Location:

Period:

29 Sep 2009 17:56:05 - 29 Sep 2009 23:11:51 (5 hours 15 minutes)

Volcanoes in 100km radius:

Curacoa(37km), Tafahi(54km), West Mata(70km), Niuatahi(76km)

Earthquakes:

10 swarms found nearby.

2004

PS20040904.1(54.9km)

3 Sep

1 day 8 hours

6 earthquakes

2007

PS20071102.1(39.9km)

2 Nov

2 hours

5 earthquakes

2009

PS20090306.1(50.8km)

6 Mar

13 hours

5 earthquakes

PS20090929.1(54.1km)

29 Sep

2 days 19 hours

35 earthquakes

PS20090929.3(196.0km)

29 Sep

3 days 13 hours

28 earthquakes

PS20090929.2(109.8km)

29 Sep

6 hours

7 earthquakes

PS20091004.1(66.5km)

4 Oct

1 day 0 hours

6 earthquakes

2015

PS20150330.2(42.9km)

30 Mar

10 hours

6 earthquakes

2017

PS20171104.2(55.5km)

4 Nov

7 hours

5 earthquakes

2026

PS20260322.1(40.4km)

22 Mar

1 day 5 hours

8 earthquakes

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Analysis of the September 2009 Seismic Swarm East of Hihifo, Tonga

On 29 September 2009, a seismic swarm designated PS20090929.4 was recorded 74 km east of Hihifo, Tonga. The sequence began at 17:56 and concluded at 23:11, encompassing 11 earthquakes over a span of five hours and fifteen minutes. All events occurred at a focal depth of 10 km. Magnitudes ranged from 4.5 to 6.0, with the initial event registering 6.0 at 17:56:05, followed rapidly by a 5.6 event at 17:57:26. Subsequent shocks included magnitudes of 5.4, 5.3, 5.1, 5.2, 5.1, 5.4, 4.5, 5.1, and a final 5.5 at 23:11:51. This tightly clustered activity reflects typical swarm behavior, characterized by multiple events of comparable size without a single dominant mainshock.

The location lies within the Tonga subduction zone, where the Pacific Plate descends beneath the Australian Plate along the Tonga Trench. This convergent boundary forms part of the Pacific Ring of Fire and generates frequent intermediate-depth and shallow seismicity. The 10 km depths recorded during the swarm indicate rupture within the upper plate or near the plate interface, consistent with crustal adjustment above the subducting slab. Historical records show elevated seismic rates in the region, driven by ongoing plate convergence at rates exceeding 15 cm per year.

Since 2000, four swarms have been documented in the broader Tonga area according to internal classifications, occurring in 2004 (one swarm), 2007 (one swarm), and two in 2009. Such episodic clusters underscore the zone’s capacity for distributed stress release rather than isolated large events. The 2009 activity aligns with patterns observed along other subduction margins, where fluid migration or aseismic slip can trigger rapid sequences of moderate earthquakes.

Geological context for Tonga includes active volcanism on the Tofua arc and back-arc spreading in the Lau Basin, both influenced by the same subduction dynamics. Shallow swarms like PS20090929.4 may relate to strain accumulation along the megathrust or secondary faulting in the overriding plate. No surface rupture or tsunami was associated with this particular sequence, reflecting its modest energy release compared with great subduction earthquakes that have historically affected the archipelago.

Continued monitoring by regional seismic networks remains essential for distinguishing swarm activity from foreshock sequences that might precede larger events. The 2009 swarm provides a clear example of how subduction-zone tectonics produce short-lived, high-frequency earthquake clusters without immediate escalation.

References: USGS Earthquake Catalog Tonga Meteorological and Geological Services reports Global CMT catalog for regional tectonics