Seismic Swarm PS20200126.2 East of New Zealand's North Island

An earthquake swarm designated PS20200126.2 occurred east of New Zealand's North Island, beginning at 19:24 on 25 January 2020 and concluding at 10:20 on 26 January 2020. Over 14 hours and 56 minutes, the swarm produced five earthquakes. This sequence reflects typical clustered seismicity in a tectonically active subduction setting.

The events unfolded as follows. The first shock registered magnitude 5.3 at 19:24:52 on 25 January at a depth of 17 km. Two additional events followed shortly after at 21:19:59 (magnitude 5.3, 10 km depth) and 21:20:07 (magnitude 5.1, 15 km depth). Activity resumed the next morning with a magnitude 5.1 event at 09:43:35 on 26 January (10 km depth), closing with a magnitude 4.5 shock at 10:20:56 (10 km depth). Depths remained shallow throughout, consistent with upper-crustal faulting above the subduction interface.

Such swarms represent periods of elevated but non-mainshock-aftershock seismicity. They often arise from fluid migration, slow slip episodes, or stress transfer along the plate boundary without a single dominant rupture. In this case, the tight temporal clustering and comparable magnitudes illustrate swarm characteristics rather than a classic foreshock-mainshock sequence.

Regional geology explains the occurrence. The area lies along the Hikurangi subduction margin, where the Pacific plate descends beneath the Australian plate at rates of approximately 4–5 cm per year. This convergent boundary generates frequent earthquakes, volcanic activity, and slow-slip events. The North Island's eastern offshore zone experiences both interface thrust earthquakes and shallower crustal events on splay faults. Historical records document recurrent moderate-magnitude sequences in this setting, underscoring its persistent seismic hazard.

Since 1 January 2000, only two swarms have been identified in the same offshore sector, with the first occurring in 2001. The 2020 swarm therefore represents a relatively rare but geologically expected episode within the margin's long-term behavior.

Seismic monitoring in New Zealand relies on dense networks operated by GNS Science, which rapidly detect and locate events. Continued observation of swarm patterns aids in refining hazard models for the Hikurangi margin, where large interface earthquakes remain possible over longer timescales.

References

GNS Science. Hikurangi Subduction Zone Overview. https://www.gns.cri.nz USGS Earthquake Catalog. Regional seismicity data for New Zealand. https://earthquake.usgs.gov Wallace LM et al. (2009). Characterizing the seismogenic zone of a major plate boundary subduction thrust. Geochemistry, Geophysics, Geosystems.