Map to help zero in on areas prone to landslides, earthquakes

From earthquakes to landslides, Seattle is rife with natural land mines. Now, residents and officials alike can analyze those risks down to the neighborhood level, thanks to the nation's most-detailed geologic map.

Developed over the past five years by University of Washington and federal scientists, the new map allows anyone to zoom in on their block, and see whether they're sitting on solid or shaky ground.

Improved understanding of the region's geology is also helping researchers figure out how much rain it takes to trigger landslides — and how to provide early warnings.

"To understand the hazards, we first have to understand the geology," said UW geologist Kathy Troost, unveiling the map at a briefing Thursday in Seattle's Carkeek Park, where railroad tracks were slammed by mud during the winter of 1996-97.

Pounding rain in January followed snow in December, triggering nearly 400 landslides in Seattle alone and causing more than $100 million in damage. A family of four on Bainbridge Island perished when a slurry of rock and mud buried their waterfront home.

Spurred by the destructive season, U.S. Geological Survey scientists began mapping and studying landslide-prone areas. At the same time, Troost and her colleagues were updating a geologic map from the 1950s.

Both groups were aided by remarkable collections of data.

Maps online


A version of the new Seattle geologic map is available at:

http://geomapnw.ess.washington.edu. Click on "online interactive mapping."

A technical description of soil types is available by clicking on the link below the map. Generally, soils described as "over consolidated" are resistant to earthquake shaking. These include Vashon till (Qvt) and anything older — or listed after it on the chart.

Soils described as "fill" are likely to shake most in an earthquake.

Landslide maps are at: http://pubs.usgs.gov/of/2005/1405

The USGS prototype warning system is at: http://landslides.usgs.gov/advisories

The city of Seattle has landslide information at: www.seattle.gov/dpd/emergency/landslides

Troost relied heavily on decades of geologic core records kept by a Seattle city worker.

"Every commercial building, every bridge, every sewer line — somebody drilled a hole in the ground," she said. Combined with records from highway construction and other major projects, the trove included data from 36,000 cores.

The city's landslide records go back even further — to the late 1800s.

"I don't know of any other city that had the foresight to save this kind of data," said USGS scientist Ed Harp.

The core records filled in gaps and led to reclassification of about half the land area, Troost said.

The new map also adds fault zones that were discovered over the past 10 or 20 years, including the Seattle fault.

Drawing on the new map and aerial surveys with a type of radar that peers through vegetation, Harp and his colleagues have produced a more detailed picture of the region's landslide hazard zones. They include the usual, waterfront suspects, from Duwamish Head on Alki to the Magnolia bluffs, as well as a number of steep inland slopes.

By comparing rainfall records with the historic data on landslides, USGS scientists also have developed a model that allows them to predict when the ground is getting so saturated that it's likely to give way. Total rainfall, duration and intensity all factor in, as do conditions like the snow-rain combination that caused such mayhem a decade ago.

The forecast system is still in development, said USGS scientist Rex Baum. But eventually, he hopes to expand it into a color-coded warning system that will give homeowners enough advance notice to evacuate their belongings — and themselves.

Warnings also would allow homeowners to check their drainage systems and perhaps avoid problems, said Alan Justad of Seattle's Department of Planning and Development. City engineers are studying the new maps to see if building codes or regulations should be tweaked, he added.

Insurance companies will probably use the new map, too, said Karl Newman, president of the Northwest Insurance Council. Currently, rates for earthquake insurance are about the same across Seattle, he said. Eventually, companies might raise rates in areas where loose soils would intensify shaking and lower them for neighborhoods with more stable soils.

For residents who want to use the maps to assess their personal risk level, Troost offers a word of caution: The information is online, but descriptions of soil types haven't been translated from scientific jargon to plain English.

She hopes to provide that translation, if she can find funding for the work. In the meantime, if you want a detailed interpretation, you'll have to study up on geology — or consult an expert.

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