Rupture diectivity effect in the 2008 Ms 8.0 Wenchuan earthquake

We study the near field earthquake ground motion records and engineering damage caused by the great 2008 Mw7.9 Sichuan Wenchuan earthquake. Statistical analysis indicates: Although the earthquake has a thrust faulting mechanism, the dominating unilateral rupture propagation caused a distinct rupture directivity effect both on ground motion recordings and on the engineering damage. In detail, (1) The peak ground acceleration (PGA) in the rupture forward direction is bigger than that of the backward direction, and the difference between the forward and backward depends on the closest rupture distance; (2) The mean response spectra in the forward direction is higher than that of the backward direction, especially for periods higher than 2 sec, and the maxim ratio of forward and backward average spectra may reache 4; (3) For strong ground motion duration, the difference between the forward and backward direction sometimes reaches 2∼4 times on average; (4) The distribution of building collapse ratio and human mortality ratio in direction along and perpendicular to the fault strike are an additionally prove to the directivity effect.