Two-dimensional Numerical Simulation Of Amplification Effect In Trapezoidal Sedimentary Basin

Seismic damage investigation of many earthquakes in history shows that sedimentary basins have a strong amplifying effect on ground motions,so buildings located in basins are often severely damaged when earthquakes occur.Many cities in each country are located in the basin area,such as Mexico City,the capital of Mexico,Tokyo,Beijing,Taipei and so on.These cities have a large population and all kinds of buildings are dense.Once an earthquake occurs,it will cause immeasurable huge losses.The current seismic design code of building in China has not put forward clear requirements for the amplification of the special ground to ground motion in the basin,and the building structures located in the basin tend to underestimate the impact of seismic action,which brings hidden dangers to such building structures.The study of the influence of basin on ground motion can make us more clearly understand the law of its influence on the characteristics of ground motion and provide a reference for seismic fortification and urban planning in the basin.There exist lots of research about the effect of the basin,this paper adopts the method of explicit finite element method combined with transmission artificial boundary,first discusses the simple SH wave with different incident angles of incidence and Angle trapezoid sedimentary basin basement rocks and the change of the thickness of covering soil layer amplification coefficient distribution,different situations,such as the surface response spectrum and the development of surface wave.Then,using the seismic waves recorded in the wider seismic band as input,the differences of response spectra at different locations on the surface of the basin are analyzed.Then,by changing the thickness of the overburden,the influence of the change of the overburden thickness on the amplification of the surface peak acceleration and the influence on the response spectrum was studied.Finally,the parameters influencing the amplification effect of the basin are dimensionless,and the ratio of depth to width,the ratio of wave velocity between soil layer and bedrok medium,the ratio of basin depth to the dominant frequency wavelength of incident wave,and the ratio of basin opening width to the dominant frequency wavelength of incident wave are defined.The influence of the above infinite parameters on the basin amplification coefficient was studied.The main results of this paper are as follows:1.The magnification coefficient of observation points located on the overlying soil layer inside the basin is obviously greater than that of observation points located on the bedrock outside the basin.It can be observed from the synthetic seismogram that surface waves are generated at the edge of the basin and propagate to the center of the basin along the horizontal direction.2.With the change of the incident Angle of the seismic wave,the distribution of the peak acceleration amplification coefficient is also changing.The maximum amplification factor does not appear in the case of vertical incidence of seismic wave,but occurs in the case of about 10° incident.In addition,the variation of the dip Angle of bedrock in the basin also has a great influence on the distribution of the peak acceleration amplification factor.The larger the dip Angle of bedrock,the closer to the edge of trapezoidal basin,the smaller the dip Angle of bedrock,the closer to the center of trapezoidal basin.3.The influence of incident Angle and dip Angle of bedrock on the development of surface wave in the basin is studied,and the variation curve of peak acceleration of surface wave with dip Angle of bedrock is given.The results show that the surface wave composition is more abundant when the bedrock dip Angle is smaller than when the bedrock dip Angle is larger.4.The difference of response spectrum of receiving points in different positions of the basin when the dip Angle of bedrock changes is studied.The variation of bedrock dip Angle has no obvious effect on the response spectrum,but the amplitude of ground motion at the observation point in the center of the basin with a long period(T>ls)is obviously larger than that at other positions.Taking the actual ground motion as the input,the response spectrum ratio curve shows that the point located in the center of the basin can magnify the ground motion about 3 times with the period T=1.6s.5.Finally will impact basin amplification effect of three kinds of factors,including the geometry of a basin,the basin of material physical properties and incident wave frequency is defined as the following dimensionless parameters,such as:deep wide than basin,basin soil layer and bedrock medium wave velocity ratio,the depth of the basin and the ratio of the incident wave frequency wavelength,basin opening width ratio of the wavelength and incident wave frequency,etc.It is found that the larger the depth to width ratio of a basin is,the more obvious the basin focusing effect is,and vice versa,the basin edge effect is more significant.The change of the ratio of the basin opening width to the main frequency wavelength of the incident wave also affects the distribution of the surface acceleration peak amplification factor.With the increasing of the ratio,the point with the largest amplification factor in the basin gradually moves from the center of the basin to the edge of the basin,showing more and more obvious edge effect.The most significant parameter affecting the amplification factor is the ratio of wave velocity between the soil layer and the bedrok medium.The larger the difference of wave velocity,the larger the amplification factor of each observation point in the basin is,on the whole,greater than that of the medium with a relatively small wave velocity.In addition,the larger the difference of wave velocity,the larger the refractive Angle of seismic wave at the boundary of bedrock soil layer,and the basin is more inclined to have the maximum amplification coefficient of acceleration peak at the center of the basin,showing a significant focusing effect.The main innovative work of this paper is as follows:the dimensionless parameters affecting basin amplification effect are proposed,and the law of basin amplification effect is systematically analyzed and summarized with the method of dimensionless parameters,which provides new ideas and new methods for the study of basin amplification effect in the future.