Microseismicity and Velocity Structure in the Hawkes Bay Region, New Zealand: Fine Structure of the Subducting Pacific Plate

Hawkes Bay overlies part of the shallow zone of subduction of the Pacific plate beneath the Australian plate in the North Island of New Zealand. A temporary dense network of 11 seismograph stations placed in the region recorded more than 350 micro-earthquakes in two months, providing a data set for velocity and structure inversion. The hypocentres define a seismic band 11–12 km thick, dipping 6°±2° beneath Hawke Bay and increasing to more than 20° towards the west side of the region. This seismic band is thought to delineate the crust of the subducting Pacific plate. S to P converted phases for deeper events are consistent with a fairly large impedance contrast across the plate interface at the top of the band. P and S arrival times were inverted to give a simultaneous determination of hypocentres and velocity structure. Results indicated a thin layer of low (∼4.7 ksm−1) P-wave velocity in the neighbourhood of the plate interface. We interpret this layer as a thickness of subducted sediments beneath the micro-earthquake station network. This is consistent with seismic reflection work to the south of Hawkes Bay and with sediment volume calculations. The stress distribution as given by focal mechanisms shows that the microseismicity represents primarily intraplate rather than interplate deformation, with the main band of microseismicity indicating normal faulting throughout at least the top 10 km of the subducted Pacific plate. The observation of reverse faulting at depth beneath the main band is interpreted as being due to the flexure of the subducted Pacific plate.