Slip Rate on the Wellington Fault, New Zealand, during the Late Quaternary: Evidence for Variable Slip during the Holocene

We investigate fault-offset fluvial terraces at two sites along the Hutt River, near Wellington, New Zealand, to determine horizontal slip rates for the southern part of the Wellington fault for the last similar to 100 ka. Using photogrammetric analysis of historic aerial photos, we measured the original displacements of terrace risers at Emerald Hill, which have been modified by urban development. Fourteen sediment samples from this site and from another location, Dry Creek, were analyzed using Optically Stimulated Luminescence (OSL) methods, yielding ages of between similar to 10 ka and similar to 100 ka. From our displacement-age relations, we estimate an average slip rate of 6.3 +/-(1.9)(1.2) mm/yr (2 sigma) during the last similar to 100 ka. However, slip on the Wellington fault has not been steady throughout this time. During the Holocene, there was a phase of heightened ground rupture activity between similar to 10 and 8 ka, when an incremental dextral displacement of similar to 32 m accrued. This was followed by a period of relative quiescence between similar to 8 and 4.5 ka, with similar to 5 m of displacement during this time. Another period of heightened activity followed, resulting in a further similar to 20 m of slip during the last <= 4.5 ka. Our inference of variable slip during the Holocene accords with independent evidence from paleoseismological studies at other sites along the Wellington fault. Slip on the Wellington fault, and its variability through time, may be influenced by activity on neighboring faults.