S U M M A R YThe Taupo Volcanic Zone (TVZ), New Zealand is a region characterized by very high magma eruption rates and extremely high heat flow, which is manifest in high-temperature geothermal waters. The shear wave velocity structure across the region is inferred using non-linear inversion of receiver functions, which were derived from teleseismic earthquake data. Results from the non-linear inversion, and from forward synthetic modelling, indicate low S velocities at ∼6-16 km depth near the Rotorua and Reporoa calderas. We infer these low-velocity layers to represent the presence of high-level bodies of partial melt associated with the volcanism. Receiver functions at other stations are complicated by reverberations associated with nearsurface sedimentary layers. The receiver function data also indicate that the Moho lies between 25 and 30 km, deeper than the 15 ± 2 km depth previously inferred for the crust-mantle boundary beneath the TVZ.

A deployment of 87 seismometers, including 23 broadband instruments, for a 5-month period in 1995 yielded a detailed view of the distribution and nature of the shallow seismicity (depth <20 km) within the central part of the Taupo Volcanic Zone (TVZ), New Zealand. On a broad scale, the pattern of shallow seismicity observed during this study was similar to that recorded by the permanent National Seismograph Network between 1987 and 1994. The distribution of seismicity was not uniform in either time period. Rather, it was scattered throughout the currently active portion of the Taupo Fault Belt, with a number of distinct clusters of events near the northern end of the fault belt. Specifically, in 1995, there did not appear to be any correlation between the seismicity and individual faults. With the exception of a cluster of events near Rotorua, little seismicity occurred on the western side of the TVZ. Similarly, on the southeastern margin of the TVZ, the TaupoReporoa Depression was characterised by low seismicity. Although a small group of earthquakes at Ohaaki were thought to be related to the exploitation of that geothermal system, there was no consistent relationship between geothermal systems and seismicity. Rhyolitic calderas, which mark the centres of previous intense volcanic activity, did not appear to influence the locations of the seismicity. More than 80% of well-recorded earthquakes occurred at depths of <6 km, and none occurred at depths >10 km. We argue that the upper 6 km of the crust represents the seismogenic zone, and that the base of this zone occurs approximately at the limit of the convective geothermal regime that occupies the upper crust in the TVZ.

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