Backcountry skiers are being warned to beware of potential avalanches due to aftershocks associated with a 6.5 magnitude earthquake that occurred northwest of Stanley on March 31.
The earthquake occurred at the end of a snowstorm that dropped 2-3 feet of snow in the surrounding mountains, and the avalanche danger was rated as High on the day of the quake. According to a press release from the Sawtooth National Forest, many Stanley residents reported hearing the rumble of avalanches coming from the nearby Sawtooth Mountains when the earthquake occurred.
“Clearing weather on April 1 revealed incredibly widespread avalanching in the Sawtooth Mountains and in the mountains closer to the epicenter near Banner Summit,” the Forest Service stated. “While some of those avalanches may have occurred naturally prior to the earthquake, evidence suggests that most of them released as a direct result of the earthquake.”
The agency stated that a major earthquake occurring during a period of elevated avalanche danger resulting in widespread avalanching is a very rare and historic event for Idaho.
Chris Lundy, an avalanche specialist with the Sawtooth Avalanche Center, 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,” Lundy said in the release. “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, we climbed higher into the mountains to take advantage of good visibility and look at what 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.”
Avalanche Center Director Scott Savage said that in the northern Sawtooth Mountains near the epicenter, hundreds of steep slopes had slid.
“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,” Savage said.
He said initial observations suggest the earthquake triggered avalanches to the south at least 60 miles away, not quite to Ketchum.
The Forest Service said aftershocks could cause additional avalanches.
The U.S. Geological Survey has reported a continuous series of aftershocks since the initial quake, and stated that an even larger quake cannot be ruled out. Two tremors were felt in the Wood River Valley on Sunday morning—a magnitude 3.8 quake at 5:26 a.m. and a magnitude 3.6 quake at 8:10 a.m.
“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 Forest Service stated Friday.
The Avalanche Center states that six human-triggered slab avalanches had been reported in the area since March 28. On Tuesday, the avalanche hazard was rated as Considerable in the local mountains.
The Idaho Transportation Department reported that the initial quake caused “extensive” landslides in the Canyon Creek section of state Highway 21 west of Banner Summit. The department had already closed the road due to avalanche danger when the earthquake occurred. It said crews would not clear the highway until the danger of aftershocks abated. The section remained closed as of Tuesday afternoon. The ITD stated it would reassess the situation at 5 p.m. Tuesday.