A seismic swarm designated S20230308.1 was recorded 124 km east of McCarthy, Alaska, beginning at 07:32 on 8 March 2023 and concluding at 07:15 on 10 March 2023. Over 47 hours and 42 minutes, the swarm comprised 70 earthquakes. The sequence initiated with a magnitude 4.7 event at 11 km depth, followed by numerous smaller events predominantly below magnitude 2.0. Depths ranged from surface level to 26 km, with the majority occurring between 2 km and 12 km.
The temporal distribution showed highest activity in the first 12 hours, including a magnitude 2.9 event at 11:28 on 8 March and a magnitude 2.6 event at 13:41 the same day. Subsequent activity declined steadily, with isolated events continuing into 10 March. Shallow foci dominated the later stages, consistent with swarm characteristics where fluid migration or stress redistribution sustains clustered seismicity without a single dominant mainshock-aftershock pattern.
The McCarthy region lies within the eastern Chugach Mountains and adjacent Wrangell-St. Elias area of south-central Alaska. This zone experiences ongoing deformation from the collision of the Yakutat terrane with the North American plate margin. Convergence rates exceed 5 cm per year, producing distributed crustal faulting and moderate seismicity. Historical records indicate that earthquake swarms occur intermittently, with three documented episodes since 2000: two in 2014 and one in 2023. These events reflect localized release of accumulated strain along secondary faults rather than primary plate-boundary megathrust activity.
Regional geology features accreted terranes, active volcanism in the Wrangell Mountains, and extensive glacial cover that can influence shallow seismicity through loading changes. Depths observed in swarm S20230308.1 align with typical crustal earthquakes in this compressional regime, where brittle failure occurs above approximately 20–25 km.
Analysis of the provided event parameters reveals a classic swarm signature: an energetic onset, rapid decay in magnitude, and persistence of low-level activity over two days. No events exceeded magnitude 4.7, and the cumulative energy release remained modest. Such patterns are common in the area and rarely escalate to damaging levels, though they contribute to refined understanding of local stress fields.
Continued monitoring by regional networks supports improved characterization of swarm recurrence and potential links to deeper tectonic processes along the plate margin.