Seismic Swarm VS20200403.1: Analysis of Activity Near Karluk, Alaska

Seismic swarm VS20200403.1 was recorded 83 km NNW of Karluk, Alaska, on Kodiak Island. The sequence began at 17:47 on 2 April 2020 and concluded at 05:09 on 7 April 2020, spanning 107 hours and 22 minutes. During this period, 61 earthquakes were registered, with magnitudes ranging from -1.8 to 1.6 and depths predominantly between 0 and 7 km. One event reached 29 km depth. Most events clustered in the first three days, with the largest magnitude (1.6) occurring on 4 April at 07:00:00.

The swarm exhibited typical characteristics of low-magnitude, closely spaced events without a dominant mainshock-aftershock pattern. Activity showed temporal clustering, particularly between 3 and 4 April, followed by a gradual decline. Negative magnitudes indicate microseismicity detectable only by sensitive instruments, consistent with background tectonic strain release in the region.

The location lies within the Alaska-Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. Kodiak Island sits above the megathrust interface responsible for major historical events, including the magnitude 9.2 Great Alaska Earthquake of 1964. This tectonic setting produces frequent shallow crustal seismicity alongside deeper intraslab events. Updated USGS models confirm ongoing convergence and associated seismic hazard in the Kodiak segment, with the area classified as having high potential for both moderate swarms and larger ruptures.

Historical swarm data for the region since 2000 indicate four documented episodes: one in 2002, two in 2019, and the present 2020 sequence. These events reflect episodic strain release along the subduction margin rather than magmatic processes. The 2020 swarm aligns with patterns observed in prior years, featuring predominantly shallow depths and low magnitudes that do not indicate escalation toward larger tectonic failure.

SeismoSight internal classification identifies this sequence as a distinct swarm based on its spatiotemporal distribution and lack of a clear triggering mainshock. Such swarms contribute to long-term monitoring of stress accumulation along the plate boundary and support refinement of regional seismic hazard assessments.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center regional reports
SeismoSight internal swarm classification records