Earthquake Swarm Analysis South of Tonga, February 2010

An earthquake swarm occurred south of Tonga between 07:00 on 22 February 2010 and 06:05 on 23 February 2010. In 23 hours and 4 minutes, six events were recorded. The sequence began with a magnitude 6.0 earthquake at 15 km depth, followed by a magnitude 5.6 event at 35 km depth roughly 24 minutes later. Subsequent shocks included magnitudes 5.5 at 35 km, 5.0 at 30 km, 4.9 at 35 km, and a final magnitude 5.3 at 35 km depth.

This swarm unfolded along the Tonga subduction zone, where the Pacific plate descends beneath the Australian plate at one of the world’s fastest convergence rates. The region forms part of the Pacific Ring of Fire and hosts the Tonga Trench, reaching depths exceeding 10 000 m. Intermediate-depth seismicity between 30 km and 70 km is common because of the steep slab geometry and active dehydration reactions within the downgoing plate.

Seismic swarms in this setting often reflect stress transfer along the slab interface or within the overlying crust rather than a single mainshock-aftershock sequence. The 2010 events clustered at shallow to intermediate depths consistent with typical activity in the southern Tonga arc. No damage or tsunami was reported, reflecting both the offshore location and moderate magnitudes.

Since 1 January 2000, four swarms have been documented in the same area, occurring in 2001, 2004, 2006, and 2009. These episodes illustrate the persistent, episodic nature of seismicity driven by continuous plate convergence and fluid migration along the subduction interface.

The Tonga-Kermadec system remains one of the most seismically active subduction zones on Earth, with well-documented historical events including great earthquakes and associated volcanic activity along the arc. Continued monitoring supports improved understanding of slab dynamics and regional hazard assessment.

References USGS Earthquake Hazards Program Global CMT Catalog Tectonophysics literature on Tonga subduction zone