On April 18, 1906, the world awoke abruptly to the existence of the San Andreas Fault. A 7.8 earthquake ripped open the fault's northern segment just south of San Francisco, shaking many of the city's buildings to rubble and sparking fires that incinerated the rest. At least 1,000 people lost their lives; the city sustained nearly a half-billion dollars in property damage. It was one of the worst catastrophes in U.S. history; it still ranks among the top 10 deadliest natural disasters to strike on U.S. soil.
Some modern seismologists admit to morbidly wishing for earthquakes, if only to have a chance to observe the forces they have spent their lives studying. A hundred years ago, geologists were no different. The 1906 temblor split a crack in the earth nearly 300 miles long; the earth on either side had slipped north and south, opening up 24-foot gaps in fences at places where they crossed the fault. "It afforded," as geologist Andrew Lawson wrote at the time, "an exceptional opportunity for adding to our knowledge of earthquakes."
Without fancy instruments, radiocarbon dating, or even knowledge of plate tectonics, Lawson, who chaired the Department of Geology at the University of California at Berkeley, dispatched teams of geologists and students to study the force that laid waste to San Francisco. They followed scarps, valleys and tumbled-down rocks from Humboldt County, across the Bay to Mussel Rock south of the city and through Southern California's mountains, all the way to the Coachella Valley, southeast of Los Angeles. By the autumn of 1908, Lawson's team had constructed a near-perfect rendering of the San Andreas Fault, but for one segment: While Lawson's chief geologist on the scene, Harold Fairbanks, speculated that the fault continued down to the Salton Sea, he could not follow an accurate line to confirm it. The far southeastern segment remained a mystery.
A century after Lawson completed his report, geologist Ken Hudnut sits in his office at the U.S. Geological Survey and uses Google Earth to examine some of the same features that Lawson's crew explored on foot and horseback. A tall, sandy-haired 47-year-old with a teacher's straightforward demeanor, Hudnut heads up what's known as the Southern San Andreas Fault Evaluation, or SoSAFE. The project aims to log the past 2,000 years of earthquakes along the fault's southern stretch to get a better sense of what the fault segment might do in the future, and when it might be active. The last major earthquake on the southern segment happened in 1857; geologists' best guess is that it was centered at a place called Fort Tejon, just north of Los Angeles. And if geology follows any pattern at all, it appears that this segment of the fault is due to rip again soon -- right about now, in fact.
"We didn't appreciate that until recently," Hudnut says. "For the longest time, we thought that 1857 section of the fault had a recurrence interval of anywhere from 150 to more like 300 years." Now scientists think that window could be as narrow as 100 years. They're also beginning to suspect that, over time, the fault has been more prone to massive rips than moderate ones. "Historically, we've seen that it can produce a 7.8," Hudnut says. "But we also have to wonder whether it can break in even bigger earthquakes." On the logarithmic moment magnitude scale -- a refinement of the mathematical formula Charles Richter devised to compare earthquakes in 1935 -- a magnitude 8 earthquake releases 32 times the energy of a 7, and is 1,000 times more energetic than a 6. Hudnut thinks the southern San Andreas may have enough energy stored up to produce an 8. "We have no reason to doubt it, but we can't prove it -- yet."
California has some of the strictest building codes in the world. Since the Long Beach earthquake in 1933, the state has passed rigorous standards for construction, particularly for schools. When a 7.3 quake hit Landers, Calif., in 1992, an elementary school one-half mile from the epicenter held up so well that it served throughout the day as the neighborhood emergency shelter. But even if most buildings stay standing, a 7.8 on the southern San Andreas could still devastate the cities of Southern California, possibly rendering them uninhabitable for weeks, even months, as Hurricane Katrina did New Orleans. The reason: Many of the supply lines and transportation arteries that feed into Los Angeles come through the San Bernardino Mountains, right along the fault. An earthquake of that magnitude, at that location, could shred them all.
SoSAFE is part of the USGS-run Multi-Hazards Demonstration Project, an effort to bring geologists, economists, public health officials and politicians together to figure out how to prevent natural disasters like earthquakes from evolving into catastrophes -- and determine whether it's worth spending billions preparing for something that may never happen. On Nov. 13, another offshoot of the Multi-Hazards project, a consortium of schools, scientists and government agencies called "the Earthquake Country Alliance," will stage its first public relations event: the Great Southern California ShakeOut. This region-wide drill will simulate a 7.8 earthquake on the southern San Andreas -- using a possible location, and potential magnitude, of the long-awaited Big One.
At 10 a.m. that Thursday morning, teachers will order schoolchildren to duck, cover and hold on under their desks, pretending that books and other objects are flying across the room. Business owners will put out their cigarettes and practice turning off the gas. People at home will fill bathtubs with the water that, in an actual earthquake, might provide the region's only drinkable supplies. Utility companies will hoist temporary towers to replace imaginary downed transmission lines; emergency workers will practice responses to calls for help from trapped people who don't exist.