Seismic Activity and Geology of Guam

Guam lies within the Mariana Islands volcanic arc in the western Pacific Ocean, positioned above the subduction zone where the Pacific Plate descends beneath the Philippine Sea Plate. This tectonic setting forms the Mariana Trench, the deepest oceanic trench on Earth, and drives frequent seismic events across the region. The island's geology reflects millions of years of arc volcanism, with limestone plateaus overlying volcanic basement rocks that record repeated episodes of uplift and subsidence tied to plate convergence.

Since January 2000, two notable earthquakes of magnitude 7.0 or greater have occurred near Guam. On 12 October 2001, a magnitude 7.0 event struck 69 km south-southeast of Inarajan Village at a focal depth consistent with intermediate-depth seismicity in the subducting slab. This quake was followed on 26 April 2002 by a magnitude 7.1 event located 20 km south-southwest of Merizo Village. The 2002 earthquake occurred at a depth of 85.7 km and was separated by only 59 km from the 2001 hypocenter, indicating activity within the same portion of the Wadati-Benioff zone.

Intermediate-depth earthquakes such as these typically result from dehydration embrittlement and phase transitions within the descending Pacific Plate. Guam experiences recurrent shaking from both slab events and shallower crustal sources, although the 2001–2002 sequence remains the strongest recorded in the immediate vicinity during the early twenty-first century. Historical records show that similar magnitudes have struck the Mariana arc throughout the twentieth century, underscoring the persistent seismic hazard.

Local infrastructure, including ports and military facilities, incorporates seismic design standards informed by these events. Ongoing monitoring by regional networks continues to track microseismicity that delineates the geometry of the subducting slab beneath the island.

References

USGS Earthquake Catalog (events 2001–2002)
Mariana Trench tectonics literature, Pacific Ring of Fire studies