Evaluation of factors controlling earthquake-induced landslides caused by Chi-Chi earthquake and comparison with the Northridge and Loma Prieta events

Over 10,000 landslides were triggered by the September 21, 1999, Chi-Chi Earthquake. A large number of these landslides have been mapped from SPOT images and a smaller number were described in a detailed field investigation. Geographic information systems (GIS) was used to conduct a spatial characterization of the slope failures, including distribution of type, size, slope angle, bedrock geology, ground motion, and distance from earthquake source. The most abundant landslides were shallow, disaggregated rock and soil slides. Landslides occurred primarily in Tertiary sedimentary rocks, which are well known for their susceptibility to landsliding in many parts of the world. Landslide concentration values diminish beyond epicentral distances of 40 and 70 kin from the epicenter and the surface projection of the fault plane, respectively. Ground motion was found to be the most significant factor in triggering the shallow landslides in the Chi-Chi earthquake. Overall, 74% of all slope failures occurred in regions with vertical ground motions greater than 0.2g and 81% of all slope failures occurred in the region with mean horizontal peak ground accelerations (PGA) greater than 0.15g. These factors were used to compare landslides generated by the Chi-Chi earthquake to the landslides triggered by the 1989 Loma Prieta earthquake and the 1994 Northridge earthquake. The major difference in distribution of landslides between the Chi-Chi earthquake and the two California earthquakes was the distribution of slope angles. In the Chi-Chi earthquake, 90% of the failures occurred on slopes steeper than 45degrees, while more than 80% of failures occurred on slopes less than 50degrees in California, apparently reflecting much steeper and tectonically more active geologic setting in Taiwan. (C) 2003 Elsevier Science B.V. All rights reserved.