Late quaternary displacement rate, paleoseismicity, and geomorphic evolution of the Alpine Fault: Evidence from Hokuri Creek, South Westland, New Zealand

A 400 +/- 100 m offset of Lake McKerrow, South Westland, New Zealand, combined with dated (15.6 ka) glacial lake silts, requires an Alpine Fault displacement rate of 26 +/- 7 mm/yr. Moraines associated with Hokuri Creek (assumed to be 17 +/- 2 ka) are offset by 440 +/- 40 m and require a displacement rate on the Alpine Fault of 26 +/- 6 mm/yr. Slickensides, fault exposure, and offset topography are consistent with an almost pure dextral sense of movement on a vertical or subvertical fault. Locally, a small vertical component of up-to-the-west movement is observed. Folding in late Quaternary sediments indicates active tilting of sediments at up to 0.4 degrees/ka and variations in local uplift/subsidence rates of up to 4 mm/yr. At one locality c. 1 km northwest of the Alpine Fault and near the core of an anticline, uplifted shells require an uplift rate of 1.4 +/- 0.5 mm/yr relative to sea level. Displaced river channels provide estimates of the last two coseismic displacements on the fault of 9 m (penultimate) and 8 m. This suggests characteristic earthquake behaviour with a recurrence interval of 330 +/- 90 yr and probable M(w) > 7.5. Radiocarbon dating suggests the last coseismic displacement occurred just after 370 +/- 150 cal yr B.P.