From the local vista point known as Twin Peaks, Mary Lou Zoback, a senior scientist with the U.S. Geological Survey (USGS), looks out on a breathtaking view of San Francisco--the gilded dome of City Hall, the diagonal stripe of Market Street, the little neighborhoods marching up and down steep hillsides. Slowly she pivots, taking in the sailboats on the bay, the Golden Gate Bridge, the shimmering surface of the Pacific Ocean. Just out there--she points--a couple of miles offshore, lies the place where, early in the morning of April 18, 1906, the earth's crust cracked like an eggshell, unleashing what--even in the aftermath of 9/11 and Hurricane Katrina--stands as one of the greatest disasters in U.S. history.
Trees were "lash't as tho by a gale," bystanders reported, and fields undulated "like the waves of an ocean." Buildings swayed, clocks stopped, church bells rang, water mains burst, gas lines broke, electrical wires snapped and sparked. Then came the flames, which for three days burned out of control as firefighters stood helplessly by.
Today, 100 years later, the damage that resulted from the great quake seems nearly as shocking as it did then: some 28,000 buildings destroyed, more than 3,000 people killed, at least 225,000 more--roughly half the population of the city of San Francisco at the time--left homeless. But, more shocking still, was the fact that no one, not even scientists, could explain why, without warning, such fury had erupted from the earth below.
That quickly changed, however, as geologists, led by Andrew Lawson of the University of California, Berkeley, raced into the field, making observations that established the existence of a fault line that parallels the California coast for more than 700 miles. They named the fault the San Andreas, after a jewel-like lake that lay within the rift zone less than 10 miles south of what was then America's largest and richest Western city.
In a two-volume report published in 1908, Lawson and his team went on to elaborate a new model of earthquake formation--the elastic-rebound theory--that holds up to this day. For years, they correctly surmised, stress had been ratcheting up along the San Andreas until finally it became so overwhelming that the earth's crust snapped like an overextended rubber band. Moreover, the buildup and release of strain appeared to be recurrent, resulting over time in a succession of earthquakes "of greater or less violence." These pioneering researchers provided the first big clue that earthquakes occur in cycles--that in the area around San Francisco Bay, earthquakes are as certain, if not as regular, as the seasons.
It's this certainty that lends urgency to the efforts by Zoback and her colleagues to remap the San Andreas and its subsidiary faults, to amass new clues to its murky prehistory and to re-create in cyberspace the primordial violence of the 1906 quake. In addition to being the centennial of the last Big One, April 18, 2006, marks the approximate midway point in the countdown to the next Big One--100 years of stress accumulation in one of the world's most earthquake-prone regions. The more scientists learn about the ways in which that stress may be released, the more ominous the next earthquake cycle seems.