Hazard survey using multitemporal satellite imagery

The World Bank has reported Taiwan as the most vulnerable country in terms of the percentage of exposed areas and multiplicity disaster. During the past decade, a number of natural disasters have occurred in Taiwan, especially two critical ones, i.e. Chi-Chi earthquake and Typhoon Morakot. In the first event, the earthquake was with a magnitude of 7.3 on the Richter scale and caused seriously damage on Sep. 21, 1999. The second one is Typhoon Morakot that accumulated more than 2,700 mm precipitation in 72 hours over a large coverage in Aug. 2009. The typhoon caused floods, landslides, and debris flow in the Central and South Taiwan. Therefore, the survey for the landscape change for hazard monitoring is an important task. Satellite imagery that provides capabilities to monitor landscape with large coverage area is an excellent tool for disaster survey. Landscape changes due to the earthquake and the typhoon are detected in this study. First of all, satellite images are orthorectified. Then the normalized difference vegetation indices (NDVI) are calculated to detect the vegetation changes. Considering the slope derived from a digital elevation model (DEM), the landslide areas are estimated. For Chi-Chi earthquake, we analyze the changes over last 11 years in the Central Taiwan. It is found that the landslides keep on growing for 11 years with little vegetation recovery since the earthquake in Da-Jia River areas. Another reported case is about Typhoon Morakot, which produced a huge landslide that destroyed whole Shiao-Lin village with 565 casualties.