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 natu re 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 sh allow seismicity observed during this study was similar to that recorded by the permanent National Seismograph Network between 1987 and 1994. The dist ribution of seismicity was not uniform in either time period. Rather, it wa s 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 correlat ion between the seismicity and individual faults. With the exception of a c luster of events near Rotorua, little seismicity occurred on the western si de of the TVZ. Similarly, on the southeastern margin of the TVZ, the Taupo- Reporoa Depression was characterised by low seismicity. Although a small gr oup of earthquakes at Ohaaki were thought to be related to the exploitation of that geothermal system, there was no consistent relationship between ge othermal systems and seismicity. Rhyolitic calderas, which mark the centres of previous intense volcanic activity, did not appear to influence the loc ations of the seismicity. More than 80% of well-recorded earthquakes occurr ed at depths of <6 km, and none occurred at depths >10 km. We argue that th e upper 6 km of the crust represents the seismogenic zone, and that the bas e of this zone occurs approximately at the limit of the convective geotherm al regime that occupies the upper crust in the TVZ.