YAKIMA — It’s estimated at least a million Washingtonians crawled under desks, quickly left buildings or practiced other precautions as part of a statewide earthquake drill on Thursday.
But if that estimate is accurate, another 6 million residents didn’t participate, including many in Central Washington.
And that concerns Stephen Reidel.
“I think the problem with Eastern Washington is we don’t take earthquake hazards seriously,” said Reidel, an adjunct professor of geology at Washington State University’s Tri-Cities campus. “The bottom line is, some day we are going to have a major earthquake.”
Evidence suggests a major earthquake strikes the Yakima Valley about once every 10,000 years. And while the threat is nowhere as great as in the Puget Sound region, Reidel and others say the risk of serious damage and injury should one occur here is still serious enough that people should take precautions.
The largest earthquake in recent Washington history was in Chelan County, said Renate Hartog, a research scientist with the Pacific Northwest Seismic Network at the University of Washington. That was in 1872 — a blink in geologic time.
With an estimated magnitude of 6.5 to 7.5, the quake cracked chimneys in Olympia and shattered windows as far as Victoria, B.C.
In 1936, a quake between 5 and 5.5 magnitude severely damaged buildings in Milton-Freewater, Ore., and parts of Athena, Ore., and Walla Walla.
Closer to home, the ridges that define the Yakima Valley are evidence of fault lines associated with the Yakima fold and thrust belt, where slabs of earth and bedrock have been shoved as underground plates move.
And the fault lines are still active.
According to the U.S. Geological Survey, there have been 13 earthquakes in the Yakima area in the past 30 days. The biggest ?was Oct. 12 when a magnitude 3.4 quake was recorded about 20 miles south of Toppenish.
That was one of the more powerful quakes in recent years, Reidel said.
And that’s part of the problem. Large parts of the fault line have not moved, which Reidel said suggests that pressure may be building up and could lead to a major release.
Researchers have found evidence of such a quake dating back 10,000 years near the Tri-Cities, based on soil found in the fault, which he said suggests such events have a likelihood of happening once every 10 millennia.
That’s not a timetable but rather a measure of probability, in the same way meteorologists talk about 100-year storms, he said.
And like storms, the quake probability remains the same each year, regardless of how long it was since the last quake, Reidel said.
And a magnitude 7 or greater earthquake in the Yakima Valley, while rare, would have serious consequences.
A state Department of Natural Resources study estimates that if a magnitude 7.1 quake — the same intensity as the one that ?hit Mexico City in September — along the 35-mile fault line on Toppenish Ridge would injure 191 people, damage 1,678 buildings, leave 325 people homeless and cause almost $340 million in damage, mostly in Yakima County, but also Benton and Klickitat counties.
Reidel said much of downtown Yakima is built on river sediment, which is more likely to amplify the effects of an earthquake, while buildings to the east are more on basalt bedrock, which is more stable.
Yakima County’s Public Services Department does advise builders about the seismic risks in a particular area, so builders can adjust their designs accordingly.
Thursday’s Great Washington ShakeOut Drill was an annual drill intended to raise awareness about what to do before, during and after an earthquake. Participants were instructed to drop, cover, and hold on for at least 60 seconds.
