Seismicity, structure and tectonics of the Wellington region, New Zealand

Abstract

The Wellington region lies on the border between the Pacific and Australian plates, the former being subducted. The plate interface lies at shallow depth under the region (10–60 km) so that detailed and long-term monitoring of the seismicity of the zone where large thrust earthquakes are expected to occur is possible. Hypocentres for events recorded by a 12 station seismic network for the period 1978–1982 (7231 events) have been refined by the use of a three-dimensional velocity model derived from a simultaneous inversion of arrival-time data from 129 local events for both hypocentres and velocity. The hypocentres show that the subducting Pacific Plate is defined by a NW dipping zone of relatively intense activity. The plate interface, as given by the upper envelope of this activity, is offset vertically, by 7 km on average, along a NW—SE line through Cook Strait parallel to the direction of absolute motion of the Pacific plate rather than to the direction of convergence. The strike of the interface at deeper levels also changes across this line. This distortion may partly reflect effects of the transition to continent-continent-type convergence further south. Within the subducting Pacific Plate the seismic activity defines two parallel zones separated by about 20 km, the lower zone being much less active and perhaps merging with the upper zone at a depth of 80–90 km. Composite focal mechanisms of events in the upper zone imply predominantly down-dip tension with a compression axis perpendicular to the plate interface. The mechanism in the lower zone also implies down-dip tension but the compression axis has rotated to be horizontal and along strike indicating a component of lateral shearing within the plate. These two types of mechanism correlate well with those found previously for events at depths of 150 km or more. Thus it seems that the large negative buoyancy of the subducted lithosphere dominates the internal stress field throughout the length of the seismically active region, the lithosphere acting as a stress guide. There is no evidence for a significant number of events with mechanisms indicating thrusting along the plate interface. The seismic velocity in the region corresponding to the upper seismic band is close to that typical for oceanic crust, which here is probably considerably thicker than normal. There is no evidence for a substantial increase in velocity down-dip as would be expected if a pervasive basalt to eclogite phase-change were taking place. The velocity deeper in the subducting lithosphere suggests the presence of anisotropy as often observed in the oceanic mantle. Within the overlying Australian Plate (here taken as including the ‘accretionary bordera’) the seismic activity can be separated into three regions whose boundaries are related to both surface geology and structure in the underlying plate interface. Shallow (0–5 km) seismic velocities in the Australian Plate vary substantially in accordance with the surface rock type. They are lowest, especially for S-waves, in the SE region, commonly described as part of an accretionary border. However, at deeper levels in this region the velocity is not especially low indicating that no substantial amounts of accreted sediments have been incorporated. Thus the description of this region as a compressed and deformed pre-subduction borderland of the Australian Plate is perhaps more accurate. Composite focal mechanisms for activity in the Australian Plate imply largely strike-slip faulting with an east-west oriented compression axis, parallel to the direction of plate convergence. This indicates that the plate interface is currently locked but probably down to depths of only 20 km where there is a concentration of activity (seismic ‘front’) in the subducted plate. The lack of a significant number of events with interplate thrust mechanisms in the locked region could reflect very strong coupling due to the buoyancy of the thicker than normal subducted crust and/or a late stage in the ‘seismic cycle’. The seismic evidence, combined with geodetic observations, indicates that strain is now accumulating in the region, probably to be released by faulting within the Australian Plate (such as on the Wairarapa fault which slipped during a magnitude 8+ event in 1855) or on the plate interface itself. The offset found in the plate interface might serve to limit the rupture areas of such events, as may have been the case in 1855. It might also be the focus of possible precursory activity. These results will provide a basis for long-term monitoring of the seismicity of the Wellington region so that comparisons of activity throughout the seismic cycle can be made.