Multi-Component Ambient Seismic Noise Tomography of the Auckland Volcanic Field

Abstract

Auckland, New Zealand's most populous region (1.6 million), is situated over an active volcanic field of at least 53 volcanoes. The mechanism driving volcanism in the Auckland Volcanic Field is not well understood. We aim to increase knowledge of the structure of the crust across the Auckland Volcanic Field through multicomponent ambient noise tomography. We hope this will contribute toward understanding possible structural controls on volcanism. We first estimated the orientation of borehole seismometers by polarization analysis of estimates of the Rayleigh wave Green's function, retrieved from ambient noise cross-correlation. Knowing the orientations, we then computed multi-component estimates of Rayleigh wave Green's functions. We performed frequency-time analysis on the Green's functions to extract Rayleigh wave dispersion information. We then used Rayleigh wave group arrival times to perform a reversible-jump Markov chain Monte Carlo inversion for Rayleigh wave speed maps. We took dispersion curves from the Rayleigh wave speed maps, and performed a damped inversion of each dispersion curve for a 60-layer 1D shear speed model. Lastly, we then stitched the 1D models together, creating a pseudo-3D shear speed model. The 3D model is a considerable improvement compared to previous tomographic models, being more robust and having improved resolution. The model best resolved the upper-most crust but has also been able to resolve some features as deep as 25 km. One of the main features in the model is a NNW-SSE boundary slightly east of the Junction Magnetic Anomaly and the volcanic centres of the AVF. This boundary more clearly defines an eastern high speed region and a western low speed region in the upper 5 km divided. We interpret the high speed region as the Waipapa Terrane, and the low speed region as the Murihiku Terrane. Deeper we see a reversal, with a high speed region west of the boundary and a low speed region in the east. We interpret the high speed region to be the continuation of the Dun Mountain-Maitai Terrane, and the low speed region in the east an effect of metamorphic weakening of lower parts of the Waipapa Terrane. We identify a long low speed zone along the NNW-SSE boundary, and other features indicating possible connections to faults and the Takapuna gravity anomaly.