Seismic Swarm PS20071102.1: Analysis of a Tonga Earthquake Cluster

On 2 November 2007, a seismic swarm designated PS20071102.1 occurred approximately 100 km north-northeast of Hihifo, Tonga. The sequence began at 12:40 local time and concluded at 14:53, encompassing five earthquakes within a span of two hours and thirteen minutes. All events registered at shallow depths between 10 km and 33 km, with magnitudes ranging from 4.3 to 5.8.

The individual events unfolded as follows: a magnitude 5.7 quake at 12:40:10 (10 km depth), followed by a 5.2 event at 12:47:51 (10 km), a 5.8 shock at 13:30:44 (10 km), a 5.7 event at 13:30:47 (33 km), and a final magnitude 4.3 tremor at 14:53:19 (10 km). These closely timed occurrences reflect typical swarm behavior, where multiple events cluster without a single dominant mainshock.

Tonga lies along the Tonga-Kermadec subduction zone, where the Pacific Plate descends beneath the Australian Plate at rates exceeding 15 cm per year. This tectonic setting produces frequent shallow to intermediate-depth seismicity, often in the form of swarms and aftershock sequences. The 2007 swarm aligns with this pattern, occurring within the overriding plate or near the plate interface at shallow crustal levels.

Historical records indicate limited swarm activity in the region since 2000. Only one prior swarm was documented, occurring in 2004. Such infrequent clustering underscores the predominantly mainshock-aftershock character of Tongan seismicity rather than prolonged swarm episodes.

This event provides insight into localized stress release along the northern Tonga arc. The tight temporal grouping and consistent shallow depths suggest fluid migration or aseismic slip may have contributed to triggering successive ruptures. No significant damage or tsunami was associated with the sequence, consistent with the moderate magnitudes involved.

References

• SeismoSight internal swarm classification records
• USGS earthquake catalog for Tonga region tectonics