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Abstract

The Australian- Pacific plate boundary on the South Island of New Zealand comprises a zone of distributed deformation along active right lateral strike slip faults of the Marlborough Fault System. The four main strike slip faults that make up the Marlborough Fault System are the Hope, Clarence, Awatere and Wairau faults and of these, the Wairau Fault is minimally studied. The Wairau Fault is a northeastward extension of the Alpine Fault beyond the branch point for the distributed zone. It is prone to Ms 7.2-7.7 earthquakes with displacements of 7 meters. Slip rates vary from 3.6-6.7 mm/yr across the Wairau Fault, according to previous studies. The fault splits into two parallel traces near Wairau Valley that continue separately to Cloudy Bay. A 2006 paleoseismology study on the northern trace at Wairau Valley, by Zachariasen et al. revealed four earthquakes within the last 5321-5611 yrs years associated with 23 meters of displacement, providing a slip rate of 3.6 mm/yr - 4.7 mm/yr on that trace. LiDAR coverage reveal geomorphic landforms offset by 30 meters along the southern trace of the Wairau Fault near Wairau Valley, that haven't been mapped and haven't been accounted for in slip rate calculations. A shutter ridge with a measurable south facing scarp, 2.3-3.9 m in height is evident along the southern fault trace. The ridge developed from at least three earthquake events. Ages determined from scarp diffusion modeling of the shutter ridge are compatible with the earthquake time intervals from previous studies (Zachariasen et al, 2006; Barnes and Pondard, 2010). There are contrasting results of modeling of scarps along the the two traces including variations in degradation coefficients between the northern and southern fault trace. This indicates that the fault strands rupture independently and that the penultimate event occurred along the southern 6 fault trace. Diffusion ages from the shutter ridge suggest a maximum displacement of 10 m per event since 6023 yrs, and a maximum slip rate of 9.67 mm/yr for the Wairau Fault. One concern is that irregularities in the scarp morphology might obscure evidence for older events, leading to an overestimate of slip rate and displacement. Using a conservative age estimate from the diffusion age of the valley bordering the shutter ridge, the slip rate is 4.2 to 5.8 mm/yr for the Wairau Fault. An implication is that previous studies may underestimate the slip rate and seismic hazard for the Wairau Fault. Therefore, this study identifies a need for more paleoseismology research along the Wairau Fault in order to realistically assess the earthquake hazard for townships in the Marlborough region of South Island, New Zealand.

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