A notable seismic swarm designated PS20050117.1 occurred in the western Pacific Ocean, centered approximately 142 km north-northeast of Fais Island in the Federated States of Micronesia. The sequence began at 20:17 UTC on 16 January 2005 and concluded at 10:50 UTC on 17 January 2005, spanning 14 hours and 32 minutes. During this interval, seven earthquakes were recorded, with magnitudes ranging from 4.1 to 6.6 and focal depths between 10 km and 60 km.
The event initiated with two closely spaced moderate-to-large shocks: a magnitude 6.6 earthquake at 20:17:52 on 16 January at 24 km depth, followed immediately by a magnitude 6.5 event at 20:17:55 at 33 km depth. Subsequent activity included a magnitude 4.1 shock at 05:28 on 17 January (30 km depth), a magnitude 5.0 at 05:43:53 (10 km depth), and a magnitude 5.1 at 05:44:02 (60 km depth). The swarm concluded with a magnitude 6.1 event at 10:50:32 (12 km depth) and a magnitude 5.9 aftershock at 10:50:39 (40 km depth). This temporal clustering of events without a single dominant mainshock is characteristic of swarm behavior in tectonically active subduction settings.
The region lies along the Caroline Islands chain, situated near the boundary between the Pacific Plate and the Philippine Sea Plate. The Yap Trench, a convergent margin where the Pacific Plate subducts westward beneath the Philippine Sea Plate, drives the area's seismicity. This tectonic interaction produces intermediate-depth earthquakes and occasional shallow crustal events, consistent with the observed depth range of the swarm. Fais Island itself rests on a volcanic edifice associated with the Caroline hotspot track, though the swarm epicenters align more closely with the trench-related deformation zone to the north.
Historically, the western Caroline Islands have experienced recurrent seismic activity due to ongoing plate convergence at rates of approximately 6–8 cm per year. Instrumental records document multiple magnitude 6+ events in the vicinity since the mid-20th century, often occurring in clusters that reflect stress transfer along the subduction interface. The 2005 swarm fits this pattern, highlighting the persistent seismic hazard in this remote segment of the Pacific Ring of Fire.
No significant surface rupture or tsunami generation was associated with these events, owing to their moderate magnitudes and offshore locations. Regional monitoring networks, including those operated by the United States Geological Survey, continue to track such activity to refine understanding of subduction zone dynamics in Micronesia.
USGS Earthquake Catalog (earthquake.usgs.gov)
Pacific Plate boundary studies, Geological Society of America
Federated States of Micronesia seismic hazard assessments