Seismic Swarm PS20210304.1 Northeast of Gisborne, New Zealand

An earthquake swarm designated PS20210304.1 was recorded 246 km northeast of Gisborne, New Zealand, from 13:27 on 4 March 2021 to 22:51 on 7 March 2021. Over 81 hours and 24 minutes, the sequence included 34 events, beginning with a magnitude 7.3 earthquake at 10 km depth. Subsequent activity featured multiple events of magnitude 5.0–5.6, predominantly at depths of 8–15 km.

The swarm exhibited a rapid onset with six events above magnitude 5.0 within the first two hours, followed by sustained moderate shaking through 7 March. Notable later shocks included a magnitude 6.3 event at 13 km depth on 6 March and a magnitude 5.6 at 15 km on 7 March. Depths remained shallow throughout, consistent with crustal faulting in the region.

This activity aligns with New Zealand’s position along the Hikurangi subduction zone, where the Pacific plate subducts beneath the Australian plate. The zone produces frequent seismicity, including both mainshock-aftershock sequences and swarms driven by fluid migration or slow slip events. Gisborne lies near the northern extent of this margin, where oblique convergence generates thrust and strike-slip faults.

Historical records since 2000 show four prior swarms in the area: two in 2001, one in 2011, and one in 2016. The 2016 swarm preceded a magnitude 7.0 earthquake 175 km northeast of Gisborne. The 2021 swarm itself initiated with a magnitude 7.3 event 182 km northeast of Gisborne, 45 km from the swarm centroid, underscoring the potential for larger events within or adjacent to swarm clusters.

Such sequences provide insight into stress transfer along the subduction interface and upper-plate faults. Monitoring continues through regional networks to assess ongoing hazard, as shallow depths amplify ground shaking for nearby coastal communities.

References

SeismoSight internal swarm classification PS20210304.1
GNS Science New Zealand earthquake catalogue
USGS Earthquake Hazards Program records