Seismic Swarm S20130914.2: Analysis of Activity Near Atka, Alaska

Seismic swarm S20130914.2 occurred 82 km south-southwest of Atka, Alaska, in the central Aleutian Islands. The sequence began at 00:27 UTC on 14 September 2013 and concluded at 10:45 UTC on 17 September 2013, spanning 82 hours and 17 minutes. During this interval, 87 earthquakes were recorded.

The Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate, forms one of Earth’s most active tectonic margins. This setting produces frequent earthquake swarms and volcanic eruptions along the island arc. Atka lies within this dynamic region, characterized by intermediate-depth seismicity associated with slab subduction and occasional shallow crustal events linked to volcanic processes.

The swarm exhibited a range of magnitudes and depths. The largest event reached magnitude 5.9 at 15:42 UTC on 14 September at a depth of 20 km. Additional notable shocks included a magnitude 5.4 event at 17:09 UTC the same day (24 km depth), a magnitude 5.2 shock at the swarm’s onset (19 km depth), and several magnitude 4+ events distributed through 15 and 16 September. Depths predominantly clustered between 10 km and 25 km, with isolated shallower and deeper outliers reaching 1 km and 38 km respectively. Lower-magnitude events (1.1–2.8) filled the sequence, illustrating the typical decay pattern of swarm activity rather than a classic mainshock-aftershock sequence.

Activity peaked on 14 September with multiple events above magnitude 4.0 within a few hours, followed by a gradual decline over the subsequent days. By 17 September, only a single small event remained. This temporal distribution aligns with known swarm behavior in subduction zones, where fluid migration or stress transfer along the plate interface can trigger clusters without a dominant mainshock.

Since 1 January 2000, five swarms have been documented in the broader Atka region. Prior activity includes one swarm in 2008 and four swarms in 2013, indicating episodic clustering of seismicity. Swarm S20130914.2 represents one of the 2013 sequences and underscores the persistent seismic productivity of the central Aleutians.

Such swarms provide valuable data for understanding stress accumulation and release along the subduction interface. Continued monitoring by regional networks remains essential for assessing potential links to volcanic unrest at nearby centers such as Atka Volcano.