Map shows more geologic hazards

SEATTLE – Earthquake experts say Seattle now has the nation’s most precise geologic hazards map, and it indicates a greater risk of damage from quakes and landslides than was known before.

Scientists and engineers have been working on the project for years, using up-to-the-minute technology and the city’s extensive record of landslides.

Though some areas were found to be less hazardous, the new map shows an overall increase in Seattle-area land at higher risk, said University of Washington geologist Kathy Troost.

Scientists produced the new map after analyzing findings from 36,000 boreholes, sifting soil, applying equations to determine slope stability and digging through nearly 100 years of city records.

Turns out the existing map, based on incomplete data collected in the 1950s and used in setting some of the city’s building and land-use codes, has wrong just about as often as it was been right.

“It was a huge surprise to see that 50 percent of the land area is now mapped differently,” said Troost, who has been working with colleague Derek Booth since 1998 to collect much of the geologic information used in the new map.

One unexpected discovery was hilltop areas with weak, unstable soil in West Seattle and Beacon Hill. Mapmakers had expected the hilltops would be mostly bedrock or sturdy, consolidated glacial deposits.

In upland valleys with weak deposits, “The material could potentially liquefy during an earthquake,” Troost said.

The new map also better incorporates the Seattle Fault Zone – a shallow fault underneath downtown Seattle that could produce a destructive earthquake and a tsunami in Puget Sound.

“Hazard mapping is the foundation of all our disaster mitigation efforts,” said Ines Pearce, director of Seattle’s Project Impact, a public-private partnership that promotes community-based quake preparedness.

The new map is the result of a five-year collaborative effort involving UW scientists, the U.S. Geological Survey, the city of Seattle and private experts in engineering.

The first of four quadrants that make up the map, showing the northwest section of the city, is now on the Web. The three other sections will be made available soon.

Troost warns that descriptions of soil types haven’t been translated from scientific jargon. Much of the precision of the new map is due to the city’s retention of landslide observations dating back nearly a century.

Using a mathematical analysis of existing slopes plus the historical record, agency scientists produced a map of “shallow” landslide risk, the most common type of rainfall-driven slope failure.

Using a sophisticated computerized topographic imaging technique – a kind of radar known as LIDAR that can see through trees and vegetation – revealed previously hidden areas of high risk.

Engineering firm Shannon & Wilson worked with the city to create a map of landslide probability and recurrence. The USGS team also developed a map of deep landslide risks that threaten some of the bluffs on Puget Sound, using a three-dimensional analysis of groundwater flow.