Description: From Orr and others, 2024: "A large ice and rock avalanche took place on the east flank of Iliamna Volcano on August 5, 2021, at 07:21 AKDT (15:21 UTC). Afterward, satellite imagery and oblique aerial photographs showed a dark-colored flow deposit on Iliamna Volcano’s east-facing Red Glacier. This avalanche initiated less than 1 km [0.6 mi] from the volcano’s summit and traveled at least 4 km almost due east, generating seismic and acoustic signals recorded locally and regionally. High-frequency signals were clearly recorded on Iliamna Volcano’s seismic network. Farther away, the KENI infrasound array at Kenai, and the HOM infrasound station in the City of Homer, Alaska, detected acoustic waves from the event.
"Red Glacier has hosted many avalanches historically and prehistorically (Waythomas and others, 2000); the most recent ice and rock avalanche of comparable size to this event occurred in June 2019 (Toney and others, 2021; Orr and others, 2023). These mass flows are generally composed of mostly ice and snow with smaller amounts of rock (involved through entrainment or as part of the original failure region). Avalanches on Red Glacier are highly mobile, traveling at mean speeds of ~50 meters per second (m/s) [~110 miles per hour] (CaplanAuerbach and Huggel, 2007) and peak speeds, estimated via numerical modeling and seismic force inversion, of greater than (>) 70 m/s [160 miles per hour] (Schneider and others, 2010; Toney and others, 2021). Judging by the size of the deposit and the amplitudes of the seismic and infrasound signals, the August 2021 event appears to be smaller than the June 2019 ice and rock avalanche."