Seismic Swarm VS20200814.1: Analysis of Activity Near Karluk, Alaska

Seismic swarm VS20200814.1 was recorded 88 km north of Karluk, Alaska, on Kodiak Island. The sequence began at 04:58 on 14 August 2020 and concluded at 19:51 on 17 August 2020, spanning 86 hours and 53 minutes. During this interval, 68 earthquakes were registered.

The events exhibited low magnitudes, ranging from -0.6 to 2.6, with the majority below 1.0. Depths varied between 1 km and 15 km, indicating shallow crustal activity. Notable events included a magnitude 2.3 quake at 21:11 on 14 August at 2 km depth, a magnitude 2.5 event at 22:09 on the same day at 7 km depth, and a magnitude 2.6 shock at 16:05 on 15 August at 4 km depth. The swarm displayed clustered timing, with heightened activity during the evening of 14 August and mid-afternoon on 15 August.

This swarm aligns with patterns observed in the region since 2000. Eight prior swarms have occurred, distributed across 2002 (1 swarm), 2016 (1 swarm), 2019 (2 swarms), and 2020 (4 swarms). Such sequences reflect episodic releases of strain in a tectonically dynamic setting.

The Kodiak Island region lies within the Aleutian subduction zone, where the Pacific Plate converges with and descends beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic interaction generates frequent seismicity, including both mainshock-aftershock sequences and swarm-type activity. Historical context includes the 1964 Great Alaska Earthquake (magnitude 9.2), whose rupture extended across the Kodiak segment and produced widespread deformation. Ongoing monitoring by regional networks confirms persistent background seismicity driven by plate boundary processes, with swarms often linked to fluid migration or localized stress adjustments rather than large-scale fault rupture.

Data from the swarm indicate predominantly shallow foci consistent with upper-crustal deformation above the subduction interface. The absence of a dominant mainshock and the gradual decay of activity are characteristic of swarm behavior in subduction-related volcanic arcs and forearc settings.

Continued observation of this area remains essential given its position within one of the most seismically active margins on Earth. The VS20200814.1 sequence provides additional insight into the temporal and spatial distribution of microseismicity that precedes or accompanies larger tectonic events.