Seismic Swarm VS20120308.1 Near Pedro Bay, Alaska

A seismic swarm designated VS20120308.1 occurred 65 km east-northeast of Pedro Bay, Alaska, from 03:10 UTC on 8 March 2012 to 04:54 UTC on 15 March 2012. Over 169 hours and 43 minutes, 152 earthquakes were recorded. Analysis of the first 100 events shows predominantly shallow foci between 1 km and 9 km depth, with magnitudes ranging from 0.6 to 3.4. The largest events reached magnitude 3.4 at 05:26 on 9 March and magnitude 3.2 at both 02:52 on 9 March and 14:29 on 9 March. Most activity clustered between 1 km and 4 km depth, consistent with shallow crustal processes.

The swarm location lies within the Cook Inlet basin of south-central Alaska, part of the active Aleutian subduction zone where the Pacific plate converges with the North American plate at approximately 6 cm per year. This tectonic setting produces frequent earthquakes and volcanic activity along the arc. Pedro Bay sits near the western margin of the basin, adjacent to the Bruin Bay fault system and within the influence of regional strike-slip and thrust structures. Historical seismicity in the area includes both mainshock-aftershock sequences and episodic swarms, often linked to fluid migration or stress transfer along pre-existing faults.

Geological mapping indicates the subsurface comprises Mesozoic sedimentary and volcanic rocks overlain by Quaternary glacial and volcanic deposits. Nearby volcanoes such as Iliamna and Redoubt demonstrate ongoing magmatic influence, though the 2012 swarm exhibited no clear volcanic signature in its depth distribution or temporal evolution. Shallow events suggest activation within the upper crust rather than deeper slab-related seismicity.

The temporal pattern of the first 100 events shows an initial burst of activity on 8 March, followed by sustained elevated rates through 9 March and a gradual decline thereafter. Depths remained consistently shallow except for one outlier at 112 km, likely representing a separate tectonic event unrelated to the swarm. Magnitudes stayed below 3.5, typical of swarm behavior where energy is released through numerous small ruptures instead of a single large shock.

Such swarms contribute to understanding regional stress accumulation in the subduction margin. Continued monitoring by the Alaska Earthquake Center and USGS networks provides updated catalogs that refine fault models and improve hazard assessments for nearby communities and infrastructure.

References

• Alaska Earthquake Center, University of Alaska Fairbanks (updated catalogs through 2023)
• USGS Earthquake Hazards Program, Advanced National Seismic System
• USGS Geological Map of the Iliamna Quadrangle, Alaska (Professional Paper series)
• Tectonic framework descriptions from the Alaska Division of Geological & Geophysical Surveys