SCIENCE SATURDAY: Earthquakes in Chewelah? We can feel them but don’t expect “San Andreas”

(BRANDON HANSEN/Chewelah Independent)

Residents of Chewelah and much of the Inland Northwest had the unnerving feeling of the ground moving in April as a 6.5 magnitude quake struck the Boise, Idaho area near Challis.

The quake was the largest to hit Idaho since 1983 and was felt in six states. In 1983, the Borah Peak earthquake hit 6.9 on the Ritcher Scale and killed two while causing millions of dollars worth of damage.

In Chewelah, buildings shook and many people took notice of the weird wave motion the ground was making. Kathryn Patton said, “My office chair started rolling with me in it,” while Kristin Grooms said “Totally felt it after noticing noise coming from my chandelier.”

Other people in Stevens County also felt the shaking. Loretta Corter said, “yes, I live in the Ford, Wellpinit line and it knocked things off my shelves and moved my couch.”

Other locals tried to make some jokes about the situation. Aaron Alvarado said, “Sorry everyone, my bad, I let my pet lizard out again – Godzilla says sorry.”

The earthquake caused no reported damage in Stevens County. In Idaho, there was some damage done to structures, while some landslides and avalanches were also triggered. The event was kind of a “once in a generation” quake for the area – and a reminder that Idaho and the Rocky Mountain region are seismically active.

“There were notably large ones greater than magnitude 6 in 1944, 1959, 1983 and this 2020 quake,” said Earthquake Geologist Austin Elliot at the USGS Earthquake Science Center Center. “The state is indeed generally a seismically active area. There are several large well known faults that form some of its most iconic mountain ranges – the Sawtooths, the Lost River Range, the Beaverhead Mountains – as they stretch out the crust.”

Elliot said that even though the plate boundary is way to the West along the coast, slow extension of the North American continent is happening across the whole intermountain west all the way over to Wyoming, resulting in quakes in the Rocky Mountain region.

Chewelah also felt some shaking from the 2001 Nisqually earthquake in Seattle, but that was a much deeper earthquake – about 31 miles down which tends to dampen shaking, whereas the 2020 Idaho earthquake was just six miles down.

It also appears to be felt more by those in the Colville Valley than those surrounding residents up in the hills. This has something to do with the ground that residents are built on.

“The sedimentary rock beneath you in a flat valley is less compact and rigid than the solid rock of the surrounding mountains, so seismic waves move through it more slowly, amplifying their intensity,” Elliot said.

He also added that the contents of a building – such as having a lot of glassware – would make shaking more apparent as opposed to a less cluttered household. He also said building construction determines shaking as well.

Now earthquakes aren’t really something of a concern for people in Northeastern Washington; the area just doesn’t have the potential for large quakes. Residents, might, however, feel waves from other Northwest regions that are more active.

“Eastern Washington is not particularly seismically active compared to the regions surrounding it,” Elliot said. “So most of the quakes you’ll feel will be ones that are worse somewhere else. Those big ones only happen every couple of decades, but every now and then there are smaller tremors — even less likely to be widely felt — that probably rattle somewhere around you every couple of years or so.”

There have been 10 major earthquakes in Washington State since the 1700s, with the last being the 2001 quake that was 6.8 on the Ritcher Scale. In 1946, a 7.3 quake hit the Straight of Georgia in Puget Sound. It was estimated in the 1700s that a 8.6-9.2 magnitude earthquake struck the Puget Sound area.

Spokane, has a pattern of relatively minor earthquakes occurring about every 20 to 25 years. This is due to some minor fault lines. The biggest quake that happened in Spokane has been a 4.0 magnitude earthquake.