Seismic Analysis of Earthquake Swarm VS20190820.1 Near Karluk, Alaska

An earthquake swarm designated VS20190820.1 was recorded 84 km north of Karluk, Alaska, on Kodiak Island. The sequence began at 06:01 on 20 August 2019 and concluded at 04:11 on 22 August 2019, spanning 46 hours and 10 minutes. During this interval, 61 earthquakes were detected, with magnitudes ranging from -0.5 to 1.9 and focal depths between 0 and 10 km. The events clustered in two main phases on 20 August, with the largest shock reaching magnitude 1.9 at 21:59.

The swarm exhibited typical characteristics of low-magnitude, shallow seismicity. Initial activity included events of magnitude 1.1 at 06:01 and 1.6 at 21:26, followed by a dense cluster of smaller tremors through the evening. Activity tapered significantly after midnight on 21 August, with only isolated events recorded until the final tremor on 22 August. Depths remained consistently shallow, consistent with crustal processes rather than deeper subduction-related rupture.

Kodiak Island lies within the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at approximately 6 cm per year. This tectonic setting produces frequent seismic activity, including both great megathrust earthquakes and smaller swarms along the margin. The region experienced the magnitude 9.2 Great Alaska Earthquake in 1964, whose rupture extended beneath Kodiak Island and generated widespread uplift and subsidence. Ongoing convergence continues to load the plate interface, resulting in background seismicity and occasional swarm episodes.

Historical records maintained by SeismoSight indicate only two prior swarms in the area since 2000: a single-event swarm in 2002 and another in 2016. The 2019 sequence represents the third such episode, underscoring the relatively infrequent nature of swarm activity in this specific locale despite the high overall seismicity of the Aleutian arc.

Such swarms are often attributed to fluid migration or localized stress perturbations within the overriding plate. The shallow depths and small magnitudes observed in VS20190820.1 align with these mechanisms rather than mainshock-aftershock sequences driven by a single large rupture. No damage or felt reports were associated with the events, reflecting their microseismic scale.

Continued monitoring by regional networks remains essential for distinguishing swarm behavior from potential precursors to larger events along the subduction interface. The 2019 swarm provides additional data for refining models of crustal deformation in this tectonically active region.

References

SeismoSight internal swarm classification records Alaska Earthquake Center regional seismic catalog USGS tectonic framework of the Aleutian subduction zone