Seismic Swarm S20021207.1: Earthquake Activity Near Cantwell, Alaska

Seismic swarm S20021207.1 occurred 42 km east-northeast of Cantwell, Alaska, within the central Alaska Range. The sequence began at 20:27 UTC on 6 December 2002 and concluded at 17:45 UTC on 27 December 2002, spanning 501 hours and 17 minutes. During this period, 548 earthquakes were recorded. This event marked the first swarm documented in the region since systematic monitoring began in 2000.

The swarm location lies along the Denali Fault system, a major right-lateral strike-slip structure that forms part of the boundary between the Pacific and North American plates. The fault accommodates significant lateral motion and has produced large earthquakes throughout the Holocene. Shallow crustal seismicity in this area commonly reflects both tectonic loading and post-seismic relaxation following the magnitude 7.9 Denali earthquake of 3 November 2002, whose rupture passed within tens of kilometers of the swarm epicenters.

Analysis of the first recorded events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.3 to 3.2, with the majority below 2.0. Depths were generally shallow, concentrated between 1 km and 10 km, although several events reached 17–23 km. Early events on 6–7 December were mostly below magnitude 1.0 and occurred at depths of 0–5 km. Activity intensified on 9–11 December, producing the largest events of the initial sequence, including magnitudes 2.5, 2.8, and 3.2. These larger shocks clustered at depths of 1–4 km, suggesting a concentration of slip within the uppermost crust.

Temporal patterns show a gradual increase in both rate and magnitude during the first five days, followed by a sustained high rate of small events. Most hypocenters remained within a compact volume, consistent with swarm behavior driven by fluid migration or localized stress perturbations rather than a single mainshock-aftershock cascade.

Since 1 January 2000, eight swarms have been identified in the broader Cantwell–Denali region, confirming that episodic swarm activity is a recurring feature of the fault zone. The 2002 sequence established the baseline for subsequent monitoring and highlighted the continued seismic productivity of the Denali Fault following the 2002 mainshock.

References

• Alaska Earthquake Center, University of Alaska Fairbanks – Regional seismicity catalog and swarm documentation.
• U.S. Geological Survey Earthquake Hazards Program – Denali Fault rupture mapping and aftershock studies (2002–2003).
• Quaternary fault and fold database of the United States – Denali Fault system description and Holocene slip rates.