Earthquake-induced deep-seated landslide and landscape evolution process at Hungtsaiping, Nantou County, Taiwan

Abstract

The Chi-chi earthquake (magnitude 7.6) on September 21, 1999 triggered numerous landslides in central Taiwan. A particularly large, deep-seated landslide occurred at Hungtsaiping, 15 km north of the epicenter. Field investigation and landscape evolution analysis revealed an irregular pattern of displacement vectors, indicating that this failure mechanism is highly complex. This paper presents the results of site investigations in conjunction with simulations of landscape evolution associated with this landslide. Analysis of landscape evolution was based on topographic maps produced at seven different times as well as aerial photographs taken between 1966 and 2006. Landslide site investigation included field reconnaissance, geomorphologic analysis, geophysical exploration, borehole logs, and laboratory experiments. Comprehensive geologic investigation led us to conclude that the materials involved in the landslide are colluvial deposits. Landscape evolution involved at least three large landslide events, two previous rockslides (the first triggered during the 1916 Nantou earthquake and another event occurring in 1934) as well as the 1999 colluvium slide (triggered during the Chi-chi earthquake). By taking into account the source of the collapsed mass, we were able to reproduce the three landslide events using a 3D discrete element model. Calibration of the parameters used in the numerical model was based on the strength characteristics of the rock mass as well as the morphology of the landslide deposits.