Earthquake Swarm S20241208.1 Near Adak, Alaska

A significant earthquake swarm, designated S20241208.1, occurred 102 km south-southwest of Adak, Alaska, in the Aleutian Islands. The sequence began at 19:57 UTC on 8 December 2024 and concluded at 00:06 UTC on 15 December 2024, spanning 148 hours and 8 minutes. During this period, 168 earthquakes were recorded, highlighting ongoing seismic unrest in a tectonically dynamic region. The swarm initiated with a magnitude 6.3 event at 18 km depth, followed rapidly by additional shocks including two magnitude 4.9 events within the first hour. Subsequent activity featured multiple events exceeding magnitude 5.0, such as a second magnitude 6.3 quake on 9 December at 19 km depth, alongside several magnitude 5.2 shocks clustered in the early hours of that day. Depths for the first 100 events predominantly ranged between 4 km and 40 km, with the majority occurring at or near 10 km, consistent with shallow crustal deformation. Analysis of the initial 100 events reveals a pattern of high-frequency smaller-magnitude aftershocks interspersed with larger triggers. Magnitudes decreased progressively after the peak activity on 9 December, shifting toward events between magnitude 2.0 and 3.5. This distribution suggests stress redistribution along fault structures following the main shocks. The location lies within the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–8 cm per year. This setting produces frequent seismic sequences and volcanic arcs across the central Aleutians. Adak Island and surrounding waters have long been recognized for elevated seismicity due to this plate boundary interaction. Historical records indicate limited swarm-type activity in the immediate vicinity since 2000. Only two prior swarms have been documented: one in 2008 comprising a single event cluster and another in 2017 with comparable limited extent. The December 2024 sequence represents the most extensive swarm recorded in the area during this timeframe, underscoring episodic rather than continuous swarm behavior. Such swarms in subduction environments often arise from fluid migration or aseismic slip that facilitates brittle failure. The predominance of shallow depths in the recorded events aligns with typical patterns observed along the Aleutian megathrust and associated splay faults. Continued monitoring remains essential given the region's capacity for larger megathrust earthquakes. References
SeismoSight internal swarm classification data
USGS Earthquake Catalog (regional tectonic framework)