Location:

Period:

30 Mar 2015 07:56:53 - 30 Mar 2015 18:02:10 (10 hours 5 minutes)

Volcanoes in 100km radius:

Curacoa(79km), West Mata(94km), Tafahi(96km)

Earthquakes:

M 7.0+:

M 8.1; 2009 Samoa Earthquake; (29 Sep 2009) (91km from the swarm center)

9 swarms found nearby.

2004

PS20040904.1(52.6km)

3 Sep

1 day 8 hours

6 earthquakes

2007

PS20071102.1(44.0km)

2 Nov

2 hours

5 earthquakes

2009

PS20090306.1(37.4km)

6 Mar

13 hours

5 earthquakes

PS20090929.1(68.9km)

29 Sep

2 days 19 hours

35 earthquakes

PS20090929.4(42.9km)

29 Sep

5 hours

11 earthquakes

PS20090929.2(71.5km)

29 Sep

6 hours

7 earthquakes

PS20091004.1(87.8km)

4 Oct

1 day 0 hours

6 earthquakes

2017

PS20171104.2(55.9km)

4 Nov

7 hours

5 earthquakes

2026

PS20260322.1(8.8km)

22 Mar

1 day 5 hours

8 earthquakes

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Seismic Swarm PS20150330.2 Near Tonga: Event Analysis and Regional Context

The seismic swarm designated PS20150330.2 occurred on 30 March 2015 in the central Pacific, centered 115 km east-northeast of Hihifo, Tonga. The sequence began at 07:56 local time and concluded at 18:02 the same day, spanning 10 hours and 5 minutes. During this interval, six earthquakes were recorded, with magnitudes ranging from 4.5 to 6.5 and focal depths between 9 km and 14 km.

The sequence initiated with a magnitude 6.0 event at 07:56:53, followed rapidly by a magnitude 6.4 shock at 08:18:01. Subsequent events included a magnitude 4.5 at 08:45:11, a magnitude 6.5 at 08:48:25, a magnitude 5.2 at 08:50:42, and a final magnitude 5.8 at 18:02:10. All events clustered within a compact spatial and temporal window, characteristic of swarm behavior rather than a classic mainshock-aftershock sequence.

This location lies along the Tonga Trench, where the Pacific Plate subducts westward beneath the Tonga microplate at rates exceeding 20 cm per year. The trench constitutes one of Earth’s most active subduction zones, generating frequent intermediate-depth and shallow seismicity. The shallow focal depths observed in the swarm align with crustal faulting above the plate interface.

Since 1 January 2000, seven swarms have been documented in the immediate vicinity. These include single swarms in 2004 and 2007, together with five swarms recorded in 2009. The elevated swarm count in 2009 coincided with the aftermath of the magnitude 8.1 Samoa earthquake of 29 September 2009, whose epicenter lay approximately 91 km from the 2015 swarm center. That great earthquake ruptured a normal-faulting structure on the Pacific Plate seaward of the trench, triggering widespread aftershock activity that persisted for months.

The Tonga subduction system has produced some of the largest instrumentally recorded earthquakes in the southwest Pacific. Ongoing plate convergence continues to load the megathrust and adjacent crustal faults, sustaining elevated seismic hazard. The 2015 swarm, while energetic, remained within the historical range of activity documented for this segment of the trench.

References
USGS Earthquake Catalog (events 2000–2015)
Global CMT Catalog (2009 Samoa earthquake parameters)
SeismoSight internal swarm classification PS20150330.2