Calibrating Alaska’s natural seismograph: preliminary results of the sedimentary response to the 2018 Anchorage Earthquake in lakes and fjords of south-central Alaska

Abstract:

South-central Alaska is a region of high seismicity that is frequently impacted by intraslab earthquakes. Intraslab earthquakes, which occur at mid-crustal depths, do not produce the characteristic surficial faulting, land-level change, or tsunami deposits typically associated with megathrust earthquakes, but instead generate a more subtle signal in the geologic record. The emerging field of lacustrine and fjord paleoseismology utilizes relatively small and well-defined basins with unique depositional characteristics (e.g., varve formation), in combination with earthquake-generated turbidites as paleoseismic proxies, to construct earthquake histories that are sensitive enough to record intraslab events. The 2018 Anchorage Earthquake resulted in high-intensity shaking across the upper Cook Inlet and demonstrated the region’s vulnerability to intraslab earthquakes. Recent work on Eklutna Lake by Van Daele et al. (2020) has confirmed the presence of earthquake-generated turbidites as a result of the 2018 earthquake across the lake’s two sub-basins. The results of their study provided the first inputs necessary to begin calibrating south-central Alaska’s natural seismograph. However, important questions remain, including the minimum MMI necessary to trigger turbidity currents, which depositional environments are most susceptible to failure by seismic triggers, and the potential of relative turbidite thickness as an indicator of epicentral direction. We present preliminary results from fieldwork carried out in the summer of 2020 that employed short-barrel gravity cores, MCS Sparker profiles, and high-resolution Chirp data collected on several lakes and fjords throughout south-central Alaska. The goal of this project will be to characterize the sedimentary response of south-central Alaska’s lakes and fjords to the 2018 Anchorage Earthquake. As a first step, short-barrel gravity cores are used to investigate the spatial extent of turbidity currents triggered by the 2018 Anchorage Earthquake, and if possible identify a correlating acoustic signature.

Ref: Van Daele, M., et al., 2020. The sedimentary record of the 2018 Anchorage earthquake in Eklutna Lake, Alaska: Calibrating the lacustrine seismograph. Seismological Research Letters, 91(1), pp.126-141.