Seismic Travel Time Tomography of the Auckland Volcanic Field

Abstract

The Auckland Volcanic Field (AVF) is a monogenetic intraplate volcanic field located right beneath New Zealand’s biggest city, Auckland. Despite the potential hazard it imposes, much of the structure and driving mechanism in the AVF are remain to be known. A high resolution three dimensional geophysical model is important to better understanding of the volcanism in the AVF. However, past geophysical studies, in particular regarding the model of deeper structures are generally lack in resolution. This research aims to infer three dimensional geophysical structure of the Auckland Volcanic Field, with focus on the seismic velocity structure deduced from P-wave travel time tomography. For this purpose, the Fast Marching Method, a grid based eikonal solver, is implemented. Capable of consistently finding the first arrival travel time for specific source-receiver paths, this method has low computing cost, and is stable, and robust. The first part of this research is to perform synthetic checker-board tests to assess the potential resolution that can be achieved. The size and geometry of the checker-board pattern is the key in inferring the scale and coverage of the resolvable features. We found that subsurface features on the order of 60 km can be resolved to depth reaching 300 km while smaller features on the order of 30 km can be resolved at least to 80 km depth. The second part of this study is to use real earthquake data in order to probe into the subsurface structure of the Auckland Volcanic Field. The adaptive stacking method is implemented to obtain the travel time residuals of 131 teleseismic events. The outcome shows structural heterogeneity in the AVF, consistent with the Junction Magnetic Anomaly associated with the Dun Mountain Ophiolite Belt that extends to at least 80 km depth.