Dangerous ground: Hard lessons learned since the 2001 Nisqually quake

Feb 27, 2011

Dangerous ground: Hard lessons learned since the 2001 Nisqually quake

A decade ago, scientists knew Western Washington was vulnerable to much nastier shakes than the magnitude 6.8 that rattled Seattle on Feb. 28, 2001. But it's what they've learned in the last 10 years since the Nisqually earthquake that really has some researchers spooked.

Seattle Times science reporter

The biggest surprise about the Nisqually earthquake was that it wasn't worse.

Even a decade ago, scientists knew the Northwest was vulnerable to much nastier shakes than the magnitude 6.8 that rattled Seattle 10 years ago Monday. But what they've learned in the past 10 years really has some researchers spooked.

"It used to be, I never really thought much about it," said Oregon State University marine geologist Chris Goldfinger. "Now I sit in my building — the kind that pancakes in an earthquake — and I'm thinking: I hope it doesn't go now."

Goldfinger's studies suggest megaquakes of the type that devastated Sumatra and set off a deadly tsunami may jolt the Northwest much more often than previously thought. In the Puget Sound region, scientists have added more than 10 active faults to a list that numbered two in 2001. And new analyses show the ground under downtown Seattle isn't nearly as stable as folks used to think.

"The hazard has certainly gone up, but it's a reflection of our increased knowledge," said Brian Sherrod, a U.S. Geological Survey (USGS) scientist stationed at the University of Washington.

Technologies that reveal hidden faults and measure the scrunching of the Earth's crust down to the millimeter have propelled a decade of remarkable discovery in the Northwest. Old-fashioned fieldwork played a part, too. In their search for signs of earthquakes past, Sherrod and his colleagues dug so many trenches he's lost track of the tally.

The new insights are beginning to inform construction and emergency planning, said Craig Weaver, the USGS' regional seismic-hazards chief. "Ten years ago, we were in an exploratory phase. Now we have much more certainty about what we might expect to happen."

That includes quakes of magnitude 6.5 to 7.5 on shallow faults that fracture the landscape from Olympia to the Canadian border. There's one aimed at the heart of Tacoma. Another skirts Olympia, and a third passes between Mukilteo and Edmonds. Some of the faults appear to cross the Cascade Mountains into Central Washington.

"We're astounded by the number of big faults we're dealing with," Weaver said.

Last week's New Zealand quake underscored the danger of shallow faults near cities. Even a relatively modest-sized quake can be deadly when the shaking is not dampened by distance or depth. Seattle's worst-case scenario is a magnitude 7+ on the Seattle Fault, the biggest of the two faults geologists had penciled in on their maps 10 years ago. Extending from Bainbridge Island to Bellevue, it passes under Safeco Field.

A repeat of the Seattle Fault's last big jolt 1,100 years ago would be among the most destructive in American history. And Sherrod and others now believe the Seattle Fault is dwarfed by a much larger fault zone called the Southern Whidbey Island Fault, which may have strands that extend from Vancouver Island to Richland.

The good news about the region's shallow faults is they don't pop very often, Sherrod said. Repeat times seem to range between 700 and several thousand years. The bad news is Sherrod and his colleagues keep finding more.

"I'm working on three more right now," he said. "We haven't even really begun looking south of Olympia."

Changing "everything"

The key to unlocking the region's seismic past was the airborne laser scanner called LIDAR. The technique filters out vegetative cover and creates topographic maps that strip the landscape bare. Potential faults stand out like broken bones on an X-ray. "That completely changed everything," Sherrod said.

On the coast, Goldfinger, too, is using novel approaches to better understand the cocked gun called the Cascadia Subduction Zone, source of some of the world's biggest earthquakes. The 600-mile-long offshore fault is the place where the ocean floor is being crammed under the continental plate. By the early 2000s, it was clear the fault could produce magnitude-9 earthquakes and tsunamis on a par with Sumatra's 2004 cataclysm. The most recent was in 1700.

By examining sediment cores for evidence of underwater landslides, Goldfinger pushed the record back to include 22 megaquakes over the past 10,000 years. More controversial is his claim the sediments also show big quakes on the section of the fault off the Oregon coast strike every 240 years, instead of the 400 to 500 years now factored into risk assessments. If true, that means there's a 37 percent chance the region will be rocked within the next 50 years.

Another bit of unsettling science suggests that instead of being tucked well offshore, Cascadia's danger zone — the portion of the fault that would snap in an earthquake — may be closer to Seattle, Portland and Vancouver, B.C., than previously thought.

The evidence comes from the study of the recently discovered phenomenon called "silent earthquakes." Seismologists "listening in" on the Olympic Peninsula have detected tiny vibrations they believe are produced as the geologic plates in the subduction zone slip past each other. A network of more than 400 GPS stations across the Northwest is able to measure that slip, along with all the other tectonic wrenching that makes the region such fertile earthquake ground. A decade ago, there were only about a dozen of the stations, said Central Washington University geologist Tim Melbourne.

The slip measurements and seismic signals allow scientists to locate the portion of the subduction zone that's slipping freely, Melbourne explained. They use that information to estimate the location of the danger zone, the area where the fault is locked and building up tension that will be released with a bang.

Some experts remain unconvinced the danger zone lies within a scant 50 miles of the Interstate 5 corridor. "The jury is out; but if it's true, it has to be taken seriously because of the implications," Goldfinger said.

