Research On The Influencing Factors Of Seismic Response In Two-dimensional Basins

A large number of seismic damage investigation and theoretical research show that basin ha a significant impact on the characteristics of seismic hazards and ground motion,which is usually manifested as the amplification of ground motion,thus directly affecting the severity of earthquake disaster of the building structures in the basins.The interior of the basin is mostly flat,usually resource-rich and densely populated,such as Los Angeles,Mexico,Osaka,Taipei,Chengdu and Lanzhou are located in the basin.Domestic and international seismic hazards information and numerical simulation results show that the existence of sedimentary basin caused direct wave is amplified,multiple reflections and transformation,the basin edge caused strong interference between the formation of secondary surface wave and body wave and the focusing effect of the special geometry of the basin,while common increase the duration of ground motion,long-period ground motion amplification,it causes serious damage to buildings or structures with long natural vibration period,such as mid-rise building,long-span bridges,buildings with complex structure,dam and other lifeline engineering.A large number of 2D and 3D numerical simulations of basin seismic responses have been carried out abroad,and plentiful achievements.In this paper,based on the hypothetical two-dimensional basin model,using a combining of the Lumped Mass Finite Element Method and Multi-Transmitting Formula,from the basin sloping edge angles,medium impedance contrast ratio and incident seismic wave predominant frequencies in research,and basin amplification effects was studied in detail by introduction of basin aggravation factors?AGF?.the main research content are as follows:1.This paper introduces the development of the research progress of basin seismic effects at home and abroad,and summarizes the main methods of studying basin effects,including seismic hazards investigation and strong ground motion observation,theoretical analysis and numerical simulation.In addition,the accuracy and stability of the numerical simulation method which combines the Lumped Mass Finite Element Method with the Multi-Transmitting Formula are verified by the basic theory of wave propagation.2.The influence of basin sloping edge angles,inside and outside the basin medium impedance contrast ratio and incident seismic wave predominant frequencies on the basin ground motion response was studied respectively.The results show that,with the increase of basin sloping edge angle,the seismic response in the middle of the basin weaken,while the seismic response in the edge of the basin strengthen,and the region with the strongest seismic response in the interior of the basin moves from the middle to the edge of the basin.Moreover the numerical simulation shows that the inside and outside the basin medium impedance ratio is a significant factor affecting the seismic response in the inner part of the basin.When the medium impedance ratio is smaller,the seismic wave energy is trapped in the sedimentary layer on the upper part of the basin,and the surface earthquake response is strong,the medium impedance ratio increases,the wave transmittance increases and the ground seismic response decreases.Finally,the closer the predominant frequencies of incident seismic wave is to the natural vibration period of the basin,the more likely it is to generate resonance phenomenon.Therefore,when the predominant frequencies of incident wave is close to the natural vibration period of the site,the ground motion response of the site is the strongest.3.Basin effects are quantified and discussed through a period-location dependent“Aggravation factor”?Chávez-García&Faccioli,2000?,defined as the ratio between2D and 1D acceleration response spectra at the basin surface.This aggravation factor expresses the additional effect of the 2D response at different locations along the basin with respect to the corresponding 1D response.Through modeling and analysis of a large number of basins,the influence of control parameters such as basin width,basin sloping edge angle,sedimentary layer thickness and shear wave velocity?or impedance contrast ratio?was taken into comprehensive consideration,the basin was divided into shallow or deep basin and shorter or longer basin by introducing“shape ratio”and period T₀,and the aggravation factors of the two types of basins were given respectively.In addition,the studies shows that the basin should be divided into different regions when considering the aggravation factors of the basin,and the magnification level of different regions in the basin should be considered respectively,otherwise the amplification effect of the basin will be underestimated or overestimated.