Earthquake News

Ten years after Nisqually quake, Northwest’s seismic dangers still lurk

The level of seismic threat in the Pacific Northwest hasn't changed in the decade since the Nisqually earthquake shook buildings, unsettled nerves, and churned the ground in Western Washington. However, scientific understanding of the threat has progressed, and our ability to analyze and prepare for it has vastly improved.

When the magnitude 6.8 Nisqually earthquake struck just before 11 a.m. PST on Ash Wednesday, Feb. 28, 2001, Malone was the director of the Pacific Northwest Seismic Network, based at UW. The Nisqually earthquake was about 32 miles (ca. 51 km) deep, similar to earthquakes near Olympia in 1949 and Tacoma in 1965.

It occurred along the interface where the Juan de Fuca tectonic plate is descending beneath the North American plate. According to John Vidale, a UW Earth and space sciences professor and current head of the seismic network, these intraslab quakes have been the most common damaging earthquakes in Western Washington's known history, but the time frame in which they recur is probably the least understood.

Scientists from the University of Washington and the US Geological Survey have learned more about the three types of non-volcanic earthquakes that strike the Pacific Northwest, as well as how unnoticed "episodic tremor and slide" events increase seismic risk.  

The number of seismic sensors has tripled, as has the region that they cover. Engineering standards have improved to meet the region's seismic threats.

"We're at a disadvantage because most earthquake-prone locations across the world have a better recorded history than we have," said Stephen Malone, an emeritus research professor of Earth and space sciences at the University of Washington. 

"They aren't your average people." "There's no reason to believe another one isn't on the way," Vidale said, but he noted that if history is any indication, Western Washington won't be as seismically calm because it has been for the past five years.

An intraslab earthquake of magnitude 6.5 or more is predicted to occur during this region over the subsequent 50 years, in keeping with the geological survey.

Intraslab earthquakes are not the most adverse seismic events in the region. Shallow earthquakes on crustal faults – as well as the Seattle, Tacoma, and South Whidbey Island faults – occur rarely as frequently however can motive greater massive harm close to their supply and additionally should generate tsunamis in Puget Sound.

The final primary quake at the Seattle fault, anticipated at value 7.4, took place approximately 1,one hundred years ago, elevating the south cease of Bainbridge Island and what's now Alki Beach in West Seattle 15 to twenty toes and producing a Puget Sound tsunami. 

Then there are megathrust, or subduction quarter, quakes that normally arise off the Washington and Oregon coast and will be nearly as effective because of the value 9.1 quake that struck South Asia in 2004 and generated a lethal tsunami.

As the Juan de Fuca plate slides underneath the North American plate, a procedure known as subduction, the plates are locked collectively close to the floor and pressure keeps accumulating over centuries. When the locked quarter offers way, an immense earthquake results.

The last subduction earthquake here, in January 1700, caused a tsunami that wreaked havoc on Japan and the Pacific Basin as a whole. Because it would be farther distant from Western Washington's population centers than, say, a quake on the Seattle fault, it could be less severe. However, the continuous shaking, which might last anywhere from five to nine minutes, would wreak considerable damage across the Pacific Northwest west of the Cascade Range.

Scientists estimate that magnitude 9 earthquakes in the Cascadia subduction zone strike every 500 years on average, though they can strike on shorter or longer timescales. According to Vidale, the threat today is around half of what it will be in 200 years. "The risk isn't as high as it appears to be if the models are correct,” he said.

The Nisqually earthquake was detected by Western Washington's first sophisticated strong-motion sensors. Since 2001, the number of stations has tripled to more than 130, with another 30 on the way. A new set of sensors has been installed to monitor liquefaction, which occurs when ground water rises to the surface and liquefies the soil. In the United States, there are few precise liquefaction records, and some of them are from the Nisqually earthquake.

Since the earthquake in 2001, there have been a number of further developments, including:

  • Refinement of hazard maps and models to provide more accurate projections of future earthquake shaking.
  • New faults are documented. In 2001, the Seattle and Price Lake faults (Olympic Peninsula) were discovered. Since then, several more have been reported, including Tacoma, Frigid Creek, Canyon River, South Whidbey Island (which has probably had the largest previous quakes), Utsalady, and Ellensburg and Umtanum Ridge in eastern Washington.
  • Within 10 minutes of stronger earthquakes, shake maps and "Did You Feel It" reporting are immediately made available to the public.
  • Building codes are changing as a result of advances in seismic engineering and the adoption of the International Building Code, which has standards based on the most up-to-date seismic research.
  • Public awareness and readiness have increased.
  • Episodic tremor and slide, or slow-slip, are discovered. These unnoticed events occur every 15 months, last several weeks, and have the same energy output as a magnitude 6.5 earthquake. They're complicating an already difficult regional seismic picture.
  • Broadband seismometers that are better and more numerous to see earthquakes clearly over the entire region.

"We're more equally monitoring the Pacific Northwest, including previously unmonitored portions of eastern Oregon and eastern Washington," Malone said. "We also have better volcano monitoring in the Cascades."

Scientists in the United States and around the world are attempting to develop methods for predicting earthquakes, but "right now we can't even understand how that may work," according to Vidale. He also expressed optimism for improved forecasting abilities based on historical trends and generic probability.

It's also now possible to create an early warning system that uses data from sensors near an earthquake source to predict which way the waves will travel and how violent the shaking will be. In Japan, Mexico, and other places, such systems are already in use.

"This might provide folks anything from a few seconds to a few minutes of advance warning," Vidale added. "The question is whether the financial investment is worthwhile."

 

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