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California's most famous fault line runs 800 miles from the Salton Sea to Cape Mendocino. In the Bay Area, it passes through San Francisco and down the Peninsula. The 1906 earthquake that devastated San Francisco originated here.
The San Andreas Fault is a transform fault marking the boundary between the Pacific Plate and the North American Plate. Unlike subduction zones where one plate dives beneath another, transform faults feature two plates sliding horizontally past each other.
In California, the Pacific Plate (carrying much of coastal California including Los Angeles) is moving northwestward relative to the North American Plate at about 2 inches per year. This seemingly slow movement builds enormous stress that is periodically released as earthquakes.
The fault was discovered in 1895 by UC Berkeley geology professor Andrew Lawson, who named it after the San Andreas Valley (now Crystal Springs Reservoir) where he first identified it on the Peninsula south of San Francisco.
The San Andreas Fault enters the Bay Area from the south, passing through San Juan Bautista (where it creeps continuously) and continuing up the Peninsula.
Runs along the Crystal Springs Reservoir and through the hills west of Redwood City, Millbrae, and Daly City. This segment has been "locked" since 1906.
Passes just offshore at the Golden Gate, then comes onshore at Mussel Rock in Daly City. Continues offshore past the city and up through Marin County.
| City | Population | Distance to Fault | Risk Level |
|---|---|---|---|
| San Francisco | 874K | 0 mi | Very High |
| Daly City | 104K | 1 mi | Very High |
| Pacifica | 39K | On fault | Very High |
| Half Moon Bay | 12K | 5 mi | High |
| San Mateo | 105K | 8 mi | High |
| Redwood City | 84K | 10 mi | High |
| Palo Alto | 68K | 12 mi | Moderate |
| San Jose | 1M | 15 mi | Moderate |
Devastated San Francisco. Fire caused 80% of destruction. Estimated 3,000+ deaths. Surface rupture traced for 296 miles.
Struck during World Series game. Collapsed Cypress Structure and Bay Bridge section. 63 deaths, $6B in damage.
Ruptured the northern Peninsula segment. Damaged Mission Dolores and other structures.
Ruptured the southern San Andreas. One of the largest earthquakes in California history.
Scientists cannot predict exactly when the next major earthquake will occur, but they can estimate probabilities based on historical patterns and accumulated stress.
22%
probability of M6.7+ in next 30 years
Last major rupture: 1906
60%
probability of M6.7+ in next 30 years
Last major rupture: 1857
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