A new method for mapping and modelling buried faults within the Auckland Region, using gravity and borehole data

Abstract

Limited data is currently available on the presence and extent of buried faults within the Auckland region. This thesis presents a method for mapping and modelling buried and obscured faults using geological and geophysical data. The method integrates borehole and surface lithological data, informed by gravity survey data in a 3D modelling software to produce high-resolution models from which fault location and geometry can be derived. This method is trialled on the Waikopua North Fault, selected from the buried faults proposed in previous studies. Located just to the west of the Auckland suburb of Beachlands, the Waikopua North Fault exhibits variable surface expression and was previously thought to be a minor fault structure, with offset of less than 50 m in the Waipapa Group basement. The high resolution, integrated geological models derived in this study confirms the existence of the Waikopua North Fault as a northward continuation of the Wairoa-Waikopua Fault Complex. The integrated model (Model 4) indicates the presence of a complex zone of normal faulting, with a variable dip angle of between 40oW and 70oW, and combined offsets of up to 200 m within the Waipapa Group basement. The fault represents a potential land stability hazard in an area of increasing urban development and should be further investigated to this end. This method, though not without uncertainty, is proven to be applicable to the task of identifying and mapping buried faults in the case study area, and future work could apply the method throughout the Auckland region.