Some quakes are so powerful they leave the earth speechless. The Great San Francisco Earthquake of 1906, a massive 7.8 quake, shook the region so violently that scientists believe it cast a calming "shadow" over a half-dozen parallel faults, sending the entire Bay Area into a long seismic sleep.
In the seventy years leading up to the catastrophic day, at least fifteen quakes measuring 6.0 to 6.9 slammed the Bay Area. But since the morning of April 18, 1906, when the ground convulsed for nearly a minute, there has been precious little activity -- nothing exceeding a 6.5 on the infamous San Andreas; nor the Calaveras, which flanks I-680 through Pleasanton and Danville. The Hayward Fault, which cuts through the East Bay Hills, has remained essentially idle. Ditto the San Gregorio, Rodgers Creek, Concord-Green Valley, and Mount Diablo faults. These giants have been slumbering in relative peace for the better part of a century -- which is striking given that the region's soaring hills and deep valleys were formed by countless millennia of seismic upheaval. "The Bay Area has the highest density of active faults per square mile of any urban center in the country, and on a long-term basis it has the highest amount of earthquake energy released per square mile of any urban center in the country," says David Schwartz, a geologist with the US Geological Survey. "So we're really kind of living at ground zero."
Loma Prieta was not, in fact, a local quake. The 6.9 that rocked the Bay fifteen years ago last October was centered sixty miles south, in the Santa Cruz Mountains near Watsonville. Although it was within the San Andreas zone, seismologists believe the rupture occurred on a sub-parallel fault, not the San Andreas itself. And because a quake's energy diminishes over distance, Loma Prieta had weakened considerably by the time it reached the East Bay. It was also a short and clean rupture that, in a freak bit of good luck, struck in the early fall when landslide risk is minimal, and during game three of a Giants-Athletics World Series. Hundreds of people who otherwise would have been stuck in rush-hour traffic and crushed by the doomed Cypress freeway were at Candlestick Park or safely at home watching the ball game.
As for the toll -- 63 deaths and an estimated $10 billion in overall damage -- scientists who study the Bay's earthquake potential are unanimous: We ain't seen nothing yet. "[Loma Prieta] was merely a warning shot across our bow," says William Lettis, a Walnut Creek-based consultant in applied earth science. "It was enough to shake everybody up and say hey, earthquakes can happen, but it didn't kill very many people. ... An earthquake in the heart of the Bay Area will be ten to a hundred times worse than Loma Prieta."
It is not an overstatement to say anyone who thinks Loma Prieta was "the big one" is in a dangerous state of denial. On the moment magnitude scale -- the Richter scale successor now used to rate quakes -- each point represents a 33-fold increase in released energy, which means the great quake of 1906 was 2,320 percent stronger than Loma Prieta. If a 6.9 temblor managed to kill scores of locals, pancake an East Bay freeway, close ten bridges, and shut down the Bay Bridge for more than a month, all from a distance of sixty miles, consider the scenario a repeat of 1906 would unleash. Our world would come falling down.
It turns out the protective shadow of the 1906 quake was a mixed blessing. While Bay Area residents experienced fourteen times fewer 6.0-plus quakes in the 75 years following the big one, compared with the 75 years that preceded it, this geological cease-fire came with a price. It has led to decades of public apathy, and has allowed developers and government agencies in this fast-growing region to encroach upon the deceptively quiet fault traces.
Major Bay Area highways and BART structures cross or run dangerously close to the faults. Key gas, water, and sewage lines crisscross them, as do telephone and power cables. Cal State East Bay sits alongside the primed Hayward Fault, and UC Berkeley straddles the fault -- which famously runs between the goalposts at Cal's Memorial Stadium.
Perhaps most alarming is that authorities have placed critical lifeline facilities such as hospitals and power stations on hillsides vulnerable to landslides, or on shoreline landfill that may literally turn to liquid in a powerful earthquake. San Francisco and Oakland airports, Oakland's main sewage-treatment plant, portions of the Oakland port, the Bay Bridge toll plaza -- all are built on this type of land. Despite active retrofitting efforts, some of the East Bay's most critical structures remain vulnerable: the Bay Bridge, BART tracks and tunnels, medical centers, downtown office high-rises, and apartment buildings that house tens of thousands of local residents. As it stands, the Bay Area is seriously pushing its luck.
And now for the really bad news: We're coming out of the shadow. US Geological Survey geophysicist Tom Parsons estimates that the 1906 quake reset the Bay Area's faults, delaying their next ruptures by 17 to 74 years, depending on the fault. But he also believes the relaxation effect wore off by 1990 -- and that local faults are now busy accumulating the stress that will ultimately burst free in a deadly new barrage of seismic activity.
How likely is a killer quake? In 2002, a working group composed of researchers from the USGS, universities, and private institutions calculated a 62 percent chance that a 6.7-or-larger quake will strike on a local fault by 2032. In addition, they came up with an 80 percent probability that we'll see one or more local shakers in the 6.0 to 6.6 range over the next 28 years.