Comparison of landslide inventories from the 1994 M_w 6.8 Arthurs Pass and 2015 M_w 6.0 Wilberforce earthquakes, Canterbury, New Zealand

The 6th January 2015 M_w 6.0 Wilberforce earthquake, 30 km west of Arthurs Pass in the Upper Wilberforce catchment, produced ground shaking of at least 0.22 g, just above the expected threshold for landslide initiation based on previous work. An aerial reconnaissance confirmed that at least 200 landslides occurred within an area of 370 km². The majority of failures were of moderate volume and occurred on slopes of between 22° and 67°. Approximately 85% of failures occurred within 12 km² of the earthquake epicentre, although large landslides also occurred outside of this area, at sites where previous landslides had occurred. We compared this inventory with one obtained by a similar method in similar terrain after a M_w 6.8 earthquake in 1994, 20 km southwest of Arthurs Pass. Whilst the 1994 earthquake included a higher proportion of landslides with larger volumes close to the earthquake epicentre, the number of landslides and area that they affected was greater during the Wilberforce earthquake. In addition, the Wilberforce earthquake triggered its second largest landslide at a substantial distance (25 km) from the epicentre, in an area of moderate shaking. This landslide temporarily dammed Arahura River, causing the greatest potential geo-hazard risk of the earthquake event. The results indicate that whilst the sizes of landslides that may initiate in an earthquake generally increase with increasing earthquake magnitude, failure does not require the exceedance of a specific earthquake magnitude threshold. We suggest that landslide initiation is equally dependent on: (1) the susceptibility to failure of the terrain at the time of the earthquake and 2) the specific ground-shaking intensity (MM) experienced. These two factors should be important considerations when planning emergency responses and post-earthquake aerial reconnaissance.