ACTIVITY AND ROLE OF A PULL-APART BASIN FAULT: THE HANMER FAULT, NEW ZEALAND

We constructed detailed geomorphic and fault maps and excavated and logged paleoseismic trenches along the central portion of the Hanmer Fault, New Zealand to better characterize its rupture hazard and active fault parameters. The Hanmer Fault is a dextral-normal fault which bounds the north side of the Hanmer Basin, a pull-apart basin along the dextral-slip Hope Fault in northern South Island.

The fault is well-expressed (striking c. 105°) as a series of scarps formed across a post-glacial alluvial fan of age 12.2-12.8 kyr on which the town of Hanmer Springs has been built. Aerial photograph analysis and GPS-RTK survey-constructed micro-topographic contour maps were used in a GIS to define the fault scarps and zones of potential surface rupture hazard through part of the town. In this area the fault splays into stepping, en-echelon traces.

We excavated four trenches across four fault splays to analyse the paleoseismicity of the fault. There have been no historical surface-faulting events on the Hanmer Fault, i.e. post-1840, and there is no evidence for rupture during the 1888 North Canterbury earthquake on the nearby Hope Fault. The trenches exposed post-glacial and Holocene sediments deformed by the Hanmer Fault. Trench 1, excavated across the Barn trace revealed disrupted late Holocene alluvial deposits, peat and soil associated with at least the last 2 surface faulting events. One or two ruptures occurred since the deposition of units containing charcoal, and peat with radiocarbon ages of c. 580 ± 25 radiocarbon yr BP. Another rupture event occurred between 580 and 1188 ± 25 yr BP (uncalibrated). From these data we construct a preliminary recurrence interval of 330-615 yr. The average recurrence interval is 1-2 times that of the adjacent Conway segment of the Hope Fault (slip rate c. <23 ± 4 mm/yr; RI 180-310 yr) east of the Hanmer Basin. The dip-slip rate for the Hanmer Fault, west of Hanmer Springs is c. 1 mm/yr. Assuming an H:V ratio of 1 to 3:1 yields a total slip rate of 1.1-3.2 mm/yr. Collectively, these data imply that the Hanmer Fault has a second-order, but important role in the strain release across this part of the Hope Fault system.