Wide-angle OBS velocity structure and gravity modeling along the SAHKE transect, southern North Island, New Zealand

Abstract

We acquired wide-angle reflection / refraction seismic data using ocean bottom seismometers (OBSs) along a transect in the southern North Island of New Zealand, where the Hikurangi Plateau, an early Cretaceous large igneous province, subducts westward beneath Wellington, the capital city of New Zealand. The survey was designed to investigate the physical parameters controlling locking at the plate interface beneath the southern North Island and characterize slip processes in a major segment of the Hikurangi system. We applied first-arrival travel-time inversion in order to obtain P-wave velocity structure along the 80 kmlong profile. Before manually picking travel times, in order to improve temporal resolution, we deconvolved each recorded trace by applying a filter to transform source signature of a corresponding airgun shot to a zero-phase Ricker wavelet. The velocity structure to ~25 km depth is well resolved. In order to visually compare the velocity structure and the geometries of reflection interfaces in depth, we calculated a diffraction migration image section for the OBS array. First, we made travel-time tables of diffracted waves between shot-receiver pairs for the grid points in the 2D section using the obtained velocity model. We stacked window segments of waveform at corresponding travel times for waves diffracted at each grid point using the travel time tables. Finally, we calculated a cross correlation coefficient with the Ricker wavelet that represents the total diffracted amplitude at each grid point. The resulting image successfully depicts fine structure at shallow depths and the subducting interface, and it reveals internal structure within the subducting Hikurangi Plateau. The depth image shows a rather constant thickness of ~13 km of the subducting oceanic crust dipping to the west from offshore near the trough axis.