Seismic Swarm S20040223.1: Analysis of Earthquake Activity Near Nabesna, Alaska

Seismic swarm S20040223.1 occurred 21 km east-southeast of Nabesna, Alaska, from 22:12 on 22 February 2004 to 14:19 on 26 February 2004. Over 88 hours and 6 minutes, the sequence registered 46 earthquakes. Nabesna lies within the eastern Alaska Range, part of the tectonically complex boundary where the Pacific Plate converges with the North American Plate at rates of approximately 5–6 cm per year. This setting produces frequent seismic activity through both subduction-related thrusting and strike-slip faulting along regional structures such as the Denali Fault system.

The swarm initiated with a magnitude 1.4 event at 6 km depth. Activity intensified rapidly on 23 February, culminating in the largest event—a magnitude 4.8 earthquake at only 1 km depth—followed within minutes by additional shocks of magnitude 2.7 and 1.6. Subsequent events remained predominantly shallow, with the majority occurring between 0 and 11 km depth. Notable later shocks included a magnitude 2.6 event on 26 February at 5 km depth. One outlier reached 45 km depth, while another occurred at 23 km, indicating limited but present activity within the mid-crust.

Most events clustered tightly in time and space during the first 36 hours, consistent with swarm behavior driven by fluid migration or aseismic slip rather than a single mainshock-aftershock sequence. Magnitudes ranged from 1.1 to 4.8, with the bulk below 2.0, reflecting a low b-value typical of volcanic or hydrothermal environments in the Wrangell Mountains region. Depths concentrated near 3–7 km suggest activation along shallow crustal faults influenced by nearby volcanic systems.

Nabesna sits adjacent to the Wrangell volcanic belt, where Quaternary volcanism has shaped the landscape through andesitic stratovolcanoes and caldera complexes. Historical seismicity in the area includes moderate events associated with both regional tectonics and local volcanic unrest. The 2002 Denali Fault earthquake (magnitude 7.9), located roughly 200 km to the north, altered regional stress fields and may have contributed to heightened sensitivity in the Nabesna area by 2004.

Geological mapping indicates the swarm location coincides with fractured Paleozoic and Mesozoic basement rocks overlain by Tertiary volcanic deposits. These units host active hydrothermal systems that can facilitate swarm-type seismicity through pore-pressure changes. No surface rupture or significant ground deformation was reported following the 2004 sequence, consistent with its modest energy release.

The swarm underscores the persistent seismic hazard in interior Alaska, where even small-magnitude clusters can signal broader tectonic or magmatic processes. Ongoing monitoring by regional networks continues to track similar activity, aiding in the characterization of fault behavior and volcanic risk within Wrangell-St. Elias National Park and Preserve.

References

• Alaska Earthquake Center, University of Alaska Fairbanks
• USGS Earthquake Hazards Program
• Geological Survey of Canada – Pacific Geoscience Centre tectonic summaries