Three-Dimensional Numerical Modeling of Seismic Wave Propagation in Wudu Basin

The Mw 7.9 earthquake occurred in Wenchuan, Sichuan, China, on 12 May 2008, and it caused huge casualties, economic loss, and serious damage to engineering structures and infrastructures in the area near the fault rupture. In particular, the Wudu township far away from the fault rupture (> 100km), which is located in a sedimentary basin, also suffered serious damage, and was in the area with seismic intensity VIII-IX in the Isoseismal Map of Wenchuan Earthquake. In this study, full elastic wave field simulations in the Wudu Basin were conducted by using a three-dimensional (3D) finite element method based on the parallel computing cluster platform of the ABAQUS software. The 3D Wudu Basin model is constructed based on the the digital terrain data borehole spatial distribution data in regional geological survey. The basin model consists of Four major subsurfaces and the basin basement, All calculations were executed in the time domain, and the soil and rock were assumed to be linearly elastic. The viscous spring artificial boundary was adopted as the artificial condition, and the oblique incidenct wave was transformed into the equivalent nodal forces acting on the viscous spring artificial boundary of the finite model. In the simulation, an impulse with a duration of 0.25 s was taken as the input SV wave. The simulating results of seismic response characteristics varying greatly from different points in the Wudu Basin revealed the basin edge effect, and the basin-focusing effect. The surface wave generated after the primary S wave is trapped at the shallow part of the basin, and most of the energy is reflected from the interfaces of soil strata and focused back into the basin when the wave propagates through the deepest part of the basin. Moreover, a part of the seismic wave front turns and follows the shallow basin edge, resulting in further amplification. The phenomenon, the incident angle of the seismic waves can produce unusually strong shaking distribution, was demonstrated. It indicates that the complex Wudu Basin geometry, the fairly low velocity of the surface soil layer, and the incident angle of seismic waves dominate the amplification and wave propagation behavior, which results in extraordinary strong shaking patterns in the Wudu basin area.