Seismic Swarm PS20170717.1: Analysis of the July 2017 Western Aleutian Events

The seismic swarm PS20170717.1 occurred in a tectonically active segment of the western Aleutian Islands region, approximately 244 km west-northwest of Attu Station, Alaska. This sequence unfolded over 14 hours and 1 minute, beginning at 11:05 UTC on 17 July 2017 and concluding at 01:06 UTC on 18 July 2017. Eight earthquakes were recorded, with magnitudes ranging from 4.9 to 7.7 and focal depths consistently near 10 km.

The sequence initiated with a magnitude 6.3 event at 11:05:09 UTC on 17 July, followed shortly by a magnitude 5.1 shock at 11:23:01 UTC. Activity remained low until the mainshock, a magnitude 7.7 earthquake at 23:34:13 UTC on the same day. Subsequent aftershocks included events of magnitudes 4.9, 5.1, 5.2, and two magnitude 5.0 earthquakes, all at similar depths around 10 km. The swarm's tight temporal clustering and uniform depths indicate a localized release of stress along a fault segment within the subduction interface.

Geologically, the swarm region lies along the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–8 cm per year. This convergent margin forms the Aleutian Trench and the associated volcanic arc extending from Kamchatka through the Commander Islands to the Aleutian chain. The area's high seismicity stems from both megathrust earthquakes on the plate interface and shallower crustal faulting in the overriding plate. The Commander Islands (Komandorskiye Ostrova), located nearby, mark a transitional zone where subduction transitions to strike-slip motion along the Bering Fracture Zone.

Historical records document recurrent large-magnitude events in this sector. Notable examples include the 1965 magnitude 8.7 Rat Islands earthquake and multiple magnitude 7+ events throughout the 20th century. Since 2000, the most significant nearby shock was the magnitude 7.7 earthquake on 17 July 2017 itself, centered 72 km from the swarm's primary location in the Komandorskiye Ostrova region. This event aligns with the swarm's mainshock parameters and underscores the persistent seismic hazard.

The 2017 swarm likely represents triggered afterslip or secondary fault activation following the mainshock, a common pattern in subduction-zone sequences. Depths near 10 km place the activity within the seismogenic zone of the megathrust, where locked patches can generate substantial moment release. Updated monitoring by regional networks confirms ongoing low-level background seismicity in the western Aleutians, consistent with the long-term strain accumulation along this plate boundary.

Such swarms provide critical data for refining seismic hazard models in remote, sparsely instrumented areas. Continued observation aids in distinguishing between independent mainshock-aftershock sequences and true swarm behavior driven by fluid migration or aseismic slip.

References

USGS Earthquake Catalog (events 2017) Alaska Earthquake Center regional reports SeismoSight internal swarm classification PS20170717.1