THE 2022 CHAOS CANYON LANDSLIDE IN ROCKY MOUNTAIN NATIONAL PARK: INSIGHTS REVEALED BY SEISMIC ANALYSIS, FIELD AND REMOTE-SENSING INVESTIGATIONS, AND RUNOUT MODELING

An unusual, high-alpine, rapid debris slide originating in ice-rich glacial debris occurred on the afternoon of June 28, 2022, at the head of Chaos Canyon, a formerly glaciated valley in Rocky Mountain National Park. The slide was recorded on videos by recreationalists, and ground shaking from the event was recorded by three seismic stations 30-70 km away. We used the video and seismic records, meteorological data, field observations, pre- and post-event satellite and unoccupied aircraft system imagery, and SHALTOP runout modeling to characterize the pre-event deformation, slide dynamics, and future hazards posed by the slide. Pre-event deformation preceded the rapid failure by decades, starting around 2000, but accelerating starting in 2018, which was the warmest year on record at the nearby Bear Lake Snow Telemetry station. Deformation reached ~20 m/yr in 2021. The combination of videos and seismic records indicated that the June 28 sliding sequence consisted of multiple failures. The main rapid failure occurred 1.5 minutes after a M1.4-equivalent reservoir construction blast about 36 km to the east. It is unlikely that this blast triggered the slide, since shaking from the blast was very weak (~0.001 %g), and video timestamps indicate that the slide was already starting to mobilize before the blast. We estimated the total slide volume to be ~2M m³, with peak velocities that reached about 5 m/s and total displacement of about 200 m. After the slide occurred, we observed melting ice-rich debris (permafrost), as well as dry, conical debris mounds (molards) from melted blocks of frozen debris. We hypothesize that the rapid slide was induced by gradually increasing long-term air temperatures that thawed ice and increased pore pressures within the glacial debris. Mean annual temperatures at the landslide elevation began to regularly exceed 0°C in 2006. The post-event slide deposits are oversteepened, contain ice, and generated small debris flows in 2022. Our runout modeling constrained by historical slide mobility data indicates that future slides may pose a hazard to some popular climbing areas downslope. However, runout at the site may be shorter than our runout analysis indicates because the runout path is a boulder-rich rock glacier that lacks a distinct channel.