Idaho started shaking March 31. Why hasn’t it stopped? Geologists research Sawtooth fault
Natalia Gutierrez-Pinto and Nicole Blanchard, Idaho Statesman
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BOISE (Idaho Statesman) — The shaking started March 31.
That’s when the Sawtooth mountain range in central Idaho trembled with a 6.5-magnitude earthquake — the second strongest ever recorded in Idaho.
From that very moment, geologists rushed to the epicenter area — 45 miles west of Challis — to start collecting valuable information that could help them understand what happened.
Three months later, the rumbling still hasn’t subsided as aftershocks continue to jar the area. But now scientists have a better idea of what may have led to the major earthquake — and it’s helping them uncover some other secrets buried under Idaho’s soil.
Idaho fault discovered only a decade ago
Idaho sits on the northern end of an area that is geologically very active, known by geologists as the Basin and Range Province. The earth crust in this area is constantly stretching and breaking in multiple places, creating hundreds of geological faults over long periods of time.
“Most of those faults are very old faults, and they’re no longer moving,” said Glenn Thackray, a geoscience professor at Idaho State University who specializes in studying faults. “The fault that that broke in that first earthquake in March was thought to maybe be active but not known to be active.”
Thackray refers to a 40-mile long fault that runs north to south from Stanley Lake to Alturas Lake. He discovered the Sawtooth fault — which gets its name from the Sawtooth mountain range — only about a decade ago.
His initial thoughts after the magnitude-6.5 earthquake were that the Sawtooth fault was involved, but many questions remained to be answered.
Initial difficulties come with difficult terrain
The Central Idaho region is geologically active and interesting, but it’s poorly understood. Thackray said research funding usually goes to places that are more densely populated — where more people are at risk if a large earthquake happens.
That’s why there are not many permanent seismometers in Central Idaho.
The Stanley earthquake created a unique opportunity for Idaho geologists to understand what is happening on the surface of the earth and beneath it. They rushed to the area and deployed more than 30 seismometers that could capture the movements of the aftershocks — in spite of deep snow, difficult access and pandemic-related travel restrictions in the area.
Thackray’s initial hunch was that the Sawtooth fault was involved in the earthquake, but at that time, two pieces of information didn’t quite fit with this scenario.
First, the epicenter of the earthquake was located some 16 miles north of where they thought the Sawtooth fault ended. Second, its movement pattern was not typical of the region.
“The way that the rocks moved is very unusual for this part of the country, (which is) mostly up and down. … The main shock moved almost perfectly left to right, almost kind of a California style” earthquake, said Claudio Berti, director of the Idaho Geological Survey.
Aftershocks are first clue for solving earthquake puzzle
Aftershocks — the continued movements of the broken fault as it settles — usually happen along the same fault that caused the main earthquake.
Seismometers have been collecting data on the location and motion of aftershocks over the past three months, which is allowing geologists to get some answers. It’s a “connect the dots kind of game,” Berti said.
“We think that (the Sawtooth fault) actually now extends further north than what we thought before the earthquake,” said Boise State University seismologist Dylan Mikesell. He can tell because most of the aftershocks are happening in a north-south line, filling in the space between the north end of the fault and the epicenter.
Berti has a slightly different idea. He thinks that the Sawtooth fault might be expanding to the north, using other weak spots in the earth’s crust to grow.
Another set of aftershocks happening on an east-west line suggests that there might be another fault that runs in that direction.
“There could have been some (energy) transfer from the Sawtooth fault on to this other fault — which I don’t think that we knew about before,” Mikesell said.
Liquefaction at Stanley Lake
In May, officials of the Sawtooth National Forest announced that Stanley Lake’s beach — a popular fishing and swimming location — had collapsed after the earthquake.
That’s where Berti headed in late June to continue cataloging the quickly disappearing signs of the quakes. Berti conducted an aerial survey of the area by drone to compare the lake’s appearance to previous photographs. Then, pulling on rubber waders up to his thighs, he sloshed through Stanley Lake looking for the various disruptions the earthquake caused.
Many of the marks of the earthquake are subtle: a half-inch crack near the boardwalk that once led to the beach, a circle of fresh white sand where water jetted up and disrupted the sediment. Then there’s the obvious evidence: feet of water where a beach once stood.
The beach and delta sunk into the lake because of a physical process called liquefaction. When this happens, the soil behaves as a liquid for a short time.
Liquefaction is a relatively rare process, Berti said. It “happens when you have the right composition of sand and silt and clay, and the soil is completely saturated with water.” This is exactly what the soil looks like in Stanley Lake.
Berti compared the event with what happens when a baker measures sugar for a recipe. To ensure they have the right amount, they shake the container so sugar grains can accommodate and fill the empty spaces between them.
Similarly, the sand grains in the bottom of the lake are not particularly well-packed. Berti explained that when the earth shakes, grains “try to set into pores and spaces that are occupied by water. (When) the grains try to push on the water, the water pushes back,” disintegrating and mixing sand grains — and the soil effectively behaves like a liquid.
Scientists know very well when liquefaction happens since it leaves evident signs. One is the formation of sand boils — also known as sand blows, or sand volcanoes — which happen when the pressure built beneath the surface is released, leaving those circles of white wet sand.
Another sign is the presence of fallen trees or sunken structures. It happens because their roots — or foundations — lose grip with the soil that sustains them when it becomes liquid.
What are scientists still hoping to find?
Scientists are eager for more upcoming data that will help them reconstruct the missing pieces of Central Idaho’s geology puzzle.
Their main goal is to reconstruct the geometry of the faults in the area, especially between the north end of the Sawtooth fault and the epicenter. Apart from the data they are gathering from beneath the ground with the seismometers, they can also understand the area using satellites and airplanes.
“There’s a bunch of satellites right now that are collecting what we call InSAR data – Synthetic Aperture Radar – and that will allow us to actually see which part of the surface of the Earth has moved since the big earthquake,” Mikesell explained.
Another technique that will be applied to this area in the coming months is called LIDAR, which is key to identify faults in forested areas. “An airplane (flying at low altitude) with a laser scanner is really what it is,” Thackray said.
Using LIDAR, researchers can remove the trees digitally and recognize faults that are not evident to the naked eye.
Berti mentioned they’re also hoping to dig trenches along the newly identified faults, which allows them to see how many times similar events have happened in the past. “That gives you a better idea of the recurrence time of those main, large, surface-breaking events.”
Idahoans should prepare for more aftershocks
Big earthquakes like March’s one trigger a sequence of smaller quakes – aftershocks – that can last for a long time.
As of June 24, the USGS had registered 211 earthquakes of magnitude 3 or higher. “The aftershocks will slow down over the next, let’s say decades, but then they won’t completely disappear,” Mikesell said.
He added that “nothing is strange about this earthquake, and the aftershock sequence seems to be pretty normal as well. It’s just (that) we don’t have these earthquakes very often, (maybe) every couple of decades.”
Although predicting the exact time, place or strength of an earthquake is impossible, the USGS estimates a 99% chance of having earthquakes of magnitude 3 or higher over the next month and a 12% chance of earthquakes of magnitude 5 or higher.