Contextualizing the 2021 M8.2 Chignik and 2020 M7.8 Simeonof earthquakes: Background seismicity detected by the Alaska Amphibious Community Seismic Experiment

On July 29, 2021, the M8.2 Chignik earthquake ruptured the Semidi Segment of the Alaska-Aleutian megathrust, which had previously failed in 1938 during a M8.2 great earthquake. The Chignik earthquake may have been triggered by the neighboring July 2020 M7.8 Simeonof earthquake, which ruptured into part of the neighboring Shumagin Gap a year earlier. Contextualizing this earthquake sequence requires knowledge of background seismicity, which is usually poorly constrained offshore. Fortunately, >100 broadband seismometers were deployed in the region in 2018-2019 as part of the Alaska Amphibious Community Seismic Experiment (AACSE), providing an exceptional opportunity to characterize offshore regional seismicity prior to this sequence. We report on seismicity recorded during AACSE in the region that would later rupture during the 2020 M7.8 and 2021 M8.2 events. We leverage new machine-learning detection methods to enhance the existing hand-checked AACSE catalog (Ruppert et al., this meeting) by adding additional small seismicity that evades traditional detection methods. We use the machine-learning algorithm EarthquakeTransformer (Mousavi et al., 2020) with a grid association method to detect, pick, and associate small seismicity not reported in the existing catalog, finding roughly 40% more events. This enhanced catalog may then be expanded with match-filter based detections, and hypocenters may be improved by inverting travel times for three-dimensional velocity variations and earthquake locations. The expanded seismicity catalog reveals high-resolution patterns of microseismicity beneath the Alaska-Aleutian forearc prior to the Simeonof and Chignik earthquakes. With improved offshore observations, we find more small seismicity across the region, particularly near and to the west of the Shumagin Islands. Previously observed along-strike variation in seismicity rate persist: there are few earthquakes of any size in the Semidi segment thrust zone and outer rise, while the Shumagin segment has abundant seismicity both in the thrust zone and outer rise. Quantifying spatial and temporal patterns of microseismicity prior to these two large earthquakes may provide new insight into stress or rheological conditions in the thrust zone that prepared the plate interface for large-scale ruptures.