Steep terrain and fragile geology in the mountainous area in Taiwan makes the slope susceptible to landslides during heavy rainfall and earthquakes. In 1999, the Chi-Chi Earthquake induced a great deal of landslides in central Taiwan. It is dubious if the weakening of the slope material by the earthquake still influences slope stability and changes the landslide-triggering threshold in some rainfall characteristics. Typhoon Polly in 1992, Herb in 1996, Toraji in 2001 and Mindulle in 2004 are selected for study. The former two took place before the Chi-Chi Earthquake, while the other two occurred after it. Aerial photos are interpreted to identify landslides triggered by each of the typhoons, and individual landslide is then delineated and digitized. Based on the 40 meter resolution DEM, terrain attributes such as size, slope, slope location and specific catchment area for each landslide are calculated using and the results are statistically analyzed. In addition, SINMAP is applied to back calculate the internal frictional angle of each geologic stratum during corresponding typhoon.
It is concluded on the calculation results that (1) the fragile metamorphic strata are more subjected to landslides than the sedimentary strata. However, after the Chi-Chi Earthquake, landslides in the sedimentary strata have increased a lot. (2) Both before and after the Chi-Chi Earthquake, the number
and area of the landslides increase with increasing rainfall intensities. (3) In all the three sub-watersheds, the estimated internal frictional angles of the slope material after the earthquake tend to be smaller than those took place before the earthquake. (4) The average specific catchment area for the landslides which took place after the earthquake also tends to be smaller than those before the quake. (5) The internal frictional angles of the metamorphic rock after the earthquake are also smaller than it was before. It is shown that the Chi-Chi Earthquake weakened slope material, but since most part of the loose materials has slipped away and vegetation has colonized the once stripped ground surface, the after effects of the Chi-Chi Earthquake have been alleviated.