Seismic Swarm VS20251205.1: Analysis of Activity Near Pedro Bay, Alaska

A seismic swarm designated VS20251205.1 was recorded 65 km east-northeast of Pedro Bay, Alaska. The sequence began at 00:24 on 5 December 2025 and concluded at 15:43 on 7 December 2025, spanning 63 hours and 19 minutes. During this interval, 44 earthquakes were detected.

The events were predominantly microearthquakes, with magnitudes ranging from -1.1 to 2.4. Depths were generally shallow, concentrated between 0 and 6 km, though isolated events reached 11 km and 111 km. Notable activity included two larger shocks on 6 December 2025 at 21:33 UTC: one of magnitude 1.9 at 0 km depth and another of magnitude 2.4 at 3 km depth. The majority of events exhibited negative magnitudes, consistent with high-resolution seismic monitoring capable of detecting low-energy releases.

This swarm occurred in a region shaped by ongoing subduction of the Pacific Plate beneath the North American Plate along the Aleutian megathrust. The Cook Inlet and Lake Iliamna area, which includes Pedro Bay, forms part of the southern Alaska margin where convergence rates average approximately 6 cm per year. Crustal stresses from this interaction produce both volcanic and tectonic seismicity. The local geology features Mesozoic sedimentary and volcanic rocks overlain by Quaternary glacial deposits, with nearby Iliamna Volcano contributing to the broader volcanic arc setting.

Historical records indicate that earthquake swarms are recurrent in this portion of Alaska. Since 1 January 2000, six swarms have been documented in the vicinity, occurring in 2012 (1 swarm), 2016 (1 swarm), 2023 (2 swarms), and 2024 (2 swarms). These episodes typically involve clustered, low-magnitude events without a single dominant mainshock, distinguishing them from typical aftershock sequences.

The December 2025 swarm aligns with this pattern, showing no escalation toward higher-magnitude events. Such activity is routinely monitored by regional networks to assess potential links to volcanic unrest or fault slip, though no surface rupture or significant ground deformation has been associated with prior swarms in the catalog.

References

• Alaska Earthquake Center, University of Alaska Fairbanks – Regional seismicity reports and swarm catalogs.
• U.S. Geological Survey Earthquake Hazards Program – Tectonic framework of southern Alaska and subduction zone parameters.
• USGS Volcano Hazards Program – Iliamna Volcano geologic and seismic background.