Seismic Swarm PS20131102.1: Activity in the Tonga Subduction Zone

The seismic swarm designated PS20131102.1 occurred in a tectonically active region 173 km east-southeast of Neiafu, Tonga. Registered between 18:53 on 2 November 2013 and 10:08 on 3 November 2013, the sequence lasted 15 hours and 15 minutes and included six earthquakes. All events were shallow, with focal depths between 4 km and 10 km, consistent with the upper crustal environment of the Tonga subduction zone.

The sequence began with a magnitude 6.2 event at 18:53:46 on 2 November, followed within minutes by a magnitude 5.2 shock. Subsequent events included magnitudes 5.7, 5.1, and 4.5, all occurring at 10 km depth. The final recorded earthquake, magnitude 5.6, occurred at 10:08:54 on 3 November at a slightly shallower depth of 4 km. These closely spaced, moderate-magnitude events illustrate the clustered nature of swarm activity, where energy release occurs through multiple similar-sized shocks rather than a single mainshock-aftershock pattern.

Tonga lies along the convergent boundary where the Pacific Plate subducts westward beneath the Tonga Plate at rates exceeding 15 cm per year. This rapid subduction produces one of the most seismically active segments of the Pacific Ring of Fire. The Tonga Trench reaches depths greater than 10 km, and the associated Wadati-Benioff zone extends to more than 600 km depth. Shallow crustal earthquakes such as those in swarm PS20131102.1 typically arise from stresses within the overriding plate or along the plate interface at low to moderate depths.

Historical records maintained since 1 January 2000 indicate that swarm PS20131102.1 represents only the second documented swarm episode in the immediate area; the preceding swarm occurred in 2006. This low frequency underscores the episodic character of swarm-type activity compared with the more common isolated mainshock sequences in the region. A notable strong earthquake, magnitude 7.3, occurred on 11 November 2022 approximately 205 km east-southeast of Neiafu and 39 km from the 2013 swarm centroid, highlighting continued strain accumulation along the same segment of the subduction interface.

The combination of shallow focal depths, moderate magnitudes, and spatial clustering observed in PS20131102.1 is typical of fluid-related or stress-transfer processes within the forearc of the Tonga system. Such swarms provide valuable data for understanding short-term seismic hazard in an environment already prone to great earthquakes and associated tsunamis.

References

SeismoSight internal classification records for swarm PS20131102.1 parameters and statistics.
USGS Earthquake Catalog for regional tectonic framework and 2022 magnitude 7.3 event details.
Pacific Tsunami Warning Center historical summaries of Tonga subduction zone activity.