Earthquake Swarm S20050216.1: Seismic Activity Near Point MacKenzie, Alaska

An earthquake swarm designated S20050216.1 occurred 6 km east of Point MacKenzie, Alaska, beginning at 18:35 on 16 February 2005 and concluding at 06:58 on 17 February 2005. Over the 12-hour-and-23-minute period, 32 earthquakes were recorded, providing a detailed snapshot of clustered seismic energy release in the region.

The sequence initiated with a magnitude 4.8 event at 34 km depth. Subsequent activity included a magnitude 3.6 shock at 36 km depth and a magnitude 3.0 event at 35 km depth, alongside numerous smaller events ranging from magnitude 0.9 to 2.5. Depths remained consistent between 29 km and 41 km, indicating a compact source volume within the crust. Notable later events comprised a magnitude 4.2 earthquake at 36 km depth on 17 February at 01:57 and a magnitude 2.5 shock at 38 km depth on 16 February at 23:10.

This swarm represents the sole recorded instance in the Point MacKenzie area since 1 January 2000, following an earlier swarm in 2002. Such swarms typically reflect fluid migration or stress adjustments along faults without a dominant mainshock-aftershock pattern. Magnitudes declined overall after the initial larger events, consistent with swarm behavior driven by localized tectonic or hydrothermal processes.

The Point MacKenzie region lies within the Cook Inlet basin of south-central Alaska, part of the convergent margin where the Pacific Plate subducts beneath the North American Plate along the Aleutian megathrust. This tectonic setting produces frequent seismicity, with the subducting slab generating both shallow crustal events and deeper intraslab earthquakes. Historical records document elevated activity, including a magnitude 7.1 earthquake on 30 November 2018 located 1 km southeast of Point MacKenzie and approximately 11 km from the swarm centroid.

Seismic monitoring in this area benefits from dense station coverage due to proximity to Anchorage, enabling precise location and depth determination for events in the 30–40 km range. Depths observed during the 2005 swarm align with typical intraslab seismicity in the subducting Pacific Plate, where phase changes and dehydration reactions can trigger clustered activity.

Analysis of the temporal distribution shows peak rates in the first hours, followed by a gradual decay, with 23 events on 16 February and nine on 17 February. This pattern underscores the swarm’s short duration and limited energy release compared to mainshock sequences.

References

SeismoSight internal swarm classification S20050216.1 (2005).
Alaska Earthquake Center regional catalog (2000–present).
USGS tectonic framework reports for Cook Inlet.