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 t he Wellington region. It extends northeastward from offshore at Tongue Poin t to Waikanae and beyond, but in the north the fault trace becomes intermit tent. 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 o f 4-5 m, an average horizontal, slip rate of 1-2 mm/yr, and a recurrence in terval of 2000-5000 yr. The timing of the last event was between 150 and 11 30 cal. BP. North of the harbour the fault is characterised by lateral disp lacement of drainage features of up to 250 m, a single-event lateral displa cement 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 the re is a single rupture segment from Tongue Point to Waikanae. On this assum ption, the last movement on the fault occurred 1070-1130 yr ago. The averag e horizontal surface displacement for the event was 3.7 m (based on seven s ites) and the estimated earthquake magnitude was M-W 7.1-7.5.