Seismic Swarm PS20110711.1 Near Tonga: Geological Context and Event Analysis

The seismic swarm designated PS20110711.1 occurred in the central Pacific Ocean, centered 177 km west of Hihifo, Tonga. It began at 18:47 on 10 July 2011 and concluded at 00:54 on 11 July 2011, spanning six hours and seven minutes. During this period, five earthquakes were recorded, all at a focal depth of 10 km. The sequence included events of magnitudes 5.7, 5.2, 5.1, 4.6, and 5.2.

This activity took place along the Tonga Trench, where the Pacific Plate subducts westward beneath the Tonga microplate at rates exceeding 20 cm per year. The trench forms part of the Pacific Ring of Fire and hosts some of the world’s deepest earthquakes and highest seismic productivity. Shallow crustal events in this sector commonly arise from bending-related faulting in the downgoing plate or from strain release within the overriding plate and forearc.

Tonga’s seismic history reflects continuous subduction-driven deformation. Large interplate thrust earthquakes, outer-rise normal-faulting sequences, and occasional volcanic swarms associated with nearby island arcs have all been documented. Relative to the high background rate of individual earthquakes, clustered swarms remain uncommon in the instrumental record. Data since 2000 indicate only two prior swarms in the broader region, one in 2005 and another in 2007, underscoring the episodic nature of such clustered shallow seismicity.

The 2011 swarm’s rapid succession of moderate-magnitude events within a narrow time window and fixed depth range is consistent with localized stress transfer along pre-existing faults in the subducting slab. No larger mainshock–aftershock sequence followed, and the activity decayed within hours, typical of swarm behavior rather than a classic foreshock–mainshock pattern.

Such episodes provide useful constraints on the mechanical properties of the shallow subduction interface and outer-rise lithosphere. Continued monitoring by regional and global networks remains essential for distinguishing background subduction seismicity from potential precursory signals associated with larger megathrust ruptures.

References: USGS Earthquake Hazards Program – Tonga Trench tectonics overview (2023 update) Global CMT Catalog – subduction zone focal mechanisms IRIS Seismological Data Archive – regional event bulletins since 2000