ROCK SLOPE FAILURE DURING THE 1999 CHI-CHI EARTHQUAKE, TAIWAN: AN EXAMPLE OF TOPOGRAPHIC AMPLIFICATION EFFECTS?

Approximately 22,000 landslides were triggered in the Taiwan Central Range during the 09/21/1999 Chi-Chi earthquake (Mw=7.6). However, their spatial distribution was quite uneven, with some drainage basins being devastated by failures whilst others were essentially unaffected. An explanation for this may be topographic amplification, in which certain geomorphological configurations increase local ground accelerations, eventually triggering failure in otherwise stable slopes.

An investigation was carried out to assess topographic amplification effects along a c.10 km section of the Tachia Valley in central Taiwan. The site consists of a deeply incised gorge with high rock slopes, composed of interbedded strong sandstones and slates. Several rockslides occurred here, but preliminary slope stability analyses estimated that the required acceleration for the initiation of movement in these slopes was about two to three times the peak ground acceleration measured in a free-field strong motion station located nearby, suggesting that topographic amplification may have been an important factor. Furthermore, the crowns of the landslides are close either to the top of the ridges or slope breaks, where the topographic effect is believed to be greater.

An analysis of the conditions within the slopes during the earthquake was conducted using rock mass characterization, geotechnical testing, slope stability analyses, terrain evaluation, and analysis of strong motion data in the frequency domain. Estimated amounts of amplification required to destabilize the slopes were compared with theoretical models of the relationship between slope morphology and the wavelength of the seismic waves. The analyses reveal that significant geomorphological and seismological parameters influence topographic amplification, which need to be considered as inputs for earthquake-induced landslide hazard assessment.