Quaternary slip rate and geomorphology of the Alpine fault: Implications for kinematics and seismic hazard in southwest New Zealand

Glacial landforms at 12 localities in 9 river valleys are offset by the southern end of the onshore Alpine fault. Offsets cluster at similar to 435, 1240, and 1850 m, consistent with evidence for glacial retreat at 18, 58, and 79 calendar ka. The peak of an offset fluvial aggradation surface is correlated with the Last Glacial Maximum at 22 ka. Displacement rates derived from features aged 18, 22, 58, and 79 cal. ka are 24.2 +/- 2.2, 23.2 +/- 4.9, 21.4 +/- 2.6, and 23.5 +/- 2.7 mm/yr, respectively, with uncertainties at the 95% confidence level. The joint probability, weighted mean, and arithmetic mean of all observations pooled by rank are 23.1 +/- 1.5, 23.2 +/- 1.4, and 23.1 +/- 1.7 mm/yr, respectively. We conclude that the mean surface displacement rate for this section of the Alpine fault is 23.1 mm/yr, with standard error in the range of 0.7 - 0.9 mm/yr. The reduction in estimated long-term slip rate from 26 6 mm/yr to 23 2 mm/yr results in an increase in estimated hazard associated with faulting distributed across the rest of the plate boundary. Model-dependent probabilities of Alpine fault rupture within the next 50 yr are in the range 14%-29%. The 36 +/- 3 mm/yr of total plate motion (NUVEL-1A) is partitioned into 23 2 mm/yr of Alpine fault dextral strike slip, 12 4 mm/yr of horizontal motion by clockwise block rotations and oblique dextral-reverse faulting up to 80 km southeast of the Alpine fault, and 5 3 mm/yr of heave on reverse faults at the peripheries of the plate boundary.