Source parameters of large historical (1917-1961) earthquakes, North Island, New Zealand

We have studied 18 earthquakes of M-w > 5.5 within the North Island, New Zealand region to determine fault parameters for earthquake hazards and regional tectonic studies. Since most large (M-w > 6.5) earthquakes in the North Island occurred prior to 1961, these events provide important information to supplement studies of more recent seismicity. Our results indicate that no large plate interface earthquakes have occurred in the central and southern North Island for the past 80 years, although six strike-slip earthquakes of M-w greater than or equal to 6.8 occurred within the Australian (upper) Plate during an extremely active 25 year period that began in 1917. Only the M-w > 7.2 strike-slip events were associated with surface faulting, indicating the difficulties that may arise in attempting to identify active faults within this region. Two earthquakes (M-w = 6.9-7.1) off the northeastern North Island in 1947 appear to have occurred along the plate interface and were associated with local tsunamis having runup heights of up to 10 m. Two M-w = 6.8 events also occurred within the Pacific (lower) Plate, highlighting the hazards related to intraslab events. Slip vectors for the earthquakes studied suggest that the majority of transcurrent motion along the plate margin is accommodated within the Australian Plate, similar to the results obtained from studies of more recent, smaller earthquakes. Pure thrusting occurs along the plate interface and T axes of intraslab events indicate downdip tension in the Pacific Plate.