The southern segment of the San Andreas fault "looks like it's locked, loaded and ready to go."
—Thomas Jordan, director of the Southern California Earthquake Center
The outer part of the Earth is divided into a dozen or so large sections known as tectonic plates. These plates are in constant motion, grinding and pushing against each other. The grinding and the pushing cause mountains and deep valleys to form and volcanic eruptions and earthquakes to occur.
The boundary between the Pacific and North America plates is a wide region, running from the Pacific coast to west Texas. (Which is why earthquakes and volcanic eruptions occur across much of the western United States.) But most of the motion between these two plates is taken up in California, and so it is here that most of the earthquake activity is occurring today.
There are more than 700 earthquake faults in California. Most of these faults rupture only every few thousand years or so and most can produce only moderate earthquakes. But there are dozens of major faults that extend for distances of many tens of miles or more, and it is along these faults that damaging earthquakes occur. And the most famous of these is the San Andreas fault.
San Andreas Fault
San Andreas fault near Carrizo Plain
"It's locked, loaded and ready to go."
—Thomas Jordan, director of the
Southern California Earthquake Center
But people who do not live next to the San Andres fault should not be complacent. Destructive earthquakes can occur elsewhere in California
In the San Francisco Bay area, destructive earthquakes have occurred along the Hayward fault in 1868, the Calaveras fault in 1984 and near the San Andreas fault in 1989. The last one, known as the Loma Prieta earthquake because it occurred near that peak in the Santa Cruz Mountains, caused 62 deaths, left nearly 8000 people homeless and caused nearly $6 billion in property damage.
Map of ground ruptures produced by historical earthquakes in the San Francisco Bay area. Several major faults and cities are shown.
The San Andreas fault is part of a system of major faults that cut through California. It receives its notoriety because most of the motion between the Pacific and North America plates is taken up along this long fault. And so it is along this fault that the most frequent major earthquakes occur in California.
The San Andreas fault runs almost the entire length of California from Cape Mendocino to the eastern edge of the Salton Sea. It has ruptured as two major earthquakes in the last 150 years: The Fort Tejon Earthquake of 1857 north of Los Angeles and the Great San Francisco Earthquake of 1906. The section between the these two ruptures has had a series of six moderate earthquakes since 1857. The section south of the 1857 earthquake has not ruptured since 1680, and so it is this section that poses one of the highest seismic threats to California.
The San Andreas fault runs from near Cape Mendocino to the eastern shore of the Salton Sea.
In southern California, there have been a series of destructive earthquakes that did not originate on the San Andreas fault. In particular, the three most destructive earthquakes in this region—in 1933, 1971 and 1994—occurred on faults that were unknown until after those three earthquakes. The last one, known as the Northridge earthquake, killed 60 people, left more than 20,000 homeless and damaged more than 40,000 buildings. The property loss was at least $44 billion.
Map of ground ruptures produced by historical earthquakes in southern California. Several major faults and cities are shown.
One of the things learned about earthquakes in the last few decades is that they do not occur randomly and they do not reoccur like clockwork. Instead, earthquakes—even large ones—cluster in time and space. And when there is a cluster of large earthquakes—one that occurs over a period of decades—there is an earthquake storm.
The most famous earthquake storm happened in northern Turkey between 1939 and 1999 when 13 major earthquakes occurred along the North Anatolian fault. The North Anatolian and San Andreas faults are similar in several ways: (1) Both are strike-slip faults, meaning the motion is primarily horizontal; (2) both are about 800 miles in length; and (3) seismic energy is building at about the same rate on both faults.
For more than a century there has been a seismic lull in California. The devastating earthquakes that have occurred—1971 in San Fernando, 1989 at Loma Prieta and 1994 in Northridge—released a minuscule amount of the seismic energy that has been building. And this energy can be released in only one way: as a series of large earthquakes. And the series of large earthquakes could happen as an earthquake storm.
Audiobook, 7 CDs 9 hours
The Fascinating History and
Volatile Future of the
San Andreas Fault
Hardcover and Paperback, 254 pages
Dvorak has done earthquake science sterling service by writing what is unarguably the best, the most comprehensive and compellingly readable book about the great fault.
—Simon Winchester, author of Pacific and The Men Who United the States
It's not just Californians who should pay attention to Dvorak's exploration of earthquake science.
—Christian Science Monitor
. . . treats Californians and other tectonic enthusiasts to an enjoyable history of the Golden State's earthquakes alongside a bracing look at potential future ones.
Recounts California's precarious relationship with the tectonic boundary between the Pacific and North American plates. Recommended reading on surviving the end of civilization or the zombie apocalypse.
—Los Angeles Magazine