Peculiarities of seismic risk in hilly regions: topographic effects on hazard and vulnerability

Dominik Lang
Norwegian Geotechnical Institute, Oslo, Norway

Many of the world’s high-seismic hazard regions are characterized by hilly topography. Topographical features pose significant problems when it comes to the seismic safety of buildings and infrastructure facilities, both with respect to the seismic impact, the structure’s vulnerability and potential secondary hazards such as slope stability-related issues. The seismic ground motion characteristics are greatly influenced by the geometry of topographical features such as slopes, hilltops, ridges and canyons and their relationship with geologic materials partly overlying these features. Both, topography and geology lead in most cases to a significant amplification of the seismic ground motion and hence increased seismic demand to buildings located in these areas. This comes in addition to the fact that buildings located in hilly areas have a significantly lower structural capacity. Due to the limitations posed by the hill topography and the scarcity of flat building plots, many buildings are placed on hill slopes thus have highly irregular configurations, both in plan and elevation, making them highly vulnerable to seismic impact.

The presentation will focus on selected case studies in the Indian Himalayas, which are not only the youngest mountains, but also one of the most seismic areas in the world. Influence of topography on seismic hazard and vulnerability of buildings in the study area is illustrated using extensive numerical studies. The gross effect of these two parameters is demonstrated by comparing the probabilistic earthquake loss estimates with and without considering the topographic effects.