Seismic Swarm VS20180606.1: Analysis of Microearthquake Activity Near Adak, Alaska

The seismic swarm designated VS20180606.1 occurred approximately 40 km east-northeast of Adak, Alaska, within the tectonically active Aleutian Islands region. This area lies along the convergent boundary where the Pacific Plate subducts beneath the North American Plate, forming the Aleutian subduction zone. Subduction occurs at rates of 6–7 cm per year, generating frequent volcanic and seismic activity along the arc. Adak itself sits on Adak Island in the Andreanof Islands group, a segment known for both volcanic edifices and shallow crustal faulting associated with the subduction interface. The swarm initiated at 17:16 UTC on 5 June 2018 and concluded at 21:15 UTC on 13 June 2018, spanning 195 hours and 58 minutes. During this interval, 430 earthquakes were recorded. All events remained at microearthquake scale, with the majority exhibiting negative magnitudes between −1.1 and 0.2 and focal depths predominantly between 0 and 9 km. Negative depth values in the catalog likely reflect minor location uncertainties common in sparse seismic networks rather than true above-surface sources. Examination of the first 100 events reveals a consistent pattern of very low-energy releases. Magnitudes clustered tightly around −0.5 to −0.9, with only a single positive value of 0.2 recorded on 7 June. Depths stayed shallow, rarely exceeding 9 km, indicating activity confined to the upper crust above the main subduction interface. Temporal distribution showed no strong diurnal variation, consistent with tectonic rather than anthropogenic triggering. Regionally, the Andreanof Islands segment has produced notable large earthquakes, including the 1957 magnitude-8.6 Andreanof Islands event and the 1986 magnitude-8.0 event. These great earthquakes ruptured portions of the subduction megathrust, releasing accumulated strain over hundreds of kilometers. In contrast, the 2018 swarm represents a localized cluster of small events, typical of swarm behavior where fluid migration or minor stress perturbations trigger numerous similar-sized quakes without a clear mainshock–aftershock sequence. Historical records since 2000 document three comparable swarms in the immediate vicinity: one event in 2002, one in 2013, and the 2018 sequence itself. Each swarm remained short-lived and low-magnitude, underscoring that such clusters form a recurring but minor component of the region’s overall seismicity. The 2018 swarm did not culminate in larger events or volcanic unrest at nearby volcanoes such as Mount Adagdak or Great Sitkin. Its shallow, low-magnitude character aligns with background crustal seismicity driven by plate convergence and local faulting rather than deep megathrust slip.

References

• Alaska Earthquake Center, University of Alaska Fairbanks – Regional seismicity catalog and swarm documentation.
• U.S. Geological Survey Earthquake Hazards Program – Tectonic framework of the Aleutian subduction zone.
• National Oceanic and Atmospheric Administration – Historical earthquake database for the Andreanof Islands segment.