Seismic Swarm S20110801.1: Analysis of Activity Near Salcha, Alaska

Seismic swarm S20110801.1 was recorded 12 km west of Salcha, Alaska, beginning at 15:36 on 31 July 2011 and concluding at 14:53 on 1 August 2011. Over 23 hours and 16 minutes, the sequence included 25 earthquakes with magnitudes ranging from 0.0 to 2.3 and focal depths between 0 and 21 km. The events clustered in two primary phases, with an initial sparse set on 31 July followed by a more concentrated series in the early morning hours of 1 August, including the largest event of magnitude 2.3 at 06:13:25.

This swarm exhibited classic swarm characteristics, featuring numerous small-magnitude events without a dominant mainshock-aftershock sequence. Most events registered below magnitude 0.5, with only four exceeding magnitude 1.0. Depths remained predominantly shallow, concentrated between 1 and 7 km, consistent with activity in the upper crust. The temporal distribution showed a rapid increase in frequency around 03:00–06:00 on 1 August, followed by a decline toward the swarm's termination.

The Salcha region lies within interior Alaska's seismically active zone, part of the broader North American plate margin influenced by Pacific plate subduction and strike-slip faulting. The area belongs to the Yukon-Tanana terrane, where crustal deformation occurs along regional fault systems such as the nearby Tintina and Denali faults. Historical seismicity in the Fairbanks-Salcha corridor includes frequent small earthquakes and occasional swarms, reflecting ongoing tectonic strain accumulation. Updated assessments from regional monitoring confirm that interior Alaska experiences hundreds of events annually, predominantly at depths under 20 km, with swarms often linked to fluid migration or minor fault adjustments rather than large-scale rupture.

Geological records indicate that the region has hosted similar low-magnitude clusters in the past, contributing to the gradual release of strain without producing destructive shaking. The 2011 swarm aligns with this pattern, posing no significant hazard due to its limited energy release and shallow but contained depths.

References

• Alaska Earthquake Center, University of Alaska Fairbanks (updated regional seismicity reports through 2023).
• USGS Earthquake Hazards Program, Denali Fault and interior Alaska tectonic summaries.