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The Chi‐chi earthquake (MS = 7.7), which occurred in September 1999, seriously damaged central Taiwan. Approximately 2 years later (July 2001), the Toraji typhoon brought a heavy rainstorm (650 mm rain/day) and triggered widespread landslides in central Taiwan and parts of eastern Taiwan. Approximately 10 000 Chi‐chi earthquake‐induced landslides and 6000 Toraji typhoon‐related mass movements were delineated in an area of 2400 km2 using Satellite Pour l’Observation de la Terre (SPOT; French earth resource satellite) images. The landslide distribution could be closely related to the distribution of peak ground acceleration registered during the Chi‐chi earthquake. The study area was composed of Tertiary sedimentary and metamorphic rocks, whose age and induration increased eastward. The earthquake‐induced landslides were mostly distributed in the region between the Chelungpu Fault and the Lishan Fault to the east, whereas they were few in the region east of the Lishan Fault. The Toraji typhoon in 2001 severely damaged both regions that had been shattered by the Chi‐chi earthquake in 1999. The occurrence of earthquake‐induced landslides can be correlated with epicentral distance, and their occurrence has more influence from the rock type than from the ground motion.

An earthquake (M7. 3) took place in central Taiwan on the early morning of September 21, 1999. This disastrous ground shaking is called the Chi-chi earthquake, with its hypocenter about 10km below land surface. Earthquake-induced landslides were surveyed in order to mitigate secondary hazards. The area studied is about 375,000 ha, in which landslides were identified by aerial photo interpretation accompanied by SPOT images and field surveys. The color aerial photos used in this study are on the scale of 1/17,000 and were taken between September 22-26, 1999. More than twenty thousand Chi-chi earthquake-induced landslides have been recognized by aerial photo interpretation. The average landslide density and landslide rate are 0.1 pieces/ha and 4.7%, respectively. Over 90% of the landslides were less than one hectare in average and categorized as shallow landslides and rock falls. Most of the landslides occurred in a region between the Chelungpu active fault to the west and the Lishan fault to the east or in regions with a horizontal peak-ground acceleration (PGA) of more than 300 gal. Of the four geological regions in the area studied, the inner foothill zone had the greatest average landslide rate (11.6%) and the greatest number of landslides (9,090 landslides). The occurrence of earthquake-induced landslides has a close relation to their geology and distances from the earthquake fault. As to geologic formation and PGA, the Ptk2 (Plio-Pleistocene period) formation has the greatest landslides rate, 20% in a 501-600 gal area, and the Pcl formation (Pliocene period) has a landslide rate, 9.2% in a 401-500 gal area. By dividing into six 5-km-wide belts paralleling the Chelungpu fault, the landslide amount and its rate in the Pcl formation area have the greatest value in the 5-10km belt (about 6,000 amount) and the 10-15km belt (12.7%), respectively. In the Mkc formation area, the landslide amount and its rate have the greatest value in the 0-5km belt, about 1,400 amount, 4.7%, respectively, and then generally decrease with an increase of 5km.