Clark, K.J; Boyes, A.F.; McGrath, J.D. 2025 It’s Our Fault Hikurangi Subduction Zone Hazards Task Report FY24/25: exploring subduction earthquake records at the Wairau lagoons and Parangarahu lakes. Lower Hutt, NZ.: Earth Sciences New Zealand. GNS Science report 2025/37. 23 p.; doi: 10.21420/8V9P-3H26
Abstract
The southern Hikurangi subduction zone is a major source of earthquake and tsunami risk for the Wellington region. Previous It’s Our Fault: Nō Mātau to Te Hapa (IOF) programme research has identified four subduction earthquakes over the past 2000 years; however, this record is relatively short and limited to single-site evidence (in eastern Marlborough). To improve hazard models and better understand impacts of subduction earthquakes, further research is needed to constrain the size and frequency of past subduction earthquakes and distinguish them from earthquakes on upper-plate faults (e.g. the Wairarapa or Wellington faults). This report presents new findings from two sites on either side of Cook Strait where field work was undertaken with the aim of extending the earthquake record beyond 2000 years and confirming past earthquakes across multiple sites. Two cores from Lake Kohangapiripiri, near Wellington, show potential hints of disturbance that may be compatible with tsunami and earthquake shaking in both cores, but there are no very obvious indications of catastrophic marine incursions into the lake (e.g. such as a layer of rounded gravel, coarse sand and shell). Potential earthquake/tsunami indicators include coarse sand layers and likely lake environment changes; the strongest event evidence is dated at 5247–4195 years before present (BP), 3000–2800 years BP and c. 1500 years BP. With this research, we partially achieve the aim of extending the earthquake record <2000 years BP, but the evidence is too weak to warrant further investigation. Multiple cores were collected from throughout the Wairau Lagoons, near Blenheim, and there is evidence of likely tsunami and environmental change at c. 2000 years BP, as well as a possible tsunami and environmental change at 1600–1400 years BP. A previously identified earthquake and tsunami at c. 850 years BP is also well expressed in the new cores, enabling improved dating of this earthquake. A compilation of the new earthquake and tsunami evidence shows: • The subduction earthquake and tsunami age of 880–800 years BP can be re-modelled to 858–811 years BP. This more precise age still overlaps with an uplifted marine terrace in Palliser Bay but does not overlap with a recently revised age for the penultimate earthquake on the Wairarapa Fault. This suggests that joint rupture of the subduction interface and Wairarapa Fault is a less likely scenario than previously thought, but still points to ruptures closely spaced in time (clustered); • Multi-site evidence for a tsunami at 1500–1300 years BP. Previously identified at Lake Grassmere only, we now see probable evidence of this tsunami at the Wairau Lagoons and possible evidence for a disturbance at Lake Kohangapiripiri at this same time.; • Multi-site evidence for a tsunami at 2000 years BP. The c. 2145–1837 years BP paleotsunami at Lake Grassmere is also likely seen at the Wairau Lagoons and possibly at Puakamuri Stream on the Wairarapa coast. There may also be a broad enough spatial spread in sites to warrant tsunami modelling to match the paleotsunami sediment spatial extent to an earthquake source. These refined earthquake ages will enable refinement of the southern Hikurangi subduction earthquake recurrence interval and the probabilistic estimate of a subduction earthquake affecting the Greater Wellington region in the next 50 years. They also contribute to a better understanding of past subduction interface to upper-plate fault-rupture sequences, which will contribute to improving the modelling in hazard models such as the National Seismic Hazard Model. (auths)
