Study of rotational ground motion in the near field region

While the earthquake induced translational wave field has been recorded and studied since the nineteenth century, the rotational motion still nowadays remains poorly observed and investigated. We aim at further understanding the rotational ground motion and its relation to the translational wave field with a special emphasis on the near field, few wavelengths away from the hypocenter, where damage related to rotational motion might need to be considered. A broad picture of the available values of rotational amplitudes and their variability is obtained by gathering most of the published data on strong rotational motion. To obtain a more detailed picture, we perform a large scale 3D numerical study of a strike-slip event in the Grenoble valley, where a combination of topographic, source, and site effects produces a realistic wave field. The size of the synthetic dataset allows us to study the distribution of the rotational and translational peak amplitudes and their dependence on two effects: non-linear soil behaviour and source directivity. Finally, we compare our numerical results in terms of Peak Ground Velocity, PGV, vs. Peak Ground Rotation, PGω, with field data obtained at similar scenarios (e.g. Parkfield) by array techniques to investigate the relation between translational and rotational amplitudes which can be expected in the near-field for shallow medium-sized earthquakes. Furthermore, the spatial variations of PGV/PGω ratio show a trend that seems to be correlated with the velocity structure of the model under study.