Clark, K.J; Howell, A.; Abbott, K.W.; Norton, T.; Coffey, G.L.; Medina, I; Dahl, J.; Norton, K. 2023 It’s Our Fault Hikurangi Subduction Zone Hazard Task Report 2002–2003: Wairau Lagoons earthquake and tsunami study. Lower Hutt, N.Z.: GNS Science. GNS Science report 2023/41. 24 p.; doi: 10.21420/07F5-KR16
Abstract
The southern Hikurangi subduction interface represents one of the most significant sources of seismic and tsunami hazard for the Wellington Region. The It’s Our Fault: No Matou Te Hapa programme subduction earthquake task is focused on better understanding the hazard of the southern Hikurangi subduction zone through studies of past earthquakes and tsunami. In this report we present the preliminary results from new investigations at the Wairau Lagoons, near Blenheim, that aim to refine the existing subduction earthquake record from this site and extend the record of earthquakes and tsunami further back in time. Preceded by a period of consultation with mana whenua and obtaining permits for field work, in February 2023 we collected sediment cores from six new locations across the Wairau Lagoons. All cores underwent computerised tomography (CT) scanning and about half the cores have been split open and photographed. From one core (BL23-V7) we obtained 26 radiocarbon ages. The previously observed buried salt marsh soil associated with earthquake-driven subsidence at ~500 years BP was widely observed in cores from across the lagoons. We have not yet obtained more radiocarbon ages to confirm and refine the age of this earthquake. Most cores contain evidence of the previously observed tsunami sand and buried soil dated at 880–800 years BP. New radiocarbon ages provide a preliminary revised age for this earthquake of 858–811 years BP. This small change in the event age may help up to better understand if the subduction zone ruptured with upper plate faults in this earthquake. Prior to ~1000 years BP it appears the lagoon was more connected to the open ocean and deeper. The sediments deposited prior to the 800–880 year BP tsunami are typically homogenous silts and fine sands that are characteristic of intertidal to subtidal embayments. While we had hoped to find locations that preserved sensitive high saltmarsh environments prior to 1000 years BP in our new suite of cores, we have not found this. Despite this, anomalous deposits do exist deeper in the sediment cores and may indicate past sudden changes associated with earthquakes and/or tsunami. These warrant further study and most likely additional core collection. Future work will involve studying the cores that have not yet been opened, submitting more radiocarbon ages and focusing on the anomalous deposits identified deeper in cores BL23-V7 and BL23-V1. (auths)
