Seismicity, Stress Field and Lithospheric Structure of the Northern Hawkes Bay Region, New Zealand

Abstract

Hawke's Bay lies in the zone of subduction of the Pacific plate beneath the Australian plate in the North Island of New Zealand. A temporary network of 14 seismograph stations placed in the region recorded more than 400 micro-earthquakes, providing a data set for velocity and structure inversion. The observed seismicity distribution indicated that the subducted plate beneath and southeast of Hawke Bay has a dip to the northwest of 6 ±2 degrees. This dip increases quite rapidly to between 20 and 25° beneath the western side of the Hawke Bay syncline. A low impedance contrast, interpreted as being associated with the plate interface, was located at a depth of 20-22 kilometres beneath Hawke Bay using the observed arrival times of P, S and S-to-P converted phases. An inversion of these observed arrival times was carried out, with the simultaneous determination of the micro-earthquake hypocentres and velocity structure. This inversion indicated a layer of low P velocity (i.e. less than 5km/s) in the neighbourhood of the plate interface. The low velocity layer was interpreted as representing an appropriate thickness of subducted sediments beneath the micro-earthquake station network. Such sediment could possibly have a large effect on the coupling between the subducted and overlying plates. The seismicity distribution and the velocities indicated by the inversion work are consistent with a subducted crust of 11 to 12 kilometres thickness. Such a thickness, although greater than the 6 to 7 kms assumed for 'typical' oceanic crust, is less than the estimate of 15 kilometres made by Robinson in the Wellington region. Two solutions were developed for the mass distribution model that adequately fitted the seismic and gravity observations. In these models at least half of the observed negative gravity anomaly dominating the Hawke's Bay region is associated with the thickness of Plio-Pleistocene and Miocene sediments in the top few kilometres of the overlying plate. The remaining anomaly was attributed to the effect of relatively low density subducted crust along with the effect of a downwarp of the crust of the overlying plate beneath the region. A. consistent pattern of stress distribution was seen throughout the subduction zone; seismicity in the region appears to represent, primarily intraplate rather than interplate deformation. Focal mechanisms of events in the main band of micro-seismicity indicate normal faulting throughout at least the top 10 kilometres of the subducted Pacific plate; the mechanisms imply down-dip tension with a compression axis close to the vertical. The observance of reverse-faulting mechanisms beneath the main band of observed micro-seismicity may indicate that the effects of 'flexure' of the subducted lithosphere with subduction are important, at least in the region of the study if not further down-dip.