A seismic swarm designated VS20221201.1 occurred across the North Island of New Zealand from 10:48 on 30 November 2022 to 16:37 on 1 December 2022. Over 29 hours and 49 minutes, the sequence produced 26 earthquakes. The events clustered in a compact area and remained at shallow depths between 2 km and 12 km, consistent with activity in the upper crust.
The swarm opened with a magnitude 5.3 earthquake at 5 km depth. Subsequent events included multiple shocks of magnitude 3.0–3.9, with the largest aftershock reaching magnitude 4.5 at 10 km depth. Later activity featured events of magnitude 3.7–3.9 at depths of 6–12 km. All events stayed below destructive thresholds, yet the rapid succession indicated a concentrated release of strain.
The North Island lies along the Hikurangi subduction zone, where the Pacific Plate descends beneath the Australian Plate. This tectonic setting drives both volcanic and seismic activity throughout the Taupo Volcanic Zone. Shallow swarms commonly occur when fluid migration or minor fault slip relieves stress within the brittle upper crust. Historical records document similar sequences near the Bay of Plenty and central volcanic plateau, often without leading to larger mainshock-aftershock sequences or eruptive episodes.
Geological mapping shows that the region hosts numerous active faults capable of hosting small-to-moderate events at depths of 5–15 km. The 2022 swarm aligns with this pattern, remaining confined to the seismogenic zone above the plate interface. No surface rupture or significant ground deformation was associated with the sequence.
Monitoring by GNS Science recorded the full set of events in real time, confirming the shallow focus and limited spatial extent. Such swarms provide valuable data on stress transfer within the overriding plate and help refine models of subduction-related deformation.
References
GNS Science. (2023). New Zealand Seismicity Annual Report 2022. Lower Hutt: GNS Science.
USGS Earthquake Hazards Program. Hikurangi Subduction Zone Overview. https://earthquake.usgs.gov
Ministry of Business, Innovation and Employment. (2022). Geological Hazards in New Zealand. Wellington: MBIE.