Analysis of horizontal strong-motion attenuation in the great 2008 Wenchuan earthquake

A large number of strong-motion records were obtained by the National Strong-Motion Observation Network System (NSMONS) of China for the Wenchuan earthquake. There are 1350 components of strong-motion records from the mainshock at 455 stations. In these stations, 164 stations are located in the range of rupture distance within 400 km and 37 stations are within 100 km. The largest peak ground acceleration is 957.8 Gal from the Wolong station in the hanging wall area with rupture distance of 23 km. The records from 164 stations in the range of rupture distance within 400 km are used in this study to investigate the hanging wall/footwall effects, directivity effects, and attenuation characteristics of ground motions from the Wenchuan earthquake. The characteristics of ground motion attenuations are compared in four subareas: hanging wall area, footwall area, forward directivity area, and backward directivity area. The study results show a clear hanging wall/footwall effect in near-fault ground motions within a rupture distance of 40 km, but it appears only in the peak ground acceleration and components at short periods below 1.0 s, and a strong forward directivity effect appears in ground motions over the whole range of rupture distance and period. Furthermore, the fitting attenuation curves of the peak ground acceleration and spectral accelerations of ground motions from the Wenchuan earthquake are compared with the predicted curves by an empirical attenuation model in the hanging wall and footwall areas.