Earthquake ground motion directionality and its effects on regional risk assessment
Keywords: Engineering.

Ground motion intensity is usually characterized by response spectra for earthquake engineering purposes. Horizontal response spectra vary with orientation, a phenomenon known as ground motion directionality. This variation affects the built environment because most structures have mechanical properties that also depend on orientation due to their geometry and the arrangement of lateral load-resisting elements. The physical mechanisms that lead to directionality are currently poorly understood, and its effects on buildings are usually neglected in earthquake-resistant design and seismic risk assessment. Thus, the main objective of this dissertation is to advance the understanding of ground motion directionality with a focus on earthquake-engineering applications. Ground motion directionality is first quantified probabilistically to capture its record to record variability using a database of more than five thousand ground motion records from shallow crustal earthquakes in active tectonic regions. In general, the variation of horizontal spectral ordinates with orientation is found to increase with period. Next, the directionality characteristics of recorded ground motions are compared to those of synthetic ground motions generated using stochastic simulations, which have no predominant orientation of motion. Synthetic ground motions are found to have directionality levels comparable to those of recorded ground motions, suggesting that most directionality can be explained by finite duration effects rather than physical mechanisms causing polarization of shaking. Inspired by the deterministic polarization of S waves, which usually control response spectra at distances of engineering significance, the orientation of maximum spectral response at long periods of records from strike-slip earthquakes is found to be close to the transverse orientation at each site. An empirical model to modify ground motion models is then developed to improve the estimation of response spectra at specific orientations. This dissertation also explores the implications of ground motion directionality on two earthquake engineering applications: design spectra and regional risk assessment. First, a simple scalar measure of ground motion intensity, referred to as MaxRotD50, is proposed for earthquake-resistant design purposes. This measure is defined as the median value of the maximum spectral ordinate of two perpendicular horizontal directions computed over all non-redundant orientations. Relations to other previously used measures are derived empirically using recorded ground motions, which enable the use of ground motion models derived for any of these other measures of intensity together with MaxRotD50. The second studied effect of directionality is on regional seismic risk analysis. In these types of analyses, the specific orientations of buildings have previously not been considered. Using a variance-based sensitivity analysis, the contribution of ground motion directionality to the output variance is found to be more significant than several other sources of uncertainties considered in these types of analyses, such as those related to structural response and damage assessment. Furthermore, the layout of street networks within a city is identified as a critical aspect in determining the impact of ground motion directionality on seismic risk results, with perfect grid layouts magnifying directionality effects.