Search for temporal changes in shear-wave splitting associated with the 2012 Te Maari Eruptions at Mount Tongariro, New Zealand

We investigate changes in shear wave splitting and VP/VS ratios of local earthquakes from the GeoNet catalogue during a 16 month period beginning a year before the first Te Maari eruption at Mount Tongariro on August 6, 2012, focusing on four permanent seismographs located in proximity to the volcano. We identify four time periods bounded by sharp transitions that comprise the study period, during which moving averages of the shear-wave splitting parameters, Φ (fast direction) and δt (delay time), are fairly constant. At all stations, VP/VS is steady throughout most of the study period at 1.75. Small variations occur during the earthquake swarm at the volcano, which started a month before the first eruption, and for some low magnitude events occurring after a change in earthquake location method. Analysis of data sets in which epicentre location, hypocentre depth and event magnitude are restricted illustrates that observed temporal changes in shear-wave splitting parameters are likely due to the spatial variation of paths. This in turn is governed by the spatial distribution of seismicity and measurement quality. We think the short term variation in VP/VS ratios is due to event origin time uncertainty of low magnitude earthquakes or incorrect S-phase arrival timing for events in the Tongariro swarm. These results suggest that any volcanic processes able to cause changes in shear-wave splitting or VP/VS associated with the two eruptions during our study period were too localised to Te Maari to be observed at the seismographs studied using our methods. Dominant Φ observed during the study period are oriented approximately tangential to the Tongariro/Ngauruhoe massif at all four stations. We suggest that this may result from gravitational loading of Tongariro and Ngauruhoe mountains inducing fracturing or dilatation of tangentially oriented microcracks. There may also be some effect from layered material causing horizontal propagating rays yielding faster speed SH waves than SV at station NGZ. Measurements from shallow earthquakes in immediate proximity to the seismographs indicate the potential presence of a shallow, highly anisotropic body in the volcano.