Seismic Swarm Near Guam: Insights into Mariana Subduction Zone Activity

On March 12, 2015, a seismic swarm designated PS20150312.1 was recorded approximately 58 km east-southeast of Inarajan Village, Guam. The sequence began at 06:03 UTC and concluded at 07:49 UTC, encompassing five earthquakes within a span of one hour and 46 minutes. Event parameters included a magnitude 5.1 shock at 23 km depth at 06:03:40, followed by another magnitude 5.1 event at 10 km depth at 06:05:11. Subsequent activity featured a magnitude 5.0 earthquake at 42 km depth at 06:28:10, a magnitude 4.7 event at 44 km depth at 07:20:55, and a magnitude 5.3 shock at 14 km depth at 07:49:59. These closely spaced occurrences highlight typical swarm behavior in tectonically active margins, where multiple events cluster without a dominant mainshock. The Guam region lies within the Mariana Island Arc, formed by the subduction of the Pacific Plate beneath the Philippine Sea Plate along the Mariana Trench. This convergent boundary generates frequent seismicity as the downgoing slab releases stress through brittle failure at varying crustal depths. Depths recorded in the 2015 swarm, ranging from 10 km to 44 km, align with intermediate-depth activity within the subducting slab. The arc's volcanic underpinnings and associated fault systems contribute to elevated earthquake rates, with the overriding plate experiencing both compressional and extensional regimes. Historical records indicate persistent seismic hazard in this sector. Since 2000, notable events include a magnitude 7.1 earthquake on April 26, 2002, located 20 km south-southwest of Merizo Village, approximately 88 km from the swarm epicenter, and a magnitude 7.0 event on October 12, 2001, situated 69 km south-southeast of Inarajan Village, about 68 km from the 2015 cluster. These larger shocks underscore the potential for significant energy release along the same plate interface, often accompanied by aftershock sequences that can persist for weeks. Such swarms provide valuable data for understanding stress migration along the subduction zone. The 2015 activity, confined to a brief interval and moderate magnitudes, likely reflects localized fluid migration or aseismic slip triggering brittle failure. Ongoing monitoring by regional networks aids in distinguishing swarm patterns from foreshock sequences that might precede larger events.

References

USGS Earthquake Hazards Program catalog (earthquake.usgs.gov) Mariana Trench tectonics overview, NOAA Ocean Exploration Seismicity of the Mariana Arc, Bulletin of the Seismological Society of America (various volumes)