Research On Seismic Effects Of Three-dimensional Sedimentary Basins

The sedimentary basins can significantly amplify the seismic motions within them, which has been validated by numerous seismic hazards and observation records.Many numerical researches about the seismic responses of three-dimensional basins have been performed abroad. However, in our country, such studies remain less. In this study, three dimensional basin models are established based on the exiting data,then the seismic effects of two typical basins, that is, the Shidian basin and the Sichuan basin, are investigated by using the spectral element method. The main contents are as follows:1. The mechanisms and research status of seismic effects of three dimensional sedimentary basins, such as the basin-induced surface wave, the basin edge effect, the focusing effect, and the resonance effect are summarized in detail.2. A combination method is introduced of multi-transmitting formula(MTF) with the spectral-element method. The technique is implemented in the widely used software packages called SPECFEM2 D and SPECFEM3 D. Responses of 2D and 3D wave source problems are simulated by the modified SEM packages. the results show that SEM combined with the MTF significantly improves the absorbing efficiency of large-angle incidence body waves and surface waves out of the boundary, and removes the drifting phenomenon of the viscous boundary.3. By using the newest 3D Shidian basin model, the wave propagation process and the effect of the basin basement geometry and the impedance contrast between basin and baserock are investigated. The results show that:(1) Strong secondary surface waves are generated at basin edge. The interference between the surface wave and the body wave leads to a stripped strong motion regions near the basin edge.(2) The east-west and north-south propagating surface waves and the trapped body waves by the basin considerably prolong the strong motion duration time.(3) The interference between the horizontally propagating surface waves results in the one- or multi-dimensional resonance in the basin-internal small bowls, which strongly amplify the ground motion within the bowls. Besides, amplitudes of multi-dimensional resonance are larger than the one-dimensional case.(4) For three dimnensional basin, the excitation frequencies of different mode Rayleigh wave and Love wave are different. For Rayleigh wave, the excitation frequency of fundmental and first mode is the fundmental natural virbration frequencyof the equivalent plane layer. For Love wave, the excitation frequency of different modes is the same order natural virbration frequency of the equivalent plane layer.(5) The generation of higher mode surface wave is influenced by the sediment velocity. For the same input wave, higher mode surface wave will appear in the low sediment velocity model. However, only the fundamental mode surface wave is generated in the high sediment velocity model.(6) The ground motion distribution in the basin is strongly influnced by the source radiation pattern and the wave incident direction.(7) For the Shidian basin, the three local bowls and the basin edge area are characterized by the largest amplification factor.4. A refined three dimensional Sichuan basin model is established first time by incorporating the data of real topography, the Quaternary sediment, the crystalline basement, the near-surface sedimentary layer, and the layered crust structure.5. The seismic effect of the Sichuan basin in the 2008 Wenchuan earthquake is investigated by the above basin model. The results show that:(1) Strong ground motion disfribution in the basin is strongly influenced by the Quaternary sediment and basin basement.(2) Interference between the waves radiated from different sub-faults leads to two stripped strong ground motion area.(3) A main reason for the severe damage in Dujiangyan, Anxian, Jiangyou is the combined effects of basin edge effect,rupture directivity effect and the interference effect.(4) Compared to the rock layered model, the largest amplification factor for the horizontal component is 9.6, which locates at the Zhuwa bowls that has the deepest Quaternary sediment.(5) The vertical component ground motion is insensitive to the basin effect, and is mainly controlled by the topography effect.(6) For the simulated frequency band, the Quaternary Chengdu plain mainly amplifies ground motions above 0.3Hz, while the Sichuan basin basement mainly influnce ground motions less than 0.3Hz. By the comparison between results of this study with the real records, the intensity map, and simulated results of some others, the rationality of the established Sichuan basin model and our simulated results are further validated.