When San Andreas Unleashes the Big One ?

What Will Really Happen When San Andreas Unleashes the Big One?
Posted on May 29, 2015 by Soren Dreier
Author: Sarah Zielinski


A giant earthquake will strike California this summer. Skyscrapers will topple, the Hoover Dam will crumble and a massive tsunami will wash across the Golden Gate Bridge. Or at least, that’s the scenario that will play out on the big screen in San Andreas.

The moviemakers consulted Thomas Jordan, director of the Southern California Earthquake Center, before they started filming, but “they probably didn’t take much of my advice,” he says. While the actual threats from the Big One are pretty terrifying, they are nowhere near the devastation witnessed by Dwayne “The Rock” Johnson and his onscreen companions.

Even the largest of San Andreas’ quakes can’t produce a massive tsunami like the one that swells over San Francisco in the movie. “The really big tsunamis, like the one that hit Japan, are caused by earthquakes that generate a major displacement of the ocean floor,” Jordan says. The San Andreas fault sits far inland, and the land slips past on either side. For that reason, a quake also can’t cause the fault to split apart into a giant chasm as it does in the film. And despite the warnings of distraught movie scientists, even the largest of California’s quakes won’t be felt by anything but seismometers on the East Coast.

That doesn’t mean California is off the hook, though. While the movie may be more fantasy than reality, the Big One is coming, and it will produce plenty of destruction. “We think Southern California is locked and loaded, that the stresses have really built up, and when things start unleashing, they could unleash for years,” says U.S. Geological Survey seismologist Ned Field.

California sits at the border between two major tectonic plates—the Pacific plate, which is moving northwest, and the North American plate, which is sliding past it to the southeast. The two plates don’t just meet at a single line, and the state is crisscrossed with dozens of earthquake faults. The San Andreas is the most worrisome, because it generates the quakes that are really dangerous to California residents, Jordan notes.

The northern San Andreas leveled San Francisco in 1906, but it’s been a lot longer since the southern part of the fault ruptured. On average, Southern California has seen big quakes every 110 to 140 years, based on records of past earthquakes and studies of earthquake faults. The last big quake near Los Angeles, a magnitude 7.9, struck Fort Tejon in 1857. Farther south, near Palm Springs, the fault hasn’t ruptured in over 300 years. “Eventually the fault will have to break,” Jordan says.

While seismologists can’t predict exactly when that will happen, every few years they release a forecast for the likelihood of such an event. The latest forecast, published earlier this year by the USGS, estimates a 7 percent chance that a magnitude 8 quake will occur in California within the next 30 years. That’s about as big as earthquakes can get in California, notes Jordan—a magnitude 8.3 quake might be possible if the entire San Andreas fault were to rupture from the Mexico border up to northern California. “We don’t think that’s likely,” he says.