Howell A, Keats BS, Moratalla JM, Coffey GL, Litchfield NJ. 2022. It’s Our Fault subduction zone hazard: field investigations at Puakimuri Stream and numerical modelling of deformation and ground motions due to subduction earthquakes. Lower Hutt (NZ): GNS Science. 26 p. (GNS Science report; 2022/55). doi:10.21420/6MA4-6W73.
Abstract
The Hikurangi Subduction Zone (HSZ) is Aotearoa New Zealand’s largest, fastest-slipping active fault, with a large patch that is locked and accumulating strain immediately underneath the Greater Wellington region. Subduction earthquakes represent a major source of seismic hazard for Wellington, but, despite this clear importance, the behaviour of the southern HSZ remains poorly understood. The It’s Our Fault (IOF) Subduction Zone hazard task focuses on improving understanding of subduction earthquake hazard through: (1) field investigations to constrain the timing and magnitudes of past earthquakes and (2) numerical modelling of the effects of plausible subduction earthquakes. This report summarises work during the 2021/22 financial year: we present results from physics-based multi-cycle earthquake simulations and resulting ground motions, and preliminary field investigations of Holocene marine terraces at Puakimuri (Pukemuri) stream in south Wairarapa district. Models of synthetic earthquakes using simulator RSQSim show that a wide range of plausible subduction earthquakes can cause both uplift of the Wairarapa coast and subsidence at Mataora-Wairau Lagoon. These results further demonstrate that physics-based simulators could potentially be a useful tool for modelling seismic hazard associated with combined ruptures of the Hikurangi subduction interface and upper-plate faults in the Wellington region; such ruptures are currently not considered in available seismic hazard models. Geophysical and ground auger investigations at Puakimuri Stream confirm it as a prospective site for obtaining constraints on the timing of past earthquakes. We identify acquisition of vibracores as the most effective method for future work to constrain earthquake ages at Puakimuri Stream. Finally, we present radiocarbon ages from shells in an uplifted beach at Manurewa Point near Puakimuri Stream. The shell ages constrain the timing of the most recent earthquake at Manurewa Point to be 982–1204 years BP (95% confidence), which is similar to inferred ages for the most recent earthquake at nearby sites. Based on this age, we attribute the most recent uplift at Manurewa Point to an earthquake on the upper-plate Palliser–Kaiwhata Fault. (The authors)
