Seismic Swarm S20120807.1: Analysis of Earthquake Activity Near Paxson, Alaska

Seismic swarm S20120807.1 occurred approximately 35 km northeast of Paxson, Alaska, beginning at 10:20 on 7 August 2012 and concluding at 22:28 on 8 August 2012. Over this 36-hour period, 48 earthquakes were recorded. The sequence featured a range of magnitudes from 0.5 to 4.2, with the majority of events occurring at shallow depths between 1 and 10 km. The initial event registered magnitude 3.4 at 6 km depth, followed rapidly by a magnitude 4.2 shock at 5 km depth within four minutes. Subsequent activity included multiple events exceeding magnitude 2.0 in the first hours, interspersed with numerous smaller tremors below magnitude 2.0. Depths varied modestly, with a few outliers reaching 16–18 km, though most clustered near 5–8 km. Activity tapered significantly after the first day, with isolated low-magnitude events continuing into the evening of 8 August.

This swarm unfolded within the tectonically complex terrain of south-central Alaska, where ongoing convergence between the Pacific and North American plates drives regional seismicity. The location lies near the Denali Fault, a major right-lateral strike-slip structure extending over 2,000 km. The fault accommodates much of the lateral motion associated with the Yakutat terrane collision, producing frequent earthquakes. Shallow crustal depths observed in the swarm align with typical faulting patterns along this system, where brittle deformation occurs in the upper 15–20 km of the crust.

Geological history in the Paxson area reflects repeated episodes of fault slip and mountain-building. The Denali Fault has hosted several large events, including the 2002 magnitude 7.9 earthquake, whose epicenter was roughly 40 km to the west. That event produced surface rupture exceeding 340 km and triggered widespread aftershock sequences. Swarm activity, characterized by clustered events without a dominant mainshock, has been documented four times in the region since 2000, with the earliest recorded in 2002. These swarms likely represent episodic stress release along secondary fault strands or fluid-influenced zones adjacent to the primary Denali trace.

Insights from the 2012 swarm indicate a rapid onset followed by quick decay, consistent with swarm behavior driven by localized stress perturbations rather than large-scale rupture. Magnitudes remained moderate, and the absence of events above 4.2 suggests limited energy accumulation during this episode. Depths predominantly under 10 km point to activity within the seismogenic crust, potentially influenced by the regional fault network. Such sequences contribute to understanding strain partitioning in central Alaska, where plate-boundary forces interact with intraplate structures.

Further monitoring of similar swarms can refine models of fault interaction and seismic hazard in this high-risk corridor. The 2012 data underscore the value of dense seismic networks for capturing subtle patterns in Alaska’s dynamic lithosphere.

References

• Alaska Earthquake Center, University of Alaska Fairbanks (updated regional seismicity reports through 2023).
• USGS Earthquake Hazards Program, Denali Fault System documentation.
• SeismoSight internal swarm classification records.