Seismic Swarm S20191209.3: Analysis of Activity Near White Mountain, Alaska

Seismic swarm S20191209.3 occurred 62 km north-northeast of White Mountain in western Alaska. The sequence began at 14:26 on 8 December 2019 and concluded at 10:35 on 9 December 2019, spanning 20 hours and 9 minutes. During this interval, 28 earthquakes were recorded, with magnitudes ranging from 1.0 to 2.7 and focal depths between 0 and 7 km.

The swarm exhibited a typical pattern of clustered, low-magnitude events without a dominant mainshock. Early activity on 8 December included events of magnitude 1.7, 1.6, 2.3, and 2.2 within the first few hours, followed by numerous smaller shocks throughout the evening. Activity intensified again on 9 December, culminating in the largest event of magnitude 2.7 at 08:04, accompanied by several events of magnitude 2.0–2.5. Depths remained consistently shallow, indicating a brittle failure zone within the upper crust.

Western Alaska, including the Seward Peninsula region surrounding White Mountain, lies within a tectonically active intraplate setting influenced by far-field stresses from the Pacific–North American plate boundary. The broader Alaskan crust experiences distributed deformation, with historical seismicity linked to reactivated faults and volcanic processes. Updated geological assessments confirm ongoing low-level activity in this area, consistent with regional strain accumulation patterns observed through modern seismic networks.

Swarm sequences such as S20191209.3 provide insight into fluid migration or stress triggering in shallow crustal volumes. The tight temporal clustering and uniform depth range suggest a localized source mechanism rather than widespread tectonic rupture. No damage or felt reports were associated with these events, reflecting their modest energy release.

This swarm aligns with the broader seismic character of Alaska, where thousands of small earthquakes occur annually. Continued monitoring supports improved characterization of background rates and potential precursors in remote regions.

References

• Alaska Earthquake Center, University of Alaska Fairbanks (updated regional seismicity reports).
• USGS Earthquake Hazards Program (tectonic framework of western Alaska).