A regional analysis of paraglacial landslide activation in southern coastal Alaska

Abstract. Glaciers worldwide are retreating rapidly due to anthropogenic climate change. One consequence of glacier mass loss is the destabilization of valley walls as the support provided by the glacier changes and eventually vanishes, a process known as ''debuttressing.'' In this work, we examine the evolution of eight large, active instabilities in southern coastal Alaska, a region experiencing some of the fastest glacier retreat worldwide. At half of the sites, the glacier is still in contact with the landslide, while in the other four cases, the terminus retreated past the landslide in recent decades. One site has experienced catastrophic failure; the others have not. We use automatic and manual feature tracking on optical imagery to derive slope movement from the 1980s to present and compare this with glacier terminus retreat and thinning, precipitation, and seismic energy. We find that the majority of sites underwent a pulse of accelerated landslide motion (up to 17 times higher compared to the five years preceding the acceleration) during the studied time period and that the subsequent deformation was independent of the initial activation. In two cases, the acceleration occurred after a particularly rainy month and/or a marked increase (around two times higher than the 1960–2000 average) in glacier thinning. At two further sites, no distinct activation could be detected, though both landslides are known to be moving at velocities below the detection threshold of the methods employed here. In four cases, landslide activation coincided with the rapid retreat (up to 12 times the long term average) of a lake- or marine- terminating glacier past the instability. Our results suggest that landslides adjacent to lake- or marine- terminating glaciers may be especially susceptible to sudden activation, which we hypothesize is due to the faster retreat rates of water-terminating glaciers as well as mechanical and hydrological changes resulting from the replacement of ice with water at the landslide toe. This work shows that glacier retreat can be associated with increasing landslide hazards in various glaciological settings in Alaska, which has implications for the assessment of hazards in a warming world.