THE INFLUENCE OF GEOLOGIC STRUCTURE ON BEDROCK LANDSLIDE SUSCEPTIBILITY IN THE FJORDS OF PRINCE WILLIAM SOUND, ALASKA

The Barry Arm landslide is a recently identified, massive, slow-moving bedrock instability in an actively deglaciating valley in western Prince William Sound, about 50 km NE of Whittier, AK. Although the landslide is tens of kilometers from nearby population centers, it poses a potential tsunami risk to coastal communities and frequent boat traffic in the area. The Barry Arm landslide is the largest known slope instability in the region, but is just one of dozens of similar bedrock landslides that have been mapped in the steep-walled glacial fjords that characterize this landscape. Across western Prince William Sound, steeply dipping, km-scale folds in Cretaceous metasedimentary bedrock (Nelson et al., 1985) appear to exert a first order control on glacier orientation. These glaciers preferentially flow parallel to bedding strike, and upon retreat, leave behind damaged, de-buttressed, and over-steepened bedrock valley walls with often adversely-oriented bedding planes and pervasive fracture sets. The Barry Arm landslide, however, is not apparently failing along bedding, but rather within an anti-dip slope. Unraveling the interaction between geologic structure, lithologic strength, and a complex and ongoing history of glacial erosion is key to understanding the hazard posed by landslides in this particularly dynamic landscape. Here, we investigate rock mass characteristics and geologic structure at >50 sites within western Prince William Sound. A preliminary analysis suggests that landslides in the region have a variety of failure modes, including rotational slides on anti-dip hillslopes, translational slides on dip slopes, and wedge failures along the intersections of discontinuities. Many of the landslides we investigated are oriented similarly to the Barry Arm landslide with respect to bedrock structure. We observe discontinuities with regionally consistent orientations and identify those that may adversely affect slope stability due to their intersection with local topography. By constructing a regional inventory of structural elements, we are able to better understand the kinematic controls on existing deep-seated landslides, and better predict where similar instabilities may develop following future glacier retreat.