Seismic Swarm S20160319.1: Analysis of Earthquake Activity near Atka, Alaska

Seismic swarm S20160319.1 occurred 97 km south of Atka, Alaska, in the central Aleutian Islands. The sequence began at 09:24 on 18 March 2016 and concluded at 00:41 on 22 March 2016, spanning 87 hours and 17 minutes. During this period, 59 earthquakes were recorded, with magnitudes ranging from 1.9 to 4.5 and focal depths between 3 km and 41 km.

The swarm exhibited clustered activity primarily on 19 March 2016, when the majority of events occurred. Notable larger events included a magnitude 4.5 earthquake at 04:41 on 19 March at 16 km depth, a magnitude 4.2 at 02:31 on the same day at 10 km depth, and additional magnitude 4.2 events on 20 March. Activity tapered after 21 March, ending with a magnitude 2.6 event at 00:41 on 22 March.

This swarm is situated within the tectonically active Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. The region forms part of the Pacific Ring of Fire and hosts frequent seismicity driven by plate boundary interactions, including megathrust earthquakes and volcanic arc processes. Atka lies on Atka Island, near the active volcanoes of the Aleutian arc, where crustal stresses from subduction can trigger both tectonic and volcanic-related swarms.

Since 1 January 2000, ten seismic swarms have been documented in the area. Prior events occurred in 2008 (two swarms), 2013 (four swarms), 2015 (three swarms), and this 2016 sequence (one swarm). Such recurrent swarms reflect ongoing strain accumulation and release along the subduction interface and associated crustal faults.

Analysis of the provided event data reveals a typical swarm pattern: an initial moderate event followed by numerous smaller aftershocks without a single dominant mainshock. Depths cluster mostly between 5 km and 25 km, consistent with upper-crustal faulting above the subduction megathrust. The absence of events exceeding magnitude 5.0 suggests localized stress release rather than a large rupture.

These observations align with broader patterns of swarm seismicity in subduction zones, where fluid migration or aseismic slip may contribute to episodic clustering. Continued monitoring remains essential given the region's history of significant earthquakes and volcanic activity.

References

• SeismoSight internal swarm classification data (S20160319.1 parameters and historical statistics).
• Tectonic framework derived from standard geological descriptions of the Aleutian subduction zone.