Earthquake Swarm VS20161015.1: Seismic Activity Near Karluk, Alaska

An earthquake swarm designated VS20161015.1 was recorded 83 km north of Karluk, Alaska, on Kodiak Island. The sequence began at 22:40 on 14 October 2016 and concluded at 08:44 on 15 October 2016, spanning 10 hours and 3 minutes. During this interval, 26 earthquakes were registered, with magnitudes ranging from 0.8 to 2.3 and focal depths predominantly between 1 km and 7 km. The events clustered in time, featuring a notable peak magnitude of 2.3 at 05:36 on 15 October.

The swarm exhibited typical characteristics of low-magnitude, shallow seismicity. Initial events on 14 October included magnitudes of 1.1 at 22:40 and 23:01, followed by a gradual increase to 1.8 by 23:25. Activity continued overnight with consistent shallow depths around 4–6 km. On 15 October, a cluster of events between 00:02 and 02:44 maintained magnitudes near 1.0–1.6, culminating in the strongest shock before tapering off with smaller tremors through the morning.

Kodiak Island lies within the tectonically active Alaska-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 megathrust boundary generates frequent seismic activity, including both interface and intraslab earthquakes. The region’s geology features accreted terranes, volcanic arcs, and sedimentary basins shaped by millions of years of subduction. Shallow swarm events like VS20161015.1 often reflect fluid migration or stress adjustments within the overriding plate rather than direct megathrust rupture.

Seismicity in this sector has a well-documented history. The 1964 Great Alaska Earthquake, magnitude 9.2, originated nearby and remains one of the largest recorded events, triggering widespread uplift and subsidence across Kodiak Island. Since 2000, swarm-type sequences have been infrequent, with only one prior swarm noted in 2002. This scarcity underscores the episodic nature of clustered microseismicity amid the dominant background of isolated tectonic quakes.

Insights from the 2016 swarm highlight its compact temporal footprint and uniform depth distribution, consistent with localized crustal adjustments in a subduction forearc setting. Such sequences provide valuable data for refining seismic hazard models in remote Alaskan regions, where monitoring networks continue to improve detection of low-magnitude events.

References

• USGS Earthquake Catalog
• Alaska Earthquake Center Annual Reports
• SeismoSight Internal Swarm Classification Database