Future Shocks

Modern science, ancient catastrophes and the endless quest to predict earthquakes

San Francisco in 1906. (USGS Photo)
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Atwater kneeled in the shallows and scraped riverbank mud down onto his thighs, then smoothed the bank with the nejiri gama. Below the two and half feet of brownish tidal muck lay a half-inch band of gray sand, which was neatly draped over black peat. The peat was laced with tree roots, even though the nearest visible tree was far across the marsh. “Hoo, that’s nice, that’s fresh!” Atwater shouted. “Old dependable!” These trees grow only above the tide line and were now below it. Something, he said, had dropped this ecosystem several feet all at once; all signs point to a seafloor quake. Radiocarbon dating has shown the plants died about 300 years ago. The overlying sand sheet was the clincher: only a tsunami could have laid it down.

Atwater, 53, has been combing the region since 1986 for evidence of past earthquakes, and his work at a dozen estuaries— in addition to other scientists’ findings—has revealed not only the great 1700 earthquake and tsunami but also a dozen other major quakes over the past 7,000 years. Recent seafloor studies off the Pacific Northwest coast tell the same story. Overall, big subduction-zone quakes strike on average every 500 to 600 years. But the intervals between them range from 200 to 1,000 years. “If we can predict that we’re in a short interval, we’ve essentially used up our time. But we cant predict,” says Chris Goldfinger, a marine geologist at OregonStateUniversity. Recent studies using satellite-controlled global-positioning systems and other new technology confirm that the region’s tectonic plates are converging and locked together. In some places, the Washington and Oregon coastlines are rising by 1.5 inches a year. As Atwater points out, “That doesn’t sound like much until you multiply it by, say, 1,000 years, and you get ten feet.” And if the land has risen that far, it could drop that far when a quake comes, just like the layer of peat Atwater uncovered in the tidal estuary. “The bulge will collapse during the next earthquake, and there will be new ghost forests,” he says.

We paddled farther up the Copalis to the mouth of a small creek, where Atwater located the 1700 tsunami sand sheet’s continuation in the riverbank. With his nejiri gama, he dug out clumps of perfectly preserved ancient spruce needles, apparently cast up by the great waves. Nearby he uncovered a shard of fire-cracked rock—evidence of a cook fire. “That’s spooky,” he says. “It makes you wonder what happened to these people.” Paleoseismology has shed new light on legends by aboriginal coastal peoples such as the Yurok and the Quileute. Many stories describe times when the earth shook and the ocean crashed in, wiping out villages, stranding canoes in trees and killing everyone but the fastest or luckiest. Storytellers often explained these events as the result of a battle between a great whale and a thunderbird. “Well before settlers came here, Native peoples dealt with earthquakes,” says James Rasmussen, a councilman for the Duwamish people in Seattle. Archaeologists have now identified many sites that contain pottery and other artifacts that were submerged by rising waters. Apparently, Native people over the years moved closer to the shore or fled it as thunderbird and whale fought it out.

Today, of course, we’re not so light on our feet. A recent study estimates that ten million people on the U.S. West Coast would be affected by a Cascadia subduction-zone quake. Three hundred years of tectonic pressure has now built up. The shaking from such a quake, lasting two to four minutes, would damage 200 highway bridges, put Pacific ports out of business for months, and generate low-frequency shock waves possibly capable of toppling tall buildings and long bridges in Seattle and Portland, Oregon. A tsunami of 30 feet or more would reach parts of the PacificCoast in little over half an hour. Of special concern to WashingtonState officials are places like the coastal resort town of Ocean Shores, on a long sand spit with a narrow access road that serves 50,000 visitors on a summer day. Here, the highest ground—26 feet above sea level—would hold only “about 100 people who are very good friends,” says Tim Walsh, state geological hazards program manager. He suggests that the town consider “vertical evacuation”—building multistory schools or other public structures in which people on the top floors could escape a tsunami, assuming the buildings themselves could withstand the impact. To flee a tsunami, people need warnings, and the U.S. government has set out Pacific Ocean monitors to pick up signals from known danger spots, not only in the Pacific Northwest, but in Japan, Russia, Chile and Alaska as well. This system is designed to transmit warnings to countries across the basin within minutes. Similar networks are planned for the Atlantic and Indian oceans.

