ASSESSING SUBMARINE LANDSLIDE SUSCEPTIBILITY IN GLACIER BAY, ALASKA

Submarine mass wasting events can damage underwater structures and propagate waves that can inundate towns and impact human populations in coastal areas. Susceptibility to submarine landslides can be pronounced in degrading cryospheric environments, where existing glaciers can contribute high volumes of sediment, and cycles of glacial advance and retreat can damage and destabilize slopes. Despite the importance of submarine landslides to assessing tsunami hazard, these features can be difficult to study because of limited data and access in underwater environments. Here, we present a method to quantify and map the submarine landslide susceptibility of sediment-covered slopes in Glacier Bay, Glacier Bay National Park and Preserve, Alaska, using multibeam-sonar bathymetry and historic maps of glacial extents over the last ~250 years. To determine susceptibility, we mapped an inventory of >7,000 landslide scarps in submarine sediments from the bathymetric Digital Elevation Models (DEM) and filtered the inventory by landslide size. We then assessed landslide spatial concentrations, accounting for the age of exposure of submarine slopes by glacial retreat. We found a positive correlation between landslide concentration and deglaciation age, which we interpreted as a mean landslide accumulation rate over the period of record. Local deviations from this mean concentration are indicative of differences in susceptibility. We accounted for the effect of material and morphometric properties by estimating submarine bedrock and sediment using a morphometric model and assessing the relationship between slope angle and landslide incidence. Finally, we supplemented our susceptibility assessment with a geomorphic component by mapping active submarine fans and deltas, which are known to produce landslides. Our map of submarine landslide susceptibility thus incorporates three components: age-adjusted landslide concentration, slope, and geomorphology. We find that areas of high susceptibility correlate broadly with areas of high sediment input and availability, locations of fans and deltas, steep sediment-covered glacial fjords and troughs, and possibly areas where glacial retreat was most rapid. Areas of high submarine landslide susceptibility in Glacier Bay moderately correspond with locations of known high-hazard subaerial slopes, but more research is needed on both submarine and subaerial landslides in actively degrading cryospheric environments to better understand the landslide and tsunami hazard implications.