Implications of considering finite fault rupture properties in seismic hazard and risk assessment

Tools for scenario-based seismic hazard and risk assessment are well documented and are generally used for predicting or reproducing the consequences of earthquakes. Traditionally, such evaluations were based on simplistic earthquake models combining a finite fault plane with one or more ground motion prediction equations (GMPE). However, more sophisticated tools exist for the simulation of earthquake ground motion, considering the effects of slip distribution, rupture propagation and other aspects of rupture dynamics. Whereas these methods are well tested and widely used in seismic hazard assessment, their influence on earthquake damage and loss estimates is not well known. In the current study, the effects of implementing stochastic finite fault ground motion simulations in seismic hazard and risk assessment are evaluated, both in terms of ground motion estimates and corresponding structural damage and losses. The investigations are applied to the city of Dehradun, located in the Indian Himalayas, for which a detailed building stock database and vulnerability model is available. Ground motion simulations are first calibrated against the 1991 Mw 6.8 Uttarkashi earthquake, which was recorded by a number of strong motion stations nearby. Afterwards, a number of relocations of the event are considered in order to investigate the difference between ground motions and losses estimated using the simplified GMPE-based approach and those estimated through simulations at varying distance and azimuth to the source. Furthermore, magnitude dependencies of these differences are studied. Results indicate that there are large differences between ground motion and risk estimates derived by the two methods especially in the direction of rupture propagation, which persist also at several fault lengths distance. The same observation is made, but to a much smaller extent, when the rupture propagates away from the site of interest. It is therefore strongly recommended to always consider rupture directivity and orientation to the test bed when providing ground motion estimates for seismic damage and loss assessment studies. In cases where these parameters are not well constrained, a range of scenarios with varying rupture directivity should be considered, thereby allowing for a more thorough estimation of the range of possible ground motions and losses.