Mapping the risk

The most likely quake to strike the Puget Sound region is another one like Nisqually. The USGS puts the probability at higher than 80 percent over the next 50 years. Similar to earthquakes in 1949 and 1965, the Nisqually originated more than 30 miles down, where the Juan de Fuca plate bends sharply as it dives under the continent.

Even though it wasn't exceptional, the 2001 quake provided some startling insights into what's likely to happen the next time the ground starts moving, said USGS scientist Art Frankel. Much of Seattle sits atop a basin, where shaking was twice as hard as expected during the 30-second Nisqually quake. Frankel and others had hypothesized the basin might trap and amplify vibrations in unexpected ways, and Nisqually proved it.

Frankel folded that knowledge into a state-of-the-art computer model he worked on for more than 10 years. Factoring in what's known about the region's subsurface geology, the recently discovered faults and the possible range of earthquake types, he ran more than 500 scenarios through the computer. To validate the model, he compared simulated outcomes against measurements from past quakes.

The result is the nation's most sophisticated seismic-hazard map. The map identifies vulnerable areas — including the basins beneath Seattle, Tacoma and Everett — and it predicts the expected level of shaking.

"We don't have to wait until the big earthquake occurs to know where the bad places are," Frankel said.

The modeling hasn't been factored into building codes — but Frankel hopes it will be.

"It's on the edge between research and application."

Frankel already is working on an updated hazard map. He's also tackling the region's biggest seismic question mark: How long and hard will the ground shake during a magnitude-9 subduction-zone quake? The answer is vital to understanding how the tall buildings in Seattle, Portland and Vancouver, B.C., will weather the ride. Several modern high-rises in Chile built to stringent codes were hammered in last year's magnitude-8.8 quake on an offshore fault very similar to Cascadia.

"A big subduction-zone quake will be pretty bad," Frankel said. "We're trying to figure out how bad."

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Mary Ellen and I are both extremely pleased with the work that Matt and Richard did.  Please pass on my personal "thank you" to each of them. 

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Even with your discovery of unforeseen conditions that necessitated additional framing, it was still done within a very reasonable time-frame.  My overall experience with Sound Seismic has been very positive and we will recommend your services to our circle of friends and family.

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We recently had Sound Seismic do our earthquake retrofit. I cannot compliment enough the very professional job they did. Leif Jackson, the owner, sat with us for two hours answering questions and explaining how earthquakes work. He was completely above board, not promising more than he could deliver. He left interesting reading material so that we could evaluate and decide for ourselves. The most compelling was from FEMA warning people in the Puget Sound area to do everything they could to protect their homes because of our earthquake risk.

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Grateful homeowner, Mercer Island
 
 

The Sound Seismic crew arrived on time and got started on this structurally engineered and contracted job. It was estimated to take approximately one week to complete this work. The job was completed in 4 days with great professionalism and dedication to the work completed. The work passed a city inspection with no additional work required. The quality of work is excellent with great attention to detail.

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the Pearsons

Bryant neighborhood, Seattle

 

 

We had a wonderful experience with Sound Seismic.  After the devastating earthquake in Japan my husband and I were quite worried about our 3 story brick home and decided to retrofit our basement as a way to ease our anxiety and protect our family.  

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"Leif

Thanks for the great work on the house - you've done an outstanding job of prompt, low-impact work - and leaving the job site in excellent shape. Appreciate the attention to detail."

 

Wayne Dodge, Ravenna, Seattle

 

"I wanted to follow up and thank you for your good communication, strong planning and flexibility.

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Urs Koenig, Madrona, Seattle

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Dear Leif,

Matt and Richard did a great job!  It was a pleasure to meet them. And as soon as I met Matt, I knew that everything would be done 'right.' 

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David Rosenbaum, Sand Point, Seattle

 

 

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Barbara E., Capital Hill, Seattle

 

 

I wanted to let you know what a great job that your crew did on our earthquake retrofit.  They were very knowledgeable about the whole process and answered any questions we had without hesitation. Their work was excellent and they finished the job quickly.  I have done my share of remodeling in the past and I can say that they cleaned up after each day’s work more thoroughly than anyone I’ve ever encountered.  They took obvious pride in the retrofit work they did but also in the cleanup. They saw that as an integral part of the Sound Seismic service.  All in all, it was a fast and painless experience.

 

Thanks for assigning those fine young men to our job.

 

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George Murphy, Northgate, Seattle

 

 

I wanted to thank you for the wonderful job you did retrofitting my home in South Seattle. I was aware that it would be a challenge and it proved to offer more problems than we expected. You met them all and did a great job correcting the rot and damage that happened over the years since 1927, when the house was built.

 

When I arrived home yesterday the basement was clean and organized. It was the first time I did not have to spend hours cleaning up after a contractor. Your lead carpenter and his assistant were a great team. Thanks for all your efforts.

 

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Thank you very much for your excellent work on retrofitting our house.  We appreciated your careful oversight of the project, ongoing contact with the engineering firm, attention to detail, and clean and timely work.  You really rose to the challenges presented by "this old house."

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Stuart Jamieson, Montlake

 

 

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Just wanted to send you a note to express how pleased we are with the work your firm recently completed.  You did exactly what you said you would and the quality of the work is first rate.  Also, please tell Matt how much we enjoyed him.  Thanks,

Gary Johnson, Wallingford

 

 

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