ABSTRACT
The Southern Alps of New Zealand have 85% probability of suffering a major earthquake (M ≥ 8) along the rangefront Alpine fault within the next century. This is a concern for Franz Josef, a rapidly growing tourist town located in South Westland of New Zealand, which is sited astride the Alpine Fault. The hillslope overlooking the Franz Josef township has been previously considered as a site of a potential collapse during a future strong earthquake (M ≥ 8; Barth 2013). Aerial photos from 1973 to 1985 were used to create stereographic models and Digital Elevation Models to observe recent topographic evolutions of the hillslope overlooking Franz Josef. In addition, high-resolution airborne LIDAR covering a 4 km2 area and field mapping were used to investigate the geomorphology of the hillslope overlooking Franz Josef township. The aerial photogrammetric analysis shows no evident topographic modification of the hillslope from 1973 to 2014 except for a probable debris flow event in 1984. The LIDAR and field mapping data show bedrock features that can be related to deformation mechanisms acting in the slope in the 15,000/20,000 years since the slope became ice-free after the last major glaciation. Geomorphic analysis suggests that a range of landslide scales is possible (from many millions to several thousands cubic meters of material), and forms the basis for numerical modelling.
