Landslide and Megaturbidite Records Reveal a 2.5 Kyr History of Seismic Shaking in Skilak Lake, Alaska

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

On 27 March 1964, a M_w 9.2 megathrust earthquake ruptured an 800-km-long segment of the Alaska-Aleutian Subduction Zone (in south-central Alaska). In order to better understand the recurrence pattern of such large earthquakes in that region, we studied the sediments of Skilak Lake, a proglacial lake located in the area affected by the 1964 earthquake, using a combination of high-resolution seismic stratigraphy (3.5 kHz), multibeam bathymetry (50 kHz) and sediment cores (~13-16 m). Seismic profiles and bathymetric maps reveal 23 lacustrine landslide deposits caused by the 1964 megathrust earthquake. We also identified a series of six older landslide events in the subsurface, which we infer to result from multiple, coeval slope failures, and can thus be attributed to past seismic shaking. Sediment cores in two sites show varved “background” sedimentation that is occasionally interrupted by megaturbidites and slump deposits. From this megaturbidite record we can infer five large mass-wasting events, which can also be observed in the upper part of the landslide record. In order to precisely date our paleoseismic records, we organized a student crowdsourcing project, in which continuously varved core sections were counted by multiple observers. Varve counts, combined with 10 ¹⁴C ages, provide a high-resolution age framework for our 2.5 kyr-long paleoseismic record. As Skilak Lake lies in a position between a more fully locked part of the megathrust beneath Prince William Sound, and a more creeping part of the megathrust offshore the Kenai Peninsula, the results of this study may elucidate the extent of past ruptures, rupture pattern variability and interplate coupling.

Presenting Author: Maarten Van Daele

Authors