Soldier & Wood River Valley Mtns

Avalanches large enough to bury people are most likely where the wind has formed recent drifts but are also possible in areas sheltered from the wind. At lower elevations, natural wet loose avalanches are possible on steep, northerly aspects if skies remain mostly clear this afternoon. 

If you're traveling in the upper reaches of the Warm Springs Ck drainage near Dollarhide Summit where more snow fell this week, refer to the Galena Summit & Eastern Mtns zone forecast. 

COVID-19: The medical system cannot spare the resources required to care for injured backcountry skiers and snowmobilers. Search and Rescue will be delayed or unavailable. Please recreate responsibly close to home, and follow social distance requirements to protect yourself and others.  

EARTHQUAKES & AVALANCHES

On Tuesday, March 31st, a 6.5 magnitude earthquake shook south-central Idaho just before 6 PM. The quake's epicenter was in the SAC Banner Summit zone, about 10 miles north of Banner Summit proper. We received reports of people hearing significant avalanches running in the Sawtooth Mountains near Stanley, making noise for up to a minute after the earthquake ended. Wednesday, we were able to view many of the hundreds of avalanches that released on Tuesday (links). Our initial observations indicate many of the slides were triggered by the earthquake, especially in the Sawtooth Mtns. Here's a brief FAQ-style overview of the topic:

• 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 steep slopes that were impacted safe now if they didn't already slide? A few documented cases of avalanche activity occurring minutes to hours following earthquakes exist (Singh et al, 2002). Scientists continue to debate whether a 2017 avalanche in Italy that destroyed a hotel and killed several people was a delayed-earthquake event (Geggel, 2018). Limited research and evidence surrounding delayed avalanche activity after earthquakes or large explosive detonations suggests that once several hours have passed from the seismic event or explosion, the snowpack is probably as stable as it was before the earthquake or explosion. Remember that aftershocks are likely to continue jostling the snowpack for days, so it will be difficult to tell when the last "insult" occurred while you're out playing.  

Are there some slopes that, without Tuesday's earthquake, would be teetering on the edge of avalanching but did release because of the earthquake? Definitely. Can we assume that the overall snow stability, over a large scale, is now better because of the earthquake? Unfortunately, not with enough certainty to risk our safety.  We're sailing in uncharted waters. 

Experiencing a strong earthquake at the tail end of a storm that dropped 2-3 feet of snow (HIGH avalanche danger) is a very rare event. We (SAC Forecasters) are as interested as everyone else to observe and study how the snowpack reacted to the earthquake. When evaluating stability gets complex (large storm + weak layers in the upper snowpack + lingering depth hoar at the bottom of the snowpack + earthquakes), we need to base our terrain decisions on simple data and red flags: we just received feet of snow, it was windy, persistent weak layers exist in the snowpack, and the storm continued after the earthquake. If you see evidence of large avalanches, the slopes that released are obviously safer than those that didn't. Other than that, assume the earthquake did not make slopes more stable than they would have been if the earthquake didn't occur.