Why Geologists are so Concerned with the San Andreas Fault

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 loss of life they experienced were caused by quakes this size.

The San Andreas Fault has the potential to do worse. Vast amounts of energy are stored in the rocks along the fracture, which could be released with disastrous results for millions of people.

The energy stored in the San Andreas Fault

It has been more than 250 years since what seismologists consider a major earthquake on the southern part of the fault. During that time, pressures have built up along a section of the fault that geologists describe as locked.

Therefore, says Professor Yuri Fialko of the Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, “The observed strain rates confirm that the SAF [San Andreas Fault] is approaching the end of its interseismic recurrence.” To paraphrase, the rocks along the fault hold stored energies that will be released, and that could be released at any time.

When is unknowable. Geologists make estimates, publishing what they believe are the chances of a temblor at various points along the fault. Estimates cannot tell the public what to expect tomorrow morning or next year.

Planning for Earthquakes

Building codes in California assume earthquakes will happen. That is why California earthquakes tend to be less destructive than temblors in some other parts of the world. However, older structures still may need retrofitting for earthquake safety.

Another way to keep Californians safe from earthquake damage is to forbid high-density development in high-risk areas. Such locations are not always obvious. Alluvium, land built up from unconsolidated deposits, can transmit two to three times the movement from an earthquake than a structure on bedrock would have to endure.

Prediction and Preparation

Since specific earthquake prediction is still not possible, geologists advise Californians to stock supplies. People should also strap their water heaters and large pieces of furniture, and make plans to stay in touch with loved ones. After the Loma Prieta quake of 1989, voice phone service went out, and many roads were blocked.

Geologists are concerned about the San Andreas Fault because many Californians live in an area that could suffer a devastating earthquake, and yet ignore the possibility. They know that people must prepare for future earthquakes, because someday the big one will come.