Seismic Swarm PS20190621.1: Kermadec Islands Earthquake Sequence

The Kermadec Islands region forms part of the Kermadec-Tonga subduction zone, where the Pacific Plate descends beneath the Australian Plate at rates exceeding 5 cm per year. This tectonic setting produces frequent seismic activity, including earthquake swarms and occasional great earthquakes exceeding magnitude 7. The islands themselves are volcanic, situated along the Kermadec Ridge, with underlying geology shaped by subduction-driven magmatism and crustal extension.

Seismic swarm PS20190621.1 was recorded between 21:45 on 20 June 2019 and 23:00 on 21 June 2019, spanning 25 hours and 15 minutes. During this interval, seven earthquakes were registered with the following parameters: a magnitude 5.1 event at 10 km depth on 20 June at 21:45:06; a magnitude 5.2 event at 15 km depth on 21 June at 07:27:40; a magnitude 6.2 event at 14 km depth on 21 June at 08:37:16; a magnitude 5.1 event at 10 km depth on 21 June at 08:40:38; a magnitude 4.9 event at 10 km depth on 21 June at 08:50:12; a magnitude 5.1 event at 10 km depth on 21 June at 22:49:24; and a final magnitude 5.1 event at 10 km depth on 21 June at 23:00:28. The sequence clustered at shallow depths between 10 and 15 km, consistent with activity along the subduction interface or overlying crust.

Such swarms are recurrent in the Kermadec region. Since 1 January 2000, eighteen swarms have occurred, distributed across the following years: two in 2003, one in 2004, one in 2005, two in 2006, three in 2008, one in 2011, one in 2012, three in 2014, two in 2016, and two in 2019. These episodes reflect episodic stress release within the subduction system.

Notable larger events have also struck nearby. A magnitude 7.3 earthquake occurred on 15 June 2019, approximately 59 km from the swarm center, while a magnitude 7.1 event was recorded on 24 April 2023, about 93 km distant. The 2019 swarm followed shortly after the magnitude 7.3 mainshock, illustrating how moderate sequences can develop in the aftershock zone of larger ruptures.

Overall, the Kermadec subduction zone remains one of Earth’s most active plate boundaries, where repeated swarms and great earthquakes underscore ongoing tectonic convergence and associated volcanic hazards.