Seismic Swarm VS20200130.1 Near Adak, Alaska: Geological Context and Event Analysis
The Aleutian Islands arc, where Swarm VS20200130.1 occurred, forms a classic subduction zone at the convergent boundary between the Pacific and North American plates. This tectonic setting drives intense seismicity across the region, with the Pacific plate descending beneath the overriding plate at rates of approximately 6–8 cm per year. Adak Island lies within this arc, roughly 41 km west of the swarm epicenter, amid a landscape shaped by both volcanic edifices and active fault systems. The area experiences frequent earthquake swarms due to fluid migration, stress transfer along the megathrust, and volcanic processes associated with the nearby Adak volcanic complex.
Swarm VS20200130.1 began at 18:01 UTC on 29 January 2020 and concluded at 08:54 UTC on 1 February 2020, spanning 62 hours and 52 minutes. During this interval, 72 earthquakes were recorded. Event magnitudes ranged from -0.7 to 2.3, with the majority falling below 1.0 and occurring at shallow depths of 0–6 km. A small cluster of deeper events reached 25–27 km, including a magnitude 2.1 earthquake at 18 km depth late on 31 January. The temporal distribution showed peak activity on 30 January, with multiple events clustered within minutes, followed by a gradual decline. Negative magnitudes reflect the high sensitivity of the local seismic network in detecting microseismicity.
This swarm aligns with the historical pattern of seismic activity in the Adak region. Since 1 January 2000, 15 swarms have been documented, concentrated in specific years: one in 2002, one in 2013, ten in 2018, two in 2019, and the present event in 2020. Such recurrent swarms underscore the persistent tectonic strain accumulation along the Aleutian subduction interface, where episodic slip and fluid-related triggering commonly produce short-lived clusters without a single dominant mainshock.
Geologically, the swarm location sits within the Andreanof block of the Aleutian arc, characterized by oblique convergence and a well-developed volcanic front. The shallow focal depths of most events suggest involvement of the upper crust and possible hydrothermal systems linked to nearby volcanoes. Deeper events may reflect intraslab deformation within the subducting Pacific plate. No surface rupture or significant ground deformation was associated with this swarm, consistent with its modest energy release.
In summary, Swarm VS20200130.1 exemplifies the background microseismicity that defines the Adak sector of the Aleutian subduction zone. Continued monitoring remains essential given the region’s capacity for larger megathrust events.
References:
SeismoSight internal swarm catalog VS20200130.1
USGS Earthquake Catalog (Adak, Alaska region, 2000–2020)
Alaska Earthquake Center regional reports