A COMPARISON OF SUBMARINE LANDSLIDES IN TWO ADJACENT FJORDS, PRINCE WILLIAM SOUND, ALASKA

The recent recognition of the ~500 million m³ bedrock landslide in Barry Arm, Prince William Sound (PWS), Alaska, highlighted the need to better document landslides in fjords and understand their potential tsunami-generating impacts. Though subaerial landslides can be directly observed in the field and mapped using various remote sensing techniques (e.g., lidar, InSAR, and satellite imagery), submarine landslides generally require multibeam bathymetric data for detection and analyses. Here, we present an analysis of Holocene submarine landslides in two adjacent fjords in northwest PWS — Barry Arm and College Fjords — all mapped using high-resolution (~4 m) bathymetric data collected in 2020 and 2021, respectively. Both fjords share similar relief, lithology, and climate, and show evidence of frequent sediment slope failures with landslide mobilities ranging from 0.02 to 0.1 in height to length ratios. Though little is known about submarine landslide triggers in this area, the M9.2 1964 earthquake caused well-documented tsunamigenic landsliding in PWS and had an epicenter within 50 km of both fjords. Despite their similarities, College Fjord hosts a 15-km-long landslide-prone moraine that was deposited during a slow ~1,500-year-long late-Holocene deglaciation period. As a result, College Fjord exhibits an order of magnitude difference (n = 10 and n = 1, respectively) in the number of large landslides (volumes > 1 million m³) compared to Barry Arm, which lacks a similar moraine complex. Considering the shared proximity of both fjords to major earthquakes, evidence of larger and blockier failures in College Fjord suggests that the geomorphic history and availability of unconsolidated glacial and fluvial sediments controls the distribution of submarine landslides in this area, as opposed to differences in triggering. Overall, our work reveals a previously undocumented active landslide-susceptible submarine landscape with fjord-scale differences, warranting further investigation into submarine landslide hazard in PWS. In addition, despite a record of up to 2,500 years of ice-free terrain, we found evidence of only two subaerial-to-submarine bedrock landslides in the bathymetry, suggesting that large subaerial bedrock landslides that enter water may be rare in northwest PWS.