Seismic Swarm S20021113.1 Near McKinley Park, Alaska

Seismic swarm S20021113.1 occurred in a tectonically active region of central Alaska, approximately 61 km east-southeast of McKinley Park. The sequence began at 05:18 on 12 November 2002 and concluded at 19:41 on 13 November 2002, spanning 38 hours and 23 minutes. During this interval, 71 earthquakes were recorded, with magnitudes ranging from 0.6 to 3.5 and focal depths predominantly between 0 and 22 km.

The swarm unfolded in two main phases. The initial activity on 12 November featured events clustered between magnitudes 1.1 and 3.0, with many occurring at depths of 3–9 km. Notable early shocks included a magnitude 3.0 event at 05:18 and a magnitude 2.3 event at 15:51. Activity intensified overnight, transitioning into 13 November with a peak magnitude of 3.5 recorded at 04:09 at a depth of 0 km. Subsequent events on the second day maintained a pattern of shallow to moderate depths, with several magnitude 2.0–2.6 shocks distributed across 1–11 km. The sequence tapered off by late afternoon, ending with a magnitude 1.5 event at 19:41.

This swarm is situated along the Denali Fault system, a major right-lateral strike-slip structure that accommodates much of the Pacific–North American plate motion in Alaska. The fault extends over 2,000 km and has produced repeated large earthquakes throughout the Holocene. The 2002 Denali Fault earthquake (M 7.9), which occurred on 3 November just 10 km from the swarm centroid, represents the most significant recent event in the vicinity. That mainshock triggered widespread aftershock activity and secondary faulting on adjacent structures, creating conditions conducive to subsequent swarm behavior.

Geological mapping and paleoseismic studies indicate that the Denali Fault has hosted multiple prehistoric ruptures with recurrence intervals on the order of centuries. The region’s crust is characterized by complex fault networks, including subsidiary thrusts and normal faults that respond to the regional transpressional regime. Depths recorded in the swarm align with the brittle upper crust, where most seismicity in central Alaska is concentrated above 20 km.

Since 1 January 2000, seven swarms have been documented in the broader area, with S20021113.1 being the earliest. The proximity of the swarm to the Denali mainshock suggests a possible causal link through static or dynamic stress changes, a common mechanism observed in post-large-earthquake sequences worldwide.

References
USGS Earthquake Hazards Program – Denali Fault, Alaska Earthquake (2002)
Alaska Earthquake Center – Regional Seismicity Catalog
SeismoSight internal swarm classification records