Comparison of response spectra in forward and backward directions in the Wenchuan earthquake

We compare the response spectra between forward and backward direction, and the contour maps of response spectra at certain natural periods. Results indicate: (1) the response spectra in the forward direction is higher than that of the backward direction. For natural period smaller that 2 second, the average response spectra in forward direction is about two times that of the backward direction; for natural period larger than 2 second, the maximum forward-backward average response spectra ration reaches about 4. (2) The long period ground motion is much more sensitive to the directivity effect. (3) The attenuation of amplitude of response spectra in the forward direction is slower than that of the backward direction.