Impacts of the Chi-Chi earthquake on subsequent rainfall-induced landslides in central Taiwan

The influence of the Chi-Chi earthquake on subsequent rainfall-induced landslides was evaluated by comparing the occurrence of landslides in the Choushui River watershed through eight SPOT images that covered the period from 1996 to 2001. The Chi-Chi earthquake not only triggered serious coseismic landslides itself but also extensively disturbed surface strata around the epicentral area. After surface strata have been highly disturbed heavy rainfalls in 2000 and 2001 have triggered more landslides than the earthquake itself. Compared with the data obtained from images before the earthquake, it is obvious that the density of rainfall-induced landslides increased significantly after the earthquake, and the places where landslides occurred changed, as well. Although typhoon Herb in 1996 brought more precipitation than typhoon Toraji in 2001, only 9.77 km2 landslides were induced by Herb, while 48.8 km2 landslides–almost five times larger than those caused by Herb–were triggered by Toraji. Besides, landslides induced by Herb were mainly distributed in mid-slope areas, with slopes of 20–30°. After the Chi-Chi earthquake, the rainfall-induced landslides occurred mainly in places with slopes between 40° and 50°.