Seismic Swarm VS20191006.1: Analysis of Activity Near Adak, Alaska
On 6 October 2019, a seismic swarm designated VS20191006.1 was recorded 41 km east-northeast of Adak, Alaska. The sequence began at 06:15 UTC and concluded at 09:56 UTC, encompassing a duration of 3 hours and 41 minutes during which 37 earthquakes were detected. All events registered negative magnitudes ranging from -1.0 to -0.3 and occurred at very shallow depths of 0 to 1 km. The swarm represents one of 13 such episodes documented in the region since 2000, following prior occurrences in 2002, 2013, and 2018.
This swarm exhibited a rapid onset with multiple events clustered in the initial minutes, followed by a gradual decline in frequency. Early activity included repeated magnitudes near -0.8 and -0.5, while later stages featured isolated events such as the final recorded quake of magnitude -0.8 at 09:56. Depths remained consistently near-surface throughout, consistent with the internal classification parameters applied to swarm identification.
The Adak region occupies a tectonically active segment of the Aleutian arc, where the Pacific plate subducts beneath the North American plate at rates of approximately 6–7 cm per year. This convergent boundary produces frequent seismicity and volcanism across the Andreanof Islands. Historical records indicate that the area has experienced several large-magnitude events, including the 1957 Andreanof Islands earthquake of magnitude 8.6, which generated a significant tsunami. Ongoing monitoring by regional networks continues to track microseismic patterns that may reflect stress adjustments along the subduction interface or associated fault structures.
Previous swarms in the Adak vicinity since 2000 total 13 episodes, with the 2018 sequence accounting for the majority at ten events. Such recurrent activity underscores the dynamic nature of the local crust, where swarms often occur without associated larger mainshocks. The 2019 swarm aligns with this pattern of short-duration, low-magnitude clustering.
References
USGS Earthquake Catalog Alaska Earthquake Center Annual Reports NOAA Tsunami Database