Analysis of Earthquake Swarm S20021201.1 in Central Alaska

Earthquake swarm S20021201.1 occurred 85 km west-northwest of Paxson, Alaska. The sequence began at 23:17 on 30 November 2002 and concluded at 02:37 on 7 December 2002, spanning 147 hours and 20 minutes. During this period, 158 earthquakes were recorded.

The swarm unfolded in a tectonically active region of central Alaska shaped by ongoing convergence between the Pacific and North American plates. The Denali Fault system dominates local deformation, producing strike-slip motion that accommodates much of the lateral component of plate motion. Shallow crustal faulting prevails in the area, consistent with the predominantly low-depth events observed in the swarm.

Analysis of the first 100 recorded events reveals a compact magnitude range, with values spanning 0.7 to 3.3. The largest event measured 3.3 on 1 December at 17:59. Depths clustered between 0 and 19 km, with the majority occurring at 5 km or less, indicating activity within the upper brittle crust. Temporal distribution showed elevated rates in the first 48 hours, followed by a gradual decline, typical of swarm behavior driven by fluid migration or aseismic slip rather than a single mainshock-aftershock cascade.

No events exceeded magnitude 3.3 within the examined subset, underscoring the low-energy character of the sequence. Depth variations remained modest, suggesting a localized source volume without significant downward migration.

Historical records indicate that eight swarms have occurred in the region since 1 January 2000. Swarm S20021201.1 represents the earliest documented episode in this catalog.

The combination of shallow focal depths, modest magnitudes, and clustered timing aligns with known swarm patterns along the Denali Fault corridor. Such sequences contribute to long-term strain release without producing damaging ground motion.

References SeismoSight internal swarm classification records USGS Earthquake Catalog (Alaska region) Alaska Earthquake Center tectonic summaries