Seismic Swarm S20030101.1: Analysis of Activity Near McKinley Park, Alaska

Seismic swarm S20030101.1 occurred 89 km east-southeast of McKinley Park, Alaska, beginning at 02:48 on 1 January 2003 and concluding at 04:19 on 3 January 2003. Over 49 hours and 30 minutes, the sequence produced 49 earthquakes, with magnitudes ranging from 0.7 to 2.5 and focal depths predominantly between 0 and 18 km. The events clustered tightly in time and space, characteristic of swarm behavior rather than a typical mainshock-aftershock sequence.

The swarm initiated with two events above magnitude 2.0 within the first three minutes, followed by a sustained series of smaller shocks throughout 1 January. Activity continued at a moderate rate on 2 January before tapering on 3 January. Depths remained shallow for the majority of events, suggesting rupture within the upper crust. This pattern aligns with fluid migration or stress redistribution along pre-existing faults in a tectonically active region.

The location lies within the broader influence of the Denali Fault system, a major right-lateral strike-slip structure extending across central Alaska. The fault accommodates lateral motion between the Pacific and North American plates, contributing to the region's high seismicity. Historical records indicate that the first swarm in the local catalog since 2000 occurred in 2002, with 13 additional swarms documented through subsequent years. The 2002 Denali Fault earthquake (Mw 7.9) produced widespread surface rupture and aftershock sequences that may have influenced subsequent clustered activity in the area.

Geological mapping shows the swarm epicenters near segments of the Denali Fault and subsidiary structures within the eastern Alaska Range. The crust here consists of accreted terranes intruded by Mesozoic and Cenozoic plutons, with ongoing deformation driven by oblique convergence. Shallow focal depths observed in the swarm are consistent with brittle failure in the seismogenic zone above approximately 15–20 km.

Analysis of the magnitude-time distribution reveals no single dominant event exceeding magnitude 2.5, supporting classification as a swarm. Most events occurred at depths of 1–8 km, with occasional deeper outliers up to 18 km. Such shallow clustering often indicates involvement of hydrothermal fluids or aseismic slip transients that promote small, frequent earthquakes without a clear triggering mainshock.

This sequence provides a useful case for examining swarm dynamics in strike-slip regimes. Continued monitoring by regional networks helps distinguish these events from background seismicity and assess any potential links to larger fault segments.

References

USGS Earthquake Hazards Program – Denali Fault overview and regional seismicity reports.
Alaska Earthquake Center – Catalog of central Alaska earthquakes since 2000.
SeismoSight internal swarm classification database.