Study reveals Spokane Fault; statewide Shakeout set for today

Be prepared to drop to the floor, crawl under something sturdy and cover your head this morning, if only for a minute or so.

Today is the Great Washington Shakeout, an exercise designed to give you practice on what to do if an earthquake hits.

Earthquakes are a constant source of concern in Western Washington, where fault lines along the Pacific Coast and just west of the Cascades offer regular reminders that a catastrophic quake – aka “the Big One” – may be just a matter of time.

But Spokane’s geology is as rock-solid as its populace and safe from quakes, right?

Maybe not. A new study by geologists suggests the city might not be as quake-free as some residents might think, turning up a new seismic fault in North Spokane.

Remember that string of tremors on the North Side in 2001? Recent analysis of satellite data reveals those quakes, technically known as a seismic swarm, are a result of a previously unknown feature geophysicists are calling the Spokane Fault.

“We don’t know anything for sure,” Paul Bodin, professor of geophysics at the University of Washington and manager of the Pacific Northwest Seismic Network, said of the Spokane Fault.

Because no one thought there was a seismic fault in north Spokane, there weren’t any monitoring units close at hand when the swarm hit in 2001. To map the fault, geologists and geophysicists used satellite data of the area before and after the swarm, which revealed the surface shifted upward and eastward. Those maps were included in an article titled “InSAR Evidence for an active shallow thrust fault beneath the city of Spokane” in the Journal of Geophysical Research. InSAR stands for interferometric synthetic aperture radar data, the kind used to map the fault.

The article explains how members of the U.S. Geological Survey and the Seismic Network used the data to measure the shifts and show the movement is consistent with some other seismic activity in the Columbia Plateau. There are other previously known faults in the area, including the Latah Fault, which skirts the west side of Spokane. Evidence of the Spokane Fault was probably obscured by the glacial floods that swept out of what is now Montana more than 11,000 years ago, the article says.

The quakes were 0.3 kilometers to 2 kilometers deep, which suggests the fault is relatively close to the surface. It’s a reverse thrust fault, where a rock layer below is pushing against a layer above, a type that is also found in Seattle, Olympia and Tacoma. That’s different than the Cascadia subduction zone, where one tectonic plate is sliding beneath another off the Pacific Coast and producing a catastrophic magnitude-9 quake about every 300 to 600 years.

But quakes from shallow faults can be deadly, too. A previously unknown fault near Christchurch, New Zealand – which like Eastern Washington is an area of relatively low seismic activity – produced a magnitude-6.2 quake in 2011, the journal article said. That quake killed more than 180 people and damaged or destroyed more than 100,000 homes.

Bodin said he wouldn’t advise Spokane residents to be overly worried about the fault line, but he does believe it’s time to study it.

“Its connection with previous known or suspected faults is something we don’t know, and we need to work on,” he said.

The USGS and the seismic network have since installed seismic monitors in several locations around Spokane, and the agency had planned to present the findings on the Spokane Fault to the public later this month. That event is on hold, however, because nearly all the USGS staff has been on furlough since Oct. 1 because of the partial shutdown of the federal government.

Not on hold, however, is the Great Washington Shakeout, which is organized in part by the state Military Emergency Management Division and the state Seismic Safety Committee. More information on that event can be found at www.shakeout.org/Washington.