Seattle Fault quake could devastate city
SEATTLE – A magnitude 6.7 earthquake on the Seattle Fault could kill 1,600 people, buckle freeway bridges, sink ferry terminals and collapse the Alaskan Way Viaduct, according to a new report.
Based on computer modeling and the expertise of dozens of engineers, scientists and emergency managers, the scenario projects that more than 45,000 families would be forced out of shattered homes and nearly 10,000 commercial buildings and houses would be destroyed.
The toll on the state’s economy: $33 billion in property damage and lost income, about the same as in the country’s most costly natural disaster to date, the Northridge quake in Southern California in 1994.
“When you start talking about numbers like this, you realize the impacts are huge and we’re not ready for it,” said Craig Weaver, a U.S. Geological Survey seismologist.
The report, scheduled for presentation next Monday in a daylong workshop in Bellevue, urges state and local governments to move quicker to make highways, buildings and other structures less vulnerable to an earthquake.
The 30-mile-long Seattle Fault runs through the heart of Seattle and the suburbs east of Lake Washington. In the past 3,000 years, it has violently rearranged the landscape as many as four times, mostly recently 1,100 years ago.
Geologists estimate there’s at least a 5 percent chance of another big shaker within 50 years.
The report, a product of almost 4,000 hours of work over three years, recommends the formation of a state seismic safety board reporting directly to the governor, as well as regulations requiring improvements in the most vulnerable buildings, those made of unreinforced brick or concrete.
“We’ve been plodding along in Washington,” said Don Ballantyne, a Seattle civil engineer who specializes in earthquake-resistant designs and was a leading organizer of the project.
“This makes it clear we’re at significant risk, and we should be working hard to manage those risks.”
A magnitude 6.7 earthquake on the Seattle Fault would be up to eight times more destructive than the 6.8 Nisqually earthquake, which caused about $2 billion worth of damage in 2001, Weaver said.
Like most quakes in Washington’s modern history, the Nisqually quake originated deep underground, which dampens effects on the surface.
Scientists say the last quake on the much shallower Seattle Fault was magnitude 7.3 and raised the bluffs that ring Alki Beach and parts of Bainbridge Island.
“This scenario is not the big one,” Weaver said.
“We chose it because earthquakes of that size occur more frequently.”
Projections were derived by overlaying census data and information about buildings, population, business areas, utilities and transportation networks on maps with estimates of how intense the shaking would be.
A Federal Emergency Management Agency computer model then produced estimates of fatalities, injuries and structural damage.
The Seattle seawall would probably crumble, taking out ferry terminals and docks. Landslides would roar down steep slopes into Puget Sound, triggering local tsunamis that could swamp waterfront homes and other buildings.
Brick buildings in Pioneer Square and the Chinatown International District would tumble.
In river valleys and low-lying areas built on fill, loose soil would turn to mush, causing foundations to collapse and breaking water pipes and utility lines.
The Olympic Pipeline, which carries gasoline and jet fuel, crosses the Seattle Fault in Bellevue and passes through unstable soil in the Renton and Kent valleys.
Part of Harbor Island could slide into Elliott Bay, destroying container terminals, cranes and docks.
Up to 40 percent of schools could be unusable, and damage to hospitals could slash the number of available patient beds by 75 percent.
Traffic could be snarled for years because of damage to parts of Interstates 5, 90 and 405 as well as practically all other major highways.
To estimate the economic impact, scenario writers noted that after a magnitude 6.9 earthquake on a similar fault in 1995 in Kobe, Japan, so many businesses moved that the Port of Kobe’s ranking as a container shipment center dropped from No. 6 to No. 17 worldwide and manufacturing operations suffered for years.