Seismic Swarm S20180404.1: Activity Near Nikolski, Alaska

Seismic swarm S20180404.1 was recorded 51 km northeast of Nikolski, Alaska, in the central Aleutian Islands. The sequence began at 07:58 on 3 April 2018 and concluded at 12:14 on 5 April 2018, spanning 52 hours and 15 minutes. During this interval, 54 earthquakes were detected, all of low magnitude.

Event magnitudes ranged from 0.0 to 1.1, with the majority below 0.6. Depths were predominantly shallow, varying from -1 km to 15 km, consistent with activity in the upper crust. The largest events reached magnitude 1.1 and occurred at depths of 1 km, 4 km, and 5 km. Temporal clustering was evident, with peak activity between 04:00 and 06:00 on 4 April and additional pulses later that day. Most events exhibited depths of 0–3 km, suggesting localized stress release near the surface.

This swarm represents the second such episode in the Nikolski region since 2000, following the previous swarm in 2007. The low-magnitude, high-frequency character of the events points to a non-mainshock-aftershock pattern typical of fluid-driven or aseismic slip processes within the subduction interface.

The Nikolski area lies within the Aleutian volcanic arc, formed by the oblique subduction of the Pacific Plate beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces one of the most seismically active regions on Earth, with frequent small-magnitude swarms alongside occasional great earthquakes exceeding magnitude 8.0. The local crust includes accreted terranes and active volcanic centers such as those on Umnak Island, contributing to heterogeneous stress fields that can trigger swarm sequences.

Historical records document major events in the broader Aleutian region, including the 1957 magnitude 8.6 earthquake and the 1965 magnitude 8.7 Rat Islands event, both of which generated significant tsunami impacts. Modern monitoring by regional networks has improved detection of microseismicity, revealing that swarm activity often occurs along the volcanic front and forearc.

Ongoing subduction continues to load the plate interface, maintaining elevated seismic hazard. The 2018 swarm, though minor, illustrates the persistent background seismicity that characterizes this convergent margin.

References

SeismoSight internal classification data for swarm S20180404.1.
USGS Earthquake Hazards Program regional tectonic summaries for the Aleutian arc.
Alaska Earthquake Center annual reports on Aleutian seismicity.