Seismic Swarm PS20230616.1: Analysis of Activity South of Tonga

A seismic swarm designated PS20230616.1 occurred in the Tonga region of the South Pacific, centered 278 km south of ‘Ohonua. The sequence began at 08:11 on 16 June 2023 and concluded at 17:16 on 18 June 2023, spanning 57 hours and 4 minutes. During this period, 20 earthquakes were recorded, with magnitudes ranging from 4.5 to 6.2 and focal depths between 10 km and 40 km.

The swarm featured several notable events. The initial shock registered magnitude 6.0 at 20 km depth. Subsequent activity included a magnitude 6.2 event at 16 km depth on 16 June at 19:10, followed by a magnitude 6.1 at 22 km and a magnitude 6.0 at 40 km on 17 June near 11:26. Later shocks on 18 June included a magnitude 5.8 at 30 km depth and a magnitude 5.9 at 10 km depth. Depths clustered predominantly in the shallow to intermediate range, consistent with crustal and upper-mantle processes.

Tonga lies within the Tonga-Kermadec subduction zone, where the Pacific Plate converges with and subducts beneath the Tonga microplate at rates exceeding 20 cm per year. This setting produces frequent seismicity along the trench and volcanic arc. The swarm location aligns with the outer-rise and fore-arc regions south of the main island chain, areas prone to both interplate thrust events and normal-faulting sequences due to plate bending.

Historical records indicate nine comparable swarms in the region since 2000, occurring in 2001, 2004, 2006, 2009, 2010, 2011, 2012, 2014, and 2017. These episodes reflect episodic stress release along the subduction interface and surrounding fracture zones rather than isolated mainshock-aftershock sequences.

Such swarms contribute to understanding stress accumulation and release patterns in one of Earth’s most active convergent margins. Monitoring continues to track potential links to larger events or volcanic unrest along the nearby Tofua arc.

References

USGS Earthquake Hazards Program – Tonga regional seismicity summaries
Global CMT Catalog – Focal mechanism data for South Pacific events
Smithsonian Institution Global Volcanism Program – Tonga-Kermadec arc overview