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Multi-Fault Rupture of the Mw 7.6 Sand Point, Alaska, Earthquake

Session: The 2020 Simeonof Island, Alaska, Earthquake: Observations, Modeling and Tectonic Insights

Type: Oral

Date: 4/21/2021

Presentation Time: 05:45 PM Pacific

Description: 

In October 2020, a Mw 7.6 earthquake occurred west of the Shumagin Islands and the July 2020 Mw 7.8 Simeonof Island earthquake. The initial model showed a steeply dipping vertical strike-slip earthquake within either the overriding plate or the downgoing slab. However, observations of the near and far-field tsunami indicated that a vertical strike-slip fault could not generate the vertical coseismic observations on the Shumagin Islands and the observed tsunami. Here we propose a multi-fault rupture that contains both strike-slip motion along an intraslab fault and megathrust rupture. We invert for a kinematic slip model using high-rate GNSS and seismic velocity observations. We explore a range of rupture speeds along both faults and find much slower rupture along the megathrust than the strike-slip fault, which helps to explain the lower radiated seismic energy. Furthermore, we independently model the required seafloor uplift to generate the tsunami observations and find a patch coincident with our megathrust slip patch and wrapping around the main slip patch from the July 2020 Simeonof Island earthquake. Finally, we analyze the total slip budget in the Shumagin Gap region from the two mainshocks and associated aftership and discuss how this pertains to future hazards in the region.

Presenting Author: Brendan W. Crowell

Student Presenter: No

Authors