| Popis: |
he 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 ( |
