Evaluating triggering and causative factors of landslides in Lawnon River Basin, Taiwan

This study evaluates triggering and causative factors of landslides by comparing their occurrence in the Lawnon River Basin prior to and after rainfall and earthquake events over a five-year period (2005-2009). The landslide ratio in the study area was low (less than 4%) before 2007, and significantly increased in the wake of Typhoon Morakot in 2009. The high accumulation of rainfall was the major triggering factor. In addition, the major seismic activity of March 4, 2008 also contributed to landslide occurrence. The combined influence of rainfall and the earthquake is evaluated based on multi-variable regression analysis. Though no significant co-seismic landslides were found after the March 4, 2008 earthquake, its influence on slope stability has been observed from the apparent growth of landslide ratio in the four rainfall events following the quake. Causative factors include the higher landslide ratios occurring in sedimentary rock zones (especially sandstone formations with intercalations of shale), and dip slopes, which are demonstrably prone to plane failures.