USING LANDSLIDE SUSCEPTIBILITY MODELING TO INVESTIGATE POTENTIAL EARTHQUAKE TRIGGERS FOR THE ANOMALOUSLY LARGE GREEN LAKE ROCK AVALANCHE, NEW ZEALAND

Coseismic landslide modeling of the Fiordland region of New Zealand explores potential triggers for the Green Lake rock avalanche (GLRA). The GLRA, which occurred post-deglaciation ~14,000 years ago, contains 27 km³ of debris, making it the largest identified landslide in New Zealand and one of the largest on Earth. A number of regional characteristics are associated with its collapse, including high relief and over-steepened slopes weakened by glacial erosion and faulting. Due to its large volume, the GLRA was most likely coseismically triggered. The only work to-date suggests MM IX-X shaking from an Alpine Fault event initiated collapse. However, the Alpine Fault is >80 km from the GLRA; such high shaking intensities at this distance seem improbable. Coseismic landslide susceptibility was therefore modeled using fuzzy logic and GIS for a number of potential earthquake scenarios to identify a more likely trigger. Existing coseismic landslide inventories for the 2003 and 2009 Fiordland earthquakes were used to determine relationships between landslide occurrence, slope, proximity to faults, and shaking intensity. Modeled earthquake scenarios include a M8.0 southern Alpine Fault rupture, a M8.0 Puysegur Trench earthquake, and a M7.0 on the nearby Hauroko Fault. Coseismic landslide susceptibility is highest at Green Lake for the Hauroko Fault earthquake, reaching values of >0.9 compared to ~0.5 and ~0.6 for the Alpine Fault and Puysegur Trench earthquakes respectively. Consequently, we infer that the GLRA was initiated by a large (M~7) earthquake on the Hauroko Fault and not an M8 Alpine Fault earthquake. This may have important consequences as such large slope failures are typically associated with much larger earthquakes.