Back-analysis of the paraglacial slope failure at Grewingk Glacier and Lake, Alaska

Abstract

The relationship between rock-slope failure and glacier retreat is complex, and paraglacial failures often lack clearly identified triggers. To better understand the role of glacier retreat in rock-slope failures, we analysed the processes that led to the October 1967 Grewingk landslide in Kachemak Bay State Park on the Kenai Peninsula, Southcentral Alaska. The rock material collapsed onto the glacier toe and into its pro-glacial lake and produced a tsunami wave that swept the outwash plain. On the day of the failure, rainfall and snowmelt were well within normal ranges, and seismic records show no significant shaking. Three years prior to the 1967 failure, the slope withstood the second largest earthquake ever recorded (Great Alaskan earthquake, MW 9.2). We reassessed the volume of the failure by differencing pre- and post-digital terrain models and found a value of 20–24 × 106 m3, which is four times smaller than a previous estimate. The back analysis of the Grewingk landslide is based on remote sensing data and field measurements including aerial satellite image analysis, detailed surveying and understanding of the structural geology, a kinematic analysis, and runout modelling. Our research provides an example of a major paraglacial failure that lacks an obvious trigger and points to several geological factors and changing environmental conditions that likely promote such failures. This study further indicates that the Grewingk landslide, pre-conditioned by the geometry of faults and joints, may have reached a critical stability state due to internal processes and the potential combined effects of seismic activity and glacier retreat prior to the collapse.

A regional analysis of paraglacial landslide activation in southern coastal Alaska

“…In 1958, for example, a landslide near the terminus of Lituya Glacier, Alaska, impacted Lituya Bay, generating a tsunami that ran up 530 m on a nearby ridge (Miller, 1960). Less than a decade later, an instability near Grewingk Glacier, Alaska, failed catastrophically into a proglacial lake and caused a tsunami with 60 m runup (Wiles and Calkin, 1992;Lemaire et al, 2023a). In 2000, a large landslide entered the Vaigat Strait in western Greenland, generating a tsunami with a 28 m runup in a town 20 km away from the source (Dahl-Jensen et al, 2004).…”

“…Higman et al (2018) identified potential triggering factors at Taan Fiord, such as above-average rainfall and an earthquake, also noting how the glacier retreated over 17 km in 50 years and thinned by 400 m from 1961-1991. In the Grewingk case, the exact cause of the failure remains unknown, though it is thought that the slope was weakened by a large earthquake in 1964, a month of intense precipitation, and cycles of glacier retreat (Wiles and Calkin, 1992;Lemaire et al, 2023a). Specifically, upon the formation of the proglacial lake, retreat sped up significantly due to ice loss by calving (Wiles and Calkin, 1992).…”