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The 22 July 2020 MW 7.8 Shumagin Seismic Gap Earthquake: Partial Rupture of a Weakly Coupled Megathrust

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

Type: Oral

Date: 4/21/2021

Presentation Time: 05:15 PM Pacific

Description: 

The earthquake potential of the Shumagin seismic gap along the Alaskan Peninsula has been debated for more than 40 years. On 22 July 2020, the eastern half of the gap hosted an M_W 7.8 earthquake involving a patchy rupture of the megathrust in the depth range 20 to 45 km. The space-time slip distribution is determined by joint inversion of teleseismic P and SH waves and static displacements from regional GNSS stations. The event initiated near the epicenter of the 1938 (M_W 8.2) event, and ruptured westward, with little/no overlap with the 1938 rupture zone. The main slip patch has peak slip of ~3.8 m below the Shumagin Islands, and produced ~30 cm uplift and ~25 cm SSE horizontal displacement on Chernabura Island. The slip model predicts well the small (<1 cm) tsunami signals persisting for >10 hours observed at DART recordings along the Alaska-Aleutian arc. Aftershocks with depths of 20-40 km fringe the large-slip patches, and show westward concentration during the first month after the mainshock. Aftershocks up-dip of the 1948 M_W 7.1 event contribute to the high level of modest-size background seismicity extending to the trench in the very low seismic coupling (0.0-0.1) western Shumagin gap east of the 1946 (M_W 8.6) rupture zone. The 1917 event is the last major earthquake to rupture the eastern half of the Shumagin gap, and appears to have somewhat lower surface wave magnitude (M_SG-R 7.4) compared to the 2020 event (M_SG-R 7.7). Comparison of instrument-equalized waveforms indicates similar size contrast and differences in overall rupture duration and slip complexity. The 2020 rupture has average slip of ~1.9 m over the 3600 km² region, much less than the ~6.7 m of potentially accumulated slip deficit since 1917, consistent with geodetic estimates of low average seismic coupling coefficient of 0.1-0.4. The megathrust seaward of the 2020 event has low seismicity and may either be aseismic or capable of comparable size ruptures. Comparisons are made with other subduction zones experienced relatively deep megathrust slip in regions with moderate seismic coupling.

Presenting Author: Lingling Ye

Student Presenter: No

Authors