Comparison between earthquake rotation spectra obtained by different experimental sources

Abstract When selecting the earthquake design specification, for a given translation ground motion, the spatial variation of the motion has often been investigated as a possible source of further excitation for certain structures of large dimensions. It can be defined by the ‘rotation response spectrum’, or the ‘rotation power spectrum’. Previous evaluations of these spectra were based on records of translation motions and elaborations according to the wave propagation theory. Accelerograms recorded at some tens of km from the causative source were applied, for which a separation between wave types was possible, and the wave theory could provide a picture of the propagation pattern around the station. The present paper shows the relation between the rotation power spectrum and the cross-correlation of vertical accelerations simultaneously registered around a station. This relation is applied to evaluate rotation spectra on the basis of empirical cross-correlations derived from records collected at three ‘dense arrays’ of strong motion instruments: El Centro, SMART 1 (Taiwan), and Shizuouka (Japan) arrays. The first and second arrays made available cross-correlation of accelerations recorded a few km from the causative fault. To remove the intensity, the ratio of rotation and translation spectra are evaluated. The main conclusion is that investigations in near field conditions provide ratios of rotation spectrum to translation spectrum that are one order of magnitude higher than the ratios obtained in far-field condition. This order of magnitude is such that rotations can be significant on the predicted dynamic response of engineered structures.