Crustal shear wave tomography of the Taupo Volcanic Zone, New Zealand, via ambient noise correlation between multiple three-component networks

The central North Island of New Zealand is characterized by extension, high heat flow, and active volcanism caused by the Pacific plate subducting beneath the Australian plate off the east coast of the North Island. The Taupo Volcanic Zone (TVZ) is the current locus of rifting and active volcanism. The shear velocity structure derived here from short-period surface wave analysis fills a gap in the geophysical information obtained in previous active and passive source studies. Using ambient noise correlation techniques, we reprocess data from four three-component temporary seismic arrays that were deployed between 2001 and 2005 in the central North Island and data from permanent seismic stations operational at the same time. Special attention has been paid to the correction of timing errors and incomplete instrument response information. Low shear velocities in the top 15-20 km of the crust inferred from Rayleigh wave dispersion analysis coincide with the presumed source regions of rhyolitic magma in the TVZ and are correlated with conductive bodies inferred from a 3-D magnetotelluric survey. Based on the observed changes in shear velocity we use empirical relations to infer an average percentage of partial melt in the upper crust of < 4%. Comparison of our shear velocity model with compressional wave cross sections from active and passive source studies and shear velocities derived from previous 3-D V-p and V-p/V-s models reveals several consistent features, especially in the lower crust. The greater sensitivity of shear velocity than compressional velocity to the presence of melt and other fluids aids in linking seismic and electromagnetic observations and provides additional constraints on the distribution of magmatic structures in active continental rifting environments.