The Influence Of Local Raised Topography On Ground Motion

The results of a number of seismic surveys have shown that locally elevated terrain has a significant impact on the intensity,spatial distribution and propagation of earthquakes,especially at the top of steep slopes and gently sloping plateau areas,where there is a significant amplification effect compared to flat free sites.In order to protect the lives and property of the people in mountainous areas,the influence of locally elevated terrain on ground shaking must be taken into account in the siting of buildings and the design of engineering ground vibration parameters.From the San Fernando earthquake to the Lushan earthquake,the location of the mountainous terrain is often seen in many observation records as an area of intensity anomalies.Scholars at home and abroad have conducted detailed studies of these phenomena based on record analysis,analytical analysis and numerical modelling methods,and some qualitative conclusions have been formed.However,the quantitative analysis of the effect of raised terrain still needs to be further explored.In this paper,we introduce amplification factors to characterize the topographic effects,simplify the complex ridge topographic model into a two-layer medium raised slope topography,and quantitatively analyse the effects of SV and SH wave incidence on the slope angle,soil thickness,soil to bedrock impedance ratio and observation location on the local raised topographic effects based on the display finite element method,and carry out a comparative analysis between records and simulations for the Zigong topographic table array.are as follows.1.The effects of slope angle,soil thickness,soil-to-bedrock impedance ratio and observation location on the local raised topography effect under SV wave incidence were investigated.The results of the study show that the secondary surface waves generated by the slope are the root cause of the difference in ground vibration between the slope site and the 1D soil layer.When the thickness of the cover layer is greater than 1/4 of the slope height and the angle of the slope is greater than 45°,the ground vibration effect of the double-layered media slope is significantly amplified compared with that of the 1D soil layer.2.3 times.2.The effects of slope angle,soil thickness,soil-to-bedrock impedance ratio and observation location on the effect of local raised topography under SH wave incidence were investigated.The results of the study show that the reflected waves generated in the slope and the overburden are the root cause of the difference in ground vibration between the slope site and the 1D soil layer.When the thickness of the cover layer is less than 1/4of the slope height or the slope angle is less than 60°,the ground vibration effect at the plateau of the double-layered media slope is significantly amplified compared to that of the 1D soil layer,and the amplification factor decreases with the increase of the soil layer thickness and shows a periodic change with the increase of the distance to the top of the slope,and the period increases with the increase of the distance to the observation point.When the soil layer is thin,the effect of slope angle on the ground vibration effect is greater than the impedance ratio,and the opposite is true when the soil layer becomes thicker.The maximum value of the amplification factor is obtained under gentle slope conditions and is 1.93 times the amplification of a one-dimensional soil layer.3.Using the Zigong topographic array as the object of study,we divided the grid and built a model according to the site conditions of each station location by a selfprogrammed program,and used the explicit finite element method to simulate the impulse propagation in the Zigong topography.The analysis shows that the seismic wave incidence angle and epicentral distance are the main reasons for the difference in the spectral ratio results between the two earthquake records.The two-dimensional and threedimensional simulations can reflect the site effects of the Zigong topographic array to a certain extent,especially in the low frequency region where the simulated results and the recorded observations are in good agreement.Comparing the data from the same elevation stations,it can be seen that the amplitude of ground shaking is attenuated at the foot of the slope,and the closer to the foot of the slope,the more obvious the attenuation feature is.