Velocity Structure of the Wellington Basin, New Zealand, from Passive Imaging

We passively image shear-wave velocity structure in an area of dense urbanization. Wellington, the capital of New Zealand is a densely populated urban area located at the boundary of the convergent Australian and Pacific plates. It is prone to the effects of both large strike-slip and large subduction earthquakes. The city lies on the edge of a funnel shaped basin and is underlain by the strike-slip Wellington Fault. The fault has average single event displacements of approximately 5 meters for surface-rupturing events. These events represent a significant seismic hazard. In order to accurately model wave propagation resulting from earthquake scenarios derived from geological mapping and calculations of synthetic seismicity, we must accurately know the basin geometry and velocity structure. Geotechnical data in the basin is sparse and insufficient to provide a full 3D basin model. The dense human development precludes the use of active source seismics for imaging. We therefore apply noise-based, passive seismic imaging to define velocity structure and basin geometry. We deployed eight 40 second and six 1 second seismometers to record both earthquake and ambient noise data. The ambient noise data have been cross-correlated and stacked for every station pair. It is widely accepted that such processing is capable of revealing the Green’s Function between the two stations. However, at many of the sites, the ambient noise is saturated by a heterogeneous distribution of anthropogenic sources. By selectively cross-correlating data with a lower noise-signal, we forestall complications arising from this heterogeneous saturation. We then apply a sequence of multiple filtering and phase-matched filtering to the Green’s Function to yield dispersion measurements. The resulting dispersion curves have been inverted with a non-linear scheme using the neighborhood sampler to calculate multiple, path-averaged 1D Vs profiles that have been integrated with existing geotechnical and geological data to generate a comprehensive basin model. Concurrent work is focused on modeling complex wave propagation through this model, including the effects of the basin's drastic bounding topography.