Seismic Swarm PS20111118.1 Northeast of Gisborne, New Zealand

On 18 November 2011, a seismic swarm designated PS20111118.1 occurred approximately 206 km northeast of Gisborne, New Zealand. The sequence began at 02:43 local time and concluded at 07:51 the same day, spanning 5 hours and 7 minutes. During this interval, six earthquakes were recorded, with magnitudes ranging from 4.2 to 5.8 and focal depths between 10 km and 41 km.

The events unfolded in rapid succession. Two initial shocks at 02:43:51 and 02:43:56 registered magnitudes of 5.2 and 5.3 at depths of 41 km and 30 km, respectively. Subsequent activity included a magnitude 4.2 event at 04:14:17 (33 km depth), followed closely by paired shocks at 04:34:03 and 04:34:05 with magnitudes 5.6 and 5.7 at 33 km and 10 km. The sequence ended with a magnitude 5.8 event at 07:51:24 (12 km depth). This tight temporal clustering and variable depth distribution are characteristic of swarm activity in the region.

The location lies within the Hikurangi subduction zone, where the Pacific plate subducts obliquely beneath the Australian plate along New Zealand’s eastern margin. This tectonic setting drives frequent seismicity, including both shallow crustal events and deeper intraslab earthquakes. The North Island’s northeastern offshore area experiences elevated strain accumulation due to the transition from subduction to strike-slip faulting further south. Historical records indicate persistent low-to-moderate seismicity punctuated by occasional larger events.

Since 1 January 2000, only two swarms have been documented in this sector, with the earlier episode occurring in 2001. Notable larger earthquakes have also occurred nearby. A magnitude 7.3 event struck 182 km northeast of Gisborne on 4 March 2021, approximately 24 km from the 2011 swarm center. Another magnitude 7.0 earthquake occurred 175 km northeast of Gisborne on 1 September 2016, roughly 49 km from the swarm centroid. These events underscore the region’s capacity for significant ruptures along the plate interface and associated faults.

Swarm sequences such as PS20111118.1 typically reflect fluid migration or aseismic slip transients rather than mainshock-aftershock patterns. Depths spanning the upper to mid-crust suggest involvement of both the subducting slab and overlying plate. Ongoing monitoring by regional networks continues to refine understanding of stress transfer and recurrence patterns in this subduction environment.

References
GNS Science. Hikurangi Subduction Zone Overview. Updated 2023.
USGS Earthquake Catalog. New Zealand Region Events 2000–2024.