Analysis of Earthquake Swarm S20031210.1 in Central Alaska

Earthquake swarm S20031210.1 was recorded in a remote area 89 km east of Cantwell, Alaska. The sequence began at 14:02 on 9 December 2003 and concluded at 10:33 on 20 December 2003, spanning 260 hours and 31 minutes. During this interval, 147 earthquakes were detected, providing a detailed record of clustered seismic activity in the central Alaska Range.

The first 100 events revealed predominantly low-magnitude earthquakes, with the largest reaching 3.4 on 11 December at a depth of 7 km. Magnitudes clustered between 1.0 and 2.1, while depths remained shallow, typically between 0 and 18 km. Early activity on 9 December included events of 1.1 to 2.0 magnitude at depths of 1 to 18 km. Subsequent days showed continued low-level energy release, punctuated by occasional peaks such as the 3.0 event on 11 December at 5 km depth and the 3.3 event on 13 December at 3 km depth. Depths averaged around 6–7 km, indicating activity within the upper crust. The swarm exhibited a gradual decline in frequency after mid-December, consistent with aftershock-like decay following initial bursts.

This swarm occurred within a tectonically active corridor shaped by the interaction between the Pacific and North American plates. The location lies near the Denali Fault, a major right-lateral strike-slip structure that accommodates much of the lateral motion resulting from oblique subduction along the Aleutian megathrust. Central Alaska experiences elevated seismicity due to this plate boundary dynamics, with the Denali Fault system serving as a primary conduit for strain release. Historical records indicate that the region has hosted multiple similar swarms since 2000, totaling 19 episodes. Earlier swarms occurred in 2002 (13 events) and earlier in 2003 (6 events), underscoring recurrent clustered activity along this segment of the fault zone.

The 2002 magnitude 7.9 Denali Fault earthquake, located nearby, likely influenced local stress conditions, potentially contributing to the elevated swarm frequency observed in subsequent years. Depths recorded during S20031210.1 align with typical rupture zones along the Denali Fault, where brittle failure occurs in the uppermost 20 km of crust. Such swarms often reflect fluid migration or aseismic slip transients rather than mainshock-aftershock sequences driven by a single large event.

Continued monitoring of this area remains essential given its proximity to critical infrastructure corridors and the broader seismic hazard posed by the Denali Fault system. The characteristics of S20031210.1 illustrate the value of dense seismic networks in capturing subtle patterns of strain accumulation in interior Alaska.

References

SeismoSight internal classification data for swarm S20031210.1 parameters and event catalog.
USGS Earthquake Hazards Program reports on Denali Fault tectonics and regional seismicity.