Seismic Swarm S20160124.2 Near Pedro Bay, Alaska

The seismic swarm designated S20160124.2 occurred in south-central Alaska, centered 46 km east-southeast of Pedro Bay. It began at 10:30 UTC on 24 January 2016 and concluded at 04:39 UTC on 21 February 2016, spanning 666 hours and 9 minutes. During this interval, 777 earthquakes were recorded, with the sequence initiated by a magnitude 7.1 event at 125 km depth.

Analysis of the first 100 events reveals a rapid onset followed by a prolonged decay in activity. The initial M7.1 shock was succeeded within minutes by an M4.3 event at 110 km depth. Subsequent events clustered tightly in magnitude between 1.6 and 3.3, with the majority falling between 1.7 and 2.6. Focal depths remained consistently deep, ranging from 93 km to 127 km and averaging near 106 km. This depth distribution points to seismicity occurring within the subducting slab rather than at shallow crustal levels.

Temporal patterns show the highest rates in the first 24 hours, with events occurring every few minutes initially before intervals lengthened. Notable larger aftershocks included an M3.3 at 103 km depth on 25 January and an M4.3 at 104 km depth later the same day. No events exceeded magnitude 3.3 after the first day, indicating a swift transition to lower-energy release.

The region lies within the Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces both shallow megathrust earthquakes and intermediate-depth intraslab events. Pedro Bay sits near the volcanic arc associated with Iliamna Volcano, where dehydration reactions in the downgoing slab can trigger deep seismicity. Historical records document elevated background seismicity throughout the Cook Inlet and Lake Iliamna area, consistent with ongoing plate convergence.

A magnitude 7.1 earthquake on 24 January 2016, located 47 km east-southeast of Pedro Bay and only 4 km from the swarm centroid, stands as the strongest event in the vicinity since 2000. Its occurrence at intermediate depth aligns with the broader pattern of slab-related seismicity observed across southern Alaska.

The swarm’s characteristics—high initial productivity, sustained deep activity, and absence of significant shallow events—provide a clear example of intraslab earthquake clustering in an active subduction environment. Such sequences contribute to understanding stress transfer within the downgoing plate and help refine seismic hazard assessments for nearby communities and infrastructure.

References

• SeismoSight internal swarm classification S20160124.2
• Event parameters from provided catalog data (24 Jan–25 Jan 2016)