Seismic Swarm S20200723.3: Analysis of Activity near Perryville, Alaska

The seismic swarm designated S20200723.3 occurred approximately 106 km south-southeast of Perryville on the Alaska Peninsula. It began at 06:33 UTC on 22 July 2020 and concluded at 10:02 UTC on 26 July 2020, spanning 99 hours and 28 minutes. During this interval, 44 earthquakes were recorded, with magnitudes ranging from 1.8 to 4.6 and focal depths between 10 km and 46 km. The majority of events clustered at depths of 20–35 km, consistent with activity in the subducting slab.

The swarm initiated shortly after the magnitude 7.8 Simeonof Island earthquake of 22 July 2020, whose epicenter lay only 13 km from the swarm centroid. Early events included a magnitude 4.1 at 10 km depth followed rapidly by a magnitude 4.2 at 29 km. Peak activity featured a magnitude 4.6 event at 30 km depth roughly four hours after onset. Subsequent larger shocks reached magnitude 4.5 at 13 km and magnitude 4.3 at 32 km. Later stages showed a gradual decline in both frequency and magnitude, concluding with a magnitude 4.0 at 34 km depth.

Geologically, the Alaska Peninsula lies above the convergent boundary where the Pacific plate subducts beneath the North American plate at rates of approximately 6–7 cm per year. This subduction drives the Aleutian volcanic arc and produces intense seismicity across a wide range of depths. The region has hosted numerous great earthquakes, including events exceeding magnitude 8 in the twentieth century. Historical records indicate that aftershock sequences and swarms commonly follow large ruptures along the megathrust, reflecting stress redistribution within both the overriding and subducting plates.

The 2020 magnitude 7.8 mainshock ruptured a portion of the Shumagin gap, an area previously identified as having elevated seismic potential. Post-event monitoring revealed widespread aftershock activity extending southeast of the main rupture, encompassing the location of swarm S20200723.3. Depths recorded during the swarm align with intraslab seismicity typical of the Pacific plate at this latitude.

No prior swarms meeting the internal classification criteria of SeismoSight were identified in the catalog since 1 January 2000, underscoring the rarity of such clustered sequences outside major aftershock periods in this segment of the subduction zone. The swarm’s temporal decay and spatial concentration near the mainshock rupture support its interpretation as triggered aftershock activity rather than an independent tectonic episode.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center, University of Alaska Fairbanks
Hayes et al. (2020), USGS preliminary finite-fault model for the 2020 M7.8 Simeonof earthquake