Earthquake focal mechanisms in the central Taupo Volcanic Zone and their relation to faulting and deformation

The New Zealand Taupo Volcanic Zone (TVZ) is an area with considerable crustal seismicity, but the shallowness of the seismicity has made it difficult to determine good focal mechanisms from the permanent seismic stations. Within the central TVZ, the only focal mechanisms previously available were composite ones derived from earthquake swarms. These mechanisms were dominantly strike-slip, rather than the normal mechanisms that might be expected from the dominant southeast-northwest extension seen from deformation measurements and geology. The 1987 M(L) 6.3 Edgecumbe earthquake near the northern end of the zone had a normal mechanism consistent with the deformation, as did some large earthquakes just offshore. We present focal mechanisms obtained from a 6 month (Jan-Jun 1995) seismometer deployment consisting of 64 short-period and 23 broadband seismometers together with the 15 permanent stations within the TVZ. This close-spaced network enabled us to get focal mechanisms for a considerable number of crustal earthquakes. We present here 23 TVZ mechanisms, mainly for events around the Rotorua lakes, and southwestwards towards Lake Taupo, together with one strike-slip mechanism for an earthquake east of the TVZ. The focal mechanisms are mainly normal, consistent with the observed extension, although most mechanisms have a strike-slip component. Three earthquakes appeared to have thrust mechanisms, but a non-double-couple solution is also a possibility for two of these events. Even within a single swarm, earthquakes with normal focal mechanisms had considerable variation in the strike direction. These variations suggest that the high pore pressure has produced a very low strength region in which slip can occur in a range of directions. These normal mechanisms make unnecessary the model of extension by conjugate strike-slip events proposed by Smith & Webb in 1986. We also carried out a direct stress inversion from the first-motions of a larger set of earthquakes. The principal stress directions obtained by this method agree with the average for the 19 normal earthquakes, and both support extension in a northwest-southeast direction, in general agreement with the geodetic results.