Late Holocene Rupture History of the Alpine Fault in South Westland, New Zealand

Strata and fault relationships revealed in five trenches excavated across the recent trace of the Alpine fault at the Haast, Okuru, and Turnbull Rivers, South Westland, New Zealand, record the three most recent surface-faulting events. Using back-stripping techniques to remove the three faulting events and the sedimentary units associated with the faulting restores the cross-sections to gravel-bed floodplains at the Haast and Okuru Rivers, at about A.D. 750. Horizontal and vertical offsets of stream channels and terrace risers reveal characteristic displacements of about 8-9 m dextral and up to 1 m vertical per event. Cumulative dextral displacement is 25 +/- 3 m in the past three events. The most recent surface-rupture event was probably in A.D. 1717, and the next prior events were about A.D. 1230 +/- 50 and about A.D. 750 +/- 50. The timing of these events is consistent with past large-great earthquakes on the southern section of the Alpine fault inferred from off-fault data, but there are fewer events identified in trenches. Our three-event dataset indicates the average surface-rupture recurrence interval for the South Westland section of the fault is about 480 years, much longer than the current elapsed time of 295 years. Therefore, the Alpine fault in South Westland may not be close to rupture as is often speculated.