Seismic Swarm Activity Near Tonga: The February 2020 Event PS20200222.1

On 22 February 2020, a seismic swarm designated PS20200222.1 was recorded approximately 83 km south-southeast of ‘Ohonua, Tonga. The sequence began at 18:50 UTC and concluded at 19:46 UTC, encompassing five earthquakes within a 55-minute window. Event parameters included magnitudes ranging from 4.5 to 5.6 and focal depths between 10 km and 88 km. The largest events, both registering 5.6, occurred at 19:46 UTC at depths of 10 km and 30 km respectively.

This swarm reflects typical clustered seismicity in an active subduction environment. The timing and spatial distribution indicate rapid stress release along the plate interface without progression to a larger mainshock. Such sequences are common where the Pacific Plate subducts westward beneath the Tonga Plate at rates exceeding 15 cm per year, generating frequent intermediate-depth and shallow earthquakes.

Tonga occupies the northern segment of the Tonga-Kermadec subduction zone, one of Earth’s most seismically productive regions. The trench reaches depths greater than 10 km and hosts both shallow thrust events and deeper intraslab seismicity extending beyond 600 km. Historical records document recurrent great earthquakes, including the 2009 Mw 8.1 event south of Tonga and multiple magnitude-7+ shocks throughout the 20th century. Volcanic arcs associated with the subduction system, such as those forming the Tongan islands, further contribute to regional hazard through occasional explosive eruptions and related seismic swarms.

Since 2000, six earthquake swarms have been documented in the immediate vicinity according to internal SeismoSight classification. These occurred in 2001 (one swarm), 2006 (one), 2009 (two), 2011 (one), and 2014 (one). The 2020 swarm fits this pattern of episodic, short-duration clusters separated by years of relative quiescence, underscoring the persistent but segmented nature of strain accumulation along the Tonga trench.

Analysis of focal depths shows a mix of shallow crustal events (10 km) and deeper activity (up to 88 km), consistent with both interplate thrusting and intraslab deformation. Magnitudes remained below 6.0, limiting potential for widespread damage, although felt shaking was reported on nearby islands. No tsunami generation was associated with this sequence.

Continued monitoring of the Tonga subduction zone remains essential given its capacity for great earthquakes and the established recurrence of swarm activity. The 2020 event provides additional data for refining seismic hazard models in this rapidly converging plate boundary.

References

USGS Earthquake Catalog (events 2020-02-22)
Global CMT Project focal mechanism database
Tonga-Kermadec subduction zone summaries, GNS Science and Pacific Island regional reports