New on-fault evidence for a great earthquake in AD 1717, central Alpine fault, New Zealand

The dextral-reverse Alpine fault is the major onshore plate-boundary structure between the Australian and Pacific plates in New Zealand. No previous study of the central portion of the 200-km-long central segment has provided on-fault evidence for the most recent event (MRE). Using lidar (light detection and ranging) data coupled with field mapping, we recognized the main trace of the Alpine fault north of Gaunt Creek (South Island) as a north-striking fault scarp. We enhanced a natural exposure that revealed evidence for repeated late Holocene thrust fault movement. The north-northwest-striking fault zone is characterized by a distinct 5-50-cm-thick clay fault-gouge layer juxtaposing hanging-wall bedrock (mylonites and cataclasites) over unconsolidated late Holocene footwall colluvium. The bedrock is cut by a strath terrace and overlain by mid-Holocene (ca. 5400 calibrated C-14 yr B.P.) alluvial terrace, which has been faulted repeatedly and is conformably overlain by undeformed late Holocene colluvium and alluvium. An unfaulted peat at the base of the scarp is buried by post-MRE alluvium and yields a calibrated 2 sigma radiocarbon age of A.D. 1710-1930, which dates the MRE as post-1709. Our data are consistent with sparse on-fault data, and validate earlier off-fault records that suggest an A.D. 1717 MRE. The 1717 event had a moment magnitude of M-w 8.1 +/- 0.1, based on the 380-km-long surface rupture. Because the fault has not ruptured for similar to 300 yr, it is likely approaching the end of its seismic cycle and poses a significant seismic hazard to New Zealand.