Seismic Swarm VS20250717.1 Near Beluga, Alaska: Analysis and Geological Context

A seismic swarm designated VS20250717.1 was recorded 65 km west-northwest of Beluga, Alaska, in the Cook Inlet region. The sequence began at 16:11 on 16 July 2025 and concluded at 19:48 on 25 July 2025, spanning 219 hours and 36 minutes. During this period, 170 earthquakes were detected.

The Cook Inlet area lies within the tectonically active Pacific Ring of Fire, where the Pacific Plate subducts beneath the North American Plate along the Aleutian megathrust. This subduction drives frequent seismic activity, including both shallow crustal events and deeper intraslab earthquakes. Beluga and surrounding communities sit near the western margin of the Cook Inlet basin, underlain by Mesozoic and Cenozoic sedimentary rocks deformed by ongoing convergence. Regional faults such as the Castle Mountain Fault and structures associated with the Bruin Bay Fault system contribute to localized strain release.

Historical records document significant earthquakes in south-central Alaska, notably the magnitude 9.2 Great Alaska Earthquake of 1964, which originated in Prince William Sound and produced widespread subsidence and uplift across the Cook Inlet region. Smaller instrumentally recorded events have occurred regularly since monitoring began in the mid-20th century. Seismic swarms, defined as clusters of earthquakes without a distinct mainshock-aftershock sequence, have been observed intermittently in the area.

Analysis of the first 100 events in swarm VS20250717.1 reveals predominantly low-magnitude activity. Magnitudes ranged from -0.7 to 2.7, with the majority below 1.0. Depths were generally shallow, concentrated between 0 and 5 km, though two deeper events reached 127 km and 138 km. The sequence began with microearthquakes near 3 km depth, followed by occasional larger events such as the magnitude 2.2 at 9 km on 16 July and multiple magnitude 2+ shocks on 19–21 July. Negative magnitudes indicate events detected only by sensitive local networks. Temporal distribution showed bursts of activity interspersed with quieter intervals, consistent with swarm behavior driven by fluid migration or aseismic slip rather than a single large rupture.

Since 1 January 2000, fifteen swarms have been identified in the broader region. Prior swarms occurred in 2021 (one), 2022 (one), 2024 (ten), and 2025 (three). These episodes underscore the persistent low-level seismicity characteristic of the Cook Inlet subduction environment.

The swarm VS20250717.1 did not produce reported damage or felt shaking beyond instrumental detection thresholds. Continued monitoring by regional seismic networks remains essential for understanding subduction-related processes and assessing long-term hazard in this populated corridor of Alaska.

References

USGS Earthquake Catalog
Alaska Earthquake Center Annual Reports
Alaska Division of Geological & Geophysical Surveys, Cook Inlet Geology Publications