Earthquake Swarm S20210730.2: Seismic Activity Southeast of Perryville, Alaska

An earthquake swarm designated S20210730.2 was recorded southeast of Perryville, Alaska, beginning at 06:45 on 29 July 2021 and concluding at 08:34 on 31 July 2021. Over the 49-hour and 49-minute duration, 38 earthquakes were registered at a location approximately 137 km southeast of Perryville. The sequence featured a range of magnitudes from 2.1 to 4.8, with focal depths varying between 0 km and 69 km. The two largest events, magnitudes 4.8 and 4.5, occurred within the first two minutes of the swarm, followed by a gradual decline in event size and frequency.

The swarm exhibited a typical pattern of clustered seismicity, with initial higher-magnitude shocks giving way to numerous smaller events. Early activity included multiple events between 3.0 and 4.0 in magnitude at depths of 19–69 km. Later phases were dominated by events below magnitude 3.0, many occurring at intermediate depths around 35–47 km. A notable concentration of activity took place during the first 12 hours, after which the rate decreased steadily until the final recorded event.

This swarm occurred within the tectonically active Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. The resulting megathrust interface and associated crustal faults generate the majority of Alaska’s seismicity. Perryville lies on the southern Alaska Peninsula, a region characterized by volcanic arcs and deep sedimentary basins that amplify ground motion during larger events. Historical records indicate that the area experiences both mainshock-aftershock sequences and swarm-type activity linked to fluid migration or stress transfer along the plate boundary.

Since 1 January 2000, only three swarms have been identified in this specific sector. Prior episodes occurred in 2020 (one swarm) and 2021 (two swarms, including the present event). These infrequent swarms contrast with the steady background rate of isolated earthquakes driven by ongoing subduction.

Seismic monitoring in the region relies on the Alaska Earthquake Center network, which provides real-time detection and location for events above approximately magnitude 2.0. Depths reported for the swarm are consistent with both intraslab and interface seismicity typical of the Aleutian system. No surface rupture or significant damage was associated with this sequence, reflecting the offshore location and moderate magnitudes involved.

Continued monitoring remains essential given the potential for larger megathrust earthquakes in the broader Alaska Peninsula segment. The 2021 swarm provides additional data for refining models of stress accumulation and release along this portion of the plate boundary.

References

• SeismoSight internal swarm classification S20210730.2
• USGS Earthquake Catalog (earthquake.usgs.gov)
• Alaska Earthquake Center regional reports (aeic.alaska.edu)