Seismic Swarm S20210614.1: Analysis of Activity Near Beluga, Alaska

On June 14, 2021, a seismic swarm designated S20210614.1 occurred approximately 75 km west-northwest of Beluga, Alaska. The sequence began at 11:31 local time and concluded at 13:43, lasting 2 hours and 11 minutes. During this period, 37 earthquakes were recorded, with magnitudes ranging from -0.3 to 0.4 and focal depths between 1 km and 9 km. The events exhibited typical swarm characteristics, featuring clustered small-magnitude tremors without a dominant mainshock.

The swarm's temporal distribution showed initial activity concentrated in the first hour, with events occurring at intervals of seconds to minutes. Magnitudes remained consistently low, peaking at 0.4, while depths clustered around 2–3 km for most later events. This pattern aligns with fluid-driven or stress-release mechanisms common in volcanic or tectonically active margins.

The Beluga region lies within the Cook Inlet basin of south-central Alaska, part of the tectonically dynamic Pacific Ring of Fire. Here, the Pacific Plate subducts northwestward beneath the North American Plate at rates of approximately 6–7 cm per year along the Aleutian megathrust. This convergence generates frequent seismicity, including both crustal and intraslab earthquakes. The Cook Inlet area features a complex network of strike-slip and thrust faults superimposed on sedimentary basins filled with Mesozoic and Cenozoic strata.

Geological history of the region includes multiple episodes of arc volcanism and basin development since the Jurassic period. Quaternary glaciation further shaped the landscape, depositing thick sequences of unconsolidated sediments that can amplify seismic shaking. Nearby volcanic centers, such as Mount Spurr and Mount Redoubt, contribute to elevated background seismicity through magmatic processes.

Alaska experiences thousands of earthquakes annually, with the largest historical event being the 1964 Great Alaska Earthquake (magnitude 9.2) centered in Prince William Sound. The Cook Inlet zone has recorded numerous moderate events, including the 2016 magnitude 7.1 Iniskin earthquake, which highlighted the potential for intraslab ruptures. Updated seismic monitoring by the Alaska Earthquake Center indicates ongoing low-level activity consistent with plate-boundary deformation.

Insights from swarm S20210614.1 suggest localized stress perturbations at shallow depths, possibly linked to regional fault interactions or hydrothermal circulation. Such sequences provide valuable data for refining hazard models in this high-risk area.

References

• Alaska Earthquake Center. (2023). Regional seismicity reports. University of Alaska Fairbanks.
• USGS Earthquake Hazards Program. (2024). Tectonic setting of southern Alaska. U.S. Geological Survey.
• Plafker, G., & Berg, H. C. (1994). The geology of Alaska. Geological Society of America.