Aftershocks possible for weeks, years
An Idaho earthquake centered near Stanley caused street signs to sway for hundreds of miles in every direction Tuesday, and while it is rare for tremors in the state to be felt so clearly at such distances, a local expert said Idaho is more geologically active than many would guess.
“It was a great day for a seismologist; it’s wonderful to have a big earthquake like that where nobody gets hurt,” seismologist and University of Idaho professor Ken Sprenke said. “It was way up in the national forest out there – I doubt if there was anybody close to the epicenter at all. So a wonderful earthquake and it’s going to be a really interesting one to study.”
Sprenke said Idaho is covered in active faults that cause small but noticeable tremors all the time, but much of what keeps these events from being noticed is the state’s sparse population.
At magnitude 6.5, Sprenke said Tuesday’s tremor was the largest in Idaho since a magnitude-6.9 earthquake which originated in the same region near Borah Peak in 1983. According to the U.S. Geological Survey, that event killed two children in Challis and resulted in more than $12 million in damage. He said Idaho more commonly sees smaller events around magnitude 3 and 4 – but they happen all the time.
“I believe the year before last we had an earthquake swarm going on down in Soda Springs, Idaho, and one going on north of Challis and were feeling events from over in Montana,” Sprenke said. “We had more ‘felt’ earthquakes in Idaho than California that year – but they’re generally small, so that’s why this one is nice because it’s a nice big event.”
Sprenke said the main quake will have sent out seismic waves, weakening rocks and triggering other faults nearby to produce their own events, commonly known as aftershocks.
USGS servers went down shortly after Tuesday’s quake, Sprenke said, so many aftershocks haven’t been recorded by the agency. He said his personal seismograph is covered with hundreds of aftershocks that were big enough to be felt and these will likely continue for weeks or even years.
“We still get aftershocks from the Borah Peak event and (that was) 38 years ago,” he said.
While most aftershocks will likely be smaller than what was felt Tuesday, he said the possibility for a much larger aftershock remains – which could kick off a series of seismic happenings.
According to a release from the Idaho Transportation Department, several mountain highways saw rockfall and a few landslides following the quake. The release said aftershocks continue to cause rockfalls throughout the region and crews are actively patrolling high-risk areas and removing rocks from the roadway.
ITD said landslides near the epicenter caused the closure of at least one state highway leading to Stanley. Maintenance crews are waiting for the USGS to say that area is safe from additional slides before they begin cleanup. ITD said there has been no apparent damage to the state’s bridges, but the agency’s bridge department is conducting a more detailed analysis.
Sprenke said he is looking forward to studying Tuesday’s quake – specifically how it will create and trigger other events in the region. He said he is particularly excited to search the area for a fault scarp – or noticeable ground breakage caused by shifting tectonic plates.
He said the Borah Peak quake occurred around the time he was first hired to teach at UI. Now nearing emeritus status, he said it is fitting that the end of his tenure as active faculty coincides with another notable quake.
“Right about when I got here there was a big earthquake, and now I’m retiring in a month or two here and I get another big earthquake,” he said. “I started with a boom and I can end with a boom.”