Seismic Swarm PS20040904.1: Tonga Subduction Zone Activity in September 2004

A notable seismic swarm designated PS20040904.1 occurred near the northern Tonga Islands between 12:16 on 3 September 2004 and 20:24 on 4 September 2004. The sequence was centered 77 km east-northeast of Hihifo on Niuatoputapu Island and comprised six earthquakes recorded over 32 hours and seven minutes. Events ranged in magnitude from 4.5 to 6.2, with most hypocenters located at shallow depths of 10 km.

The swarm began with a magnitude 5.6 event at 12:16 on 3 September, followed by a magnitude 5.3 shock at 14:53 and the largest event, magnitude 6.2, at 19:04 the same day. Subsequent activity included a magnitude 5.6 earthquake at 23:18, a magnitude 4.5 event at 05:01 on 4 September, and a final magnitude 5.7 shock at 20:24 that marked the end of the sequence. All but one event occurred at 10 km depth, consistent with shallow crustal or plate-interface faulting.

Tonga lies along the convergent boundary where the Pacific Plate subducts westward beneath the Tonga microplate at rates exceeding 15 cm per year, forming the Tonga Trench. This tectonic setting produces frequent shallow to intermediate-depth earthquakes and volcanic activity across the region. Historical records document repeated seismic swarms and mainshock-aftershock sequences near the northern islands, reflecting episodic stress release along the megathrust and subsidiary faults. The 2004 swarm fits this pattern, likely representing a brief period of accelerated slip on the subduction interface without generating a damaging tsunami.

Modern monitoring by regional and global networks confirms that shallow swarms in northern Tonga commonly cluster near the trench axis and exhibit rapid onset and decay, as observed in this sequence. Depths around 10 km align with the expected location of the plate boundary in this sector, while the single deeper event at 24 km may indicate minor activity within the downgoing slab or overlying crust.

No significant surface deformation or volcanic response was associated with the swarm, underscoring its limited overall energy release compared with great subduction earthquakes elsewhere in the Tonga-Kermadec system. Such episodes nevertheless contribute to long-term strain accumulation and release along one of Earth’s fastest-moving plate boundaries.

References

USGS Earthquake Catalog
Global CMT Project
Tonga Geological Survey reports on regional tectonics