Seismic Activity in the Pagan Region of the Northern Mariana Islands

The Pagan region lies within the Northern Mariana Islands, an arc of volcanic islands formed by the subduction of the Pacific Plate beneath the Philippine Sea Plate along the Mariana Trench. This tectonic setting produces frequent intermediate-depth earthquakes, with hypocenters commonly occurring between 100 and 300 kilometers. The region’s geology features active stratovolcanoes, including Pagan Island itself, which has erupted multiple times in the Holocene and remains a locus of volcanic and seismic unrest. On 29 July 2016 at 21:18 UTC, a magnitude 7.7 earthquake struck the Pagan region at a depth of 196 kilometers. The event originated within the subducting slab, consistent with the region’s history of intraslab seismicity. Two closely spaced reports list the same mainshock, one placing the epicenter 4 km from Pagan and the other directly beneath the island, reflecting typical uncertainties in offshore locations. A prior significant event occurred on 31 October 2007, when a magnitude 7.2 earthquake was recorded 41 km from Pagan at intermediate depth. Both the 2007 and 2016 shocks illustrate the persistent seismic hazard posed by the steeply dipping slab beneath the Mariana arc. No surface rupture or tsunami was generated by the 2016 event owing to its depth and focal mechanism. The Mariana subduction zone has produced several magnitude 7+ earthquakes since 2000, underscoring its status as one of the most seismically active margins on Earth. Continued monitoring by regional seismic networks remains essential for understanding slab dynamics and mitigating risks to the sparsely populated islands.

References

USGS Earthquake Catalog (events 2016-07-29 M7.7 and 2007-10-31 M7.2)