Seismic Activity in the Pagan Region of the Northern Mariana Islands

The Pagan region in the Northern Mariana Islands lies along the Mariana subduction zone, where the Pacific Plate descends beneath the Philippine Sea Plate. This tectonic setting produces frequent intermediate-depth earthquakes and drives volcanism across the island arc. Pagan itself is an active stratovolcano whose eruptive history reflects ongoing subduction-related magmatism.

On 29 July 2016, a short-lived earthquake swarm was recorded in the Pagan region. The sequence began at 21:18 UTC and concluded by 21:39 UTC, encompassing five events within roughly twenty minutes. Magnitudes ranged from 4.8 to 7.7, with focal depths between 187 km and 267 km. The two largest events, both registering magnitude 7.7, occurred within seconds of each other at depths of 196 km and 267 km. Subsequent shocks measured 5.6, 5.3, and 4.8 at depths of 187 km, 206 km, and 197 km, respectively.

Such swarms are uncommon in the instrumental record for this segment of the arc. Available data indicate only one prior swarm since 2000, which took place in 2001. The 2016 sequence therefore represents a notable clustering of intermediate-depth seismicity.

In addition to the swarm, the Pagan region has experienced several strong earthquakes since the beginning of 2000. Two magnitude-7.7 events occurred on 29 July 2016 within 21 km of the swarm centroid. An earlier magnitude-7.2 earthquake struck on 31 October 2007 approximately 34 km from the same location. These events underscore the persistent seismic hazard associated with the steeply dipping Wadati-Benioff zone beneath the northern Mariana Islands.

The depths of the 2016 events place them well within the subducting slab, consistent with the typical distribution of seismicity in this arc segment. Intermediate-depth earthquakes in the Mariana system often occur in clusters that may reflect dehydration embrittlement or phase changes within the descending lithosphere.

Continued monitoring of the Pagan region remains important for understanding both seismic and volcanic processes along this active plate boundary. The 2016 swarm provides a compact example of how brief, high-magnitude sequences can punctuate the background seismicity of the northern Mariana Islands.

References
SeismoSight internal swarm classification PS20160729.1
USGS Earthquake Catalog (events 2016-07-29 and 2007-10-31)
Geological framework derived from published studies of the Izu-Bonin-Mariana subduction system.