Abstract

Rock avalanches are large-magnitude mass movements with high mobility and fluid-like runout; however, because of their scarcity, little information is typically available to describe the hazard posed by these events. Geologic records thus provide key data regarding rock avalanche size, timing, and dynamics. Here we present a detailed case history analysis of the Devils Castle rock avalanche located near the town of Alta in the Wasatch Mountains of Utah. The deposit is ∼1.5 km in length with a Fahrboeschung angle of 14 degrees (height-to-length ratio = 0.25). Through topographic reconstruction, we calculated a deposit volume of 1.7 million m3 with a maximum thickness of 25 m and an average thickness of 7 m. Cosmogenic surface exposure dating of six deposit boulders indicates a failure age of 14.4 ± 1.0 ka. The Devils Castle headwall displays no obvious evidence indicating precise source location and geometry; therefore, we reconstructed two plausible source volumes and performed numerical runout simulations for each. Results agree well with mapped deposit boundaries for both source scenarios; however, the east source model better represents material and dynamic characteristics of the deposit observed in the field. While the region is seismically active, the Late Pleistocene age for the rock avalanche precludes ascribing direct correlation with any currently known surface-rupturing paleoearthquakes. We identified and describe five similar events in the region highlighting the extent of the potential hazard. Individual case history analyses such as this allow us to better understand the processes and controls of large-scale mass movements in the region.

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