The 2021 Kermadec Islands Earthquake: Regional Geology and Seismic History

The M8.1 Kermadec Islands earthquake struck on 4 March 2021 at 19:28 UTC, with a focal depth of 28.9 km. This event occurred within the tectonically active Kermadec subduction zone, where the Pacific Plate descends beneath the Australian Plate, generating frequent large-magnitude seismicity along the Kermadec Trench. The Kermadec Islands region forms part of the 2,500 km-long Kermadec-Tonga arc system in the southwest Pacific Ocean. Subduction here proceeds at rates exceeding 6 cm per year, producing intermediate-depth earthquakes and occasional great events. The arc features active submarine volcanoes and back-arc spreading in the Havre Trough, contributing to the area's elevated seismic hazard. Seismic records since 2000 document several strong earthquakes near the 2021 rupture zone. On 24 April 2023, an M7.1 event occurred 59 km from the mainshock epicenter. The M8.1 mainshock itself was followed the same day by an M7.4 aftershock 54 km distant. Earlier activity includes an M7.6 earthquake on 6 July 2011 located 93 km away and an M7.0 event on 29 September 2008 situated 39 km from the 2021 hypocenter. These events cluster along the plate interface, reflecting the persistent strain accumulation and release characteristic of the subduction megathrust. The 2021 sequence highlights the Kermadec region's capacity for both great earthquakes and energetic aftershock sequences. Continued monitoring by regional seismic networks supports improved understanding of rupture processes along this remote segment of the Pacific Ring of Fire.

References

USGS Earthquake Catalog (event parameters and historical seismicity)
Global CMT Catalog (moment tensor solutions for regional events)