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Hill MP, Kaiser AE, Wotherspoon LM, Manea EF, Lee RL, de la Torre CA, Bradley BA. 2022. 3D geological modelling of Wellington Quaternary sediments and basin geometry. Lower Hutt (NZ): GNS Science. 58 p. (GNS Science report; 2022/33). doi:10.21420/TS0B-8A37.
Abstract
This project has developed two new three-dimensional (3D) models of geological and geotechnical properties for Wellington: one regional-scale model from the Cook Strait north to Porirua; and the other city-scale model encompassing the Thorndon, Wellington central business district (CBD), Te Aro and Miramar areas. The models were created using the latest geomorphic and geological map interpretations, a borehole database and new geophysical investigations. The models interpret the accumulations of loose to dense Quaternary sediment deposited on weathered Rakaia Terrane greywacke (basement) rocks. These sediments are located in multiple basin structures that vary in thickness from a few metres to several hundred metres. Shear-wave velocities have been assigned to each layer in the model to create a 3D regional velocity model for site or basin amplification studies. These basins have amplified ground motions during past earthquakes and are therefore important for ground shaking and geotechnical studies.
The Wellington Region 3D geological model combines data for Wellington City, Rongotai Isthmus, Wellington Harbour, Lower Hutt, Upper Hutt and Porirua to create maps of Rakaia Terrane greywacke elevation and Quaternary sediment thickness. The Wellington City 3D geological model uses detailed data at the city-scale for the Thorndon, Te Aro, Rongotai and Miramar basin areas. Detailed geological mapping controls the surface geology of the model, and thousands of borehole data records and results from geophysical studies control the subsurface interpretation. The model is divided into eight blocks that are defined by the Ohariu, Wellington, Aotea, Evans Bay east and west, and Seatoun faults. Within these blocks, 19 lithological units are modelled: one water, four anthropogenic fill, four Holocene sedimentary, nine glacial and interglacial sedimentary, and the basement. The glacial and interglacial sedimentary units are divided into Late Pleistocene and mid-Pleistocene time-period units and are also divided into loose, dense, a loose–dense transition layer and very dense sediments.
The two 3D geological models can be viewed in 3D modelling software and can be evaluated at surface points, in sections through the model or as blocks with material properties. The surfaces can also be mapped in two dimensions (2D) as isopach values (thickness of sediment); as elevation relative to sea-level; or, importantly for many studies, as a thickness of Quaternary sediments (also referred to as ‘depth to basement’). In Wellington, the depth to basement surface is also a reasonable proxy for Z1.0; the depth to material of shear-wave velocity >1 km/s. The modelled Quaternary sediment from this study is up to 560 m thick along the coast adjacent to Aotea Quay and over 600 m thick in the harbour opposite Ngauranga Gorge in the basins controlled by the Wellington Fault. Estimated fundamental site period or 30-m time-averaged shear-wave velocity (Vs30) can be created from forward modelling of the sediment thickness data derived from the geological model, and data from the models can be used in earthquake-induced ground-motion modelling.
The digital 3D geological models, model surfaces and volumes, as well as contours of sedimentary thickness, are available as a digital appendix. The geological models were created in Leapfrog Geo geological modelling software and are continuously updated as new data, such as borehole records, are publicly released, or as insight and research extends our understanding of the subsurface geology. Several questions about the geometry of the surfaces, deep harbour stratigraphy, thickness of Quaternary sediments and correlation between geophysical data and basement still remain, and we hope that future versions of this model with additional data will help address these. (The authors)
