Earthquake Swarm S20121204.1 Near Beluga, Alaska

Earthquake swarm S20121204.1 was recorded 18 km ENE of Beluga, Alaska. The sequence began at 13:13 on 3 December 2012 and concluded at 21:12 on 8 December 2012, spanning 127 hours and 58 minutes. A total of 105 earthquakes were registered during this period.

The swarm featured a mainshock of magnitude 5.8 at 63 km depth on 4 December 2012 at 01:42:48 UTC. Subsequent events remained predominantly shallow to intermediate in depth, ranging between 50 km and 67 km. Magnitudes clustered between 1.0 and 3.2, with the majority falling below 2.0. The first 100 events showed a rapid onset of activity following the mainshock, followed by a gradual decline in both frequency and size over the subsequent days. Notable later events included magnitudes of 3.2 on 4 December at 14:35:33 and 2.8 on 7 December at 05:00:17, both at depths near 55–58 km.

This activity occurred within the Cook Inlet basin of south-central Alaska. The basin lies above the Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate at rates of approximately 6 cm per year. Intermediate-depth seismicity in the 50–70 km range aligns with the Wadati-Benioff zone associated with this convergence. Regional geology includes Mesozoic and Cenozoic sedimentary rocks overlying accreted terranes, with active fault systems and volcanic arcs contributing to elevated seismic hazard.

Historical records document frequent moderate earthquakes in the Cook Inlet region. The 1964 Great Alaska Earthquake (magnitude 9.2) produced widespread strong shaking and triggered numerous aftershocks across south-central Alaska. Smaller swarms and clusters have been noted near Beluga and the adjacent Susitna basin in subsequent decades, consistent with ongoing plate-boundary strain release.

The 2012 swarm illustrates typical characteristics of subduction-related seismicity, with a clear mainshock-aftershock pattern and depth distribution reflecting slab geometry. No surface rupture or significant damage was associated with the sequence.

SeismoSight internal classification records.
USGS Earthquake Catalog (historical regional events).
Alaska Division of Geological & Geophysical Surveys (Cook Inlet tectonics).