Landslides triggered by the 7 August 2009 Typhoon Morakot in southern Taiwan

Landslides triggered by Typhoon Morakot with an area over 576 m2 were mapped from FORMOSAT-2 images. Recognition and calculation of the number and area of landslides show that at least 22,705 landslides with a total area of 274 km2 were recognized in the 7811 km2 study area. Among these landslides, 17,417, with a total area of 150 km2, are recognized as new ones, and 5288, with a total area of 124 km2, are classified as enlarged from pre-existing ones. Out of the 22,705 landslides, there were 22,221 recognized with an area smaller than 10 ha; 298 recognized in the range of 10 to 20 ha. Twenty-two landslides with an area of over 60 ha have also been identified, and the total area of these landslides is 20 km2, which is equal to 7.37% of the total landslide area. With respect to exposed rock type, the landslides mainly occurred in Eocene-Miocene slate and Miocene sandstone and shale. In addition, Morakot-induced landslides mainly occurred in areas with cumulative precipitation in the range of 800 to 2600 mm, and the magnitude of landslide concentration is roughly linearly proportional to the amount of cumulative rainfall.