A Preliminary Report on the Chiufenershan Landslide Triggered by the 921 Chichi Earthquake in Nantou, Central Taiwan

A large-scale dip-slope landslide, the Chiufenershan landslide, was triggered by the 921 Chi-Chi Earthquake on September 21, 1999. At the site, which covers some 200 hectares of incline, over thirty million cubic meters of rock masses were displaced, blocking streams to form two dammed lakes and causing more than 20 fatalities. Dip-slope topography, incompetent fault zone on slope base together with well-developed discontinuities in rock masses constitute the fundamental internal factors for sliding. The critical factor contributing to the landslide is the strong ground motion generated by the earthquake. The event is totally unexpected because there was no trace of daylight on the slope prior to slope failure. Based on detailed geological investigations followed by the collection of various on-site features, the classification and factors causing landslide were described and discussed.