Seismic Swarm VS20230512.1: Analysis of Earthquake Activity Near Pedro Bay, Alaska

A seismic swarm designated VS20230512.1 was recorded approximately 64 km east-northeast of Pedro Bay, Alaska. The sequence began at 16:47 on 11 May 2023 and concluded at 07:09 on 13 May 2023, spanning 38 hours and 22 minutes. During this period, 29 earthquakes were detected, with magnitudes ranging from -0.7 to 2.6 and focal depths varying between 0 km and 116 km.

The swarm featured a mix of very shallow events clustered near the surface and several deeper occurrences. Notable activity included an initial event of magnitude -0.4 at 1 km depth, followed shortly by a magnitude 2.0 earthquake at 114 km depth. Subsequent shallow events dominated the later hours of 11 May and throughout 12 May, with the largest event reaching magnitude 2.6 at 0 km depth on 12 May at 02:35. A final magnitude 0.9 event at 2 km depth marked the swarm’s end on 13 May. Most events exhibited magnitudes below 1.0 and depths under 3 km, indicating predominantly low-energy, near-surface seismicity interspersed with isolated deeper shocks.

The region lies within south-central Alaska, part of the tectonically active Pacific-North American plate boundary. This area experiences ongoing deformation from the subduction of the Pacific plate beneath the North American plate along the Aleutian megathrust, approximately 300 km to the southwest. Crustal faults and volcanic systems associated with the Cook Inlet and Lake Clark fault zones contribute to background seismicity. Pedro Bay sits near the northern shore of Iliamna Lake, in proximity to the active Iliamna Volcano, where hydrothermal and magmatic processes can influence local earthquake patterns.

Historical records indicate limited swarm activity in this specific locale since 2000. Only two prior swarms have been documented: one in 2012 consisting of a single event and another in 2016 also limited to one event. These infrequent episodes suggest that clustered seismicity remains uncommon compared with the more continuous background activity driven by regional tectonics.

Analysis of the 2023 swarm reveals a predominantly shallow, low-magnitude character consistent with possible fluid migration or minor fault adjustment within the upper crust. The presence of deeper events may reflect separate stress release along the subducting slab interface. Overall, the sequence did not produce widely felt shaking or significant surface effects, aligning with the modest energy release observed.

Such swarms provide valuable data for refining seismic hazard models in Alaska’s remote regions. Continued monitoring by regional networks supports improved understanding of how these episodic clusters relate to long-term tectonic loading.

References

• Alaska Earthquake Center, University of Alaska Fairbanks (2023 catalog data).
• U.S. Geological Survey Earthquake Hazards Program (regional tectonic summaries).
• SeismoSight internal swarm classification records.