Ground rotational motion characteristics of The 2008 Wenchuan earthquake, China

The rotational seismic motions are estimated from main-shock translational acceleration records of the 2008 Wenchuan earthquake based on the theory of elastic plane wave propagation. The time-frequency response spectrum (TFRS) of the rotational motions is calculated and its characteristics are analyzed. The results show that the dominant peak amplitude spectrum is around 0.40 s for torsional motion and around 0.10 s for rocking motion, the torsional component is richer than the rocking component in period domain, one of the ground motion components can not reflect the characteristics of the seismic motions completely; the characteristics of each component, especially rotational motions, need to be studied. TFRS can reflect the changes of amplitude of different component with period and time simultaneously, especially the change in the frequency (or period) domain characteristics of ground motion with time. The TFRS could be a great method to analyze the damage mechanism of structure indeed considering the cumulative effect of the seismic ground motions and choose the input seismic wave for seismic analysis.