Study On Seismic Effect Of Two Dimensional Sedimentary Valley Based On Spectral Element Method

The seismic effect of large and complex sedimentary valleys has a great influence on the surface seismic damage.Since the analytical method is generally suitable for solving the relatively regular terrain,the numerical simulation method is often used to explore the complex site.In this paper,two types of two-dimensional sedimentary valley models of Lanzhou basin and Valley are firstly established.On this basis,numerical simulation method combining two-dimensional spectral element(SPECFEM2D)and multiple transmission boundary(MTF)is used to study in detail when different seismic waves are incident vertically.The influence of the structure of lanzhou basin and valley on the surface seismic wave propagation,surface peak acceleration(PGA)and its amplification coefficient,as well as the amplification characteristics in frequency domain is analyzed.The main contents of the paper are as follows:1.This paper introduces the modeling method of two-dimensional lanzhou basin and valley,solving the free field with vertical input,and the computer numerical simulation method combining spectral element method and multiple transmission boundary to vertically input the plane wave into the sedimentary valley under the action of external sources.2.A two-dimensional lanzhou basin with the characteristics of a large basin with openings at both ends is embedded with a small closed basin.When nine seismic waves incident vertically from the bottom of the model,the influence law of the amplification characteristics in the time-frequency domain of the basin surface ground motion is studied.The results show that when the first natural frequency of the enclosed small basin is close to the preeminent frequency of the incident wave,the resonance phenomenon will occur,and the distribution characteristics of the amplification coefficient of ground motion acceleration will change significantly.The two-order surface waves with significant difference in wave velocity are formed by the convergence of seismic waves from the edge of a large open basin at both ends to a smaller,looser basin,and the stronger surface waves mainly appear in the edge region of the basin with a steeper dip angle.The marginal area of the small basin in lanzhou basin shows the strongest ground motion when the exogenous input occurs,and the sub-strong ground motion amplification area is above the edge of the steeper side of the large basin,while there is no edge effect near the side of the basin with a slower dip angle.3.A two-dimensional valley with complex soil layers with different structural distributions and a depression in the middle is studied.When three seismic waves incident vertically from the bottom of the model,the effects of time-frequency amplification on the ground motion of the valley surface are studied.The results show that when the inclinations of overburden layers on both sides of the valley change at different input seismic waves,the seismic waves in the horizontal direction are clearer than those in the vertical direction.63% of the input waves are the weakest,10% are the second,and 5% are the strongest.The soil layer in the depression in the middle of the valley shows a large amplification effect of ground motion.Under the input wave with a exceedance probability of 63%,the horizontal spectrum ratio is significantly different from that in the vertical direction,and the horizontal direction is larger than the vertical direction.At the same time,when the preeminence frequency is 6.5Hz,the acceleration amplification coefficient is 3.5.