Geological Sciences

San Andreas Fault

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San Andreas Fault The San Andreas Fault stretches 600 miles from the Mendocino Escarpment in northern California to the Salton Sea in southern California. It was discovered after the 1906 San Francisco earthquake when geologists found a rupture extending for 270 miles. Earthquake Websites Earthquake Safety Home Construction in Earthquake Areas		This Global Marine Gravity Map shows the Mendocino Escarpment striking across the Pacific Seafloor. Cape Mendocino is the meeting point of three plates: the North American plate to the east, the Pacific Plate to the west, and the Gorda microplate to the north. The San Andreas Fault begins here and extends south through San Rosa, San Francisco, Palmdale, and San Bernardino.

The last great earthquake on the San Andreas Fault in northern California occurred in 1906. The line of posts in this photo mark the 1906 break.		Red dots on this Landsat photo of the San Francisco Bay area mark the San Andreas fault extending from the northwest through Tomales Bay and passing only 3 miles from downtown San Francisco. San Andreas Lake can be seen to the south of San Francisco.

The break through this barn is the 1906 surface rupture along the San Andreas fault. Governor George Pardee established an Earthquake Commission immediately after the disaster. Andrew Lawson was elected chair of the committee and the very well known Grove Karl Gilbert was also a member. Geologists established that surface rupture occurred along a 270 mile stretch of the fault. Displacement averaged 10 feet with a maximum of 20 feet in some areas.		Earthquake shaking did severe damage in San Francisco and Santa Rosa. Water-saturated soil near San Francisco Bay liquefied and added to the damage.

The shaking was followed by fire that eventually burned 400 city blocks and 25,000 buildings.		The San Francisco earthquake occurred on April 18, 1906. Numerous books have been written to commemorate this tragic event. One of the best is by Peter Fradkin.

Lake Andreas lie directly astride the San Andreas Fault.		Fault creep is occurring at numerous places along the San Andreas in central California. The curb shown in this photo has been broken by fault creep.

Fault creep is also displacing the rows of vines in this vineyard.		On October 17, 1989 stress along the San Andreas created the magnitude 7.1 Loma Prieta earthquake and severely damaged the older buildings in Santa Cruz.

Soil beneath the Nimitz Freeway liquified during the Loma Prieta earthquake causing the southbound lanes to fall on the northbound lanes killing 41 people. Light traffic during the Giants-Athletics World Series saved many lives.		The Carrizo Plain provides some of the best displays of the San Andreas Fault. The Temblor Range shown in the distance separates the Carrizo Plain from the San Joaquin Valley. The white scarps visible in the distance are breaks along the San Andreas fault which defines the western flank of the range.

The offset stream pattern at Wallace Creek provides an excellent demonstration of the right-lateral strike-slip nature of this fault.		This USGS photo by Robert Wallace illustrates the extremely sharp nature of the plate boundary on the Carrizo Plain.

Red dots on this Landsat photo mark the linear trend of the San Andreas fault. The fault separates the San Gabriel Mountains from the Mojave Desert. Because of the plate boundary nature of this fault the Mojave Desert is on the North American Plate and Los Angeles is on the Pacific Plate.		Palmdale Reservoir is a sag pond on the San Andreas Fault. The San Gabriel Mountains can be seen in the distance. The last great earthquake on the San Andreas in southern California occurred in 1857. The 1857 surface rupture crosses Route 14 at Avenue S, very close to where this photo was taken.

This road cut on Route 14 through a pressure ridge shows the folding that occurs in sedimentary rocks adjacent to the San Andreas fault.		This photo taken from Devils Punchbowl State Park shows the nearly linear pressure ridge that is parallel to the San Andreas fault in the Palmdale area.

The sedimentary rocks of the Devils Punchbowl have been folded into a syncline by the forces associated with the San Andreas fault.		The San Andreas fault forms this linear valley at Cajon Pass. From here the fault continues through the San Bernardino urban area and southeast along the eastern side of the Salton Trough.

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