Journal Article

Limited Shallow Slip for the 2020 Simeonof Earthquake, Alaska, Constrained by GNSS‐Acoustic

Ericksen, Todd L., DeSanto, John B., Webb, Spahr C., Crowell, Brendan W., Brooks, Benjamin A., Nooner, Scott L., Schmidt, David A., Chadwell, C. David

2023

Overview

The 22 July 2020 Mw7.8 Simeonof earthquake was a deep megathrust event that ruptured along the Shumagin segment of the Alaska‐Aleutian subduction zone. This earthquake occurred ∼250 km from a seafloor geodetic GNSS‐Acoustic site IVB1, where we observed a velocity of 3.78 ± 1.15 cm/yr with the down‐going slab prior to the earthquake followed by 0.6 ± 0.7 eastward and −15.5 ± 0.8 cm northward coseismic offset. We computed a slip model of the coseismic rupture using the static offset at IVB1 alongside regional continuous GNSS and strong motion stations. The small static horizontal offset at the site precludes significantly shallower rupture than previously inferred from terrestrial observations, confirming that the Simeonof earthquake was a deep megathrust earthquake. The observed site velocity implies partial locking prior to the earthquake, implying significant shallow strain accumulation such that the small coseismic offset is unlikely to have relieved all of the accumulated strain since the last coseismic rupture. Plain Language Summary The 22 July 2020 Mw7.8 Simeonof earthquake ruptured east of the Shumagin Islands along the interface of the Alaska‐Aluetian subduction zone, where the Pacific plate subducts beneath the North American plate. This earthquake occurred ∼250 km from a seafloor geodetic GNSS‐Acoustic site IVB1, which acts as a seafloor GPS station and provides the only direct offshore measurement of seafloor deformation following the earthquake. In late September 2020, we observed 0.6 ± 0.7 of eastward offset and −15.5 ± 0.8 cm of northward offset at the station IVB1 and used this measurement to refine a model of the earthquake rupture area that previously only used the GPS and seismic stations installed on the nearby Aleutian Islands. The seafloor offset was not large enough to require additional rupture to the south of where previous studies have imaged the earthquake, confirming that the Simeonof earthquake was a deep megathrust earthquake. Prior to the earthquake, station IVB1 was moving with the down‐going Pacific plate with a velocity of 3.78 ± 1.15 cm/yr despite resting on the North American plate, implying that the two plates are partially stuck together. We expect that there could still be another future earthquake beneath IVB1 since the offset at IVB1 following the Simeonof earthquake was smaller than expected. Key Points We observed seafloor offset at a GNSS‐Acoustic site along the Alaska subduction zone following the 22 July 2020 Mw7.8 Simeonof earthquake Additional slip updip of the main earthquake rupture area is not required to generate the observed seafloor offset The shallow plate interface was partially locked prior to the Simeonof earthquake and may still host unrelieved strain

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Acoustics / Alaska subduction zone / Deformation / earthquake / Earthquakes / GNSS‐Acoustic / Islands / Ocean floor / Offshore / Plates / Plates (tectonics) / Rupture / Rupturing / Satellite navigation systems / Satellite observation / seafloor geodesy / Seismic activity / Subduction / Subduction (geology) / Subduction zones / Velocity