Seismic Swarm PS20011009.1: Geological Context in the Tonga Subduction Zone

The seismic swarm designated PS20011009.1 occurred on 9 October 2001, approximately 212 km south-southwest of ‘Ohonua, Tonga. The sequence began at 17:45 UTC and concluded at 20:42 UTC, encompassing five earthquakes within a span of two hours and fifty-six minutes. All events originated at a focal depth of 33 km, consistent with intermediate-depth activity along the Tonga subduction interface.

The recorded events unfolded as follows: a magnitude 5.5 earthquake at 17:45:33 UTC, followed by a magnitude 4.8 event at 20:08:13 UTC, a magnitude 5.3 shock at 20:12:02 UTC, a magnitude 5.0 event at 20:20:38 UTC, and a final magnitude 5.2 earthquake at 20:42:06 UTC. This tightly clustered pattern exemplifies a classic earthquake swarm, in which energy release occurs through numerous moderate events rather than a single dominant mainshock followed by aftershocks.

Tonga occupies a tectonically active segment of the Pacific Ring of Fire, where the Pacific Plate subducts westward beneath the Tonga microplate at rates exceeding 20 cm per year. The Tonga Trench, reaching depths greater than 10,800 m, marks the surface expression of this convergent margin. Intermediate-depth seismicity, such as the 33 km events of the 2001 swarm, arises from dehydration embrittlement and phase transitions within the downgoing slab. Historical records document recurrent swarms in this region, often linked to episodic slip along the plate interface or fluid migration within the subduction channel.

A notable larger event in the vicinity occurred on 19 March 2009, when a magnitude 7.6 earthquake struck 191 km south of ‘Ohonua, approximately 88 km from the swarm centroid. This megathrust rupture released substantially greater energy than the 2001 sequence and underscores the potential for the Tonga subduction zone to generate great earthquakes. The 2001 swarm, by contrast, remained confined to moderate magnitudes and did not trigger a larger mainshock, illustrating the variability of stress release along the margin.

Seismic swarms in Tonga contribute to the long-term accommodation of plate convergence. Monitoring of such sequences aids in delineating locked versus creeping portions of the subduction interface, informing regional hazard assessments. Continued instrumentation of the Tonga Trench region remains essential for refining models of slab dynamics and associated seismic hazards.

References

• USGS Earthquake Catalog (historical event parameters)
• Tonga Trench tectonic summaries, Pacific Plate subduction rates (GEBCO and regional plate-motion studies)