Recent vertical offset and near-surface structure of the Alpine Fault in Westland, New Zealand, from ground penetrating radar profiling

The Alpine Fault in central Westland is an oblique slip fault with a predom inantly strike-slip component of 25-35 mm/yr. The smaller vertical componen t has been variously estimated at between 5.5 and 14 mm/yr. Ground penetrat ing radar and level profiles of the Alpine Fault beside the Toaroha River i n Westland indicate that there has been vertical movement of 21.75 +/- 0.5 m of a fluvial surface dated at between 2420 and 3150 yr. This yields an av erage vertical movement of 7.8 +/- 1 mm/yr, and is consistent with an estim ate derived from regional studies at Paringa, which excludes the local effe cts of tilting. It is a little higher than uplift rates of 5.5 mm/yr previo usly estimated by Bull and Cooper, based on the current elevation of inferr ed uplifted Pleistocene marine terraces from the same general area, but is of the same order as their estimate.