The San Andreas Fault of California

A new disaster movie called “San Andreas”, from Warner Bros., is coming up on May this year, featuring “The Rock” Dwayne Johnson who plays an emergency rescue pilot who will do anything to save his daughter when a 9.0 magnitude earthquake hits California. After watching the trailer and going through the comments section on one of the leading entertainment website, I was surprised to see that many people couldn’t understand why the movie was called San Andreas. Some of them even expressed disappointment that the movie wasn’t about the popular video game GTA:San Andreas. So here, I take this opportunity to present you the notorious geological marvel – the San Andreas Fault that runs across California, and based upon which the movie is named.

View of the San Andreas Fault, one of the few transform faults exposed on land. Photo credit

The San Andreas Fault is the boundary where two tectonic plates - the Pacific Plate and the North American Plate, meet. The fault runs across the state of California from Mendocino to the Mexican border, splitting it into two parts. San Diego, Los Angeles and Big Sur lies on the Pacific Plate, while San Francisco, Sacramento and the Sierra Nevada are on the North American Plate. The fault is roughly 810 miles long and extends to depths of at least 10 miles within the Earth.

The San Andreas Fault is a transform fault which means that the plates are sliding along the fault. The Pacific Plate is moving north-west relative to the North American Plate, and it’s this movement that causes earthquakes.

The plates are moving past one another at a rate of about 1.5 inches a year, but the movement is not a steady continuous motion. For years the plates will be locked with no movement at all as they push against one another. These stuck sections of the fault store energy like springs, slowly building up strain until the rocks along the fault break and the plates slip a few feet all at once. The breaking rock sends out shock waves in all directions and it is the waves that we feel as earthquakes. During the 1906 earthquake in San Francisco, roads, fences, and rows of trees and bushes that crossed the fault were offset several yards.

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Over much of its length and particularly in the Carrizo Plain, the San Andreas fault is visible as a liner trough that looks spectacular from the air. Viewed from the ground, however, the features are more subtle. In some sections the fault hasn’t moved in many years and is covered with alluvium, or overgrown with brush. In San Bernardino and Los Angeles Counties, many of the roads run along the fault and through the trough. In Palmdale area where Highway 14 goes through the famous roadcut one can see the wrinkled and warped rock layers adjacent to the San Andreas.

California suffers thousands of small earthquakes each year but major ones occur only after long intervals. The last big earthquake to occur along the San Andreas fault was the 1906 San Francisco earthquake of magnitude 7.8. It’s difficult to predict when the next big one will strike, but its probably due in the near future. A new study by the U.S. Geological Survey suggests that California will experience a magnitude 8 or larger earthquake in the next 30 years.

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The San Andreas fault along highway 14 near Palmdale, California. Photo credit

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The fault line in Wallace Creek. Photo credit

Fault line Santa Margarita. Photo credit

Sources: Wikipedia / Geology.com / USGS