Stress and strain from earthquakes at the southern termination of the Taupo Volcanic Zone, New Zealand

We use first motion polarity data from small earthquakes recorded by a recent dense seismograph deployment to determine 16 new focal mechanisms at the southern termination of the Taupo Volcanic Zone (TVZ). These crustal earthquakes are between 11 km and 41 km deep, and have focal mechanisms which range from normal to strike-slip. We then combine these focal mechanism data with previously determined focal mechanisms to investigate earthquake strain directions. A prominent feature of the combined data is the consistency of the T-axes, which indicates that subhorizontal, ~ NNW extensional strain predominates at all depths in the crust at the southern termination of the TVZ. We then use all currently available first motion data to invert for the stress tensor orientation. First motion data from earthquakes throughout the crust show a consistent minimum principal stress (σ3) orientation, which is subhorizontal and oriented NW. The σ3 azimuth that we observe (316°) is perpendicular to the strike of recent aligned vents in the southern TVZ, very similar to the extension direction in the Ruapehu segment of the TVZ determined from structural data, and consistent with the σ3 direction determined from a stress inversion using crustal earthquakes in the central TVZ north of Lake Taupo. This suggests that the stresses driving the opening of the TVZ continue to be active at its southern termination. The stress and strain results, combined with the earthquake distribution, are consistent with a model where the TVZ is only the present day surface expression of an extensive region of active mantle corner flow that is progressively eroding thicker crust to the south. We also investigate the temporal evolution of stress around Mt Ruapehu, the southernmost large andesitic volcano in the TVZ, given previous suggestions that this may change during the eruption cycle. We find no major change in the stress tensor orientation at the time of the 1995 eruption, using first motion data from earthquakes 15–20 km from the volcano.