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Earthquake–triggered Submarine Landslides in Kachemak Bay, Alaska: New Constrains on Distribution and Timing Based on Marine Geophysical and Geological Data

Description: 

Fjords of the Kenai Peninsula were severely shaken during the 1964 Mw 9.2 Great Alaska Earthquake resulting in widespread subaqueous slope failure. Destructive tsunamis generated by underwater landslides impacted several of the coastal communities, including ~6–8 meter-high waves in Resurrection Bay. Although less destructive, Kachemak Bay and the town of Homer also experienced intense sea waves in 1964 a few minutes after intense shaking began, damaging portions of the Homer spit and harbor, and causing overall subsidence. Early reports suspected submarine landslides in Kachemak Bay played a role, but direct observational evidence was not available. New geomorphic mapping in Kachemak Bay, based on NOAA multibeam bathymetry data, identifies a complex of submarine landslides along the southern flank of the bay with mass transport deposits that extend several kilometers into the basin. The headscarps are mostly concentrated along subaqueous deltas downstream of the Grewingk and Portlock glaciers. Large earthflows, slumps, and blocky failures are also observed near the head of the bay.

In summer 2022, the USGS collected a dense grid of Chirp subbottom profiles in Kachemak Bay to characterize the Holocene sedimentation patterns and examine the spatial and temporal distribution of submarine landslides. A suite of 27 sediment cores were collected to sample the upper ~2 m of sediment and develop preliminary age models. Preliminary analyses of these integrated datasets suggest that multiple phases of failure have occurred throughout the bay in the past. The most recent complex of deltaic failures appears to have occurred synchronously and most likely during 1964 earthquake (pending results of Cs137 dating). In addition to the stacked sequences of thick MTDs that are observed in several of the depocenters, 10–12 event beds (turbidites?) are seismically imaged within a small, isolated sub-basin and may provide evidence of repeated shaking events. Lastly, we present a preliminary assessment of landslide-generated tsunami sources and discuss the potential for Kachemak Bay to contain a valuable marine paleoseismic archive.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments [Poster Session]

Type: Poster

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Daniel

Student Presenter: No

Invited Presentation: 

Authors