Sublacustrine Landslides in Several Alaskan Lakes Reveal a Long History of Strong Earthquake Shaking

Abstract:

In 1964, the “Good Friday Earthquake” ruptured a 800 km-long segment of the Alaskan-Aleutian Subduction Zone (AASZ), representing the largest measured earthquake in North America (Mw 9.2). Lake sediments potentially record such large earthquakes, since seismic shaking can generate subaquatic mass-wasting deposits. Several short cores, high-resolution seismic (3.5 kHz) and multibeam (50khz) data were collected in Eklutna, Skilak and Kenai Lakes in southern Alaska. The data reveal the presence of sublacustrine landslides related to the 1964 earthquake. The seismic profiles also show a succession of older landslide deposits in the subsurface. It has been shown that earthquake shaking with intensities of > VII can trigger such landslides and as such they are useful for paleoseismology research, especially in low-seismicity regions. In high-seismicity regions (e.g. southcentral Chile), however, the turbidite record has shown to be a much more sensitive seismometer, because frequent and strong seismic shaking is under-represented in landslide deposits in the lake records. Eklutna, Skilak and Kenai lakes are also located in a high-seismicity setting (the Prince William Sound segment of the AASZ), but they exhibit significantly larger sedimentation rates (4 mm/yr), potentially reducing the effect of under-representation.

Our multilake approach allows us to compare landslide records between lakes with different characteristics: low slope gradients and sedimentation rates cause under-representation of the landslides in Skilak Lake with respect to other independent paleoseismic records, while steeper slopes and higher sedimentation rates in Eklutna Lake produce more frequent event deposits, potentially thus resulting in a more precise sedimentary archive of past earthquake events.

Multiple lake records will also help us understand segment boundaries of megathrust earthquakes. This is crucial for inferring information on variability between ruptures and interplate coupling.