The Wanganui-Wilberg rock avalanche: deposit, dynamics and dating

The Wanganui-Wilberg landslide lies between Hokitika and Franz Josef townships, at the entrance of Harihari, on the true left bank of the Wanganui River, by State Highway 6. This apparently co-seismic landslide belongs to the class of events called rock avalanches - powerful destructive agents (Keefer, 1984) in the landscape. Other rock avalanches are numerous (Whitehouse, 1983), and widespread over the Southern Alps of New Zealand, and many appear to be co-seismic. De Mets et al. (1994) used the model NUVEL-1A to characterize the motion of the Alpine fault: 37 mm/year at an azimuth of 071° for the strike-slip and a dip-slip of 10 mm/year normal to the strike direction. Although linear when seen from the sky, the detailed morphology of the fault is more complex, called en échelon (Norris and Cooper, 1997). It exhibits metamorphosed schists (mylonite series) in its hanging wall (McCahon, 2007; Korup, 2004). Earthquakes on the Alpine fault have a recurrence time of c. 200-300 years and a probability of occurrence within 100 years of 88% (Rhoades and Van Dissen, 2002). Thought to have been triggered by the AD1220 event (determined by dendrochronology), the Wanganui-Wilberg rock avalanche deposit represents only 20% of its original volume, which was c. 33 million cubic metres. The deposit probably dammed the Wanganui River and, as a result, created a small and short-lived lake upstream. The next earthquake capable of triggering such events is likely to occur fairly soon (Yetton, 1998). Knowledge of historic catastrophic events such as the Wanganui-Wilberg rock avalanche is of crucial importance in the development of future hazard and management plans