Abstract
At 12.03 am local time on 14th November 2016 (UTC: 11.03 am 13th November 2016) a shallow magnitude 7.8 earthquake, with an epicentre located near Waiau in North Canterbury, struck the North Canterbury and Marlborough regions of NZ. The most visible consequence of the strong ground shaking was widespread landslides. A feature of the landslides from this earthquake is the large number (196) of drainage blocking landslides it generated. This was partly due to the steep and confined slopes in the area and the widely distributed strong ground shaking.
The majority of the landslide dams occurred in two geological and geotechnically distinct materials: weak sedimentary rocks (sandstones and siltstones) where first-time and reactivated rock-slides were the dominant landslide type, and; strong sedimentary rocks (greywacke and limestones) where first-time rock and debris avalanches dominated. This gave rise to two quite distinct end-member landslide dam types, large rock block slides comprised a few large blocks, and rock and debris avalanches comprised of coarse angular gravels.
Identifying the location and size of landslide dams was a priority in the post-earthquake response because of the potential public safety risks. Once dams had been located the hazard of catastrophic failure (likelihood) was assessed. Those with a higher likelihood of catastrophic failure had the consequences (risks) from failure identified. Those with a higher likelihood of failure and substantive risks were examined in more detail using field mapping and terrestrial laser scanning. These data were used to model the catastrophic failure scenarios to determine the scale of the risk so that appropriate countermeasures could be put in place to alleviate the risks.
1 INTRODUCTION
The Kaikoura Mw7.8 earthquake struck at 12.03 am local time on 14th November 2016 (UTC: 11.03 am 13th November 2016) – a shallow (15 km) magnitude 7.8 earthquake (Mw), with an epicentre located near Waiau in North Canterbury (Kaiser et al, 2017), and strongly shook the North Canterbury and Marlborough regions of NZ (Figure 1). The strong ground shaking caused widespread damage to buildings and infrastructure across the sparsely populated areas of the northeast of the South Island. The most visible consequence of the strong ground shaking was widespread landslides (Figure 1). Given the sparsely populated area affected by landslides, only a few homes were impacted and there were no recorded deaths due to landslides.
GeoNet, the geohazards monitoring programme funded by the New Zealand Earthquake Commission and run by GNS Science, has a requirement to respond to major landslide events in New Zealand using a set of established criteria (Dellow 2001, McSaveney et al. 2010). The MW7.8 Kaikoura earthquake met several of these criteria, including the presence of consequential hazard in the form of landslide dams and direct damage in excess of $1M. The landslide reconnaissance effort the first day after the earthquake quickly determined that landslide dams represented on ongoing hazard and risk to earthquake response activities. Immediate plans were made to search for, photograph and attribute all the landslide dams. Following on from this, a process for evaluating the hazard of dam failure and the risks or consequences of failure was developed. This process quickly identified landslide dams with the highest hazards and greatest risks allowing emergency managers to focus efforts to develop mitigation plans. The dams with the greatest risks, to people or road networks, were selected for additional work to acquire survey-quality data to model rapid dam failure to further inform mitigation strategies.
