Seismic Swarm S20110903.2 East of Atka, Alaska

A notable earthquake swarm designated S20110903.2 occurred approximately 180 km east of Atka in the Aleutian Islands, Alaska. The sequence began at 10:55 on 2 September 2011 and concluded at 16:04 on 5 September 2011, spanning 77 hours and 8 minutes. During this period, 58 earthquakes were recorded.

The swarm initiated with a magnitude 6.9 event at 32 km depth. Subsequent activity included multiple events between magnitudes 2.6 and 4.9, with focal depths ranging from 17 km to 54 km. Key larger shocks comprised a magnitude 4.9 at 49 km depth on 2 September, a magnitude 4.8 at 37 km on the same day, another magnitude 4.8 at 30 km on 3 September, a magnitude 4.5 at 54 km on 4 September, and a magnitude 4.6 at 39 km later that day. The majority of events clustered between 40 km and 45 km depth, consistent with typical subduction-related seismicity in the region.

This swarm represents the first such episode recorded in the area since 1 January 2000, according to internal classification records. The Aleutian arc forms part of the Pacific Ring of Fire, where the Pacific plate subducts beneath the North American plate at rates of approximately 6–8 cm per year. This tectonic setting produces frequent volcanic and seismic activity along the island chain.

Atka lies within the central Aleutian Islands, an area characterized by active stratovolcanoes and a history of significant earthquakes. The subduction zone has generated great earthquakes, including the 1957 magnitude 8.6 Andreanof Islands event and the 1965 magnitude 8.7 Rat Islands earthquake. Modern monitoring by the Alaska Earthquake Center and USGS networks provides detailed catalogs of both mainshock-aftershock sequences and swarm activity in this remote region.

The 2011 swarm exhibited characteristics typical of fluid-driven or stress-transfer episodes within the Wadati-Benioff zone, with events distributed over a compact spatial area. No surface rupture or tsunami was associated with the sequence, and activity remained offshore. Depths predominantly in the 30–50 km range align with intermediate-depth seismicity observed along the arc.

Post-2011 monitoring has continued to track background seismicity in the central Aleutians, though no additional swarms matching the internal classification criteria have been noted in available records up to the present. The event underscores the persistent seismic hazard in this tectonically active margin, where even moderate swarms can provide valuable data on subduction dynamics and stress accumulation.