Seismic Swarm S20021220.1: Analysis of Activity Near Mentasta Lake, Alaska

Seismic swarm S20021220.1 was recorded in a remote area of eastern Alaska, centered 34 km west-northwest of Mentasta Lake. The sequence began at 10:20 on 19 December 2002 and concluded at 08:20 on 6 January 2003. Over 429 hours and 59 minutes, the swarm produced 205 earthquakes, providing a clear example of clustered seismic activity in a tectonically active region.

The first 100 events exhibited predominantly low magnitudes, with values ranging from 0.5 to 3.7. Depths were shallow, mostly between 0 and 14 km, and a small number reached 19 km. Timing showed an initial burst of activity on 19 December followed by sustained but variable rates through late December. Notable events included a magnitude 3.7 quake at 3 km depth on 23 December and a magnitude 3.5 event at 1 km depth on 24 December. The distribution indicates a compact source volume with limited migration, consistent with fluid-driven or stress-triggered mechanisms common in swarm sequences.

Mentasta Lake lies within the eastern Alaska Range, where ongoing convergence between the Pacific and North American plates drives regional deformation. The Denali Fault system, a major right-lateral strike-slip structure, dominates local tectonics and has produced large earthquakes historically. The 2002 Denali Fault earthquake (Mw 7.9) occurred weeks earlier approximately 150 km to the southwest, altering regional stress fields and potentially influencing subsequent swarm activity. Shallow crustal earthquakes in this area typically reflect slip along subsidiary faults or distributed deformation within the fault zone.

Since 1 January 2000, only two swarms have been identified in the broader region, with S20021220.1 representing the first. This low frequency underscores the episodic nature of swarm behavior compared with aftershock sequences following major events. Depths recorded during the swarm align with the brittle upper crust, where temperatures permit brittle failure above approximately 15–20 km.

Continued monitoring of such sequences contributes to refined hazard assessments for nearby communities and infrastructure, including the Trans-Alaska Pipeline corridor. The compact spatiotemporal pattern of S20021220.1 offers a useful dataset for studying precursory or triggered seismicity in strike-slip settings.

References

• Alaska Earthquake Center, University of Alaska Fairbanks – regional seismicity catalog
• U.S. Geological Survey Earthquake Hazards Program – Denali Fault tectonic framework
• SeismoSight internal swarm classification records