Seismic Swarm PS20171016.1 Off Tonga: Event Analysis and Regional Context

On 16–17 October 2017, a seismic swarm designated PS20171016.1 was recorded 258 km SSW of ‘Ohonua, Tonga. The sequence began at 18:09 UTC on 16 October and concluded at 11:27 UTC on 17 October, spanning 17 hours and 18 minutes. Seven earthquakes were detected during this interval, with magnitudes ranging from 4.7 to 5.5 and focal depths between 10 and 14 km.

The swarm exhibited a rapid onset followed by a gradual decline in activity. The two largest events, both magnitude 5.5, occurred within the first 30 minutes. Subsequent shocks decreased in frequency, with the final magnitude 5.0 event marking the swarm’s termination. All events clustered at shallow to intermediate crustal depths, consistent with tectonic release along the convergent margin.

Tonga lies along the Tonga subduction zone, where the Pacific Plate descends beneath the Tonga microplate at rates exceeding 15 cm per year. This setting produces one of the most seismically active regions on Earth, with frequent moderate-to-large earthquakes and associated volcanic arcs. The 2017 swarm occurred seaward of the trench axis, likely reflecting intraslab or outer-rise faulting typical of subduction environments.

Since 2000, eight comparable swarms have been documented in the same offshore sector. These episodes occurred in 2001, 2004, 2006, 2009, 2010, 2011, 2012, and 2014, indicating recurrent clustered seismicity without progression to a major mainshock. Such patterns suggest episodic stress release along pre-existing fault networks rather than long-term seismic quiescence or acceleration.

The 2017 swarm did not generate a tsunami or reported damage, consistent with its moderate magnitudes and offshore location. Continued monitoring remains essential given the region’s capacity for great earthquakes and the potential for cascading hazards along the plate boundary.

References
USGS Earthquake Catalog
Global Centroid Moment Tensor Project
Tonga Geological Services reports