Seismic Swarm South of Tonga: Analysis of the June 2023 Event

On 25 June 2023, a seismic swarm was recorded south of Tonga in the southwestern Pacific Ocean. The sequence began at 00:58 UTC and concluded at 08:26 UTC, lasting 7 hours and 28 minutes. During this period, five earthquakes were registered, with magnitudes ranging from 4.3 to 6.2 and focal depths between 9 and 10 km.

The largest event reached magnitude 6.2 at 07:16:58 UTC at a depth of 9 km. Other notable shocks included two events near magnitude 5.4, one at 5.3, and one at 4.3. All events occurred within a compact time window, characteristic of swarm behavior where multiple earthquakes cluster without a single dominant mainshock-aftershock pattern.

This activity aligns with the tectonic setting of the Tonga subduction zone. The region lies along the convergent boundary where the Pacific Plate subducts westward beneath the Tonga Plate at rates exceeding 15 cm per year. Such rapid convergence generates frequent intermediate-depth and shallow seismicity, often accompanied by volcanic arcs including the Tonga Islands. Depths around 10 km indicate shallow crustal or uppermost mantle rupture, consistent with interface or intraslab events in this margin.

Historical records from the same internal classification show that ten swarms have occurred south of Tonga since 2000. These took place in 2001, 2004, 2006, 2009, 2010, 2011, 2012, 2014, 2017, and 2023. The recurrence suggests episodic clustering along the subduction interface, possibly linked to transient slip or fluid migration, although specific triggering mechanisms require further geophysical modeling.

Swarm sequences in subduction zones can elevate short-term seismic hazard through increased ground shaking and potential for larger triggered events. Monitoring networks such as those operated by regional agencies provide real-time detection that supports tsunami warning protocols, given Tonga’s exposure to both local and trans-Pacific tsunami risk.

Further analysis of waveform data and geodetic observations would help constrain whether the 2023 swarm involved slow slip or purely brittle failure. Continued surveillance remains essential in this highly active margin.

References:
SeismoSight internal swarm classification database.
USGS Earthquake Catalog (general tectonic framework of Tonga Trench).
Global CMT catalog (subduction zone parameters).