Litchfield NJ, Howell A, Clark KJ, Coffey G. 2021. It’s Our Fault Hikurangi Subduction Zone hazard: south Palliser Bay Holocene marine terraces. Lower Hutt (NZ): GNS Science. 47 p. (GNS Science report; 2021/28). doi:10.21420/8VDN-AE32.
Abstract:
The Greater Wellington region sits above a currently locked part of the Hikurangi Subduction Zone (interface) and, to date, no unequivocal evidence for past subduction earthquakes has been found in the region. In this study, we investigate Holocene marine terraces along the south Palliser Bay coast to test if they were uplifted in earthquakes on the Palliser–Kaiwhata Fault in the Pacific Plate or the Hikurangi subduction interface. Up to four low-lying marine terraces were mapped for the first time in this study using high-resolution maps from Light Detecting and Ranging (LiDAR) data. Heights of these terraces, and elevation differences between them, were compared with modelled earthquake-generated uplift derived from elastic dislocation models. The surface heights of individual terraces were shown to vary on a tens- to hundreds-of-metres scale and not over kilometres, which is likely to reflect coastal processes rather than tilting from earthquake uplift. Dislocation modelling shows that the terrace heights are consistent with uplift by an earthquake on either a listric (curved) Palliser–Kaiwhata Fault or the Hikurangi subduction interface. Six pits excavated on the mapped Holocene marine terraces at two sites, Te Humenga Point (three terraces) and SW Kawakawa Station (two terraces), reveal beach deposits resting on a bedrock shore platform and overlain variable thicknesses of coverbeds. Radiocarbon ages were obtained from four pits, but multiple ages from shells in beach deposits were only obtained from the youngest terrace at Te Humenga Point. Nine of ten radiocarbon ages date the youngest terrace to 920–660 cal. yr BP. The terrace morphology and tight clustering of the ages supports previous interpretations that this closely approximates the timing of an earthquake. Synchronicity of this uplift (earthquake) with previously published earthquake ages from Mataora–Wairau (Big) Lagoon (Marlborough) and the Cape Palliser – Tora (eastern Wairarapa) coast further suggests that this earthquake was a Hikurangi subduction earthquake, as vertical deformation generated by the much smaller Palliser–Kaiwhata Fault would not be that extensive. Further work is needed to investigate the fault responsible for earthquake uplift of the higher terraces, as well as to search for evidence of tsunami triggered by the earthquakes. (auth)
