by Janet Grischy

Created on: December 28, 2009

At 800 miles long and 10 miles deep or more, the San Andreas Fault is huge. It has a long history of creating devastating quakes. At least one section of the fault holds enormous energies stored up over centuries, locked in and liable to break loose.

Yet the main reason that geologists are so concerned about the San Andreas Fault is because of the large population that lives and works close to the fault.

The Nature of the San Andreas Fault

It runs along much of the length of California, passing near the great urban centers of the Los Angeles basin and the San Francisco Bay area. The northern segment of the fault runs from near Cape Mendocino to Hollister. It runs just offshore from San Francisco. The central segment runs from Hollister south to Parkfield. The southern segment runs south to the Salton Sea, bending inland at Frazier Park to curve around the mountains that ring Los Angeles.

According to plate tectonics, the San Andreas is actually a ragged joint, a crack between the Pacific plate and North American plate.

The Pacific plate sticks and slides its way northwest towards Alaska. The North American plate is moving too, at a different speed and in a slightly different direction. On part of the central segment of the San Andreas, the plates slide slowly past each other and have for centuries, in a motion called aseismic creep.

In other places the plates become locked, held together nearly motionless, while the strain at the fault line builds. Then the plates may jerk in sudden movements that rip up the land, displace roads and fences, and send out seismic waves that destroy property and take lives many miles away.

The San Andreas connects to a network of smaller faults that extend its influence into most of western California.

Historic Earthquakes

In 1906, movements on the northern segment of the fault caused the Great Earthquake and Fire that shattered San Francisco. Another destructive earthquake occurred in 1857, caused by movement on the central segment of the fault. There has been no truly large earthquake on the southern section of the fault in years.

The Loma Prieta Earthquake of 1989 measured 6.9 on the Richter scale and the Northridge quake of 1994 measured 6.7, according to the U.S.G.S. That government agency estimates that the Loma Prieta quake caused 63 deaths and cost $6 billon in damage. Northridge caused 60 deaths, and perhaps $60 billion in damage. After each quake, rumors said the numbers were low. No one could believe the damage and

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