Mass movements caused by recent tectonic activity: The 1999 Chi-chi earthquake in central Taiwan

The Chi-chi earthquake (M-S = 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 km(2) 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.