A moderate earthquake at 9:42 a.m. shook the ground in southeast Okanogan County in north of Nespelem Washington.
It was measured at a magnitude of 4.3 and was felt over a wide area, law enforcement officials said.
No damage or injuries were reported after the shaking.
The epicenter was 25 miles north of Grand Coulee.
Two aftershocks measuring magnitude 2.0 at 10:03 a.m. and magnitude 1.8 at 12:32 p.m. were recorded.
Bill Steele, of the Pacific Northwest Seismic Network at the University of Washington, said that seismographs picked up the quake, and the seismic monitoring system then alerted staff that an event had occurred.
Scientists were studying the quake data within a matter of minutes, he said.
The quake occurred about six miles below the surface. The seismic data that pinpointed the epicenter was considered to have imprecise location quality.
Professor John Vidale, director of PNSN, confirmed that the aftershock occurred and that there is an increased chance of more earthquakes in the near-term future. He said seismograph patterns were clearly caused by an earth tremor
“The chance of anything serious is very small,” he said.
The earthquake occurred in an area with known earthquake faults with the complex geology of the Okanogan Highlands, scientists said.
Those faults are capable of producing earthquakes up to a magnitude 6, but those kinds of quakes would be very infrequent on the long-term scale of geologic time, he said.
Wildfires in the region would be unrelated to the earthquake at a depth of six miles, Vidale said.
Despite the known faults, the area has little recent history of earthquake activity. A 4.6-magnitude tremor was reported near Omak in 2011 and a 3.1-magnitude earthquake was reported north of Keller in 1999. The Keller-area quake was due east of today’s quakes.
“That area doesn’t have big frequent earthquakes,” Steele said.
Lynne Brougher, public affairs officer at Grand Coulee Dam, said the shaking was easily felt. “It was maybe 30 seconds, maybe a little less than that,” she said.
Nothing inside the office at Grand Coulee was disturbed, she said.
The earthquake has triggered an inspection protocol at the dam, Brougher said, “We take all earthquakes seriously.”
The entire dam, including galleries, was going to be inspected for any damage.
“Earthquakes are pretty few and far between here in Grand Coulee,” she said.
According to the PNSN website, a magnitude 4 earthquake would be felt by many people indoors and a few outdoors during the day. Dishes, windows and doors may be disturbed. The quake would be strong enough to awaken sleepers. Walls might make cracking sounds, and the quake might feel like a heavy truck hitting a building. Standing motor cars could be rocked noticeably.
A series of mild to moderate earthquakes occurred in north Spokane in 2001, which led to the announcement last year that a previous unknown fault bisects the north side.
Researchers have named the Spokane Fault, which may have potential to produce even larger quakes than the 3.7-magnitude tremor on June 25, 2001, and a 4.0 quake on Nov. 11, 2001, scientists said.
The two larger quakes were bookends in a “swarm” of 105 smaller tremors that occurred that year in a relatively shallow layer of basalt.
Over the years, Spokane residents have reported small quakes over the years, and some were detected on a seismograph at the former Jesuit scholasticate at Mount St. Michael.
Earthquake swarms elsewhere have also occurred in recent years north of the Tri-Cities and near Maupin, Ore.
Vidale said that it was too early to know whether the Okanogan area might undergo a swarm of smaller earthquakes.
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