Tracking volcanic and geothermal activity in the Tongariro Volcanic Centre, New Zealand, with shear wave splitting tomography

We apply a simplified two-dimensional tomographic inversion of recorded delay times of shear wave splitting and a spatial averaging of fast direction of anisotropy to data from temporary seismic deployments in the Tongariro Volcanic Centre in order to identify regions of changing seismic anisotropy. We observe a region of strong anisotropy (> 0.025 s/km greater than the surrounding area) centered on Mt. Ruapehu in 1995, the time of a major magmatic eruption. This is interpreted to be due to an increase in fluid-filled fractures during the eruption. We also observe strong anisotropy (~ 0.018 s/km greater than the surrounding area) and a change in fast direction (~ 80°) at Mt. Tongariro in 2008 and examine the temporal evolution of this anomaly using clusters of earthquakes and permanent seismic stations in operation since 2004. This anomaly is attributed to a change in the geothermal system. A pronounced and unchanging feature is observed at Waiouru, even when the source and receiver locations differ. We therefore conclude that the transient features of strong anisotropy associated with volcanic and geothermal activity detected with this method are also robust.