Large-magnitude Dextral Slip on the Wairarapa Fault, New Zealand

Dextral slip associated with an 1855 Ms 8.0+ event on the Wairarapa fault near Wellington, New Zealand was reported to be 12+/-1 m along a rupture length of at least 148km (Grapes, 1999), one of the largest single-event strike-slip offsets documented worldwide. Initial results from a new study involving detailed neotectonic mapping and microtopographic surveys of offset landforms (including many beheaded, inactive streams) strongly suggest that dextral slip was as much as 50% greater than previously measured. 1855 surface ruptures were mapped with certainty where a linear scarp characterized by steep slopes (30-90°) and exposed alluvium cuts across active or inactive stream channels. The fifteen individual strands comprising the Wairarapa fault zone that we have mapped to date are 1200+/-700 m long and typically left-stepping. Slip in the stepover zones between these strands is distributed amongst two or more ruptures and intervening anticlines, a situation that causes along-strike variations in slip and which locally complicates the interpretation of 1855 displacement. We focused on seven of the best-preserved sites where low-discharge streams are disrupted by the fault zone, including five that had been previously attributed by Grapes (1999) to coseismic slip during the 1855 earthquake. One of these (Pigeon Bush) includes two sequentially displaced, now beheaded linear stream channels, oriented perpendicular to the fault scarp, that preserve distinct offsets with respect to a single deeply incised, originally contiguous gorge on the opposite side of the fault. To quantify the minimum fault displacements at each site, we made 1:500 scale topographic maps employing n = 2,000-10,000 points collected with GPS and laser instrumentation. Measured dextral slip values, here attributed to the 1855 earthquake, include 16.4+/-1.0m (Hinaburn), 12.9+/-2.0m (Cross Creek), 17.2+/-2.5m (Lake Meadows), 18.7+/-1.0m (Pigeon Bush), 13.0+/-1.5m (Pigeon Bush 2), 15.1+/-1.0m (Pigeon Bush 3), and 16.0+/-1.5m (Tauwharenikau). Reverse slip at these localities ranged from 0.5 to 3.8+/-0.5m. Tape measurement of two other offset streams in dense bush yielded two further dextral slip measurements of 13.5+/-0.5m and 17.5+/-1.5m. AMS radiocarbon dating was undertaken at two sites to test whether slip occurred during one rupture event in 1855, or possibly could have accrued as a result of two or more earthquake ruptures. At the classic Pigeon Bush site, the youngest, most proximal beheaded stream channel is partially infilled by fluvial conglomerate with abundant charcoal. Two samples at depths of 15cm and 154cm yield calibrated dates of AD 1364+/-63 and AD 1355+/-60 (all dates are 2 sigma intervals). The stream must have been offset and abandoned after this time, and with no historical record of any other local earthquake, the 18.7+/-1.0m offset at Pigeon Bush is inferred to have occurred entirely in 1855. At Tauwharenikau, an abandoned channel is underlain by gravel on the upthrown side, but this gravel is overlain by 30 cm of swamp deposits on the downthrown side. We interpret the swamp deposits to reflect post-rupture incursion of groundwater into the down-thrown block. Basal swamp grasses yield calibrated dates of AD 1709+/-26 (27% probability) or 1869+/-60 (71% probability) for one sample and AD 1723+/-49 (34% probability) or 1871+/-70 (64% probability) for another sample, evidence that the 16.0+/-1.5m of slip at Tauwharenikau occurred entirely in 1855. These initial results strongly support the assertion that the southern Wairarapa fault experienced the largest single-event strike-slip offset yet documented worldwide.