Depth and Shape of the Basement Surface Beneath Wellington City, Based on Gravity and Seismic Constraints

Abstract

A new basin-depth map for the Wellington Central Business District shows a maximum depth of 540 m near the Wellington Stadium. This is twice that previously proposed. Our new basin geometry constraints are from a residual gravity anomaly map, based on ~600 new gravity observations. Residual gravity anomalies are as large as -6.2 mGal with uncertainties <0.1 mGal. Two-dimensional gravity models constrained by boreholes that intersect basement are used to generate the basin depth map. Our gravity models indicate the location of a possible onshore extension of the recently discovered Aotea Fault on the western side of Mt Victoria. A maximum basement offset of up to 130 m and gravity anomaly gradients up to 8 mGal/km are observed across the fault. Seismic reflection data on the down thrown side of the Aotea fault provides a constraint on the gravity model. A secondary splay off the main Aotea fault trace is identified in the NW corner of Mt Victoria, and a possible extension of the Lambton fault is identified beneath the Wellington Railway Station. This new basin depth and fault trace data will provide valuable constraints to seismic hazard models for Wellington City.