Los Angeles might not end up as an island when the Big One rocks California, but any sizable seismic event on the San Andreas fault will send L.A. several meters closer to San Francisco. Scientists and the public have long expected a major quake to strike the West Coast; the U.S. Geological Survey estimates that California has a 99 percent chance before 2038 of experiencing at least a magnitude 6.7 quake—the same size as the 1994 Northridge earthquake.
But it could easily be bigger. Much bigger. If most of the San Andreas were to rupture in one event, an earthquake could reach a magnitude 8.2, says Lucy Jones, chief scientist of the Multi-Hazards Demonstration Project at the USGS in southern California.
The San Andreas fault runs about 1,300 kilometers from southern California up past the Bay Area. It forms the boundary between the North American plate, moving southeasterly, and the Pacific plate, heading to the northwest. From geologic records, scientists think that the fault usually ruptures about once every 150 years. The last big movement, however, was about 300 years ago.
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A magnitude 7.8 earthquake (which a 2008 USGS and California Geological Society report calls a “plausible event”) would shake some 10 million southern Californians, killing about 1,800 and injuring 50,000. A rumbler this size, which the USGS modeled as the ShakeOut Earthquake Scenario outreach project, would mean a fault movement of about 13 meters. Such a slip would sever roads, pipelines, railways and communications cables that cross the fault and trigger landslides. Aftershocks—some as powerful as magnitude 7.2—would rattle the region for weeks. The quake would cause some $200 billion in damage, and long-term infrastructure and business disruption would cost billions more, Jones notes.
But the San Andreas isn’t the only fault likely to slip, and seismic activity along one fault—even one thousands of kilometers away—can set off others that are getting ready to go. An offshore 6.5 quake that shook northern California in January occurred on the southern edge of the Cascadia subduction zone, which runs just offshore of the Pacific Northwest. This plate boundary could unleash at least a magnitude 9.0—the size of the 2004 Sumatra quake that spawned a devastating tsunami. Geologic records show evidence of an earthquake in 1700 that sent a tsunami all the way to Japan, and a similarly sized quake has about a one-in-10 chance of occurring in the next few decades.
Predicting earthquakes is a little bit like trying to guess your weather for the week based on knowing the climate, says geophysicist Robert Yeats of Oregon State University. Realizing that a quake will probably occur sometime soon, he adds, “doesn’t affect your holiday plans, but it’s going to affect your building codes.” The bigger buildings can be among the safest—some of the state’s skyscrapers having been built to withstand magnitudes upward of 7.8. And just because a big earthquake appears overdue, the next seismic shift might not lead to the worst-case scenario. Scientists are still learning more about the frequency of big quakes (greater than magnitude 6.0) in the geologic record, and some newer evidence suggests that smaller earthquakes might be more the norm on the San Andreas.
When the Big One does come, it might not prove as devastating as long feared thanks to modern, savvy construction and public readiness campaigns. Much greater havoc can come from even moderate earthquakes in poorer, less prepared areas of the world. The January quake in Haiti, for instance, killed nearly a quarter of a million people—a sobering example of how a sudden slip of a fault can quickly crumble cities that have not had the luxury of careful planning.
