Determining Hydrological and Soil Mechanical Parameters from Multichannel Surface Wave Analysis across the Alpine Fault at Inchbonnie, New Zealand

Combining S-wave data, resulting from surface-wave dispersion analysis with P-wave tomographic data, is a valuable tool to improve the understanding of near-surface soil properties and allows the estimation of soil mechanical parameters and the determination of the depth of a water table. To achieve this combination of methods in a complex fault zone setting, active-source seismic data were acquired at Inchbonnie, New Zealand across the Alpine Fault. This is a major transpressional strike-slip fault that has generated magnitude > 7.8 earthquakes in the past. In this study, we focus on the surface-wave component of these data, to determine elastic parameters for the shallow (~60 m) subsurface as well as the depth of the water table. We achieve this by combining S-wave velocity models from surface-wave dispersion curve inversion and P-wave velocity models obtained from traveltime tomographic inversion in a previous study. The surface-wave dispersion curve inversion is done by means of a laterally constrained inversion algorithm. As a result, we are able to obtain elastic parameters and map the water table and the geology around the Alpine Fault at Inchbonnie, New Zealand. The Alpine Fault itself appears as a relatively sharp lateral discontinuity in all investigated parameters.