Earthquakes at an Uncoupled Subduction Zone: The 2020-2021 Shumagin Gap Earthquake Sequence

Matt Herman

CSU Bakersfield

Date & Time
Online-only seminar via Microsoft Teams

Since 2020, three large magnitude earthquakes have occurred in the vicinity of the Shumagin Islands in Alaska: the 22 July 2020 Mw 7.8 megathrust event, the 19 October 2020 Mw 7.6 intra-slab strike-slip event and the 29 July 2021 Mw 8.2 megathrust event. The 2020 events occurred in the transition from high plate interface coupling east of the Shumagin Islands to low coupling near the Shumagin Islands, and our finite element models of coupling demonstrate that the slip behavior and kinematics of these events reflect this coupling transition. The 2021 Mw 8.2 megathrust earthquake occurred further to the east, rupturing a large part of the 10 November 1938 Mw 8.2 rupture zone. Slip models and aftershock activity suggest that nearly all the slip deficit that could accumulate since 1938 was recovered in the mainshock. We find using our coupling models that an uncoupled Shumagin Gap is most compatible with observations from the July 2021 earthquake. Although the Shumagin Gap appears to be uncoupled based on these earthquake sequences, the coupling situation up-dip of the 2021 earthquake remains enigmatic. Using our modeling setup, we explore implications for scenarios with various degrees of coupling on the shallow plate interface.

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