Double Trouble By Brad Carpenter On March 31, 2020, a magnitude 6.5 earthquake violently shook south-central Idaho, in the U.S. Chris Lundy, a local ski guide in the Sawtooth Mountains near where the epicentre of the earthquake occurred, happened to be servicing the backcountry hut he co-owns there. Lundy described what he experienced in his blog: “At 5:52 PM, everything started to shake, rumble, and sway… Almost immediately we heard a different sort of rumbling—the kind that comes from large avalanches. My first thought was that a massive avalanche was about to hit the hut—an irrational thought since the yurts are in a safe location.” The next day Lundy was amazed by what he saw in the surrounding mountains::
Photo 1: Earthquake triggered avalanches in the Sawtooth Mountains. Credit: Tanner Haskins, SMG.
“April 1st dawned clear and cold, and at first light, we saw our first avalanches...The snow had slid from nearly every steep slope (see photo 1)…Slabs had fractured, powder had sluffed, and hanging snowfields had come unglued from vertical rock. The aprons below the steeper terrain were covered with piles upon piles of avalanche debris.”
If there is enough snow on the ground and an earthquake occurs, avalanches can be triggered. The first time I experienced this fact was on September 4th, 2010 when the magnitude 7.1 Darfield earthquake struck. This event caused extensive destruction to the Canterbury region resulting in billions of dollars of damage. The epicentre was located near enough to the surrounding mountains to cause a widespread earthquake induced avalanche cycle similar to what Lundy witnessed. There were multiple, large avalanche events triggered from steep terrain across the region. Once I started looking into these dual-natural hazards I learned that there have been many such earthquake triggered avalanche events recorded in modern times: A June 19, 1994 avalanche at Porter Heights Ski Area was determined to have been triggered by an earthquake. The avalanche occurred at night, travelled downhill across a cat track and swept a grooming machine from the slope, tipping it upside down (see photo 2). The accident destroyed the grooming machine and fatally injured the groomer operator. NZAD 80
Photo 2: Porter Heights 1994. Photo courtesy of NZ Mountain Safety Council.
