On 12 April 2016, an earthquake swarm designated PS20160412.1 was recorded approximately 183 km north-northeast of Fais in the Federated States of Micronesia. The sequence began at 11:24 UTC and concluded at 23:27 UTC, encompassing five events over a span of roughly twelve hours. Magnitudes ranged from 4.9 to 5.4, with most events occurring at shallow depths of 10 km; one deeper shock reached 60 km.
The first two events struck nearly simultaneously at 11:24:50 and 11:24:56 UTC, each registering magnitude 5.3. A magnitude 5.4 earthquake followed at 14:51:47 UTC. The final pair occurred late in the sequence, with magnitudes 5.1 at 23:22:58 UTC and 4.9 at 23:27:49 UTC. All but one event were located at 10 km depth, indicating predominantly shallow crustal activity typical of the region’s tectonic setting.
Fais lies within the western Pacific’s Caroline Islands, situated near the boundary between the Pacific Plate and the Philippine Sea Plate. The area experiences ongoing deformation driven by subduction along the Yap Trench to the east and broader convergence associated with the Mariana system. Shallow seismicity frequently reflects normal or strike-slip faulting within the overriding plate or along transform segments, while deeper events may relate to slab dynamics. Historical records show that such swarms remain uncommon; only two comparable clusters have been documented since 2000, occurring in 2005 and 2010.
The 2016 swarm’s brief duration and moderate magnitudes align with patterns observed in other intraplate Pacific settings, where fluid migration or localized stress adjustments can trigger rapid sequences without producing a dominant mainshock. No significant damage or tsunami was reported, consistent with the events’ offshore location and limited energy release.
Geological monitoring in Micronesia benefits from regional networks operated by the United States Geological Survey and cooperating Pacific observatories, which provide essential data for understanding long-term seismic hazards in this remote portion of the Ring of Fire. Continued observation will help refine models of plate-boundary interaction and intraplate deformation around the Caroline chain.
United States Geological Survey Earthquake Catalog
Pacific Tsunami Warning Center historical bulletins
SeismoSight internal swarm classification records