Scientific ideas about the danger of earthquakes have evolved in the decade since the 2001 Nisqually earthquake shook buildings in Western Washington. Though the level of seismic danger in the Pacific Northwest hasn’t changed much, the ability to study and prepare for it has improved immensely. The Nisqually earthquake was recorded on the first modern strong-motion sensors deployed in Western Washington.
Engineering standards have improved to meet the region’s seismic risks. Scientists from the University of Washington and the U.S. Geological Survey have augmented their knowledge about the three different types of non-volcanic earthquakes that occur in the Northwest, and are learning how unfelt “episodic tremor and slip” events relate to seismic risk. The number of seismic sensors and the territory they monitor has tripled. “Most places in the world that experience earthquakes have a better recorded history than we have, so we’re at somewhat of a disadvantage,” said Stephen Malone, a UW emeritus research professor of Earth and space sciences.
Malone was director of the Pacific Northwest Seismic Network, based at UW, when the magnitude 6.8 Nisqually quake struck just before 11 a.m. PST on Ash Wednesday, Feb. 28, 2001. Similar to quakes centred near Olympia in 1949 and Tacoma in 1965, the Nisqually quake was about 32 miles deep, near the interface where the Juan de Fuca tectonic plate is descending beneath the North American plate.
These intraslab quakes have been the most common damaging earthquakes in Western Washington’s recorded history, but the time frame in which they recur is probably the least understood, said John Vidale, a UW Earth and space sciences professor and current director of the seismic network.
“They are not regular. There is no reason to believe another one is overdue to happen,” Vidale said, though he added that if history is any guide Western Washington is unlikely to be as seismically quiet as it has been for the last five years.
The geological survey estimates there is a better-than-80 percent chance of an intraslab earthquake of magnitude 6.5 or greater in this region within the next 50 years.
Intraslab quakes are not the most-damaging seismic events in the region. Shallow quakes on crustal faults – such as the Seattle, Tacoma and South Whidbey Island faults – don’t happen nearly as often but can cause further significant damage near their source and also could generate tsunamis in Puget Sound. The last major quake on the Seattle fault, estimated at magnitude 7.4, occurred about 1,100 years ago, raising the south end of Bainbridge Island and what is now Alki Beach in West Seattle 15 to 20 feet and generating a Puget Sound tsunami.
Then there are megathrust, or subduction zone, quakes that mostly occur off the Washington and Oregon coast and could be almost as powerful as the magnitude 9.1 quake that struck south Asia in 2004 and generated a deadly tsunami.
As the Juan de Fuca plate slides beneath the North American plate, a process called subduction, the plates are locked together near the surface and stress continues to build up over centuries. When the locked zone gives way,a mammoth earthquake results. The last subduction earthquake here, in January 1700, generated a tsunami that did damage in Japan and affected the entire Pacific Basin. Such a quake could be less damaging than, say, one on the Seattle fault, since it would be farther away from the population centers of Western Washington. But the prolonged shaking, perhaps five to nine minutes, would still cause significant damage throughout the entire Pacific Northwest west of the Cascade Range.
Scientists know these magnitude 9 earthquakes in the Cascadia subduction zone occur on an average of every 500 years, though they can happen on shorter or longer time scales. Vidale estimates the danger now is about half what it will be in 200 years. “The risk is not as high as it’s going to get, if the models are correct,” he said.
Since 2001, the coverage has tripled to more than 130 stations, and 30 more are in the works. A new array of sensors has been deployed to record liquefaction, which occurs when the shaking pushes ground water to the surface to liquefy the soil. There are few detailed liquefaction records in the United States, and some came from the Nisqually quake.