Stagpoole, V.M.; Black, J.; Hill, M.P.; Stratford, W.R.; Tozer, B. 2022 The 2021 pilot gravity survey of Rongotai Isthmus. Lower Hutt, N.Z.: GNS Science. GNS Science report 2021/60. 32 p.; doi: 10.21420/B2VV-5C91
Abstract:
The subsurface of the Rongotai isthmus, the land area of the Rongotai–Miramar region of Wellington, consists of a series of Quaternary sedimentary basins upon which key infrastructure for Wellington City reside. Basin depths and geometries, critical parameters for seismic microzoning and hazard modelling, are currently poorly constrained in this region. In this study, we describe a pilot study where gravity data were acquired in a busy urban environment and modelled to evaluate the method for determining basin structure at an appropriate scale for seismic microzoning for earthquake effects. A gravity profile spanning c. 4.4 km from Melrose in the west to Fort Dorset in the east, with a measurement spacing of 25–100 m, was used to assess data acquisition issues and analyse the sensitivity and accuracy of gravity models for recovering basement depth. Two boreholes close to the gravity profile provide constraint on the thickness of the sedimentary succession for gravity models, which were then used to derive the average density contrast between greywacke basement and the overlying strata and to predict the basin geometry. Three different modelling programmes were tested. Similar sediment densities, basin structures and depths were produced by all three models. The models indicate that the sediments reach a maximum thickness of c. 100 m beneath the western end of Broadway in Miramar. Sensitivity analysis indicates that gravity models can be used to determine fault offsets that exceed c. 10 m in the basin where the basement is close to the surface (<20 m). Faults that offset greywacke basement at greater depths beneath the sedimentary succession have a smoother gravity signal and can be modelled either as a fault or a steeply dipping basement topography. However, where there is geological evidence for faults, gravity models can be used to constrain their offset and refine their locations. Results of the pilot survey provide new information that can potentially be used to improve seismic microzoning for earthquake effects in the vicinity of the gravity profile on the Rongotai isthmus. Further 2D gravity profiles on the isthmus will help to constrain the 3D geometry of the basin and refine seismic microzoning for the region. (auth)
