A microearthquake study of the plate boundary, North Island, New Zealand

The seismicity, structure and tectonics of the Indian/Pacific plate boundary in the North Island of New Zealand have been studied by means of a microearthquake traverse oriented in the direction of dip of the subducted Pacific plate and extending for about 210 km. The geometry of the top of the Pacific plate is inferred from a band of concentrated microearthquake activity approximately 10 km thick which is identified with the crust of the plate. The Pacific plate has two knee-like bends, one where the top of the plate is about 25 km deep, the other below the volcanic front, where the plate is about 70 km deep. The shallower bend and subsequent restraightening of the plate are related to phase changes in the plate, the deeper bend to volcanism. Composite focal mechanisms indicate that seaward of the shallower bend the Pacific plate is being loaded by the Indian plate, whereas landward of this bend the Pacific plate is sinking under its own weight. A lack of microearthquakes in the Indian plate below the south-eastern half of the traverse is related to the Indian and Pacific plates being currently unlocked. Below the north-western half of the traverse microearthquakes in the Indian plate are concentrated in the 25-42 km depth range, shallower activity being largely confined to the vicinity of the volcanic front. Composite focal mechanisms suggest the activity at a depth of 25-42 km results from stresses set up by locking and unlocking of the plates, while the shallower activity results from local stresses related to volcanic phenomena.