Seismic Swarm S20240416.1 Near Adak, Alaska

A seismic swarm designated S20240416.1 occurred 59 km west of Adak, Alaska, beginning at 22:37 on 15 April 2024 and concluding at 14:44 on 17 April 2024. Over 40 hours and 7 minutes, the sequence produced 35 earthquakes. This event aligns with the tectonic setting of the central Aleutian Islands, where the Pacific Plate subducts beneath the North American Plate along the Aleutian Trench. The region experiences frequent seismicity due to this convergent margin, which has generated numerous historical earthquakes and volcanic eruptions.

The swarm events ranged in magnitude from 0.7 to 3.0, with focal depths predominantly between 1 km and 13 km. Notable activity included a magnitude 2.3 event at 10 km depth on 16 April at 05:04:43 and a magnitude 3.0 earthquake at 10 km depth on 17 April at 10:44:02. Shallower events near 1–3 km depth occurred intermittently, while several readings registered at negative depths likely reflect minor location uncertainties common in offshore monitoring. The sequence showed clustered timing, with peak activity during the early hours of 16 April.

Adak lies within the Aleutian volcanic arc, part of the circum-Pacific seismic belt. Subduction-driven processes here produce both shallow crustal earthquakes and deeper events associated with the Wadati-Benioff zone. The island's geology features volcanic rocks from the Pleistocene and Holocene epochs, shaped by repeated magmatic activity. Historical records indicate that the central Aleutians have hosted multiple swarms since 2000, with six documented episodes prior to 2024 occurring in 2008 (two swarms), 2017, 2020, 2021, and 2023. These swarms typically involve low-to-moderate magnitude events and may reflect fluid migration or stress adjustments along the subduction interface.

Seismic monitoring in the region relies on networks operated by the Alaska Earthquake Center and the U.S. Geological Survey. Data from these sources confirm that swarm activity west of Adak is consistent with background rates in this highly active arc segment. No significant surface deformation or volcanic unrest has been linked to the April 2024 sequence based on available observations.

The 35 events of swarm S20240416.1 contribute to ongoing characterization of seismicity patterns in the central Aleutians. Continued analysis of depth distributions and temporal clustering can improve models of fault behavior and stress transfer in subduction zones. Such insights support regional hazard assessment for nearby communities and infrastructure.