Stability back-analyses of sixteen massive, disrupted rock slides triggered by the 1999 Chi-Chi earthquake in the Tachia Valley in Central Taiwan indicated that geological and geotechnical factors alone appear insufficient to explain the distribution of failures under regional shaking. This incidence of landsliding suggests site amplification. A strong topographic effect can be identified when slope morphology and topography are considered in conjunction with strong motion data and theoretical models of topographic amplification to assess the failures and their distribution. The landslides triggered under these conditions are shallow and with a crown located near the crest of the slopes. Slope orientation and slope height provide easily observable field indices that are particularly important, as data suggest that these parameters, together with the wavelength of the seismic waves, control the amplification of ground motions. These observations could provide key inputs to assess unexpectedly high ground accelerations for landslide and seismic hazard studies.

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