The San Andreas Fault

The San Andreas Fault

The San Andreas Fault is a continental transform fault that extends roughly 810 miles (1,300 km) through California in the United States. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). The fault divides into several segments, each with different characteristics, and a different degree of earthquake risk. Although the most significant (Southern) segment only dates back about 5 million years, the oldest sections were formed by the subduction of a spreading ridge 30 million years ago.

San Andreas fault

The fault was first identified in 1895 by Professor of geology Andrew Lawson from UC Berkeley who discovered the northern zone. It is named after a small lake which was formed in a valley between the two plates. Following the 1906 San Francisco Earthquake, Lawson concluded that the fault extended all the way into southern California. In 1953, geologist Thomas Dibblee astounded the scientific establishment with his conclusion that hundreds of miles of lateral movement could occur along the San Andreas Fault.

The San Andreas fault is the foremost of a set of faults along the boundary between the Pacific plate on the west and the North America plate on the east. The west side moves north, causing earthquakes as it moves. The forces associated with the fault have pushed up mountains in some places and stretched apart large basins in others. The mountains include the Coast Ranges and the Transverse Ranges, both of which consist of many smaller ranges. The basins include the Coachella Valley, the Carrizo Plain, the San Francisco Bay, the Napa Valley and many others. The California geologic map will show you more about those.

San Andreas fault map

San Andreas plate boundary

The northern segment of the San Andreas fault extends from Shelter Cove to south of the San Francisco Bay area. This whole segment, about 300 kilometers long, ruptured on the morning of 18 April 1906 in a magnitude-7.8 earthquake whose epicenter was just offshore, south of San Francisco. In some places the ground shifted by 6 meters, ripping roads, fences and trees apart. "Earthquake trails" on the fault, with explanatory signs, can be visited at Fort Ross, Point Reyes National Seashore, Los Trancos Open Space Preserve, Sanborn County Park and Mission San Juan Bautista. Small portions of this segment ruptured again in 1957 and 1989, but quakes the size of 1906's are not considered likely today.

The creeping segment of the San Andreas fault extends from San Juan Bautista, near Monterey, to the short Parkfield segment deep in the Coast Ranges. While elsewhere the fault is locked and moves in major earthquakes, here there is constant steady movement of about 3 centimeters per year and relatively small quakes. This kind of fault motion, called aseismic creep, is rather rare. Yet this segment, the related Calaveras fault and its neighbor the Hayward fault all exhibit creep, which slowly bends roadways and pulls buildings apart.

The Parkfield segment is at the center of the San Andreas fault. Hardly 30 kilometers long, this segment is special because it has its own set of magnitude-6 earthquakes that don't involve the neighboring segments. This seismological feature plus three other advantages—the fault's relatively simple structure, the lack of human disturbance and its accessibility to geologists from both San Francisco and Los Angeles—make the tiny, colorful town of Parkfield a destination out of proportion to its size. A swarm of seismic instruments has been deployed for several decades to catch the next "characteristic earthquake," which finally came on 28 September 2004. The SAFOD drilling project pierces the fault's active surface just north of Parkfield.

The central segment is defined by the magnitude-8 earthquake of 9 January 1857, which broke the ground for about 350 kilometers from the hamlet of Cholame near Parkfield to Cajon Pass near San Bernardino. Shaking was felt over most of California, and motion along the fault was 7 meters in places. The fault takes a large bend in the San Emigdio Mountains near Bakersfield, then runs along the south edge of the Mojave Desert at the foot of the San Gabriel Mountains. Both ranges owe their existence to the tectonic forces across the fault. The central segment has been fairly quiet since 1857, but trenching studies document a long history of great ruptures that will not stop.

From Cajon Pass, this segment of the San Andreas fault runs about 300 kilometers to the shores of the Salton Sea. It splits into two strands in the San Bernardino Mountains that rejoin near Indio, in the low-lying Coachella Valley. There is some aseismic creep documented in parts of this segment. At its south end, the motion between the Pacific and North American plates shifts to a stairstep series of spreading centers and faults that runs down the Gulf of California. The southern segment has not ruptured since some time before 1700, and it is widely considered "overdue" for an earthquake of approximately magnitude 8.

San Andreas  fault earthquakes

This often comes up when when people talk about earthquake activity along the Pacific coast of the United States. Seismologists have predicted that a massive scale (8.0 or higher on the Richter Scale) earthquake will shake the region sometime within the next 30 years or so. This is the so-called "Big One" that makes many Californians understandably nervous and inspires a variety of apocalyptic disaster speculations.

But while the Big One would definitely wreak mass destruction, it would not sink part of California into the ocean, nor would it break the state off from the rest of the country. The idea comes from a misunderstanding of the seismic forces that cause earthquakes in the region.

Powerful earthquakes occur frequently along the west coast of the United States because the region is near a boundary between two tectonic plates. If you've read How Earthquakes Work, then you know that the earth's surface is made up of large, rigid plates that slowly drift over the mantle layer below. At the boundaries between plates, a number of things can happen. The Pacific plate and the North American plate simply grind against each other -- one creeps slowly northwest and one creeps southeast.

This boundary forms a fault line that extends under the ocean and on land along the west coast of the United States. The San Andreas fault in California is the piece that's on land. Smaller faults form in the crust material near the boundary line due to the forces of the plates pushing on each other.

Friction builds up along faults because the two sides are pushed very tightly together. If the force of friction exceeds the forces moving the earth, the two sides will become "locked," so they stop creeping. When this happens, tension builds up along the fault line until the force of movement is great enough to overcome the force of friction. Then the pieces of earth suddenly "snap" into place, releasing a large amount of energy that causes earthquakes in the earth's crust.

Many scientists estimate that there is enough tension built up along some locked California faults, that when they do finally slip, the earthquake will be extremely powerful. The Hayward Fault particularly concerns these scientists because it runs under heavily populated areas in and around Los Angeles.

The notion that part of California will break off was likely inspired by the San Andreas fault. After all, since the fault goes right through California, one part of the state is on the Pacific plate and one is on the North American plate. If those plates are moving in different directions, it make sense that the two pieces of California will move in different directions too.

And this is indeed the case. But, even in a massive shift along the fault, the plates travel an incredibly short distance -- a matter of feet in the most extreme shifts. The tension cannot build up to the point that one entire mass of land will shift many miles in relation to another one, so you will not see any sizable piece of land breaking away from another. Instead, the pieces of land will move away from each other very slowly, taking millions of years to make large scale changes. One end of California may slowly drift so that it is eventually under water, but this can hardly be construed as "sinking into the ocean."

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