4.3 Quake Hits Washington’s Okanogan Area

Spokane — A moderate earthquake Tuesday morning shook the ground in southeast Okanogan County and could be felt up to 25 miles away.

The quake at 9:42 a.m. 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 and 9 miles north of Nespelem in an area hit by wildfire this summer.

Two aftershocks measuring magnitude 2.1 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 scientists were studying quake data within a matter of minutes.

Residents of the region posted quake messages on social media.

The quake occurred about 6 miles below the surface. The seismic data that pinpointed the epicenter was considered imprecise, in part because there are no nearby seismographs.

Professor John Vidale, director of the Pacific Northwest Seismic Network, confirmed that the aftershock occurred and that there is an increased chance of more earthquakes in the near future.

However, “The chance of anything serious is very small,” he said.

The earthquake occurred in an area with known earthquake faults within the complex geology of the Okanogan Highlands, scientists said.

Layers of different rock are mixed and folded beneath the surface.

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, Vidale said.

Wildfires in the region would be unrelated to the earthquake at a depth of 6 miles, he 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 Tuesday’s quakes.

Lynne Brougher, public affairs officer at Grand Coulee Dam, said the shaking was easily felt there. “It was maybe 30 seconds, maybe a little less than that,” she said.

Nothing inside the office at Grand Coulee was disturbed, she said.

Because of the earthquake the entire dam, including galleries, will be inspected for any damage.

“Earthquakes are pretty few and far between here in Grand Coulee,” she said, but “we take all earthquakes seriously.”

According to the Pacific Northwest Seismic Network, 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 it 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.