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The 22 July 2020 M_W 7.8 Shumagin seismic gap earthquake: Partial rupture of a weakly coupled megathrust

Ye, Lingling and Lay, Thorne and Kanamori, Hiroo and Yamazaki, Yoshiki and Cheung, Kwok Fai (2021) The 22 July 2020 M_W 7.8 Shumagin seismic gap earthquake: Partial rupture of a weakly coupled megathrust. Earth and Planetary Science Letters, 562 . Art. No. 116879. ISSN 0012-821X.

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The earthquake potential of the Shumagin seismic gap along the Alaska Peninsula (∼162°W to ∼158.5°W) 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 of 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 GPS stations. The event initiated near the epicenter of the 10 November 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 more than ten hours observed at deep-water DART seafloor pressure recordings along the Alaska-Aleutian arc. Aftershocks with depths from 20 to 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 region of very low seismic coupling (0.0–0.1) in the western Shumagin gap east of the 1 April 1946 (M_W 8.6) rupture zone. The 31 May 1917 event is the last major earthquake to rupture the eastern half of the Shumagin gap, and has a lower surface wave magnitude (M_(SG-R) 7.4, horizontal components) compared to the 2020 event (M_(SG-R) 7.7, vertical components). Comparison of instrument-equalized waveforms for the 1917 and 2020 events 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 with co-seismic slip ≥1 m. This is 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 that have experienced relatively deep megathrust slip in regions with moderate seismic coupling.

Item Type:Article
Related URLs:
URLURL TypeDescription ItemIncorporated Research Institutions for Seismology (IRIS) ItemGlobal Centroid Moment Tensor Solutions ItemUSGS National Earthquake Information Center
http://earthquake.alaska.eduRelated ItemAlaska Earthquake Center ItemNation Centers for Environmental Information
Ye, Lingling0000-0002-8739-2072
Lay, Thorne0000-0003-2360-4213
Kanamori, Hiroo0000-0001-8219-9428
Cheung, Kwok Fai0000-0002-7435-0500
Additional Information:© 2021 Elsevier B.V. Received 29 September 2020, Revised 27 February 2021, Accepted 7 March 2021, Available online 19 March 2021. Teleseismic body wave waveforms were downloaded from the Incorporated Research Institutions for Seismology (IRIS) data management center ( Global Centroid Moment Tensor Solutions are from Earthquake information is based on the catalogs from National Earthquake Information Center at U.S. Geological Survey (USGS-NEIC) ( and Alaska Earthquake Center (, last accessed on August 26, 2020. The high-resolution digital elevation model, Sand Point V2, at the Shumagin Islands was downloaded from the Nation Centers for Environmental Information ( We thank Professor Jeff Freymueller for helpful comments on the manuscript. Lingling Ye's earthquake study is supported by National Natural Science Foundation of China (No. 41874056, 41790465, and U1901602). Thorne Lay's earthquake research is supported by National Science Foundation (Grant EAR1802364) and Yoshiki Yamazaki and Kwok Fai Cheung received support for tsunami hazard assessment from the National Oceanic and Atmospheric Administration (Grant NA19NWS4670012). CRediT authorship contribution statement: LY performed finite-fault inversion and back-projection imaging; HK conducted W-phase inversion; YY performed the tsunami data processing and modeling; LY, TL and HK conceived the project, and along with KC interpreted the results and wrote the manuscript collaboratively. The authors declare that they have no known competing finanical interests or personal relationships that could have appeared to influence the work reported in this paper.
Group:Seismological Laboratory
Funding AgencyGrant Number
National Natural Science Foundation of China41874056
National Natural Science Foundation of China41790465
National Natural Science Foundation of ChinaU1901602
National Oceanic and Atmospheric Administration (NOAA)NA19NWS4670012
Subject Keywords:Shumagin gap; Alaska Peninsula earthquakes; seismic gaps; megathrust coupling; finite-fault inversion
Record Number:CaltechAUTHORS:20210322-121325114
Persistent URL:
Official Citation:Lingling Ye, Thorne Lay, Hiroo Kanamori, Yoshiki Yamazaki, Kwok Fai Cheung, The 22 July 2020 MW 7.8 Shumagin seismic gap earthquake: Partial rupture of a weakly coupled megathrust, Earth and Planetary Science Letters, Volume 562, 2021, 116879, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:108509
Deposited By: Tony Diaz
Deposited On:23 Mar 2021 16:00
Last Modified:23 Mar 2021 16:00

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