Seismic Swarm VS20220825.1 Near Karluk, Alaska: Geological Context and Event Analysis

Seismic swarm VS20220825.1 occurred 84 km NNW of Karluk on Kodiak Island, Alaska. The sequence began at 07:30 on 24 August 2022 and concluded at 18:29 on 28 August 2022, spanning 106 hours and 59 minutes. During this period, 146 earthquakes were recorded.

The Kodiak Island region 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 seismic activity, including both large megathrust events and smaller swarms. Historical records document major earthquakes in the area, notably the magnitude 9.2 Great Alaska Earthquake of 1964, which originated nearby and generated widespread tsunamis. The local crust consists of accreted terranes and volcanic arcs shaped by millions of years of subduction, with fault systems accommodating both compressional and strike-slip motion.

Swarm activity in this region is not uncommon. Since 2000, nine documented swarms have occurred near Karluk, with the following yearly distribution: one in 2002, one in 2016, two in 2019, and five in 2020. These episodes typically involve clusters of low-magnitude events without a dominant mainshock, often linked to fluid migration or stress adjustments along subduction-related faults.

Analysis of the first 100 events from swarm VS20220825.1 reveals predominantly small-magnitude earthquakes. Recorded magnitudes ranged from -0.6 to 1.3, with the majority falling between -0.3 and 0.9. Depths clustered between 15 km and 26 km, though a few shallower events occurred at 2–4 km and isolated deeper ones reached 32 km. Timing showed an initial burst on 24 August, followed by sustained activity through 25–27 August, with events distributed across both daytime and nighttime hours. This pattern is characteristic of swarm behavior driven by distributed stress release rather than a single rupture.

The absence of events exceeding magnitude 2.0 in the sampled data, combined with consistent intermediate depths, suggests possible involvement of crustal fluids or minor magmatic processes common in subduction environments. No damage or felt reports were associated with these microearthquakes.

• Alaska Earthquake Center catalog
• USGS National Earthquake Information Center
• SeismoSight internal swarm classification records