As night fell on the last day of March, 2020, Sara and Chris Lundy were at the Williams Peak Hut in Idaho’s Sawtooth Mountains. After upwards of two to three feet of recent snowfall, avalanche danger in the region was rated "high," and the rapidly evolving COVID-19 pandemic had caused the suspension of skiing and guiding operations three weeks prior.
The pair, both experienced avalanche professionals, had ascended to the hut to shovel out and secure the structure after the big winter storm. Despite their expertise in the winter alpine environment, neither could have foreseen the event they were about to experience.
At 5:52 p.m. a magnitude 6.5 (M6.5) earthquake struck Idaho, centered 19 miles northwest of Stanley. It was the second-largest earthquake ever recorded in the state after the M6.9 Borah ‘quake of 1983.
As the shaking built, Sara and Chris ran out onto the deck of the hut. The deck beneath their feet moved in waves as the trees shook to and fro and the mountains around them roared and rumbled in a vast thundering as the peaks shedded mantles of soft, unstable snow. It was unlike anything they had ever heard before.
“Almost immediately, we heard the rumbling sound of avalanches reverberating in the mountains," recounts Chris. "Even though we were in a location that was safe from avalanches, we felt very vulnerable as the mountains shook around us."
After the initial uncertainty and terror as to what they were hearing and feeling, it became gradually apparent what they had witnessed. When the next morning dawned clear and cold, the two toured above the hut to gain a vantage on the surrounding terrain, hoping to get eyes on the results of the unique event.
What they found was the aftermath of the incredibly-widespread avalanche cycle that had been triggered by the earthquake. Virtually every steep slope had avalanched in some way. Chris later estimated that there were “hundreds of miles” of slab avalanche fracture lines throughout the Sawtooths. Luckily there were no casualties involved.
It seemed the Lundys had witnessed a once-in-a-lifetime event; but just exactly how unique was it?
How Much Magnitude Makes the Snow Move
Existing research suggests that the event the Lundys experienced was exceedingly rare. In the specialized world of snow and avalanche science, the most commonly-cited study is a 2010 paper by Podolskiy, Nishimura, Abe, and Chernous in the Journal of Glaciology, which was only able to identify 22 cases of earthquake-triggered, or seismogenic, avalanches worldwide between 1899 and 2010.
Read through the lens of the 2010 study, one factor that makes the event in the Sawtooths stand out was the extremely weak snowpack structure in place leading up to the earthquake and the resulting types of avalanches that were triggered. While other seismogenic avalanche events may consist of full-depth (i.e. glide) avalanches or catastrophic combinations of snow, ice and rock avalanches (as in the Everest region in 2015 and Huascaran, Peru in 1970), the avalanches triggered in the Sawtooths in March were predominantly slab and dry-loose avalanches running in the upper layers of the snowpack. So what do these observations from the Sawtooths tell us about seismogenic avalanches in general?
A brief review of the mechanics of avalanches may help illustrate the larger issues at play. Once an unstable snowpack structure is in place, such as a cohesive layer of snow overlying a weak layer or potential sliding surface, the trigger of an avalanche is usually the addition of a dynamic compressive force.
In the case of a natural avalanche, this may come from an increase in loading due to rapid snow accumulation or warming; in an artificial avalanche, the cause could be explosives, a snowmachine, or a skier, to name a few. Regardless of the cause, the balance in the snowpack structure is tipped. An initial failure occurs along the shear plane, and a slab of snow begins to slide along the weak layer below. Or, as McClung and Schaerer write in The Avalanche Handbook, “An avalanche is initiated when an applied force causes the applied shear stress in the snow to approach, equal or exceed the shear strength.”
The avalanche triggers mentioned in The Avalanche Handbook affect the snowpack “top-down,” at the surface or within the snowpack at (or near) potential weak layers. But what about natural earthquake-triggered slab avalanches? While the avalanche itself will display the same initial shear failure, the triggering force itself is entirely unique.
According to the 2010 study, “A high-rate tensional force oriented normal to the shear plane is possible only during an earthquake." In other words, earthquakes are the only known natural event that can affect the snowpack with a combination of back-and-forth shaking, waves, and/or vibrations forces coming “bottom-up” from ground level. Tests performed with explosives placed at ground-level during summer and then detonated mid-winter beneath an overlying snowpack are likewise generally ineffective at triggering slab avalanches.
Bill Glude, an avalanche forecaster who splits his time between Southeast Alaska and Japan, has witnessed many earthquakes in snowy environments during his career. “It’s one of those things where what people think should happen doesn’t match what does happen," Glude says. "Earthquake-triggered avalanches are rare. The smallest 'quakes I have seen trigger avalanches are in the 7.0 range, and even then only near the epicenter, and that size of earthquake also triggers landslides.”
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“The big M9.0 Touhoku, Japan earthquake in 2011 triggered avalanches in Hakuba some 300km away, but the snow conditions were so weak that day that natural- and human-triggered slides were already releasing," he continues. "The run-of-the-mill M5.0 earthquakes never trigger avalanches, even when the snowpack is weak and I go out looking for them. My guess is that somewhere around M6.0 is the threshold.”
Glude’s experiential assessment predicts a higher threshold than the conclusions of the Podolskiy paper, which includes a confirmed avalanche event caused by an M5 earthquake and goes even further with “statistical studies that indicate minimum magnitudes of M1.9 - M3.”
Other variables further complicate the question, such as the different types of seismic waves that earthquakes produce, the duration of shaking, and the amplification of an earthquake’s peak ground acceleration over mountainous terrain.
It is also noteworthy that since the Podolskiy paper was published, there have been at least three more events to add to the list: the Sawtooths in 2020, Tohoku, Japan in 2011, Canterbury, New Zealand in 2010. Furthermore, beyond these documented seismogenic avalanches, more have certainly occurred around the world which were not detected or recorded. This gap in knowledge points to a related, growing field of avalanche research—remote avalanche detection, via technologies such as remote seismic sensors and radar imagery.
Earthquake-Caused Avalanches and You
When it comes to protecting human life and property, the fundamental problem remains that predicting earthquakes is still in the realm of science fiction. In the future, research on seismogenic avalanches and remote avalanche detection may offer a better understanding of these destructive natural hazards. Ultimately, when an earthquake large enough to trigger widespread avalanches occurs, other natural disasters such as landslides, rockfall, and tsunamis can likewise be initiated, which have been—and will, unfortunately, remain—substantial threats to human safety and property.
If the ground does start shaking while you’re in the mountains, get off, out from under, and away from avalanche terrain. While the event that Sara and Chris Lundy witnessed in the Sawtooths in 2020 was very rare, it serves as a reminder of the true scope of potential hazards backcountry users accept when they step into nature.
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