Seismic Swarm PS20060508.1 in Tonga: Geological Context and Event Analysis

Tonga lies along the Tonga Trench, a major subduction zone where the Pacific Plate converges with the Australian Plate at rates exceeding 15 cm per year. This tectonic setting produces frequent seismicity, including deep-focus events and occasional earthquake swarms driven by slab dehydration and stress transfer within the subducting lithosphere. The region forms part of the Pacific Ring of Fire and has a well-documented history of megathrust earthquakes capable of generating tsunamis.

The swarm designated PS20060508.1 occurred in this environment from 11:56 on 7 May 2006 to 10:09 on 8 May 2006, spanning 22 hours and 12 minutes. Five earthquakes were recorded during this interval, with magnitudes ranging from 4.9 to 5.4 and focal depths between 10 km and 40 km. Event times and parameters are as follows: 7 May 2006 at 11:56:30 (M5.2, 40 km), 15:21:46 (M5.1, 35 km), 22:06:23 (M5.4, 33 km), 22:06:26 (M5.0, 35 km), and 8 May 2006 at 10:09:00 (M4.9, 10 km). These events clustered near the epicentral area of the preceding M8.0 Tonga earthquake of 3 May 2006, located approximately 23 km from the swarm centroid.

Historical records indicate that only one swarm has occurred in the Tonga region since 1 January 2000. This makes the 2006 sequence the sole documented swarm in the catalog, highlighting its rarity relative to the high background rate of isolated earthquakes. The swarm followed closely after two major events on 3 May 2006: an M8.0 earthquake 23 km from the swarm center and an M7.8 event 37 km away. A subsequent M7.0 earthquake on 30 March 2025, located 61 km SSE of Pangai and 24 km from the swarm center, further underscores the persistent seismic productivity of the area.

Swarm sequences in subduction zones such as Tonga are typically attributed to fluid migration or afterslip following large mainshocks rather than precursory activity. The shallowing of focal depths observed in the final event of the sequence may reflect stress adjustment within the overriding plate or upper portions of the subducted slab. No larger events were associated with this swarm, consistent with its classification as a low-magnitude cluster.

References

USGS Earthquake Catalog
Global CMT Project
Tonga Geological Survey reports