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Seismic Response Analysis of Valley Topography

SHENG Zhi-qiang1,LU Yu-xia1,2,3,SHI Yu-cheng1,2,3,LIU Kun1,2,3,WAN Xiu-hong1(1.Lanzhou Institute of Seismology,China Earthquake Administration,Lanzhou,Gansu 730000,China; 2.Open Laboratory of Loess Earthquake Engineering,CEA,Lanzhou,Gansu 730000,China; 3.Geotechnical Disaster Prevention Engineering Technology Research Center of Gansu Province,Lanzhou,Gansu 730000,China)  
The earthquake damage phenomenon indicates that the distribution of earthquake ground motion differs significantly in various parts of bedrock valley areas.The amplitude of ground motion has an important direct impact on the extent of the seismic hazard;larger amplitudes result in more severe disasters.Many towns located in the valley areas are widely distributed over Shanxi,Gansu,and Sichuan provinces and sustain severe damages from earthquakes.Research on this type of area has recently increased with particular emphasis on valley terrain.Many scholars have reported that input data is a significant problem for calculating the parameters of ground motion during strong shocks in such cities because microzoning is often closely related to the characteristics of soil strata and the geomorphological conditions.Nevertheless,the conditions of terrain and geology are interdependent with each other and have an impact on ground motion;it is widely accepted that terrain has an important effect on ground motion.Common valley terrain can be divided into three categories including V valley,inverted trapezoidal valley,and trapezoidal valley.V valley and inverted trapezoidal valley are widespread and are the topics of numerous studies.The bottom of V valley is narrow,and many geological disasters such as rockfalls occur in trapezoidal valleys.The inverted trapezoidal valley is more suitable for residency;therefore,research on this type of valley terrain is important.Because the drainage systems and valley landforms are generally not symmetric,homogeneous terrain models with various slope angles,depth-to-width ratios,and asymmetry of valley are used to analyze the variation of peak ground acceleration(PGA) in valley areas.By comparing the PGA,seismic spectra,and acceleration time history,the relationship between ground motion and topography is analyzed in this study.The results show that 1) the ground motion amplitude at the valley bottom appears to reach maximum value when the slope angles are approximately 40°~50° and later decreases.The minimum is achieved when the angle is changed from 65° to 80° on the basis of spectrum ratio.These results are consistent with published results on the Wenchuan earthquake.The wave in the high-pitched slope valley decays slowly,which it could be due to diffraction of P,SV,and Rayleigh waves.2) No obvious variation of PGA on various valley areas is observed when using a small depth-to-width ratio model,regardless of high-frequency and low-frequency impulse input.With an increase in depth-to-width ratio,an input of high-frequency impulse causes significant variation,and the PGA in the valley center is larger than that at the valley bottom.Diffraction of P,SV,and Rayleigh waves is higher with an increase of depth-to-width ratio,and the distribution of PGA is complex.In addition,the orographic effect is stronger when the wavelength was close to the width of the bottom of valley and slope.3) The amplification factor of earthquake ground motion increases with slope angle and slope height.In addition,the valley asymmetry has an influence on the PGA at the valley bottom.The maximal PGA at the valley bottom is often close to the lower slope side,and the smaller PGA appears to correspond to the side with the larger slope angle;the amplification factors of both sides is increased due to free face changing with the increase in slope angle and slope height.4) Ground motion is consistently amplified on or near the top of the hill areas compared with that at the bases.The protruding topography has amplification effect on ground motion while the hillside has a constrained effect.In addition,a larger slope angle and height relates to more severe damages to residential areas;the effect is less intense with distance from mountains.The amplification factor of the valley between two mountains is related to the distance of the two mountains.If the distance is shorter,the factor is smaller.
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