A seismic swarm designated S20210820.1 was recorded 88 km east of McCarthy, Alaska, from 01:35 on 20 August 2021 to 07:57 on 22 August 2021. Over 54 hours and 21 minutes, 36 earthquakes were detected. The events ranged in magnitude from 0.7 to 4.1, with focal depths between 2 km and 15 km. The largest event reached magnitude 4.1 at 01:48:15 on 20 August at a depth of 6 km. Subsequent notable shocks included magnitudes 3.4 and 2.8, occurring early in the sequence.
The swarm exhibited a typical pattern of clustered, low-to-moderate magnitude activity without a single dominant mainshock. Initial high-frequency events gave way to sporadic smaller tremors over the following days. Depths remained predominantly shallow to intermediate, consistent with crustal faulting in the region.
The swarm location lies within the eastern Alaska Range and adjacent St. Elias Mountains, part of the tectonically active northern Pacific margin. This area experiences ongoing deformation from the collision and subduction of the Yakutat microplate beneath the North American plate. The interaction produces thrust faulting, strike-slip motion along regional structures, and elevated seismicity. The Wrangell volcanic belt, situated nearby, reflects long-term subduction-related magmatism, though the swarm itself aligns more closely with crustal fault zones than volcanic processes.
Historical seismicity in this sector of Alaska is well documented, with frequent small earthquakes reflecting the high strain rates of the plate boundary. Depths in the 2–15 km range match typical crustal seismogenic zones influenced by the regional stress field.
Since 1 January 2000, four swarms have been identified in the vicinity. Earlier episodes occurred in 2012 (two events) and 2013 (two events). The 2021 swarm represents the most recent and largest in event count within this record. Such swarms are characteristic of the region, often linked to fluid migration or stress transfer along pre-existing faults rather than major tectonic ruptures.
The temporal distribution shows peak activity within the first 12 hours, followed by a gradual decline. Magnitudes generally remained below 2.0 after the initial day, indicating rapid release of accumulated strain without escalation. Depth variations suggest involvement of multiple fault segments at shallow crustal levels. No events exceeded magnitude 4.1, and the sequence concluded within 54 hours, typical of swarm behavior in this geological province.
These observations contribute to understanding episodic seismicity patterns along the Yakutat–North America collision zone. Continued monitoring supports improved characterization of background activity versus potential precursors in this remote, high-strain area.