Series of small earthquakes detected in Washington and Oregon

At least 13 small earthquakes have been recorded over the past two days in Oregon and Washington.

There were no immediate reports of injuries or damage from the quakes, which ranged in magnitude from 0.2 to 3.8. The most recent was detected at 4:43 a.m. Wednesday outside of Entiat, in Washington’s Chelan County, according to the Pacific Northwest Seismic Network.

One of the biggest quakes in the state’s recorded history hit near Entiat in 1872, when Ulysses S. Grant was president and long before the first seismometer was installed in the Pacific Northwest. It knocked people off their feet at Snoqualmie Pass, cracked the lighthouse on Dungeness spit, and broke windows and glassware from Walla Walla to Seattle. Scientists now estimate the magnitude ranged between 6.5 and 7.5.

The Puget Sound region is prone to earthquakes because it lies near the edge of the North American tectonic land plate and the Juan de Fuca, a large oceanic plate. The 700-mile boundary, a fault known as the Cascadia Subduction Zone, runs from Northern California to Canada. Scientists say the Juan de Fuca plate is trying to force its way under the North America plate, which they also say is ultimately inevitable. 

Last year at this time, thousands of tiny tremors had pushed parts of Washington and Vancouver Island westward. It’s a near-annual event that backs some scientists’ expectations that a big earthquake may hit the Seattle area harder than their previous models suggested.

The waves of seismic activity are part of a process known as episodic tremor and slip, which is thought to increase stress on locked faults — areas where tectonic plates cannot move past each other, University of Washington earth-sciences professor Ken Creager has said.

Scientists believe an episode of tremors could someday trigger a so-called megaquake on the Cascadia Subduction Zone. That fault, one of the biggest in the U.S., can unleash earthquakes as strong as magnitude 9.0.

While older models suggest that the locked zone is mostly shallow and offshore, the location of these tremors indicates that in a big earthquake, layers of rock jerking past each other may take place closer to Seattle than previously thought.

The Pacific Northwest last saw a megaquake about 300 years ago. Scientists widely expect the region to experience a similar event every 500 years on average.

While the timing of the big one is unknown, here are some tips to help you prepare, come what may.

Seattle Times staff reporter Sandi Doughton contributed to this report.