Description: From Orr and others, 2023: "Although Iliamna Volcano maintained an Aviation Color Code and Volcano Alert Level of GREEN and NORMAL throughout 2019, AVO observed seismicity episodes suggestive of ice and rock avalanches, and two large avalanches were confirmed in satellite images. Avalanches like these are common on Iliamna Volcano, where they are composed mostly of ice and snow (as much as 80 percent by volume) (Schneider and others, 2010), and they are highly mobile, traveling at mean speeds of approximately 50 meters per second (m/s) [110 miles per hour] and reaching peak speeds, estimated via numerical modeling, of more than 70 m/s [160 miles per hour] (Caplan-Auerbach and Huggel, 2007; Schneider and others, 2010).
The first of the two large ice and rock avalanches took place on the south flank of the volcano in late March. This first avalanche started at an elevation of 2,800 m [9,200 ft] and ran for about 3 km [1.9 mi], reaching a final elevation of 1,450 m [4,760 ft]. A seismic signal inferred to be from the avalanche was detected on stations as far as approximately (~) 100 km [60 mi] away at 18:00 UTC (10:00 AKDT) on March 26, with a recorded duration of 3-4 minutes. This timing for the ice and rock avalanche was roughly confirmed by Landsat satellite images taken before and after the seismic signal, although the signal of the avalanche did not appear in infrasound data.
"The second, much larger ice and rock avalanche took place on the east flank of Iliamna Volcano at 00:03 UTC on June 21 (June 20 at 16:03 AKDT) (Toney and others, 2021). An oblique aerial photograph taken the same day and Copyright 2019 DigitalGlobe, Google Earth Copyright 2019 DigitalGlobe, Google Earth satellite imagery acquired the next day showed the flow deposit on Iliamna Volcano’s east-facing Red Glacier. Red Glacier has hosted many debris avalanches in prehistoric and historical time (Waythomas and others, 2000); its most recent avalanche of comparable size took place in May 2016. The June 21 rock and ice avalanche initiated less than 1 km [0.6 mi] from the volcano’s summit and traveled east for about 8 km [5 mi]. Measurements taken from satellite imagery estimate that the deposit covered an area spanning approximately 7.1×10^6 m^2 [76,000,000 ft^2]. Assuming an average deposit thickness of 0.5 m, this yields a volume of about 3.6×10^6 m^3 [39,000,000 ft^2].
"The second ice and rock avalanche on Iliamna Volcano, like the July 15 ice and rock avalanche on Mount Spurr, generated energetic seismic and acoustic signals that were recorded both locally and regionally (Toney and others, 2021). One local seismic station recorded at least 100 minutes of precursory seismicity (J. Caplan-Auerbach, Western Washington University, written commun., 2019). High-frequency signals associated with the event itself were recorded on local and regional (greater than 100 km [60 mi] away) seismic networks, and LP seismic signals were recorded more than 600 km [370 mi] away. Pre-avalanche seismicity was also documented during Red Glacier avalanches in 1994, 1997, 2003, and 2016 (Caplan-Auerbach and Huggel, 2007; J. Caplan-Auerbach, Western Washington University, written commun., 2016). The infrasound array in Dillingham detected acoustic waves from the event, as did the infrasound arrays in the Alaskan cities of Sand Point and Fairbanks."