Omnidirectional input energy spectrum and directionality of ground motions

Abstract

It is well known that the intensity of horizontal ground motion can vary significantly with changes in orientation. Studying the directionality of earthquakes is of significance for anticipating possible geological disasters and structural damage. Currently, most studies analyze the directionality of bidirectional ground motions using acceleration response spectra, but there have been no studies employing input energy spectra for this purpose. This paper proposes the omnidirectional input energy spectrum, which provides a detailed and clear view of seismic directionality characteristics by displaying input energy values across different horizontal orientations and periods. The effects of factors such as constitutive model, rupture distance, fault type, and epicenter distance on the omnidirectional input energy spectrum were investigated. The directionality characteristics of several earthquakes were analyzed. It was found that the choice of constitutive model has a minor impact on the directionality of the omnidirectional input energy spectrum. The ductility level and damping ratio have little effect on the directionality of the omnidirectional input energy spectrum. For the strike-slip event, the predominant orientation turns from fault-normal to fault-parallel as the rupture distance increases, and the predominant orientation is not always close to the transverse orientation relative to the epicenter. The predominant orientation of the hanging wall of the thrust fault is close to the fault-normal direction at the medium rupture distance, while that of the normal fault earthquake does not exhibit a clear pattern with changes in rupture distance. The predominant orientation of Chi-Chi earthquake tends to shift from fault-normal to fault-parallel with the increase of epicenter distance, whereas the Turkey earthquake and the Central Italy earthquake have no obvious regularity.