Seismic Imaging of the Central Taupo Volcanic Zone Using Double-Difference Tomograpy

Abstract

We image the crustal seismic properties beneath a 30x50 km area encompassing the Wairakei, Mokai, Rotokawa, Ngatamariki, and Ohaaki geothermal fields, as part of a multi-disciplinary research program to investigate the untapped deep geothermal resource north of Taupo. A broadband seismometer array comprised of 38 seismometer sites was deployed across the region between 2009 and 2011, recording more than 500 earthquakes. We use the travel-times of these earthquakes to derive 3D P-wave and S-wave velocity structure, using double-difference seismic tomography. The derived 3-D Vp and Vp/Vs volumes highlight a high level of spatial and depth heterogeneity of the seismic properties beneath the study region, and provide new information on large-scale structures that may influence the deep geothermal resources. This study of seismic properties is complementary to magnetotelluric studies by GNS Science investigating electrical conductivity in the same region (e.g. Bertrand et al., 2012). The relocated seismicity provides the best dataset to date of earthquake locations in the TVZ; we observe areas where the inferred brittle-ductile transition appears to be several kms deeper than the previously assumed 6-8 km.