Seismic Swarm S20021218.1 Near McKinley Park, Alaska

A seismic swarm designated S20021218.1 was recorded 81 km east-southeast of McKinley Park, Alaska, between 00:10 on 18 December 2002 and 17:03 on 19 December 2002. The sequence lasted 40 hours and 52 minutes and comprised 36 earthquakes with magnitudes ranging from 0.9 to 2.3. Depths were predominantly shallow, between 0 and 10 km, consistent with activity along upper-crustal fault structures in the region.

The swarm exhibited typical characteristics of clustered microseismicity, with events distributed throughout the 40-hour window and no single dominant mainshock. Peak magnitudes of 2.3 occurred twice, at 04:13 on 18 December and 23:30 on the same day. Most events registered below magnitude 2.0, reflecting low-energy release distributed across numerous small ruptures rather than a single large failure.

The location lies within the Alaska Range, where ongoing convergence between the Pacific and North American plates drives regional deformation. The Denali Fault, a major right-lateral strike-slip system, traverses the area and accommodates much of the lateral motion. Historical large events, including the magnitude 7.9 Denali earthquake of 3 November 2002, demonstrate the fault’s capacity for significant slip and widespread aftershock sequences.

Seismic swarms have occurred repeatedly in this sector since 2000. Ten such swarms have been identified in the catalog through the present, with the earliest documented example being the December 2002 sequence. These clusters typically involve shallow events and short durations, suggesting fluid migration or localized stress adjustments along subsidiary faults rather than primary plate-boundary rupture.

The tectonic setting features a combination of thrust and strike-slip faulting. Crustal shortening across the Alaska Range produces north-vergent folds and faults superimposed on the dominant Denali system. Earthquake depths rarely exceed 15–20 km, indicating a relatively thin seismogenic zone above the deeper subduction interface farther south.

Monitoring by regional networks provides continuous detection of events above magnitude 1.0, enabling detailed characterization of swarm statistics. The 2002 sequence remains notable as the first recorded swarm in the modern catalog for this specific source area and offers a baseline for comparing subsequent activity.

References

• USGS Earthquake Catalog (ANSS Comprehensive Catalog)
• Alaska Earthquake Center regional seismicity reports
• Geological Survey of Canada and USGS tectonic framework maps for the Denali Fault system