I recently compiled some of the stories and pictures on this blog into an article for the SunBreak warning about the Northwest’s ongoing history of earthquakes and what it means for the threat of another damaging quake. My sense was that I’d dug up stories about pretty much every significant quake on the record, but the article led me to look for any other ones I’d not yet noticed.
I found out about an earthquake swarm in Spokane in 2001 that lasted several months, from either late May or late June on into November, if not longer. In the wake of the Nisqually quake, it got some attention. I don’t know exactly what it means for Spokane’s earthquake threat, but it’s at least worth knowing about. On July 1, 2001, the Associated Press said seismologists thought the quakes might mean “the newly discovered Latah Creek fault could be rumbling to life again after a 1.5 million-year slumber.”
There were quite a few small quakes on June 25, causing damage to some brickwork, broken plates, and other fairly minor problems, and University of Washington geophysicist Bill Steele said, “We have to say it could be a precursor of a bigger thing to come. Due to modest historic seismic activity in the area, it’s a very low probability. But we can’t rule that out.”
In response, the Pacific Northwest Seismographic Network installed some digital seismographs in Spokane. Bob Derkey, a geologist with the state Department of Natural Resources in Spokane , said he’d only mapped the Latah Creek fault in winter 2000. It runs in a nearly straight line from Steptoe Butte southeast of Spokane to Tum Tum northwest of the city.
Steele said, “We just can’t predict, but if the faults there are of significant size, we couldn’t rule out magnitude 6 event or larger. It’s a low probability, but certainly possible.”
In August, the Spokane Spokesman Review reported on another seismograph being installed hours after a 1.5 quake registered as the 33rd earthquake in all; a 3.7 on June 25 may have started the swarm. A 4.0 happened in the morning on November 11, Veteran’s Day. In November, around Thanksgiving, the AP reported:
The temblors come day and night. Schools hold duck-and-cover drills. Workers in downtown high-rises study evacuation plans.
Since May 24, Spokane has been in the throes of what experts call an ” earthquake swarm.” More than 75 temblors have been recorded, and dozens more could not be measured because of a lack of seismographs.
There have been no injuries or major damage — other than bricks falling from chimneys and items off of shelves — but nerves are fraying.
“We’ve felt every single one of them,” said Cindy Burrows, who works on the 19th floor of the Bank of America Financial Center, downtown’s tallest building. “The building doesn’t sway. It jumps.”
One quake caused such a jolt she had to hang onto her desk, Burrows said. . . .
Spokane, a city of 190,000, has had no major quakes in its 120-year recorded history and wasn’t regarded as being in an earthquake zone. Because of that, there were no seismographs in the city when quakes started hitting in May. Now there are four.
Nearly all the quakes have been centered on the city’s north side, a few miles north of the Spokane River. The quakes have been shallow, sometimes only a mile or two deep, and noisy.
They are announced by loud cracks, sounding like explosions or the pounding of heavy equipment. In the tense atmosphere after the Sept. 11 terrorist attacks on the East Coast, such explosions have been particularly disconcerting.
Ground movement generally lasts only a few seconds.
Gonzaga Prep High School, directly over some of the temblors, is holding earthquake drills for its 970 students.
“Get away from the windows, get into the lowest position and stay there,” said dean of students Roger Cilley. “We don’t want them running out of the building.” . . .
The lack of recorded major earthquakes in the Spokane area is not a source of comfort, because the area’s recorded history is just a blink in geologic time, Tom Yelin of the USGS said.
While quakes have been going on for six months, the strongest occurred on Nov. 11, a magnitude 4 temblor that was followed by quakes measuring 3.1 and 3.3 over the next few hours.
“There’s no way to say if it is a harbinger” of a bigger quake, Steele said. “We can’t rule it out and can’t say if it is.”
Lack of information about location and length of the fault is a problem, Steele said.
“We don’t understand the fault’s parameters, so we don’t know what the maximum earthquake would be on that structure,” Steele said.
But the Spokane quakes are so shallow that even a magnitude 5 could cause plenty of damage, Steele said.
Spokane’s downtown includes a large number of historic buildings, built long before modern earthquake codes were developed.
If you want to learn more, the PNSN has a more extensive page giving an overview of the “2001 Spokane Earthquake Sequence,” as the page calls it.