A TERRAIN CLASSIFICATION SYSTEM FOR COSEISMIC LANDSLIDE HAZARD ANALYSIS; LEBANON, A CASE STUDY

Coseismic landslides have caused vast amounts of damage throughout history, and are often responsible for more damage than all other seismic hazards combined (Kramer, 1996). To better assess coseismic landslide hazard on a regional scale a new terrain classification system is proposed. First, the landscape is divided into classes based on possible modes of failure, defining kinematically admissible regions, followed by mode-of-failure specific hazard analyses (strength admissibility) within each region. Terrain susceptible to rock failures, shallow and deep soil failures, and lateral spreading were identified and assessed. Terrain classification was modeled after the work of Iwahashi and Pike (2007), which has been shown to relate to engineering properties of the landscape (e.g. Vs₃₀, Yong et al., 2012) using slope, curvature, and roughness. In this work topographic slope, calculated using Ritter’s algorithm (1987) is used to demarcate landslide failure modes. Topographic roughness, calculated as the standard deviation of slope, and vegetation coverage, determined from NDVI values, are then applied to each slope class as a first order approximation of landslide susceptibility. Coseismic hazard is then calculated for each susceptible terrain class using a failure-mode-specific algorithm. Results from this method of hazard analysis are maps of terrain classes, coseismic hazard maps for individual mode of failure, and a composite hazard map.

Lebanon has been used as a case study for this method using 15-meter DEM data and Landsat 8 imagery. Mass movements are estimated to cause $10-15M in damages annually in Lebanon (Khawlie, 2000), and seismic hazard is driven the by presence of a thrust fault offshore of Beirut, and the left-lateral restraining bend that bisects the country. Qualitative and semi-quantitative hazard assessments for rock failures, shallow and deep soil failures, and lateral spreads have been made for Lebanon along with maps for each type of failure. Terrain classification allowed for the rapid assessment of potential modes of failure and relative susceptibility, which were finalized by coseismic hazard calculations for each considered mode of failure.