Seismic Swarm S20021211.1: Analysis of Activity Near Paxson, Alaska

Seismic swarm S20021211.1 occurred 85 km west-northwest of Paxson, Alaska, beginning at 11:15 on 10 December 2002 and concluding at 15:44 on 14 December 2002. Over 100 hours and 28 minutes, the sequence produced 122 earthquakes. This event marks the first swarm recorded in the region since 1 January 2000, with nine additional swarms documented through subsequent years.

Analysis of the initial 100 events reveals predominantly shallow foci, with depths clustered between 0 and 20 km. A single outlier reached 67 km. Magnitudes ranged from 0.8 to 2.8, with the majority falling between 1.0 and 2.0. The largest events included magnitudes of 2.8 at 16:16 on 12 December, 2.7 at 07:32 on 11 December and again at 20:00 on 11 December, and 2.6 at 23:35 on 11 December. Temporal distribution showed elevated rates during the first 48 hours, followed by a gradual decline, consistent with swarm behavior rather than a classic mainshock-aftershock sequence.

The swarm unfolded in south-central Alaska within the northern continuation of the Pacific-North American plate boundary. This tectonically active corridor accommodates convergence through the Aleutian megathrust and accommodates lateral motion along major strike-slip structures, most notably the Denali Fault system. The fault zone trends east-southeast through the Alaska Range and has produced large historical earthquakes, including the magnitude 7.9 event of 3 November 2002 whose rupture extended hundreds of kilometers and generated widespread aftershock activity. Shallow crustal seismicity in the Paxson vicinity reflects both the regional stress field and local fault segmentation within the broader Denali system.

Crustal structure in the area comprises accreted terranes and Mesozoic to Cenozoic sedimentary and volcanic rocks overlying a complex basement. Ongoing convergence drives north-directed compression, while the Denali Fault accommodates right-lateral shear. Seismicity rates remain elevated relative to most continental interiors, with both isolated events and episodic swarm activity occurring along secondary structures.

The 2002 swarm provides a clear illustration of swarm-type behavior in this setting: numerous events of similar magnitude without a dominant mainshock, shallow hypocenters, and rapid onset followed by decay. Such sequences are recognized throughout the Denali Fault corridor and contribute to long-term strain release along the plate boundary.

Continued monitoring by regional seismic networks supplies essential data for refining hazard assessments in south-central Alaska, where population centers and infrastructure lie within reach of both the megathrust and the Denali Fault.

References
Alaska Earthquake Center, University of Alaska Fairbanks – regional seismicity catalogs and Denali Fault studies.
U.S. Geological Survey Earthquake Hazards Program – tectonic framework of south-central Alaska and Denali earthquake sequence documentation.
Global Centroid Moment Tensor Project – focal mechanisms and regional stress orientations.