SEATTLE—The Cascadia Subduction Zone may get most of the attention, but as Friday's earthquakes north of Seattle show, the monster fault off the coast isn't the only seismic threat.
Western Washington is also crisscrossed by more than a dozen large, shallow faults—cracks in the Earth's crust capable of unleashing damaging earthquakes. Seattle, Tacoma, Everett, Olympia and Bremerton all sit uncomfortably close to crustal faults. And new evidence suggests that in the aggregate, those faults might rupture more frequently than previously thought.
The magnitude 4.6 quake that struck early Friday morning near Monroe originated more than 17 miles down on a previously unknown fault and in an area seismologists don't understand well. The focus, or point where the fracture started, was several miles beneath the Southern Whidbey Island Fault zone—a wide swath of fractures that cuts diagonally across the state from Victoria, B.C, to the Tri-Cities area on the Columbia River.
"This was probably on an entirely different fault zone," said Brian Sherrod, earthquake hazards chief for the U.S. Geological Survey in Seattle. "It's a good reminder that we live in an area with lots of active faults."
Washington's major crustal faults can generate quakes as big as magnitude 7 to 7.5, Sherrod said. While that's far less powerful than a magnitude 9 subduction zone megaquake, a quake of that size near any city in the state would be devastating.
Geologists used to think large crustal fault quakes in Washington struck very rarely—only every thousand years or so, on average. But Sherrod and his colleagues recently analyzed all the geologic data from 15 faults in the Puget lowlands, and found evidence of 21 quakes of magnitude 6.6 or greater in the last 4,000 years. After statistical analysis, the researchers concluded that means a big quake rocks one fault or another every 250 years on average.
"When you aggregate these faults and treat them as one source the region has to deal with, the recurrence interval is pretty darn short," Sherrod said.
The most recent shallow-fault earthquake Sherrod and his team have uncovered in the Puget Sound region struck on what's called the Utsalady Point Fault on Whidbey Island between 300 and 500 years ago.
"So we're in the window of opportunity" for another one, Sherrod said. "Prepare your family, prepare your house, because you never know when it's going to hit."
The east side of the state is not immune, either. One of the biggest quakes in Washington's recorded history, estimated between magnitude 6.5 and 7.5, struck in 1872 on a shallow fault near the town of Entiat.
The quake broke windows in Victoria, B.C., cracked chimneys in Olympia and shook loose landslides that reportedly dammed the Columbia River.
The SWIF is the state's largest crustal fault. The best-known is the Seattle Fault, which extends from Bremerton to the Cascade foothills, crossing under Seattle's Sodo area and roughly tracking Interstate 90. The fault last ruptured around 1,100 years ago, thrusting coastlines on Bainbridge Island and West Seattle more than 20 feet into the air, triggering a tsunami in Puget Sound and setting off massive landslides on Mercer Island. One analysis estimates a magnitude 6.7 quake on the Seattle fault today could cause $33 billion in damage and kill as many as 1,600 people.
Washington's shallow faults are the result of a kind of slow-moving tectonic train wreck. California, which is creeping northward at a rate of about 2 inches a year, smashes into Oregon which smashes into Washington. Pinned up against unyielding bedrock to the north in British Columbia, Washington crumples and cracks under the north-south compression.
"Think of the crust up here as a piece of glass that's shattered and there's faults running through it in many directions," Sherrod said.
Washington's most common big quakes are of a different type altogether. They originate very deep underground—20 to 30 miles or more. Instead of being caused by ruptures on the offshore subduction zone or a crustal fault, they are caused by cracks within the oceanic plate, which is diving under, or subducting, the North American continent. The 2001 Nisqually earthquake was one of these, as were the large, damaging quakes near Olympia in 1949 and 1965.