BY KAREN BOSSICK
Avalanche forecasters had warned Wood River Valley residents about high avalanche danger Wednesday following a spring storm that dumped a foot to three feet of snow from Galena Summit to Banner Summit west of Stanley.
But they didn’t figure a 6.5 magnitude earthquake into the equation.
When the quake struck in the Banner Summit area 20 miles northwest of Stanley, the 63 residents of Stanley not only heard the rumble of the quake but they heard the rumble of avalanches tumbling down the nearby Sawtooth Mountains.
Clearing weather the next day, when temperatures dipped to minus 7, revealed “incredibly widespread” avalanching in the Sawtooth Mountains and in the mountains closer to the epicenter near Banner Summit, said forecasters for the Sawtooth Avalanche Center.
While some of the avalanches may have occurred naturally prior to the quake, the majority were released because of the quake.
Chris Lundy was in the Sawtooth Mountains near Williams Peak when the earthquake struck.
“When the earthquake hit, all of the trees shook and snow was falling out of them. That image will remain in my mind forever,” he said. “Almost immediately we heard the rumbling sound of avalanches reverberating in the mountains. Even though we were in a location that was safe from avalanches, we felt very vulnerable as the mountains shook around us.”
The next day, Lundy said, he and his colleagues climbed higher into the mountains to survey what had happened.
“Avalanches were everywhere. Snow that had previously been clinging to the steep, rocky faces of the Sawtooths had been rattled off. Crown lines from slab avalanches were countless. Even on lower angled slopes that were not steep enough to avalanche we found cracks in the snow as if the whole snowpack had shattered,” he said.
The avalanche center’s director Scott Savage said the earthquake triggered avalanches in the Smoky Mountains near Ketchum at least 60 miles away from the epicenter. People soaking in Warfield Hot Springs out Warm Springs Road near Sun Valley reported watching a wet loose avalanche release, knock down a few trees and cross the creek near them, even though they were 63 miles from the epicenter.
“When the skies cleared and the proverbial—and, maybe, literal—dust settled the day after, the sheer carnage the earthquake caused quickly became apparent,” he said. “In the northern Sawtooth Mountains near the epicenter, hundreds of steep slopes had avalanched. As you moved into the Smoky Mountains about 45 miles south of the epicenter, the number, type, location and appearance of the natural avalanches still made it obvious that something unique triggered the slides.”
And aftershocks could cause more instability in the snow, he added, cautioning backcountry skiers and snowmobilers.
The earthquake occurred as the result of a slip occurring on a steep dipping fault within the Intermountain Seismic Belt. The quake is nearly 10 miles north-northeast of the Sawtooth fault, a 36-mile fault that extends along the eastern base of the Sawtooth Range.
The quake spurred landslides across Idaho 21 north of Lowman, prompting the Idaho Department of Transportation to close the highway between Lowman and Stanley. It will be many days before the section is safe enough to reopen, department spokesmen say.
Rocks also spilled onto Idaho Highway 75 near Stanley, Idaho 55 between Horseshoe Bend and Cascade and Idaho 52 east of Emmett.
In Boise the quake cracked the stone cross on top of the Cathedral of St. John the Evangelist on 8th Street, even though the quake there was closer to a 5.0 magnitude given the distance.
Earthquakes & Avalanches—Frequently Asked Questions
- ●Do earthquakes cause avalanches? Yes. They occur frequently enough that there is a formal Snow, Weather, and Avalanche Guidelines (Greene et al, 2016) trigger code for earthquake-caused avalanches: NE. Significant avalanche activity has been attributed to large earthquakes in at least 22 cases in the Himalayas, Japan, Russia, the Andes, New Zealand, Alaska, northwestern Canada, in the mainland United States, and in other locations (Podolskiy et al, 2010).
- ●How do earthquakes cause avalanches? The working model for avalanche release involves small cracks initiating within a weak layer and then growing to a critical size where the weak layer collapses vertically. After the weak layer collapses, the slab releases and moves downslope. The motion caused by earthquakes likely causes many cracks to initiate in the weak layers, ultimately producing avalanches. Loose snow avalanches can be "shaken" off of very steep faces, similar to explosives-released loose snow slides.
- ●Have people been killed by earthquake-triggered avalanches? Yes. Fairly recently, an April 2015 magnitude 7.8 earthquake in Nepal's Himalaya Mountains triggered several large avalanches. One swept through the Mount Everest Base Camp, killing at least 22 climbers and support crew members. This is the deadliest known natural disaster on Mount Everest.
What about aftershocks? Several aftershocks occurred in the minutes and hours following the major earthquake Tuesday. Some of the recorded aftershocks were stronger than previously documented earthquakes that produced avalanche activity. The USGS expects many MW 3-4 aftershocks in the coming days, and an event larger than the initial MW 6.5 cannot be ruled out. It's worth considering this "objective hazard" when deciding to ski or ride in significant avalanche terrain this week.
- ●Are slopes stable now? Not necessarily. While the earthquake triggered avalanches on many slopes that were teetering on the edge of sliding, steep slopes that did not release could still be waiting for a skier or snowmobiler to find the perfect spot on or near a slope to trigger an avalanche. The SAC has received reports of 2 human-triggered slides in the 2 days following the earthquake. The snowpack will take some time to adjust to the feet of recent snowfall. Where evidence of large avalanches exists, the slopes that released are obviously safer than those that didn't. Other than that, we have to assume the earthquake did not make slopes more stable than they would have been if the earthquake did not occur.
CAUTION!
Skiers remotely triggered an avalanche near Galena Summit on Wednesday and possibly in the mountains west of Smiley Creek as well. Avalanche forecasters say the snowpack needs more time to recover from 1 to 3 feet of new snow made all the more unstable by strong winds.
Backcountry skiers and snowmobilers who get themselves in a pickle may not be able to count on rescuers as first responders deal with the surging spread of coronavirus in the Sun Valley area.