Late quaternary movement on the Ohariu Fault, Tongue Point to MacKays Crossing, North Island, New Zealand

The Ohariu Fault is one of the major active dextral strike-slip faults in the Wellington region. It extends northeastward from offshore at Tongue Point to Waikanae and beyond, but in the north the fault trace becomes intermittent. Extrapolation of the trend of the Ohariu Fault across Porirua Harbour suggests a 1.5 km right step. South of Porirua Harbour the fault is characterised by lateral displacement of drainage features of up to 450 m, a single-event lateral displacement of 4-5 m, an average horizontal, slip rate of 1-2 mm/yr, and a recurrence interval of 2000-5000 yr. The timing of the last event was between 150 and 1130 cal. BP. North of the harbour the fault is characterised by lateral displacement of drainage features of up to 250 m, a single-event lateral displacement of 2.9 m, a horizontal slip rate of 0.6-1.9 mm/yr, and a recurrence interval of 1530-4830 yr. The timing of the last event was between 1070 and 2310 cal. BP. Although the possibility exists that Porirua Harbour is a pull-apart basin marking separate rupture segments on the Ohariu Fault, the data suggest there is a single rupture segment from Tongue Point to Waikanae. On this assumption, the last movement on the fault occurred 1070-1130 yr ago. The average horizontal surface displacement for the event was 3.7 m (based on seven sites) and the estimated earthquake magnitude was M-W 7.1-7.5.