Spatial and temporal analysis of landslides in Central Taiwan after 1999 Chi-Chi earthquake

Abstract

The hilly Western Foothill of Taiwan Island is highly prone to landslides, especially during the typhoon season in the summer of 1999. The Chi-Chi earthquake (MW = 7.6) resulted in tremendous amount of landslides in Central Taiwan. The impact of this earthquake not only makes the geology more fractured but also changes the morphology in the Western Foothill area. The geohazards in this area, including debris flows and flooding after 1999 are closely related to the spatial and temporal behavior of these landslides. Focusing on the Ta-Chia River, Wu River and Chuo-Shuei River, this study investigated the behavior of landslides in Central Taiwan after 1999. Four major typhoon events, i.e., Toraji (2001), Mindulle (2004), Sinlaku (2008) and Morakot (2009) were included in this study. The study comprises two major parts, i.e., the analysis of catchment landslides and the study of time effects. The first part includes correlation analysis with rainfall and geology, and the second part consists of time effects modeling of the impact of 1999 Chi-Chi earthquake. Compared to typhoon Toraji, fewer and fewer reactivated landslides were induced by the subsequent typhoon events. This finding reveals the self-healing effect after the 1999 Chi-Chi Earthquake. However, the results show a close correlation between the distribution of landslides and the distribution of rainfall. A conceptual model was developed to investigate the time effects of earthquake impact on the correlation of rainfall and landslides. The analysis results suggest that the impact of the 1999 Chi-Chi earthquake decays to 50% in about 1–3 years and to 10% in about 10 years. This study can provide helpful details of the landslide investigation as an aid to hazard estimation and catchment management.