Seismic Swarm S20211011.2: Analysis of Earthquake Activity Near Chignik, Alaska

A seismic swarm designated S20211011.2 was recorded 114 km east of Chignik, Alaska, beginning at 09:10 on 11 October 2021 and concluding at 01:46 on 14 October 2021. Over 64 hours and 35 minutes, the swarm comprised 67 earthquakes. The sequence initiated with a magnitude 6.9 event at 51 km depth, followed by numerous smaller aftershocks ranging from magnitude 2.1 to 4.2, with focal depths predominantly between 40 km and 74 km.

The largest events included the initial 6.9 quake and subsequent shocks of 4.2, 3.6, 4.1, and 3.4. Depths remained consistent with intermediate-depth seismicity typical of subduction environments, though a few shallower events occurred near 14–27 km. The swarm exhibited a rapid onset followed by a gradual decline in frequency and magnitude, characteristic of fluid-driven or stress-transfer processes rather than a classic mainshock-aftershock sequence.

Chignik lies on the Alaska Peninsula within the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent intermediate-depth earthquakes along the Wadati-Benioff zone. The region has experienced multiple great earthquakes, including the 1964 magnitude 9.2 event whose rupture extended into the eastern Aleutians. Ongoing volcanic activity at nearby centers such as Mount Veniaminof and Aniakchak Caldera further indicates active mantle processes that can influence swarm occurrence through fluid migration.

Historical records since 2000 document only one prior swarm in the immediate vicinity, also occurring in 2021, underscoring the relative rarity of such clustered activity compared with isolated events. The October 2021 swarm aligns with patterns observed in other subduction margins where episodic tremor or swarm sequences release accumulated strain without producing a dominant mainshock.

Seismic monitoring in this remote area relies on regional networks operated by the Alaska Earthquake Center and the U.S. Geological Survey. Data from these instruments enable precise location and depth determination, supporting hazard assessment for coastal communities and maritime operations in the Gulf of Alaska.

References

• Alaska Earthquake Center, University of Alaska Fairbanks (seismic catalog data).
• U.S. Geological Survey Earthquake Hazards Program (tectonic framework of the Aleutian subduction zone).