Analysis of three-dimensional strong ground motions along principal axes was carried out based on K-NET records. By applying moving window technique to ground motion records, the time-dependent characteristics of ground motions along principal axes were examined. It is observed that the three rotation angles between conventional coordinates and principal axes are the function of time and the angles keep stable for a short time period after the arrivals of P and S waves, respectively. This indicates that the directly arrived P and S waves have their respective specific input directions. The rotation angle around vertical axis is almost zero after S wave arrives, which means that the principal axes vary only in horizontal plane after S wave arrives. Mean rotation angle was calculated for the stable time period after the arrival of S wave. Making use of the mean rotation angle and the maximum variance along principal axes of all observation stations, the spatial distribution characteristics of principal axes was investigated. A clear relationship between the two horizontal principal directions and the direction of source was not found. On the other hand, although the radiation patterns of SH and SV waves have a simple form of sin(2Φ) and cos(2Φ), respectively, in the moment release plane, they become very complicated in horizontal plane for general source mechanism. With the source mechanism of earthquakes, the theoretical solutions of SH and SV waves were obtained. Under the assumption of horizontal layer structure, the vibration directions of SH and SV waves are transverse and radial with respect to source direction, respectively. Because SH and SV waves share the same source spectrum and ray path, the correlation between SH and SV waves exists and the principal axes of SH and SV waves can be obtained theoretically. The results of principal axes based on theory and that based on observation data were compared.
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