Location:

Period:

19 Oct 2019 08:40:14 - 20 Oct 2019 08:08:49 (23 hours 28 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.3; 205 km ESE of Neiafu, Tonga; (11 Nov 2022) (89km from the swarm center)

3 swarms found nearby.

2009

PS20090929.3(152.4km)

29 Sep

3 days 13 hours

28 earthquakes

2013

PS20131102.1(90.7km)

2 Nov

15 hours

6 earthquakes

2019

PS20190920.1(48.7km)

20 Sep

3 hours

5 earthquakes

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Seismic Swarm PS20191019.1 Near Tonga

Seismic swarm PS20191019.1 occurred approximately 200 km east-northeast of Neiafu, Tonga, beginning at 08:40 on 19 October 2019 and concluding at 08:08 on 20 October 2019. Over this 23-hour 28-minute period, eight earthquakes were recorded, all at a focal depth of 10 km. Magnitudes ranged from 4.3 to 5.4, with the largest event (M5.4) initiating the sequence. Subsequent events included two M5.3 shocks within minutes of each other, followed by an M4.3, an M5.1, two M5.0 events, and a closing M5.1. The tight temporal clustering and uniform shallow depth indicate a localized release of stress without progression to a larger mainshock.

This activity aligns with the broader tectonic setting of the Tonga region. The swarm site lies within the Tonga subduction zone, where the Pacific Plate converges with the Tonga Plate at rates exceeding 15 cm per year. Rapid subduction produces intense seismicity along the trench and overlying plate, with frequent shallow crustal events. Historical records show that earthquake swarms have been documented in the area since 2000, with three episodes recorded: one each in 2009, 2013, and 2019. These swarms typically consist of multiple moderate events over short durations and do not follow standard mainshock-aftershock patterns.

On 11 November 2022, a strong M7.3 earthquake struck 205 km east-southeast of Neiafu, approximately 89 km from the 2019 swarm center. This event underscores the persistent seismic hazard in the region, where both interplate and intraplate ruptures occur regularly. The 2022 earthquake was felt across Tonga and generated a small tsunami, consistent with the shallow focal depths common along the arc.

Seismic monitoring in Tonga benefits from regional networks that detect events down to moderate magnitudes. The 2019 swarm, with its eight events, represents a modest but notable episode within the long-term pattern of clustered activity. Such swarms contribute to understanding stress transfer along the subduction interface and may precede or follow larger ruptures, although no direct causal link has been established with the 2022 M7.3 event.

Continued observation remains essential given the high convergence rate and history of great earthquakes along the Tonga-Kermadec system. The shallow depths observed in the 2019 swarm are typical of crustal seismicity above the subducting slab and highlight the potential for ground shaking in nearby island communities.

References

USGS Earthquake Catalog
Global CMT Catalog
Tonga Geological Survey reports