Seismic Swarm in Central Alaska: February 2017 Event Analysis

Central Alaska lies within a tectonically active zone shaped by the ongoing subduction of the Pacific Plate beneath the North American Plate along the Aleutian Megathrust. This convergent margin drives regional deformation, with additional influence from the right-lateral Denali Fault system that accommodates lateral plate motion. The resulting crustal stresses produce frequent seismicity, including both mainshock-aftershock sequences and episodic earthquake swarms. Depths of events in this region commonly range from the near-surface to approximately 15 km, reflecting brittle failure within the upper crust.

On 13 February 2017, a seismic swarm designated S20170213.1 was recorded in central Alaska. The sequence began at 07:17 and concluded at 21:50, encompassing 24 earthquakes over 14 hours and 33 minutes. The largest event reached magnitude 5.3 at a depth of 8 km. Subsequent activity consisted of smaller-magnitude earthquakes between 0.6 and 1.4, with focal depths predominantly between 6 km and 12 km. The temporal distribution showed the highest rate of events in the first several hours, followed by a gradual decline, consistent with swarm behavior driven by fluid migration or localized stress perturbations rather than a single dominant rupture.

Such swarms remain uncommon in the instrumental record for this area. Since 1 January 2000, only three swarms have been identified in central Alaska, occurring in 2002 (two events) and 2004 (one event). These earlier episodes similarly featured modest magnitudes and short durations, underscoring the limited frequency of swarm-type activity compared with typical aftershock sequences following larger regional earthquakes.

The 2017 swarm did not produce reported surface rupture or significant ground deformation, aligning with the modest energy release and shallow-to-mid crustal depths involved. Monitoring by regional seismic networks continues to track background rates, providing context for interpreting future clusters in an area where the interplay between the subduction interface and crustal faults sustains persistent, if variable, seismicity.

References

USGS Earthquake Hazards Program – Alaska Seismicity Alaska Earthquake Center – Regional Tectonic Summary Incorporated Research Institutions for Seismology (IRIS) – Crustal Structure of Alaska