Seismic Swarm PS20090320.1: The March 2009 Tonga Earthquake Sequence

The seismic swarm designated PS20090320.1 occurred in the Tonga region of the southwest Pacific Ocean, centered approximately 293 km south of ‘Ohonua. Registered between 18:17 on 19 March 2009 and 17:38 on 21 March 2009, the sequence lasted 47 hours and 20 minutes and included ten earthquakes. The largest event reached magnitude 7.6 at a depth of 31 km, with subsequent events ranging from magnitude 4.7 to 5.4 at depths between 10 km and 48 km.

This swarm unfolded along the Tonga subduction zone, where the Pacific Plate converges with and subducts beneath the Tonga Plate at rates exceeding 15 cm per year. The region lies within the Pacific Ring of Fire and features the Tonga Trench, one of Earth’s deepest oceanic trenches. High seismicity here results from intense plate interaction, producing frequent intermediate-depth and deep-focus earthquakes as well as shallow events along the megathrust interface.

The sequence began with the magnitude 7.6 mainshock on 19 March at 18:17:40 UTC, followed by nine aftershocks distributed over the next two days. Depths clustered predominantly around 35 km, consistent with the seismogenic zone of the subducting slab. Notable events included a magnitude 5.4 shock at 48 km depth on 20 March and several magnitude 5.0 events at shallower and intermediate depths.

Since 2000, only three comparable swarms have been recorded in this sector of the Tonga subduction zone, occurring in 2001, 2004, and 2006. The 2009 swarm stands out for including a magnitude 7.6 event located 73 km from the swarm centroid and 191 km south of ‘Ohonua. Such sequences reflect episodic stress release within the subducting slab rather than typical mainshock-aftershock patterns.

Tectonic activity in Tonga remains among the highest globally, with the potential for both damaging shallow earthquakes and deep events exceeding magnitude 8. The 2009 swarm underscores the persistent seismic hazard along this convergent margin and the value of continuous monitoring for understanding slab dynamics.

References

USGS Earthquake Catalog (2009 events)
SeismoSight internal swarm classification PS20090320.1