Shallow seismicity of the central Taupo Volcanic Zone, New Zealand: its distribution and nature

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 Taupo-Reporoa 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.