Seismic Swarm in the Maug Islands Region, Northern Mariana Islands (November 2023)

A seismic swarm designated S20231125.1 occurred in the Maug Islands region of the Northern Mariana Islands from 09:32 UTC on 24 November 2023 until 20:40 UTC on 27 November 2023. The sequence lasted 83 hours and 7 minutes and comprised 45 earthquakes. All events were recorded at shallow depths, predominantly 10 km, with magnitudes ranging from 3.9 to 4.9. The largest events reached magnitude 4.9 on 24 November at 14:11 and again on 25 November at 20:54.

The swarm began with a rapid succession of events on 24 November, including multiple magnitude 4.6–4.8 shocks within the first several hours. Activity continued at a steady rate through 25 and 26 November before tapering on 27 November. No single dominant mainshock was evident; instead, the sequence displayed the characteristic clustering of moderate-magnitude events typical of swarm behavior.

The Maug Islands lie within the Mariana volcanic arc, formed by the subduction of the Pacific Plate beneath the Philippine Sea Plate at the Mariana Trench. This tectonic setting produces frequent shallow seismicity and volcanic activity along the arc. The islands themselves are the emergent remnants of a submerged volcanic caldera. Historical records indicate that the region experiences recurrent earthquake swarms driven by fluid migration and stress transfer within the overriding plate and along the subduction interface.

Since 1 January 2000, only one swarm has been documented in the Maug Islands area according to SeismoSight internal classification, with the present sequence representing that event. Such swarms are consistent with the broader seismotectonic regime of the Northern Mariana Islands, where moderate-magnitude earthquakes occur regularly but rarely produce damaging ground motion at the surface owing to their modest size and offshore location.

The November 2023 swarm provides additional data on the frequency and style of clustered seismicity in this segment of the Mariana arc. Continued monitoring will help refine understanding of stress accumulation and release patterns along this active convergent margin.

References
SeismoSight internal swarm catalog S20231125.1
USGS Earthquake Hazards Program – Mariana Islands regional tectonics
Global Volcanism Program – Maug Islands volcanic history