Seismic Swarm PS20160901.1: Activity Northeast of Gisborne

Seismic swarm PS20160901.1 was recorded 175 km northeast of Gisborne, New Zealand, beginning at 22:04 on 31 August 2016 and concluding at 02:57 on 2 September 2016. Over 28 hours and 52 minutes, the sequence produced 21 earthquakes. The events clustered in a tectonically active offshore region where the Pacific plate subducts beneath the Australian plate along the Hikurangi margin.

The sequence opened with a magnitude 5.8 event at 15 km depth. Activity intensified the following day, culminating in a magnitude 7.0 earthquake at 19 km depth at 16:37 on 1 September. This mainshock was followed within minutes by multiple aftershocks exceeding magnitude 5.0, including events of 5.4, 5.5, and 6.1, with focal depths predominantly between 8 km and 19 km. Later events tapered to magnitudes between 5.0 and 5.5 through the evening and into the early hours of 2 September.

The Hikurangi subduction zone forms the primary structural control on seismicity in this area. Oblique convergence drives both interface thrust earthquakes and shallower crustal faulting within the overlying plate. Historical patterns show that earthquake swarms recur in this segment of the margin, with three documented episodes since 2000—in 2001 (two swarms) and 2011 (one swarm). These swarms typically involve moderate-magnitude events distributed over hours to days rather than isolated large shocks.

A magnitude 7.3 earthquake occurred 182 km northeast of Gisborne on 4 March 2021, approximately 43 km from the 2016 swarm centroid, underscoring the persistent seismic productivity of the region. The 2016 magnitude 7.0 event itself lies only 13 km from the swarm center, consistent with localized stress release along the plate interface.

Such sequences provide insight into the mechanics of slow-slip and triggered seismicity that characterize the northern Hikurangi margin. Depths recorded during the swarm align with both the subduction interface and overlying crustal faults, illustrating the vertical range of active deformation.

References
SeismoSight internal swarm classification PS20160901.1
GeoNet New Zealand Earthquake Catalogue
GNS Science reports on Hikurangi subduction zone tectonics