SCIENCE / SEISMOLOGY : Seattle Was Site of Big Ancient Quake, Study Says

Hundreds of years before European settlers founded Seattle, an earthquake fault under what is now the Kingdome shook the area with a mighty force, lifting parts of the Puget Sound shore by 20 feet, triggering massive landslides and pushing a tidal wave toward Canada.

That scenario, reconstructed by a team of geologists and botanists and published today in the journal Science, gives some sense of the potential seismic punch of a recently discovered earthquake fault under one of the West Coast’s largest urban areas and busiest ports.

Robert C. Bucknam of Golden, Colo., and other risk-assessment geologists from the U.S. Geological Survey estimated that the temblor they have reconstructed probably occurred about 1,000 to 1,200 years ago and would approximate a magnitude 7 quake today.

This same slip fault, they warned, is still capable of generating earthquakes of magnitude 7 or greater.

Craig Weaver, who coordinates the USGS earthquake hazards program in the Northwest, said: “This is a real reminder that Seattle is big-time earthquake country.”

Earthquakes are not unknown in northwestern Washington: A seismotectonic map published by the USGS in 1985 first described the fault, along with the epicenters of hundreds of small and moderate quakes in the area. A 7.1 temblor shook Puget Sound southwest of Tacoma in 1949 and a 6.5 quake hit south of Seattle in 1965.

However, these quakes occurred on the far edges of the urban area. John Adams of the Geological Survey of Canada said that he and other scientists were surprised to find evidence that a fault directly under Seattle might be so destructive.

In a Science commentary on the findings, Adams observed that Seattle and Vancouver, Canada, “may be at greater risk than anticipated,” but he stressed that scientists need more information about the history, stresses and strains on the fault to accurately assess the potential danger.

Estella B. Leopold, a University of Washington botany professor enlisted by Bucknam to study and date some of the organic material retrieved in core samples, said: “Every one of us is being very cautious about what this means. What we are saying is that this happened once--more than once, according to (some) data--and it can happen again. . . . If I were building, say, a nuclear power plant, I’d be very careful.”

Bits and pieces of evidence of a major ancient quake near Seattle had been uncovered in the past decade--in particular, signs that prehistoric tidal marshes suddenly became freshwater or marine environments. This indicated great shifts in the earth but did not prove conclusively what caused them.

Bucknam’s team sought to explore the question by focusing on the fault under Seattle.

Known formally as the Seattle Fault, but referred to by some as the Kingdome Fault because it runs under that domed sports stadium, the fracture crosses beneath footings for the floating bridge on Interstate 90 and crosses unnervingly close to historic, unreinforced brick buildings and downtown skyscrapers.

Deducing Puget Sound’s seismic history was difficult because bedrock, which would show the traditional, well-defined fault scarps, is particularly deep in the region. In many areas, the scientists said they found it buried under 1,000 feet or more of soil and other material.

However, Bucknam and his team did find some telltale bedrock jutting incongruously over the surface at Restoration Point on Bainbridge Island, roughly seven miles across the sound due east of downtown Seattle.

There was no visible rupture, no great crack in the bedrock, but land north of this sight showed evidence of great subsidence and land south of it showed signs of significant uplift. Something had tilted the floor of Puget Sound.

Spurred by this evidence of an ancient earthquake, the USGS crew teamed up with botanists, zoologists and other geologists to try to reconstruct how the temblor affected the area. The scientists examined rock outcroppings, mummified plants, core samples of beach soils and tree rings in sunken forests at the bottom of Lake Washington, just east of the city.

Their detective work told them a remarkable story about a prehistoric earthquake.

Core samples of deep soils north of the fault found ancient tidal plants that had been killed by a sudden flood of saltwater and buried under a layer of sand--signs of a sudden subsidence that dropped the shore underwater and a concurrent tsunami, or tidal wave. That was consistent with the idea of a sudden tilt in the floor of the sound.

Rings from an uprooted Douglas fir tree buried at that site showed that it died in the same season of the same year as a grove of trees swept into Lake Washington by a massive landslide. Sediments on the lake bottom also show signs of significant shaking disturbances between 940 and 1,280 years ago.

“Previously, Puget Sound has been regarded as immune from tsunami damage,” said Brian Atwater of the USGS in Seattle and a professor of geological sciences at the University of Washington. “Now, here’s evidence that here in town earthquakes can set off some sort of surge of water. What that means is if you live or recreate by the water and you feel a quake, get yourself to high ground.”

Scientists also have found evidence of roughly concurrent large rock avalanches in the nearby Olympic Mountain range, as well as signs of large, anomalous ground water eruptions along the Pacific Coast.

Times researcher Doug Conner in Seattle contributed to this story.