The Big One

There probably won't be an earthquake in Southern California today, but five million people will cower under furniture anyway. The Great ShakeOut, the biggest disaster drill in U.S. history, will show us just how prepared the Golden State will be when the big one finally hits.

The problem, of course, is that the big one never comes. California has over 300 faults running beneath its surface, including the massive San Andreas fault, yet the quake to end all quakes has yet to occur. In 1980, a federal report declared the likelihood of a major earthquake striking California within the next thirty years "well in excess of 50 percent." Seismologists predicted a 1993 earthquake in Parkfield, California — which lies along the San Andreas Fault — but the quake did not come until 2004. Earthquake prediction is a tricky practice, and one that, for all their gadgets, measurements, and years of study, scientists have not yet mastered.

When scientists talk about a "big" California quake, they are generally speaking of anything higher than 6.7 on the Richter scale. (The Richter scale is logarithmic, meaning that each number represents a 10-fold increase in magnitude.) Although small quakes can create major damage if they occur in a densely populated area — a 1960 earthquake in Morocco registered only 5.75 but destroyed the entire city of Agadir and killed 12,000 people — earthquakes above a 6 are almost guaranteed to be bad.

Earthquakes can't be stopped; they are a natural process that occurs when too much stress builds up between convergent tectonic plates under the Earth's surface. The plates suddenly slip and the ground rumbles with the release of friction. Geologists have found evidence of earthquakes in California that go back thousands of years, although the first strong, documented earthquake occurred in Los Angeles in 1769. A violent earthquake in the 7.9 range toppled trees and buildings around Fort Tejon — a mountainside army base — in 1857. As severe as the quake was, the state was so sparsely populated at that time that only two people died. The Santa Cruz Mountains and surrounding areas — San Francisco, San Jose, Santa Clara, and Santa Cruz — took a 6.5 magnitude shock on October 8, 1865. Mark Twain witnessed the event and wrote about it in his memoir, Roughing It: "[T]he ground seemed to roll under me in waves, interrupted by a violent joggling up and down, and there was a heavy grinding noise as of brick houses rubbing together. Never was solemn solitude turned into teeming life quicker."

But the disaster Californians remember most vividly, even though most weren't even born yet, was the earthquake that struck San Francisco at 5:12 a.m. on April 18, 1906. The first shockwave registered 8.3 on the Richter scale and shook the city for a full 45 seconds. Many buildings, including San Francisco's city hall, collapsed almost immediately. Seventeen aftershocks came within an hour and fires raged for three days afterward, destroying 500 city blocks. In photos, 1906 San Francisco resembles a war zone; buildings are left half-standing, the streets are littered with debris, barely anything is recognizable. With an estimated 3,000 deaths, 1906 was the deadliest earthquake in California's history. Economists have even traced the following year's economic crash to the quake; millions of dollars went to rebuild the city, putting a strain on the money supply.

San Francisco was hit again on October 17, 1989, during the third World Series game between the city's two teams: the Oakland Athletics and the San Francisco Giants. Measuring 6.9 on the Richter scale, it severed electrical and gas lines, caused part of the Bay Bridge to fall off, and collapsed a one-mile stretch of an elevated Oakland freeway, trapping cars between layers of asphalt and concrete. 37,000 people were injured, 1,000 people were left homeless, but only 63 deaths were directly attributed to the earthquake.

California's fault zones can match the rest of the world's in terms of earthquake magnitude, but when it comes to human casualties they barely register a blip. "They're practically nothing," says Richard Allen, an associate professor of earth and planetary science at University of California at Berkeley. Part of the reason can be attributed to the U.S.'s superior earthquake preparation — California has strict building codes designed to prevent structures from collapse, and events like the Nov. 13 ShakeOut teach individuals what to do in an emergency. For the most part, though, the low death tolls can be attributed to luck. "We haven't had a big earthquake beneath one of our metropolitan centers yet," Allen says. "For example, in '89, the quake started beneath the mountains. There was some damage in Santa Cruz and San Francisco, but San Francisco was more than 60 miles away. When an earthquake occurs in densely populated urban areas, the fatalities will be much higher."

The closest California has come to an "urban" earthquake in recent decades was the 6.7 magnitude 1994 quake in Northridge, a suburb roughly 30 miles from downtown Los Angeles. Parking structures collapsed, overpasses fell down, 9,000 people were injured but only 57 people were killed. Again, most of the shaking occurred in the mountains.

The big one is coming, promises Allen, there's no question about it. Southern California hasn't had a major upset since Fort Tejon in 1857 and is due any day — or decade — for something of magnitude 6.7 or higher. Northern California is ready for one too; the Hayward Fault, which runs along the east side of the San Francisco Bay, averages a major earthquake once every 140 years. The last one occurred in 1868, exactly 140 years ago. The U.S. Geological Survey (USGS) puts the odds of a magnitude 7 earthquake occurring within the next 30 years at 60%. Thirty years may seem a long time to residents, but it's barely a tick of the clock when it comes to the Earth. "We know it will occur," says Allen, " the question is simply when."

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