Seismic Swarm S20170124.1 Near Adak, Alaska: January 2017 Analysis

A notable earthquake swarm, designated S20170124.1, occurred west of Adak in the Aleutian Islands of Alaska. The sequence began at 22:07 UTC on 23 January 2017 and concluded at 21:07 UTC on 26 January 2017, spanning 71 hours. During this period, 126 earthquakes were recorded at a location 86 km west of Adak.

Analysis of the first 100 events reveals predominantly low-magnitude activity. The initial quake registered magnitude 2.8 at a depth of 9 km, followed closely by another of the same magnitude at 11 km. The largest events included a magnitude 4.2 at 10 km depth and a magnitude 4.4 at 10 km depth later in the sequence. Other notable magnitudes reached 4.1, 3.6, 3.4, and 3.2, with the majority falling between 1.6 and 2.9. Depths ranged from 3 km to 21 km, clustering most frequently around 5–10 km, consistent with shallow crustal processes.

This swarm represents one of only two documented in the region since 1 January 2000, with the prior event occurring in 2008. Such sequences highlight episodic seismic behavior along the Aleutian arc.

The Adak area lies 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. This setting produces frequent seismicity, including both mainshock-aftershock sequences and swarms. The islands form a volcanic arc resulting from partial melting of the subducting slab, with Adak situated on Adak Island amid a chain extending over 1,900 km from the Alaska Peninsula toward the Kamchatka Peninsula.

Historical records indicate that the Aleutian subduction zone has generated some of the largest earthquakes globally, including the 1964 Great Alaska Earthquake (magnitude 9.2) and the 1957 Andreanof Islands event (magnitude 8.6). Smaller swarms like S20170124.1 often reflect fluid migration or stress adjustments within the overriding plate or along the megathrust interface, though precise mechanisms require detailed modeling beyond basic catalog data.

Seismic monitoring in this remote region relies on networks operated by the Alaska Earthquake Center and the U.S. Geological Survey. Depths in the observed range suggest activity within the upper crust or near the plate interface, typical for intermediate-depth events in subduction environments. Continued surveillance remains essential given the zone's capacity for significant seismic and tsunami hazards affecting nearby communities and maritime routes.

The 2017 swarm's temporal clustering and magnitude distribution align with patterns seen in other subduction-related swarms worldwide, underscoring the value of long-term catalogs for identifying recurrence trends in areas with limited historical swarm frequency.