Palaeotsunami Deposits On The Southern Hikurangi Margin; Contributions From Lake Grassmere, Marlborough

Abstract

The Hikurangi subduction margin poses the most significant hazard for New Zealand, with particular threat for populated regions on the east coast. Strain accumulation on the margin suggests likely future scenarios of ruptures of > Mw 9, with great potential for large tsunamis. The southern section of the Hikurangi subduction margin is particularly poorly constrained, due to the lack of historic earthquakes and limited evidence of palaeotsunami and palaeoearthquake. Furthermore, the unknown rupture extent into the Cook Strait has major implications for tsunami height.
The shallow coastal embayment of Lake Grassmere in Marlborough is located at the southern end of the Hikurangi margin and has potential for preserving a record of prehistoric subduction earthquakes. In this thesis, I conduct a multiproxy study of sediment cores from the lake edges that includes; grain size analysis, microfossil analysis and radiocarbon dating. From these results, I delineate the palaeoenvironmental evolution of Lake Grassmere through the late Holocene. Two anomalous deposits are
identified and characterised by a densely packed shell hash of articulated and disarticulated bivalves in sand, with sharp, erosive contacts that are laterally extensive over 1.7 km inland. The deposits display many affinities with globally derived characteristics of palaeotsunami deposits and therefore I attribute a tsunami source to both units. Consideration is given to the possible evidence of coseismic vertical uplift within the cores and the beach ridge sequence. Radiocarbon dates place Tsunami 1 at 2089 -1875 cal BP and Tsunami 2 at 1509-1314 cal BP. Regional palaeoseismology presents no suitable synchronous upper plate ruptures and therefore I suggest that the simplest explanation is a subduction earthquake on the southern Hikurangi margin. Attributing this fault source has major implications for developing understanding of the southern section of the margin and I recommend next steps that should be taken to further this study.