Study Of Hill Topography Effect On Ground Motion

Effects of topography on the ground motion are in the field of local site conditions affecting the ground motion. After Wenchuan earthquake, the main shock and many aftershocks are recorded. Combining the ground motion record of hill topography from"MingDeng NO.1"artifical explosion and the topography dependent earthquake damage data of Wenchuan earthquake, the hill topography effect on the ground motion is analysed and studied in this paper, and some understanding and conclusions are acquired.By processing and analysing the hill topography effect data, the basic characteristics of acceleration , RMS acceleration , 90%duration and spectrum of the hill vibration are acquired. For considerable quantity of aftershocks, it is found that the ground motion intensity on the base of the hill is stronger than that on the hill crest, which is abnormal. In order to explain this phenomenon, the effect of overlying soil layer on the gorund surface around the base of the hill, the spectral characteristics of input earthquake motion and the predominant frequency of the hill, on the hill viberation is studied. When there exists soil layer on the horizontal ground surface around the base of the hill, and the input earthquake motion intensity is lower, the soil layer is not in the apparent nonlinear state, so the frequency contents of the input earthquake motion in different frequency bands are amplified differently; meanwhile the ground motion on the surface is amplified. However, the hill filters the high frequency components of the input earthquake motion beyond its predominant frequency because of its vibration characteristics, which leads its seismic response lower than that of the ground surface.The seismic response of the hill depends on the frequency components of the input earthquake motion. When the frequency components are close to the predominant frequency of the hill, the resonance response happens, and the input earthquake motion is amplified. When the frequency components are higher than the predominant frequency of the hill, the higher frequency of input earthquake motion is filtered, and the response of the hill is inhibited. However, there is almost no effect on the frequency components lower than the predominant frequency of the hill.There exists disadvantage while using the traditional Fourier spectrum ratio method to confirm the hill topography effect. By analysing the response spectrum ratio of the record data, the RMS response spectrum ratio method is put forward. It is proved that the randomicity and inconsequence of the Fourier spectrum ratio method caused by using the amplification amplitude at some frequency point to confirm the hill topography effect are eliminated. TheRMS response spectrum ratio method is reasonable in confirming the amplification effect on the energy of some frequency band in the input earthquake motion.The polarization effect of the hill seismic response is much relevant to its geometry. The isolated round hill is hard to polarize, but it is easy for the narrow and long mountain ridge. The two horizontal component acceleration record data are decomposed along the strike and transverse direction of the mountain ridge, and the characteristics of acceleration, RMS acceleration, 90%duration and spectrum of the mountain ridge vibration in these two directions are acquired. By analysing the spectrum characteristics of the input earthquake motion and the mountain ridge vibration characteristics in the strike and transverse directions, it is found that the polarization effect of the mountain ridge is quite relevant to the frequency component of the input earthquake motion, and the seismic response intensity in the transverse direction is not alwayes stronger than that in the srtike direction. Set Douchuan Mountain as an example, the seicmic response in the transverse direction is greater than that in the strike direction under the large earthquake input, however, the comparison result is converse under the small earthquake input. It is because that the frequency components are abundant in both the low-intermediate and intermediate-high frequency bands for large earthquakes, and the predominant frequency in the transverse direction belongs to the low-intermediate frequency band, so the resonance happens in this direction. However, for small earthquakes the frequency component is abundant in the high frequency band, and there is almost no low frequency component, which is hard to cause the transverse polarization, but the strike polarization, because the predominant frequency in the strike direciton of the mountain ridge belongs to the high frequency band. Further more, the polarization of the mountain ridge vibration is relavent to the energy content in different frequency bands.By simulating the two types of typical topography, scarp and mountain ridge, in two dimensions, the seismic response characteristics of different slop topography models are disscussed under the vertical and oblique incidence of P and SV waves, and the effect of overlying soil layer of rock topography models is analysed. It is proved that the topography effect on the seismic response of ground surface is obvious.As to the scarp model, the seismic response is inhibited seriously on the foot of the slop in the horizontal direction, and the larger slop, the greater inhibition. The ground motion intensity on the slop becomes stronger with the increasing elevation. The amplification of seicmic response in the front of the scarp crest is obvious, and the smaller slop, the greater amplification. However, the seismic response amplification becomes weaker as the distance away from the slop crest increases, and in the two sides far away from the slop, the seismic response level is close to the ground motion of the horizontal ground surface.As to the mountain ridge, the intensity of seismic response is dependent on the angle between the propagation direction of incidence wave and the slop surface, and the larger angle, the stronger intensity in the horizontal direction and the weaker intensity in the vertical direction. By comparing the seismic response history at different positions under the oblique incidence, the travelling wave effect of the ground surface is obvious.The effect of soil layer on the seismic response at ground surface of two types of topography is analysed, and the results show that the intensity of seismic response at the soil layer covered on the horizontal ground surface may be stronger than that on the soil layer covered on the slop. There dosen't exist obvious amplification between the seismic response on the slop with and without soil layer. It is because that the vertical incidence becomes oblique incidence relative to the soil layer covered on the slop, and the seismic response amplification of soil layer depends on the incidence angle. As to the SV wave, the amplification coefficient of soil layer decreases with the increasing incidence angle, and finally decreases to the minimum at the critical angle.