Identifying obscured faults in urban areas: insights from Tāmaki Makaurau Auckland, Aotearoa New Zealand

Abstract

Locating faults can be challenging in urban environments and low strain rate regions, where surface expressions of faulting are erased by anthropogenic and surface processes since the last earthquake. While urban development can provide challenges, it can also provide rich resources such as geotechnical boreholes and high-resolution LiDAR topography. The Tāmaki Makaurau Auckland area of Aotearoa New Zealand is the country’s most populated region and lies in a low strain rate setting, where intense urbanisation and Quaternary volcanics mask potential faults. We developed a new evidence-focused workflow to identify and assess the reliability of obscured structures in Auckland, using borehole, geophysical and outcrop data. From a database of >8200 boreholes we mapped the present-day top of the Waitematā Group, whether exposed or concealed, which we term the Waitematā Group erosion surface (WGES). We assume significant present-day differences in elevation between WGES blocks are the result of faulting, and thus identify 46 post-Miocene structures, including 10 likely and 25 possible faults. We provide a new GIS database of borehole and fault data to facilitate use in hazard assessments and urban development. This work shows how data associated with urbanisation can be leveraged to complement traditional methods of identifying obscured faults.