Seismic Swarm S20021106.1 Near Mentasta Lake, Alaska: Analysis and Regional Context

The seismic swarm designated S20021106.1 occurred in a tectonically active region of interior Alaska, approximately 17 km west-northwest of Mentasta Lake. This sequence began at 03:26 on 5 November 2002 and concluded at 23:23 on 28 November 2002, spanning 571 hours and 56 minutes. During this period, 352 earthquakes were recorded, with the first 100 events providing key insights into the swarm's characteristics. Analysis of the initial events reveals predominantly shallow focal depths, typically between 0 and 5 km, with occasional events reaching 15–16 km. Magnitudes ranged from 1.0 to 3.9, indicating low-to-moderate energy release consistent with swarm behavior rather than a mainshock-aftershock sequence. Notable early events included a magnitude 3.5 quake at 1 km depth on 5 November at 03:26, followed by a 3.7 event at 2 km depth later that day. Subsequent activity showed clustering, with several magnitude 3+ events on 6 and 8 November, including a peak magnitude 3.9 at 2 km depth. Depths remained shallow overall, suggesting activity within the upper crust along local fault structures. This swarm represents the first of three documented seismic swarms in the region since 1 January 2000, highlighting episodic seismicity in an area influenced by the broader tectonic framework of southern Alaska. The Mentasta Lake vicinity lies within the Eastern Alaska Range, where ongoing deformation results from the interaction between the Pacific and North American plates. The Denali Fault, a major right-lateral strike-slip system, traverses the region and accommodates significant lateral motion. Historical seismic records show that the area experienced heightened activity following the magnitude 7.9 Denali Fault earthquake of 3 November 2002, which ruptured segments of the fault system nearby. Geological studies of the Denali Fault indicate a long-term slip rate of several millimeters per year, with paleoseismic evidence of recurrent large earthquakes over Holocene timescales. The 2002 mainshock triggered widespread aftershocks and secondary sequences across central Alaska, consistent with the timing and location of swarm S20021106.1. Shallow seismicity in this setting often reflects brittle failure in the upper 10–15 km of crust, influenced by both tectonic loading and possible fluid migration or stress transfer. The swarm's temporal distribution—intense initial activity tapering over three weeks—aligns with patterns observed in other intraplate or fault-related swarms worldwide. Most events exhibited magnitudes below 3.0, underscoring the swarm's role as a low-level release of accumulated strain rather than a precursor to larger rupture.

References

• United States Geological Survey Earthquake Catalog (USGS)
• Alaska Earthquake Center Regional Seismicity Reports
• Geological Society of America publications on Denali Fault tectonics