Seismic Swarm PS20110630.1: Analysis of Activity in the Tonga Region

On 30 June 2011, a seismic swarm designated PS20110630.1 was recorded approximately 242 km south-southwest of ‘Ohonua, Tonga. The sequence began at 04:30 and concluded at 14:16 local time, encompassing five earthquakes over a span of nine hours and forty-five minutes. This event occurred within the highly active Tonga subduction zone, where the Pacific Plate descends beneath the Tonga Plate along the Tonga Trench, generating frequent seismic activity due to plate convergence rates exceeding 15 cm per year.

The individual events unfolded as follows. The initial shock registered at 04:30:54 with a magnitude of 5.6 at a depth of 35 km. Roughly four and a half hours later, at 09:09:18, a magnitude 5.7 event occurred at the same depth. Two closely timed shocks followed at 09:47:03 (magnitude 5.4, depth 2 km) and 09:47:09 (magnitude 5.5, depth 35 km). The sequence ended with a magnitude 5.0 event at 14:16:18, again at 35 km depth. Depths clustered predominantly in the intermediate range, consistent with interface seismicity along the subducting slab.

Tonga’s geological setting features one of the world’s most seismically productive margins. The trench extends over 2,000 km and reaches depths exceeding 10,000 m, facilitating megathrust earthquakes and associated swarms. Historical records indicate persistent activity driven by slab dehydration and stress transfer within the Wadati-Benioff zone. The 2011 swarm aligns with this pattern, reflecting episodic strain release rather than a single mainshock-aftershock sequence.

Since 1 January 2000, five swarms have been documented in the broader region. These occurred in 2001, 2004, 2006, 2009, and 2010, each registering a single swarm event. Such recurrent swarms underscore the Tonga arc’s capacity for clustered seismicity, often linked to fluid migration or aseismic slip along the plate interface.

This swarm contributed to ongoing monitoring of the Tonga-Kermadec system, which remains one of Earth’s most active tectonic boundaries. No significant surface deformation or tsunami generation was associated with the 2011 sequence, typical for moderate-magnitude events at these depths.

References
SeismoSight internal swarm classification PS20110630.1
USGS Earthquake Catalog (regional tectonics)
Global CMT Project (subduction zone parameters)