Seismic velocity structure of the central Taupo Volcanic Zone, New Zealand, from local earthquake tomography

Abstract

The 3-D distribution of P-wave velocity (Vp) and the P-wave/S-wave velocity ratio (Vp/Vs) are derived for the crust in the central Taupo Volcanic Zone (TVZ), New Zealand, by tomographic inversion of P- and S-wave arrival time data from local earthquakes. Resolution in the seismogenic mid-crust (4–6 km) is good, but poorer above and below these depths. The 3-D velocity model has several Vp anomalies as large as ±5% in the mid–lower crust (4–10 km) and more than ±10% in the upper crust (0–4 km). The model achieves a 55% reduction in data variance from an initial 1-D model. Young caldera structures, Okataina, Rotorua, and Reporoa, are characterised by low Vp anomalies at a depth of about 4 km and these coincide with large negative residual gravity anomalies. We attribute these anomalies to large volumes of low Vp, low-density, volcaniclastic sediments that have filled these caldera collapse structures. Although there are no Vp anomalies which suggest the presence of molten or semi-molten magma beneath the TVZ, a large, high Vp anomaly of more than +15% and a high Vp/Vs anomaly are observed coincident with a diorite pluton beneath the Ngatamariki geothermal field. However, Vp anomalies cannot be seen beneath the largest geothermal fields, Waimangu, Waiotapu, and Reporoa, and, consequently, if such anomalies exist, they must be below the resolution of our data. A prominent Vp contrast of 5–10% occurs at a depth of about 6 km beneath the boundary between the Taupo–Reporoa Depression and the Taupo Fault Belt (TFB), coincident with the eastern limit of the seismic activity beneath the TFB. We interpret this velocity contrast as being caused by the presence of extensive, non-molten, intrusives beneath the Taupo–Reporoa Depression. We suggest that the high-velocity material beneath the Taupo–Reporoa Depression is isolated from regional extension in the TVZ, and from the resulting faulting and seismicity, which occurs preferentially within the weaker material of the TFB. We are unable to determine whether greywacke, which forms the basement beneath the eastern most part of the TVZ continues further west, or is replaced by a volcanic rock such as andesite which has similar Vp and density. Vp/Vs anomalies are much smaller than Vp anomalies and generally have little spatial relationship to the Vp pattern. There is a widespread decrease in Vp/Vs, from >1.76 to 1.70–1.73, between 4 and 6 km depth over much of the study area and Vp/Vs is high southwest of the Okataina caldera, where Vp is low. Hypocentres calculated using the 3-D velocity model differ little from those obtained using a 1-D model with station terms, however, some groups of earthquakes are more tightly clustered. Following relocation, there is a slight decrease in the estimated thickness of the seismogenic zone, with 73% of hypocentres between 4 and 7 km depth and a slight increase in the depth of the brittle–ductile transition from 6 to 6.5 km.