New shaky ground in Seattle

People and buildings in Seattle's Interbay neighborhood, northern Montlake and a large area northeast of the University of Washington appear to be at greater risk from severe shaking damage in an earthquake than previously thought, a new analysis reveals.

Some scientists believe that the new seismic hazard assessment for Seattle may cause engineers to rethink how to rebuild the Route 520 bridge and municipal officials to consider new building regulations for areas likely to be hardest hit by a quake.

"This does give us greater clarity," said Barbara Graff, director of emergency management for the city of Seattle. But the new scientific findings alone, Graff said, are unlikely to immediately prompt any push for changes in building regulations.

"Seattle already has adopted some of the strictest and most current building codes," she said. Graff said the new analysis will be used immediately to assist with public education and outreach efforts, such as the city's much-hailed home retrofitting program.

Scientists at the U.S. Geological Survey, working with colleagues at the UW, gathered various earthquake studies and data done over more than a decade. The information was used to run 540 quake simulations on a network of computers, over the equivalent of 25 days of computing time, to create what the scientists believe is one of the most sophisticated analyses ever done of a community's seismic hazard.

"It took a lot to meld all this information," said Art Frankel, project leader and a seismic mapping expert with the Geological Survey in Golden, Colo. As a result of the new approach, Frankel said, they discovered new areas of higher risk in the city.

"We had suspected that Interbay was high risk, but it hadn't been quantified before," he said. The model succeeded in confirming that the other well-known high-risk areas, such as Harbor Island and much of industrial South Seattle, will experience more violent shaking and "liquefaction" -- when the soil turns liquid -- he noted.

"We needed to do this because the area has so many different kinds of earthquake sources," Frankel said. It's still technically an experimental analysis, he said, so it's not clear yet how it will be received by policymakers or regulatory agencies.

The Puget Sound basin is at risk from three quake types: The violent, shallow and so far distantly prehistoric quakes from the likes of the Seattle Fault; the massive ones produced by the Cascadian Subduction Zone Fault off the coast (the last one was in 1700); and the deep, generally milder but more frequent ones such as the 2001 Nisqually Quake.

The "Seattle Urban Seismic Hazard Map," incorporating the site-sampling fieldwork of UW geologist Kathy Troost and many other colleagues, is based on a three-dimensional model of how all the different quakes would shake the region.

"I believe it's the first time three-dimensional simulations have been used to produce a seismic hazard map," Frankel said. The Seattle map is intended to greatly increase the ability to determine how the quake threat varies according to geology, soil type, proximity to a fault and other factors.

Tim Walsh, chief hazards geologist for the state Department of Natural Resources, said the Geological Survey's theoretical model used to create the map also revealed some surprises for what's known as the Seattle Basin -- a deep hole in the bedrock under central and South Seattle full of loose sedimentary rock.

"Using this model, they were also able to show how the seismic waves" -- the destructive forces in a quake -- "get trapped in the Seattle Basin so that the duration of the strong ground shaking, in certain areas, will be much longer than earlier predicted," Walsh said. "That's in the model but hard to show on a static map."

A related analysis also indicates the potential for a destructive tsunami on Lake Washington, based on new data showing a fault cutting across the bottom of Lake Washington from Seward Park to Mercer Island, said Craig Weaver, senior seismologist for the Geological Survey in Seattle. He said this appears to be evidence of a large major quake long ago that created a localized tsunami, or seiche, in the lake.

The size of the rupture on the lake bottom indicates that such a Lake Washington tsunami could be anywhere from 9 to 18 feet high, he said.

Patrick Clarke, chief of the state Transportation Department's design unit in charge of the Evergreen Point Bridge, said he and his colleagues are considering the new findings in light of the need to rebuild the bridge. Right now, Clarke said, the bridge would be designed to withstand a maximum 11-foot wave.

Making design and construction changes to fit a worst-case scenario may not be reasonable, he said, when the kind of quake the scientists are talking about is estimated to strike maybe every 2,500 years.

"The whole thing here becomes a probability game," Clarke said. "We need to be able to see if the probability of this thing happening warrants changing the building codes or not."

The seismic hazard map does factor in the probability of specific events. The Seattle Fault -- which actually is a series of many faults running east-west from Bremerton across Puget Sound, through Seattle and Redmond into the foothills of the Cascades -- has been estimated to produce a major quake perhaps every 1,000 years.

The Cascadian Subduction Fault, a massive fault off the coast of Washington and Oregon similar to the Sumatran Fault, which produced the 2004 Indian Ocean tsunami, is thought to rupture every 500 years or so. Scientists have determined that its last major quake was in January 1700. Deep quakes like the 2001 Nisqually take place maybe every 50 years.

Frankel said the new map, despite its sophistication and all the work he and his colleagues put into it, must still be viewed as an imperfect and incomplete assessment of the local seismic risk.

"You could use this for initial design considerations, but I'm not sure I'm qualified to say if they should be used for changes to the building codes," Frankel said. What he is comfortable saying, as a scientist, is that more study is definitely in the best interest of Seattle residents.

On Saturday, Frankel and other quake experts will join Graff and other officials from local, state and federal agencies at a 9:30 a.m. news conference to debut the new seismic hazard map. The event, at Lowe's Home Improvement, at 12525 Aurora Ave. N., is aimed at encouraging city residents to be prepared for disaster.

WHAT IT MEANS

Some scientists say the new Seattle seismic hazard assessment may spur changes:

Engineers may need to rethink how to rebuild the 520 bridge.

Municipal officials may need to consider new building rules for areas in the higher danger areas.

NEW HIGH RISK AREAS:

NORTHERN MONTLAKE

UNIVERSITY VILLAGE AREA

INTERBAY AREA