loading page

Constraining the earthquake recording threshold of intraslab earthquakes with turbidites in southcentral Alaska's lakes and fjords
  • +2
  • Drake M Singleton,
  • Daniel Brothers,
  • Peter J Haeussler,
  • Robert C Witter,
  • Jenna C Hill
Drake M Singleton
United States Geological Survey

Corresponding Author:dsingleton@usgs.gov

Author Profile
Daniel Brothers
United States Geological Survey
Author Profile
Peter J Haeussler
United States Geological Survey
Author Profile
Robert C Witter
United States Geological Survey
Author Profile
Jenna C Hill
United States Geological Survey
Author Profile

Abstract

Intraslab earthquakes do not produce primary paleoseismic evidence at the Earth’s surface, making efforts to develop an event chronology challenging. However, the strong ground motion from intraslab events may initiate gravity-driven turbidity flows in subaqueous basins; the resulting deposits (turbidites) can provide a paleoseismic proxy if the conditions that initiate these flows are known. To better constrain the initiating conditions, we use two recent intraslab earthquakes in southcentral Alaska, the Mw 7.1 November 30, 2018, Anchorage and the Mw 7.1 January 24, 2016, Iniskin earthquakes, as calibration events. Through a multi-lake investigation spanning a range of shaking intensities and based on a combined geological and geophysical dataset, we document the occurrence, or absence, of earthquake-generated turbidity flows from these two earthquakes. The 2018 and 2016 earthquakes are recorded by centimeter-scale turbidites that can be differentiated from climatically generated deposits, as well as other seismic sources (i.e., the 1964 Alaska megathrust earthquake) based on deposit thickness, sedimentological properties, and deposit age. We show that a Modified Mercalli Intensity (MMI) of ~V-V1/2 is the minimum shaking intensity required to generate localized sediment remobilization from deltaic slopes, and a MMI of ~V1/2 is required to produce a deposit of sufficient thickness that a seismic origin can be confidently assigned. Deltaic slopes are the major source of remobilized sediment that record the 2018 and 2016 events, however sediment from non-tributary sourced basin slopes may become remobilized in steep-sloped, high sedimentation areas, and under elevated shaking intensity. The documentation of seismically generated deposits in quick succession (~2 years) with diagnostic features that can be assigned to the seismic source highlights the utility of using recent earthquakes as calibration events to investigate the subaqueous response to strong ground motion. 
02 Nov 2023Submitted to ESS Books
03 Nov 2023Published in ESS Books