New maps detail quake risk zones

By Sandi Doughton

Seattle Times science reporter

Map |=20 How much will the ground shake? (PDF)

Earthquake maps

The new = USGS=20 seismic maps require a little decoding. Seismologist Joan Gomberg = suggests=20 picking the map labeled "10 percent probability," which predicts the = amount of=20 shaking from an earthquake with a 10 percent chance of occurring over = the next=20 50 years.=20

Ground motions are presented as a percentage of = gravity,=20 with higher numbers reflecting stronger shaking.

The maps are available at http://earthquake.usgs/gov/regional/pacnw/hazmap/seattle/index.php= .=20 The files can take five to 20 minutes to download. For a hard copy, = contact=20 Gomberg at gomberg@usgs.gov.

New maps from the U.S. Geological Survey bring Seattle's earthquake = risk into=20 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=20 south of the Interstate 90 bridge. The fault appears to have ruptured = about=20 1,000 years ago, thrusting upward as much as 12 feet and creating a kind = of=20 sloshing tsunami that engineers will have to consider as they plan a = replacement=20 for the Highway 520 floating bridge.

The new analysis also found that the Interbay area and parts of the=20 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=20 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=20 represent the most sophisticated, most complicated set of ground-motion = modeling=20 we've done anywhere in the country."

Though scientists have long had a general picture of which = neighborhoods are=20 at most risk, earlier earthquake maps basically lumped the entire city = into a=20 single category, said USGS seismologist Joan Gomberg. "They were = basically one=20 color."

The new versions factor in variations in soil type, past earthquake = damage,=20 and the entire spectrum of possible earthquakes that could strike the = region.=20 The result is a multihued mosaic that predicts how much the ground is = likely to=20 shake, with detail down to the neighborhood level.

The maps will help government planners and builders, and also they = will be=20 valuable to homeowners trying to weigh earthquake risks against the cost = of a=20 retrofit, which can cost $5,000.

Seattle and the Puget Sound region are vulnerable to three types of=20 earthquakes.

Deep earthquakes, like the magnitude 6.8 Nisqually quake of 2001, = occur every=20 50 years or so.

A network of shallow faults, including the Seattle fault and its=20 newly-discovered offshoot in Lake Washington, generates quakes every few = thousand years that range in magnitude but scientists predict would be = more=20 damaging because they are closer to the surface.

Then there are rare behemoths, like the one that unleashed the deadly = 2004=20 Indian Ocean tsunami, that occur off the Pacific Coast roughly every 500 = years.

Rather than consider each earthquake type and scenario separately, = scientists=20 fed all the data into computer models and performed 540 simulations, = checking=20 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=20 "10 percent" version. It depicts the amount of ground shaking during the = most=20 probable earthquake =97 one that has a 10 percent chance of occurring = within the=20 next 50 years. Other maps show the predicted effects of bigger = earthquakes,=20 which have a lower chance of occurrence.

The new information probably won't lead to any changes in local = construction=20 rules because Seattle already applies the strictest codes, said Barb = Graff, the=20 city's emergency-management chief.

But the findings could affect plans for a new floating bridge across = Lake=20 Washington.

When the underwater fault slipped, it probably generated waves up to = 12 feet=20 high, said USGS geologist Brian Sherrod. "It was pretty nasty."

The new floating bridge hasn't been designed yet, but earlier studies = have=20 shown it can be built to handle waves about that size, said Patrick = Clarke,=20 who's in charge of the bridge design for the Washington Department of=20 Transportation.

"We're going to take this information and look at it carefully," he = said.

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

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