Geologists have spotted at least five other fault zones in the region, from the Canadian border south to Olympia. The faults bear signs of half a dozen ruptures over the past 2,500 years, and one fault, the Utsalady, just north of Seattle, might have ruptured as recently as the early 1800s. The evidence amassed so far suggests an average repeat time for a major shallow continental earthquake from centuries to millennia. The USGS has mounted a campaign to map the faults in detail. To do this, scientists use what they call active-source seismics—creating booms, then tracing vibrations through the earth with instruments to detect where subterranean breaks interrupt rock layers. Friendly Seattleites almost always let them dig up their lawn to bury a seismometer, and let them hook it to their electricity. Some neighbors even compete to land one of the instruments, out of what USGS geophysicist Tom Pratt calls “seismometer envy.”
To create the vibrations, scientists have used air guns, shotguns, sledgehammers, explosives and “thumpers”—piledriver- type trucks that pound the ground with enough force to rattle dishes. (A few years ago scientists had to apologize in the morning paper after one nighttime blast alarmed residents who thought it was an earthquake.) The USGS also made the most of the city’s demolition of its aging Kingdome stadium with explosives in 2000. “We said to ourselves: ‘Hey, that’s gonna make a big boom!’ ” says Pratt, who helped plant 200 seismometers to monitor the event.
One day, Atwater and USGS geologist Ray Wells took a ferry to Restoration Point. The flat lower terrace is now a golf course, and on the cliff above people have built expensive homes. From here, the scientists pointed out the invisible path of the fault under Puget Sound toward Seattle, past a ten-mile strip of shipping-container piers, petroleum tank farms and industrial plants, to the city’s passenger ferry docks—the country’s busiest. As the fault reaches land, it crosses under the waterfront Alaskan Way Viaduct, a 1950svintage raised double-decked highway that almost collapsed in the 2001 Nisqually quake and is guaranteed to pancake with anything bigger. (Many geologists avoid driving on it.) Next, the fault passes crowds of skyscrapers up to 76 stories high, and under the two new stadiums housing the Seattle Seahawks football team and Mariners baseball team. It cuts beneath I-5, proceeds under a steep knoll topped by the headquarters of Amazon.com and forms the southern shoulder of I-90, and heads out to the rapidly growing suburbs around LakeSammamish.
That is just the Seattle fault; the others zigging across the region could well be connected to it. Many scientists say it is even possible that the faults’ activities are connected by some grand mechanism to the great subduction-zone quakes out at sea, for many of the inland quakes seem to have occurred around the same times as those on the seafloor. But the inland mechanics are complicated. According to one currently popular theory, Washington is being pushed by Oregon northward, up against Canada. But Canada is not getting out of the way, so Washington is folding like an accordion, and sometimes those folds—the east-west faults—break violently. “Most people don’t want to come right out and say it, but it is all probably linked together in some way we don’t understand,” says the USGS’s Art Frankel.
Geophysicists recently created a stir when they discovered that the deeper part of the ocean slab, subducting from the west under southern British Columbia and northern Washington, slips with uncanny regularity—about every 14 months—without making conventional seismic waves. No one knows if this “silent” slip relieves tension in the offshore subduction zone or increases it—or if it could somehow help trigger inland quakes. This spring, geophysicists funded by the National Science Foundation will drop instruments into eight deep holes bored into the Olympic Peninsula, west of Seattle, in hopes of monitoring these subtle rumblings. In addition, 150 satellite-controlled global-positioning instruments will be set out across the Northwest to measure minute movements in the crust.
In any event, Seattle is one of the world’s worst places for an earthquake. Ascenario released last month by a joint private- government group estimates the damage from a 6.7 magnitude shallow crustal quake at $33 billion, with 39,000 buildings largely or totally destroyed, 130 fires burning simultaneously and 7,700 people dead or badly hurt. Part of the city sits on a soft basin of poorly consolidated sedimentary rocks, and like a bowl of gelatin this unstable base can jiggle if shocked, amplifying seismic waves up to 16 times. The harbor sits on watery former tidal mud flats, which can liquefy when shaken. One computer model shows a ten-foot tsunami roaring from Puget Sound over the Seattle waterfront to mow down cargo and passenger docks, and advancing toward the U.S. Navy shipyards in Bremerton. Even one major bridge collapse would paralyze the city, and engineers predict dozens. Seattle has a lot of high ground—some hillsides are so precipitous that driving up city streets can make ears pop—so landslides, already common in heavy rains, are predicted by the thousands.
The city is getting ready, says Ines Pearce, a Seattle emergency manager. A stricter building code was adopted last year. Raised-highway supports are being retrofitted to keep them from crumbling. Firehouse door frames are being reinforced to keep trucks from being trapped inside. Some 10,000 residents have been organized into local disaster response teams. Schools have removed overhead flush tanks and other hazards, and students duck under their desks in monthly “drop, cover and hold” earthquake exercises reminiscent of 1950s atomic bomb drills. But the preparations may not be enough. Tom Heaton, a California Institute of Technology geophysicist who first theorized the subduction threat to the Pacific Northwest and is now analyzing Seattle’s infrastructure, says that even resistant structures may not survive a major crustal quake or one from the subduction zone. “Earthquake engineers base their designs on past mistakes. No one’s ever seen ground shaking like what would occur in a giant earthquake,” he says.
Down in the basement of his home, on a leafy Seattle street, Brian Atwater pointed out where he spent $2,000 in the 1990s to reinforce his wooden house frame and bolt it to the concrete foundation, to better secure it. During the Nisqually quake, cracks broke out all over his plaster walls, and his chimney got twisted and had to be replaced. But the house didn’t go anywhere. If something worse comes along, he hopes the reinforcing will allow his family to escape alive and salvage their possessions.
But there are some risks Atwateris willing to abide. On the way back from fieldwork one night recently, he was driving toward his house when he swung his pickup truck away from I-5—the obvious route—onto the dreaded Alaskan Way Viaduct. Wasn’t he nervous? “I’d rather take my chances here,” said Atwater, bumping along high over the lights of docks and ships in the harbor. “People over on I-5, they drive too crazy.”