New maps from the U.S. Geological Survey bring Seattle's earthquake risk into sharper focus and add some unsettling details to what was...

New maps from the U.S. Geological Survey bring Seattle’s earthquake risk into sharper focus and add some unsettling details to what was already a grim picture.

Recent surveys discovered a probable fault line running under Lake Washington south of the Interstate 90 bridge. The fault appears to have ruptured about 1,000 years ago, thrusting upward as much as 12 feet and creating a kind of sloshing tsunami that engineers will have to consider as they plan a replacement for the Highway 520 floating bridge.

The new analysis also found that the Interbay area and parts of the University District, both built largely on fill and loose soil, are more vulnerable to earthquake damage than previously thought.

But the news isn’t all bad, said USGS senior seismologist Craig Weaver. Much of south Seattle stands on firm ground. The risk in north Seattle, while moderate, isn’t as bad as experts had feared.

The new maps are the culmination of a decade of study, Weaver said. “They represent the most sophisticated, most complicated set of ground-motion modeling we’ve done anywhere in the country.”

Though scientists have long had a general picture of which neighborhoods are at most risk, earlier earthquake maps basically lumped the entire city into a single category, said USGS seismologist Joan Gomberg. “They were basically one color.”

The new versions factor in variations in soil type, past earthquake damage, and the entire spectrum of possible earthquakes that could strike the region. The result is a multihued mosaic that predicts how much the ground is likely to shake, with detail down to the neighborhood level.

The maps will help government planners and builders, and also they will be valuable to homeowners trying to weigh earthquake risks against the cost of a retrofit, which can cost $5,000.

Seattle and the Puget Sound region are vulnerable to three types of earthquakes.

Deep earthquakes, like the magnitude 6.8 Nisqually quake of 2001, occur every 50 years or so.

A network of shallow faults, including the Seattle fault and its newly-discovered offshoot in Lake Washington, generates quakes every few thousand years that range in magnitude but scientists predict would be more damaging because they are closer to the surface.

Then there are rare behemoths, like the one that unleashed the deadly 2004 Indian Ocean tsunami, that occur off the Pacific Coast roughly every 500 years.

Rather than consider each earthquake type and scenario separately, scientists fed all the data into computer models and performed 540 simulations, checking the outputs against actual measurements.

“What it gives you is an average view,” Gomberg said.

For example, the most useful map for homeowners is what geologists call the “10 percent” version. It depicts the amount of ground shaking during the most probable earthquake — one that has a 10 percent chance of occurring within the next 50 years. Other maps show the predicted effects of bigger earthquakes, which have a lower chance of occurrence.

The new information probably won’t lead to any changes in local construction rules because Seattle already applies the strictest codes, said Barb Graff, the city’s emergency-management chief.

But the findings could affect plans for a new floating bridge across Lake Washington.

When the underwater fault slipped, it probably generated waves up to 12 feet high, said USGS geologist Brian Sherrod. “It was pretty nasty.”

The new floating bridge hasn’t been designed yet, but earlier studies have shown it can be built to handle waves about that size, said Patrick Clarke, who’s in charge of the bridge design for the Washington Department of Transportation.

“We’re going to take this information and look at it carefully,” he said.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com