Recent Research on Directionality of Earthquake Ground Motions

Abstract

Earthquake ground motion intensity varies significantly with changes in orientation. Although historically this has been ignored or not properly accounted for, this variation is actually significant and has multiple, important, implications in earthquake resistant design and when evaluating the seismic performance of the built environment. A common misconception is that strong directionality only occurs in the near field, while, in reality, a strong directionality occurs even at large distances from the rupture where it cannot be attributed to directivity. Recent studies to quantify and model directionality will be presented. This includes probabilistic models of two different metrics to quantify directivity in earthquake ground motions. Novel results show an interesting negative correlation between the level of polarity in a ground motion and the duration of the ground motion, meaning the level of polarization tends to decrease as the duration of the motion increases. It will be shown that for most structures, which typically have two principal axes in the horizontal direction that are perpendicular to each other, the probability that orientation-independent measures of ground motion intensity such as RotD50 are exceeded in one of the principal axes of the structure is higher than 90% when RotD50 occurs at the site and therefore the mean annual rate of exceedance the ground motion intensity in the structure is actually significantly higher than the mean annual rate of exceedance of RotD50. An alternate measure on intensity referred to as MaxRotD50 will be presented and discussed. The new measure of intensity is particularly well suited for earthquake-resistant design where a major concern for geotechnical and structural engineers is the probability that the design ground motion intensity is exceeded in at least one of the two principal horizontal components of the structure. The presentation will also include new emerging orientation-dependent ground motion models that allow to make estimates of ground motion intensities at specific orientations and will show that the variability in these models in many cases is smaller than that of orientation-independent measures of intensity. Finally, some applications will be presented to illustrate the importance of directionality.

Biography

Eduardo Miranda obtained his Civil Engineer degree from the National Autonomous University of Mexico, UNAM. He obtained his MSc and PhD degrees in Structural Engineering at the University of California at Berkeley. From 1993 to 1999 he was a Professor at the Graduate School of Engineering at UNAM. He has been a faculty member at the Department of Civil and Environmental Engineering at Stanford University since 2000 where he is currently a full professor. He is the author of more than 100 peer-reviewed publications and recipient of several awards one of which is the Moisseff award from the American Society of Civil Engineering His research focuses on Earthquake Engineering with emphasis on Performance-Based Design.