Urban Geophysics and Seismic Hazard Assessment in the Wellington CBD

Abstract

Many of our cities are built on sedimentary basins. Both the depth and shape of the basin will influence, and generally enhance the level of shaking. Our focus is the Wellington CBD, where we use seismic reflection and gravity methods to assess the depth and shape of the basin. An initial gravity survey of the Wellington CBD is interpreted to show sedimentary thicknesses of up to 450 m in the CentrePort area, and 200 m in much of Thorndon. But these interpretations depend on an assumed density contrast between sedimentary and basement rock. We use both P and S wave seismic reflection imaging, at discrete localities in the CBD to constrain or benchmark, our gravity interpretation. To obtain basement depths from two way travel time we adopt shear-wave velocity versus depth data furnished from logged, >100 m deep, bore holes in the Wellington CBD, and from surface wave dispersion studies. We report new results from three sites: Wellington Girls College hockey field, Waitangi Park and Miramar Polo Ground park. The gravity survey of the Wellington CBD highlights the importance of the subsurface expression of the Lambton Fault, which strikes NE beneath the railyards, and dips steeply to the SE. This is the effective geophysical edge of the Wellington Basin. Together with the parallel Aotea fault, the Lambton fault has created a bath-tub shape for the Wellington Basin which will have consequences for amplified seismic wave shaking due to basin edge-effects, constructive wave interference and resonance.