Susceptibility mapping of landslides and subsidence in the Philippines is conducted by the Mines and Geosciences Bureau (MGB) with an emphasis on field-based hazard observation. This study was employed to create a systematic approach to hazard susceptibility mapping that is more consistent and replicable to multiple users. Weights of Evidence (WOE) modeling is a statistical method that highlights trends by quantifying the relationship between dependent and independent variables through indicative weights. WOE has applications in the field of medicine, environmental sciences, and mineral potential mapping.
Hazard inventories for the municipalities of El Nido, Palawan as well as General Luna and Del Carmen, within Surigao del Norte were compiled from field assessments and historical records from MGB. Interferometric Synthetic Aperture Radar (IfSAR) data and satellite imagery were also analyzed to identify landslide and sinkhole features. The inventories were split into training and testing subsets and rasterized. Hazard conditioning factors such as slope and geology were selected and converted into thematic maps in raster format. The maps were imported into the ILWIS platform wherein the WOE operations were performed.
Analysis of resulting weights showed that karst lithology contributed the most to landslide occurrence, with factors such as steep slopes and bare soil cover also contributing to recorded landslide events. For subsidence hazard, higher sink densities promoted sinkhole development. Geology and proximity to drains and lineaments also influenced subsidence. Overall reliability of the resulting hazard models was validated using ROC curves. The landslide susceptibility maps had success rates of 79.8% and 80.6% for both study areas, respectively. For subsidence hazards, the study areas in Surigao del Norte had a success rate of 83.9%. The subsidence map of El Nido had a success rate of 90.8%. These indicate high reliability for the method employed for hazard susceptibility assessments and a high degree of reproducibility for future hazard susceptibility zonation.
Keywords: Weight of evidence, karst, geohazards, Philippines
Themes: Geohazards & Disaster Risk Reduction, Geoheritage & Geotourism, Environmental Geology & Climate Change