Analysis of Earthquake Swarm S20111209.1 near Glennallen, Alaska

Earthquake swarm S20111209.1 occurred in south-central Alaska, centered 16 km NNW of Glennallen. The sequence began at 01:25 on 9 December 2011 and concluded at 05:25 on 12 December 2011, spanning 75 hours and 59 minutes. During this interval, 57 earthquakes were recorded.

The swarm initiated with a magnitude 4.5 event at a depth of 16 km. Subsequent activity consisted predominantly of smaller events, with magnitudes ranging from 0.8 to 2.3. Notable later shocks included a magnitude 2.3 event at 18 km depth on 12 December at 02:24 and a magnitude 1.9 event at 20 km depth earlier that day. Depths throughout the swarm clustered between 9 km and 28 km, with the majority occurring between 15 km and 25 km. The temporal distribution showed higher frequency in the first 24 hours, followed by a gradual decline.

This swarm represents the second documented episode in the region since 2000. The prior swarm took place in 2002, indicating relatively infrequent clustered seismicity in the immediate area.

The Glennallen region lies within the tectonically active margin of south-central Alaska, where ongoing convergence between the Pacific and North American plates drives regional deformation. The Copper River Basin and adjacent Wrangell Mountains overlie a complex transition zone influenced by both subduction-related stresses and strike-slip faulting associated with the Denali Fault system. Crustal earthquakes in this setting commonly occur at depths of 10–30 km, consistent with the observed swarm depths.

Seismicity in south-central Alaska reflects long-term plate-boundary dynamics that have produced both moderate swarms and larger independent events. The 2011 sequence remained below magnitudes capable of widespread damage, yet it illustrates the persistent background of clustered microseismicity that characterizes the area.

References
SeismoSight internal swarm classification S20111209.1
USGS Earthquake Catalog (regional tectonics summary)
Alaska Earthquake Center (historical context for south-central Alaska)