Investigations Of Mountain Topographic Effects Based On Spectral Element Method

Spectral element method(SEM)is a numerical simulation method implicitly fulfilled physical free-surface boundary condition,which is very suitable for simulating the ground motion on the local geometrically complex topography.In this paper,the simulation was conducted based on SEM is investigate 2D to 3D local topographic effect.In the first part,we investigate the combined effects of steepness,frequency,angle of incidence and velocity on the topographic amplification.The results show that(1)Topographic effect heavily depend on mountain size and seismic wave frequency for the complex interference between uphill surface wave and direct wave,thus parametric study may draw a unreliable conclusion without sensitivity analysis.Non-vertical plane wave does not produce higher amplification but can significantly modulate the distribution of amplification on mountain surface.(2)The subsurface low velocity layers can obviously amplify ground motion in excess of bedrock topographic amplification.The 2D simulation applied to Weigan slope in Qingchuan area show that(1)Bedrock and soil layering has coupling effect on the amplification of grond motion,the mutiples constructive interference between seismic waves trapped and reverberating in soil layers and scattered waves produced by irregular ground surface further aggravate the amplification of ground motion.(2)In addition to shear wave velocity,contrasts and depth of layers also affect spectral ratio curve,we effectively simulate the real seismic response of Weigan vertex through the reasonable construction of underground medium.In second part we constructed 3D topography surface using high-resolution digital elevation data based on SEM and investigate seismic response of realistic topography.In general,the ground motion is obvious at mountain tops and ridges but weak at valleys.The amplification effect in Weigan hill increases with the height.However,in Mt.Dong and Mt.Shizi,the amplification effect is not positive correlated to height.The synthetic acceleration records show that amplification in Mt.shizi is obvious in three directions however amplification in Mt.Dong and Weigan slope is just occurrs in NS direction and vertical direction.The acceleration response spectrum indicate that the amplification effect occurs mainly at relative higher frequency band.Amplification of arias intensity is similar with that of peak acceleration,but the amplification of relative energy duration shows no significant regular pattern,of which amplification factor is varies between 1.4 and 1.6.These conclusions are consistent with site monitoring results,which proved to be a useful method to guide prevention of seismic hazards.Further efforts were made to analyze topographic effect in considerations of the focal mechanisms and focal locations.Topographic effects are not only related to topography itself but also controlled by source radiation pattern.The “back slope effect” can be found in some slopes which on the opposite side of site path.For the Mt.shizi and Weigan slope,the position of maximum peak ground acceleration(PGA)amplification varies with source locations and focal mechanisms but the overall topographic effect show less variations.However,the Mt.Dong appears to be more sensitive to source characteristics.Different sources may lead to opposite topographic.Seismic data recorded at Mt.Dong,Mt.Shizi and Weigan slope are analysed computing directional spectral ratio and lead to the conclusion that amplification tends to polarize along a certain direction.According to simulation results and geological informations of Qingchuan County,the amplification effect may be related to the potential landslide direction of the slope.