Earthquake Swarm VS20240610.1: Seismic Activity in the Rat Islands, Alaska

An earthquake swarm designated VS20240610.1 occurred in the Rat Islands region of the Aleutian Islands, Alaska. The sequence began at 23:23 UTC on 9 June 2024 and concluded at 04:45 UTC on 13 June 2024, spanning 77 hours and 21 minutes. During this period, 85 earthquakes were recorded, with magnitudes ranging from -0.3 to 1.8 and focal depths between 2 and 12 km. The majority of events clustered in the first 24 hours, followed by a gradual decline in frequency and intensity.

The Rat Islands lie within the western Aleutian arc, a tectonically active zone formed by the oblique subduction of the Pacific Plate beneath the North American Plate. Convergence rates average approximately 6–7 cm per year, producing a combination of thrust faulting along the megathrust and strike-slip motion on subsidiary structures. This setting generates frequent seismic swarms, often linked to fluid migration or stress transfer within the overriding plate and along the subduction interface. Depths recorded in the swarm align with typical upper-crustal activity in the region, where brittle failure occurs above the deeper ductile transition.

Historical records indicate that earthquake swarms are recurrent in the Rat Islands. Since 1 January 2000, eleven swarms have been documented: two in 2003, one in 2006, four in 2014, two in 2021, and two in 2022. These episodes vary in duration and event count but share similar magnitude and depth characteristics, underscoring persistent background seismicity driven by plate-boundary dynamics. The 1965 Rat Islands earthquake (Mw 8.7) remains the largest historical event in the immediate area, highlighting the potential for great earthquakes alongside more frequent swarm activity.

Analysis of the VS20240610.1 sequence shows predominantly microseismic events, with only a few exceeding magnitude 1.5. Early events on 9–10 June reached the highest magnitudes (up to 1.8), while later activity consisted almost entirely of events below magnitude 0.5. Depths remained shallow throughout, consistent with the brittle upper crust. Such patterns are characteristic of swarm behavior, where events occur in rapid succession without a single dominant mainshock.

This swarm adds to the long-term seismic catalog of the Aleutian subduction zone, providing additional data on short-term clustering that can inform regional hazard assessments. Continued monitoring by networks such as the Alaska Earthquake Center supports improved understanding of stress evolution along this highly active plate boundary.

References

USGS Earthquake Catalog
Alaska Earthquake Center, University of Alaska Fairbanks
Global CMT Project
NOAA National Centers for Environmental Information