THE WELLINGTON-MOHAKA FAULT: UPDATED PALEOSEISMOLOGY, SLIP RATE, AND HAZARD

The Wellington fault represents NZ’s greatest seismic risk. The fault extends ~170 km from Cook Strait at the southern end of North Island, through the urban corridor of the national capital, to the Manawatu gorge, where it continues north as the Mohaka fault for ~290 km to the Bay of Plenty. These dextral faults accommodate the oblique component of motion across the Hikurangi subduction margin. The faults are postulated to show a decreasing rate of slip from S to N related to a decrease in locking across the margin and increasing influence of extension in the Taupo Volcanic Zone. No historical earthquakes of M >6 have occurred on this fault system. Therefore recently, greater attention has been given to their slip rate and paleoearthquake records. The 75 km long Wellington-Hutt Valley segment of the Wellington fault (W-HV) is the best studied with a slip rate of 6-7.6 mm/yr. The last 2 events occurred on it between 1640-1440 AD and 1290-940 AD, yielding an average recurrence interval of 500-700 yr. The W-HV segment has the potential to deliver a M~7.6 earthquake with SED of ~4.2 m to the Wellington region. The next segment north of the W-HV is the Tararua Ranges (TR) segment, which traverses those ranges for ~53 km. No slip rate and very few paleoearthquake data exists for this mountainous, bush-covered segment. An estimate of total dextral offset of 7 ±1 km comes from displaced river drainages. A pulse of aggradation at Totara Flats (1650-1480 AD) has a similar age to the MRE on the W-HV segment. North of the TR segment 7 trenches at 4 paleoseismic sites on the straight, 43 km Pahiatua-Woodville (P-W) section have yielded at least 3 distinct paleoearthquakes during the last 2500 yr. Two new slip rate measures of 4.6-7.2 and 3.6-4.9 mm/yr for this section come from trench data combined with surveys of offset streams. New work on a 27 km section of the southernmost Mohaka fault gives a slip rate of 5-7 mm/yr. Trenching studies show that 3 paleoearthquakes have occurred on it at similar times to the P-W section. In summary, the paleoearthquake results from 3-4 segments of this fault system yield similar slip rate along strike to the north, and similar (overlapping) event ranges for the last 2-3 events. These results have significant implications to our treatment of the seismic hazard from future surface-rupturing earthquakes of M>7 in southern North Island.