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Megathrust Earthquakes Controlled by Incoming Plate Geometry: The 2020 M>7.5 Shumagin, Alaska, Megathrust Earthquake Doublet

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

Type: Oral

Date: 4/21/2021

Presentation Time: 06:00 PM Pacific

Description: 

In 2020, an earthquake doublet, a M7.8 on Jul 22 and a M7.6 on Oct 19, hit the Alaska-Aleutian subduction zone. To our knowledge, this is the first documented earthquake doublet, of considerable size, involving a megathrust event and a strike-slip slab-breaking event, with both events occurred at depths within the megathrust locked zone. The first and largest event, M7.8, ruptured the 100-year Shumagin seismic gap, and the second event, M7.6, broke the incoming oceanic slab. We used InSAR wrapped phase and GPS offsets and an improved bayesian geodetic inversion to estimate the fault slip distribution for the Shumagin earthquake doublet. The geodetic inversions revealed the Shumagin seismic gap is multisegmented, and the M7.8 earthquake ruptured the eastern segment from 14 down to 44 km depth. The western and shallower segments of the Shumagin seismic gap did not fail and remain potential seismic and tsunami hazard sources. The models of the M7.6 event constrained the rupture depth, 23-37 km, within the depth range of slip during the M7.8 event. We conclude that the rupture areas of the M7.8 and M7.6 events were controlled by the megathrust down-dip structure, with slip along a 26-degrees dipping segment bounded up-dip by a megathrust dip change. Elastic Coulomb stress change models indicated that the M7.6 event could have been triggered by the co-seismic and/or post-seismic slip caused by the M7.8 event. This earthquake doublet provides an opportunity to improve our knowledge of the mechanics of the subducting oceanic lithosphere in the Alaska subduction zone.

Presenting Author: Yu Jiang

Student Presenter: Yes

Authors