompoc, San Luis Obispo, Santa Cruz, and Monterey are on the Pacific Plate. Bakersfield, Sacramento, Santa Rosa, Ukiah, Garberville, Eureka and Crescent City are on the North American Plate.

The SAF does not go through San Francisco. The fault lies about 3.7 miles west of Point Lobos in the Pacific Ocean. The location of the epicenter of the 1906 Earthquake is not known for certain but is believed to be onshore in the area of Daly City, Pacifica or San Bruno. Earlier claims that the epicenter was in the Olema Trough of Marin County are not consistent with modern studies.

San Francisco is a few miles east of the SAF but owing to the complexity of the plate boundary, the city should be considered to be sitting more or less on the plate boundary. Finer distinctions are arbitrary and misleading. For example, SF sits approximately half way between the SAF and the Hayward Fault. The Hayward Fault is an active fault that is part of the plate boundary.

The SAF does not go through Los Angeles. It lies about 40 miles north of the city.

Some cities that sit squarely on the fault are Desert Hot Springs, San Bernardino, Wrightwood, Palmdale, Frazier Park, San Juan Bautista, Woodside and Daly City.

The SAF goes through (or very near) the three major transportation and utility corridors leading to Los Angeles: I-10 in San Gorgonio Pass and the Coachella Valley (Riverside County), I-15 in Cajon Pass (San Bernardino County), and I-5 in Tejon Pass near the junction of Los Angeles, Kern and Ventura Counties).

Parkfield, CA (Monterey County) lies about 1/4 mile from the fault. The area contains more seismometers than anywhere else in the world. Parkfield calls itself "The Earthquake Capitol of the World."

The two largest earthquakes in California's recorded history were the 1857 Fort Tejon Earthquake and the 1906 San Francisco Earthquake. Both temblors were around magnitude 7.9 but these are only estimates because seismographs didn't exist in 1857 and were sparsely deployed in 1906. Much larger ones have happened in prehistoric times.

The surface indicators of the SAF are ridges, scarps (long cliffs or lines of cliffs), offset channels and ponds that are lined up along the fault.

The San Fernando Earthquake (1971), the Northridge Earthquake (1994) and the Loma Prieta Earthquake (1989) did not take place on the San Andreas Fault (arguable in the case of the Loma Prieta Earthquake). They happened on nearby major faults.

The SAF was rather casually named for San Andreas Lake (San Mateo County) sometime around 1906. The fault runs the length of the lake. San Andreas Lake was originally named Lake San Andres by Gaspar de Portolo during his expedition to the area in 1769. Its name gradually changed to "San Andreas" when California state geologist Andrew Lawson began calling it that after the 1906 San Francisco earthquake.

In most places the SAF is accessible on public roads by ordinary cars. It is not accessible in the San Bernardino Mountains or where it lies offshore west of California.

The SAF runs the length of Tomales Bay in Marin County.

In English, a synonym for "earthquake" is "temblor" (not tremblor"). "Temblor" is a Spanish noun that means "a shaking or trembling". The Spanish word for earthquake is "terremoto" that literally means "Earth moves" or "Earth's motor".

The finest exposure of the SAF is in the Carrizo Plain National Monument (San Luis Obispo County) on good dirt roads. The mountains east of the fault in the Carrizo Plain are called the "Temblor Range".

In recent historic times, the most active fault in California is not the SAF. It is the San Jacinto Fault in Riverside and San Bernardino Counties. The most seismically-active region of California is the Cape Mendocino area.

A fault is a crack in a rock along which motion has taken place. The overwhelming majority of faults are small, unnamed, inactive and unimportant. Some are microscopic.

The boundary between two tectonic plates is a very large fault and therefore deserving of a name and of study.

Were it not for plate tectonics and the SAF, California would be much smaller and much flatter than it is today (and much less interesting).

It is possible to build a house that can withstand the strongest earthquakes, but it is not possible to secure the ground beneath the house.

The 1994 Northridge Earthquake and the 2003 Bam (Iran) Earthquake were about the same magnitude, 6.7. The reason that 41,000 people died in Bam and only 51 people died in Northridge was because the houses in Ban were built of unreinforced mud that collapsed on the villagers. Tough building codes in Los Angeles helped prevent deaths in the Northridge Earthquake.

The magnitude of an Earthquake (e.g. 6.7) is a single number related to the total energy of the earthquake. But being a single number, it does not tell the whole story. It does not tell what direction the ground moved (up-down, side-to-side), how fast the ground moved or how long the shaking lasted. Two earthquakes with identical magnitudes could produce very different damages. Damage also depends on the quality of construction and the properties of the underlying rock and soil.

The SAF rarely produces tsunamis because it is a transform fault and almost entirely on land. Offshore faults associated with the SAF along the plate boundary can produce tsunamis. Earthquakes along the northern SAF could trigger earthquakes in offshore faults.

The SAF is the main plate boundary but there are many other major active faults that are essentially parallel to the SAF and along which plate motion occurs (e. g. the Hayward and Calaveras faults.)

Copyright 2006 David K. Lynch. All rights reserved.