Seismic Swarm PS20250330.1: Tonga Earthquake Sequence and Regional Context

A seismic swarm designated PS20250330.1 occurred near the Ha’apai island group in Tonga on 30 March 2025. The sequence began at 12:18 local time and concluded at 21:51, spanning 9 hours and 32 minutes. Six earthquakes were recorded within a compact area 27 km east-northeast of Fangale’ounga. The events ranged in magnitude from 4.4 to 7.0 and originated at depths between 10 km and 29 km.

The sequence opened with a magnitude 7.0 earthquake at 12:18:50 (29 km depth), followed by a magnitude 6.2 event at 15:04:54 (17 km depth). Subsequent shocks included magnitudes 5.0 (15:26:13, 10 km), 5.9 (15:33:49, 27 km), 4.4 (15:42:20, 10 km), and 5.2 (21:51:41, 10 km). The temporal clustering and progressive shallowing of foci are characteristic of swarm behavior rather than a classical mainshock-aftershock pattern.

Tonga lies along the Tonga-Kermadec subduction zone, where the Pacific plate converges with the Australian plate at rates exceeding 15 cm per year. This tectonic setting produces one of the world’s most seismically active regions, with frequent intermediate-depth and shallow crustal events. The Ha’apai archipelago sits above the shallow portion of the megathrust, where locking and episodic slip release generate both isolated large earthquakes and occasional swarms.

Since 1 January 2000, four prior swarms have been identified in the immediate vicinity: two in 2006, one in 2009, and one in 2013. The 2006 swarm coincided with the M8.0 Tonga earthquake of 3 May and its M7.8 aftershock, whose epicenters lay 13 km and 26 km from the 2025 swarm centroid, respectively. The most recent strong event prior to the swarm was a magnitude 7.0 earthquake on 30 March 2025 located 61 km south-southeast of Pangai, approximately 20 km from the swarm center.

These repeated swarm episodes suggest localized stress heterogeneity within the shallow megathrust or overlying crust, possibly influenced by slab dehydration or seamount subduction. Continued monitoring is warranted given the proximity to populated islands and the historical precedent for large-magnitude events in the same segment.

References
USGS Earthquake Catalog (events since 2000)
Tonga Geological Survey regional tectonic summaries
Global CMT and ISC Bulletin for 2006 Tonga sequence parameters