It’s been three months since a 6.5 magnitude earthquake struck northwest of Stanley on March 31. Since then, Stanley residents have felt many more rumbles and geologists have deployed to central Idaho, investigating what happened and what it can tell us about Idaho’s seismic nature.
Dylan Mikesell, a seismologist and Boise State professor was in his fifth-floor dorm room when he felt the shaking.
“It was shaking our blinds and banging blinds against windows, and stuff like that,” he said.
Mikesell and his colleagues quickly got together a group for a Zoom call to talk about what happened. Initial reports from the United State Geological Survey showed the quake north of Stanley was along an un-mapped fault.
“This showed it was interacting with the Trans-Challis Fault system, which we thought was no longer active,” Mikesell said.
The researchers worked to locate as many seismometers as possible from regional agencies that night. Within a couple days, they had driven seven sensors to the Stanley area, placing them in a large circle around the epicenter.
Since the initial earthquake, there have been thousands of aftershocks, some over 4.0 magnitude that have been felt in Boise. Each shock gives scientists another clue about how the fault lines are behaving beneath the earth’s surface.
Idaho's permanent seismometer network can usually only pick up earthquakes stronger than 2.5 magnitude, Mikeshell said. And to really get an accurate "CT scan" of the faults, scientists want to be able to pinpoint even the smallest aftershocks, so the stronger remote sensors help gather that data.
“We’re monitoring how the ground is moving up and down, north and south and east and west,” Mikeshell said.
Since late March, the aftershocks have moved south, toward the city of Stanley and the Sawtooth Fault. But it’s not just movement the researchers are watching.
Stanley Mayor Steve Botti said he can hear the aftershocks before he feels them.
“It’s a strange thing, it’s kind of spooky, it’s just ambient noise,” he said.
The Boise State geologists placed infrasound sensors in the area, too, to record sounds from the earthquakes, including those that aren’t heard by humans.
Newspaper reports of earthquakes often include anecdotes of people saying they heard sounds before feeling an earthquake, but this hasn’t been studied extensively scientifically. So Mikesell and the Boise State team are looking into this, and they think the Sawtooth Mountains might have something to do with it.
“Those mountains are like little speakers that are converting mechanical energy in the earth’s crust into a little sound wave,” Mikesell said.
Other research teams are doing field work near Stanley, too. Claudio Berti, the director of the Idaho Geological Survey, headed to the mountains, as well, but he was looking for physical evidence of the earthquake. His team found avalanches and some slides, but no ruptures on the surface.
That was, until the Forest Service posted a picture at Stanley Lake, showing that a large chunk of the shoreline had fallen into the water. Now a land area of two or three football fields is no longer there — it slid into the lake. Months later, Berti stood on the shore and pointed out canoers navigating fallen trees.
“Those guys would not be canoeing three months ago, they would’ve been walking and now they’re paddling,” he said.
Berti wants to understand what happened at Stanley Lake, and based on some initial surveys, he thinks it underwent a process called liquefaction.
Because of the shaking during the earthquake, the sediment of sand that had made up the delta started behaving, momentarily, like a liquid.
Pointing to a fallen tree, now partially submerged in water, Berti explained how the ground around it starting acting like water, so the roots couldn’t keep the tree standing and it fell.
Investigating how the Stanley Lake delta liquified is important, Berti said, because if a shoreline can sink during a rumble, a bridge or a house might, too. That has Stanley residents on edge.
“We all just kind of get like a grimace on our face when we feel it shake,” said Mandy Clark. She owns the Mountain Village Resort in Stanley.
“We’re all kind of on edge any time the wind blows over the building.”
The initial quake, captured on security cameras in the Village wine shop, knocked a handful of bottles off shelves, but nothing broke. Minimal damage was thankfully the story for most folks in town.
Clark said the shaking was more intense at her cabin about four miles west of town that evening.
“It was terrifying,” she remembers. “The sound that was coming from the earth, it was just like a freight train going through. It shook all cupboards open, all drawers open, things off the top shelves, pictures off the wall.”
“We all remember as a child going under your desk,” Clark continued, “but as an adult, we kind of forgot what we did and a lot of people panicked or didn’t do the right thing.”
No injuries were reported that day, but the almost daily tremors since have some people in town thinking more about emergency procedures.
“We have talked with Custer County Emergency Services,” Botti said. He’s in his third year as Stanley Mayor. He says discussing earthquake preparedness while the issue is top-of-mind seems like the right thing to do.
“Like, what should people be thinking about? Should [people] be inspecting your house? Should we have earthquake preparedness ordinances, building codes?” he asks.
Botti says those kinds of ideas move slowly, even as regular shaking continues to remind residents.
The possibility of getting cut off if roads are impassable or liquified in a large earthquake is certainly real. Occasionally, Stanley — with only three highways in and out — is cut off by heavy snowfall or avalanches. Botti says the longest stretch he remembers is five days.
“Even that length of time is a great concern because, you think about medical emergencies,” he said. “If we’re cut off, there are all kinds of things like that that could happen that would be difficult for us to deal with.”
Back at Mountain Village, Clark said she knows some tourists are hoping to feel an aftershock. But the coronavirus shutdown cost her most of April’s expected revenue, so she’s glad folks are coming back at all. Businesses in Stanley make the majority, if not all their annual income between Memorial and Labor days each summer.
“We’re definitely nervous that there’s going to be another shutdown and that we will not be able to, you know, operate business as usual or people are going to stop traveling because of restrictions,” Clark said.
Between the continuing vibrations and the virus, the summer tourism season in Stanley is shaping up to be one to remember.
Follow Troy Oppie @GoodBadOppie and Rachel Cohen @racheld_cohen on twitter for more local news.
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