In WashingtonState, officials are trying to educate a public that has regarded the threat casually—but may now pay a lot more attention with the Indian Ocean tsunami as an object lesson. A few weeks before the disaster, Atwater and Walsh drove to Port Townsend, a Victorian-era seaport on the Strait of Juan de Fuca, about midway between Seattle and the open ocean, where they ran a tsunami workshop that was attended by only a handful of emergency officials and a few dozen residents. Walsh pointed out that a tsunami might take a couple of hours to reach Port Townsend, which has nearby cliffs for retreat. The town is dotted with blue-and white tsunami warning signs. Unfortunately, they are a popular souvenir. “Just please stop stealing the signs,” Walsh chided the audience as he handed out free paper replicas of the signs.

“A lot of people think of tsunamis as some kind of cool adventure,” Walsh said after the meeting. He remembered that following a big 1994 seafloor quake off Russia’s Kuril Islands, surfers in Hawaii headed for the beaches. Afilm crew actually set up at the surf line on the Washington coast, hoping to catch a giant wave that, fortunately for them, never came. Walsh said, “I think they won’t be doing that next time.”

Brian Sherrod, a geologist with the USGS in Seattle, has rush-hour traffic to thank for one discovery. Recently he led some visitors under Interstate 5, a ten-lane raised artery traversing the city’s downtown, as thousands of northbound cars and trucks thundered overhead. He pointed to the ground beneath one of the massive concrete supports, where the ruptures of an earthquake fault in prehistoric times had tortured the usually flat sediment layers into broken waves, then smashed and bent them backward so that lower ones were shoved over the upper—as if someone had taken a layer cake and slammed a door on it. This is one of many scary signs from Seattle’s past, though one of the few visible to the naked eye. “I spotted this when I was stopped in Friday-afternoon traffic,” said Sherrod, pointing to the southbound lanes, 50 feet away at eye level. “I was singing real loud to the radio. Then I stopped singing and yelled, ‘Holy sh-t!’ ”

Earthquakes have long been a fact of life in Seattle. Each year, inland Washington gets a dozen or so quakes big enough to feel, and since 1872, about two dozen have caused damage. Most cluster under the Puget Sound lowland, the heavily developed run of bays, straits, islands and peninsulas running through Seattle south to Olympia. Larger-than-usual quakes in 1949 and 1965 killed 14 people. In the past few decades, building codes have been upgraded and a network of seismometers installed across Washington and Oregon. Those instruments showed that most of the smaller quakes are shallow readjustments of the earth’s crust—rarely a big deal. The more sizable events, like quakes in 1949 and 1965, typically emanate from depths of 30 miles or more. Fortunately, this is far enough down that a lot of energy bleeds from the seismic shock waves before they reach the surface. The most recent big deep one was the February 28, 2001, Nisqually quake— magnitude 6.8, as measured at its 32-mile-deep point of origin. It damaged older masonry buildings in Seattle’s picturesque Pioneer Square shopping district, where unreinforced bricks flattened cars; at the vast nearby cargo harbor, pavement split and sand volcanoes boiled up. Though damage was some $2 billion to $4 billion statewide, many businesses were able to reopen within hours.

One of the first hints that monstrous quakes take place near Seattle’s surface, where they can do catastrophic dam- age, came when companies were hunting for oil under Puget Sound in the 1960s, and geophysicists spotted apparent faults in the sound’s floor. Into the 1990s, these were presumed to be inactive relic faults; then scientists looked more closely. At Restoration Point, on populous BainbridgeIsland across Puget Sound from downtown Seattle, one USGS scientist recognized evidence of what geologists call a marine terrace. This is a stair-step structure made of a wave-cut sea cliff topped by a flat, dry area that runs up to several hundred feet inland to a similar, but higher cliff. Restoration Point’s sharp, uneroded edges, and ancient marine fossils found on the flat step, suggested the whole block had risen more than 20 feet from the water all at once. Several miles north of the point lies a former tideland that apparently had dropped at the same time. These paired formations are the signature of what’s known as a reverse fault, in which the earth’s crust gets shoved up violently on one side and down on the other. This one is now called the Seattle fault zone. It runs west to east for at least 40 miles, under Puget Sound, downtown Seattle (cutting it in half) and its suburbs, and nearby lakes.

Along the Seattle fault on the east side of the city, Gordon Jacoby, a Columbia University tree-ring specialist, has identified another ghost forest—under 60 feet of water in Lake Washington. The trees did not sink; they rode off a nearby hill on a gigantic quake-induced landslide in the year 900, apparently at the same time that Restoration Point rose. Yet more evidence of that devastating event surfaced a decade ago several miles north of the Seattle fault. The city was digging a sewer, and Atwater spotted in one of the excavations an inland tsunami deposit—the first of many tied to that quake. The tsunami came when the fault thrust up under Puget Sound, sending out waves that smashed what is now the booming metropolitan waterfront.


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