Research On The Dynamic Site Effects Of Regional Ground Fissures In Xi'an Area

Ground fissure is a special geological structure.Human life and construction have been restricted and threatened by ground fissures since which was discovered in 1920 s.In Xi'an,the development of ground fissures is dense,which was found to be up to 14,with an extension of more than 160 km,and covering about 250km2.The buildings passing through the ground fissures have been severely damaged,which seriously restricts the construction and development of the city.So far,there are many researches on the formation,distribution and activity of ground fissures,but for the dynamic response characteristics and the seismic fortification are less and not systematic.The tectonic movement of Xi'an is very strong in Fenwei basin.Intensive ground fissure group brought the city threat like earthquark,therefore,the dynamic response characteristics of ground fissure site should be studied as soon as possible.This paper is based on the previous project,carried out reconnaissance along for F₄?F₁₀?F₁₁?F₁₂?F₁₃?F₁₄ and Changan-Lintong fault fissure.To obtain the micro-seismic data at the site of a appropriate ground fissure site.Through the analysis of 16 lines of microtremors data,we obtained the dynamic response characteristics of ground fissures site.According to the geological conditions of round fissures site,We established the Chanba River alluvial plain numerical model and the Honghu platform before the Loess Plateau numerical model.Open SeeS finite element software was used as the numerical simulation platform to study on the dynamic response characteristics of ground fissure sites under ground motion.By comparing the results of the microtremors analysis and numerical simulation,we obtained the relationship between the microtremors response and the acceleration response.At last,combining the relevant regulations of seismic fortification in Xi'an,the paper puts forward the suggestion of basic acceleration of seismic fortification design of ground fissure field.Through the research,this paper has achieved the following results.?1?Through Fourier spectrum analysis,it can be found that the excellent frequency of ground fissure site is concentrated in 2.54³.95 Hz,the average value is 3.05 Hz.There is no obvious relation between the frequency of the Fourier spectrum and the location of the measured points,and it was not affected by the distance.The stratigraphic characteristics reflected by the Fourier spectrum of each line in the ground fissure field correspond to the ground.The predominant period of response spectrum is about 0.23 s,and the frequency is 4.35 Hz,which is greater than the Fourier's remarkable frequency.There was no significant relation between the frequency of the response spectrum and the location of the measured points.?2?There is an obvious amplification effect of the ground fissure site,and the peak value of the Fourier peak,the peak of the response spectrum and the peak intensity of the Arias all show that: it is larger in the vicinity of the ground fissures,which gradually decreases against the increase of distance,and tends to be stable.The Dynamic amplitude of hanging wall is significantly greater than that of footwall.There is little difference between the amplitudes of the four platforms in the Honghu platform before the Loess Plateau,and it is smaller than the Ba and alluvial plain.We can obtain the attenuation curve of the amplification factor of the Fourier spectrum,the attenuation curve of the amplification factor of the response spectrum,the attenuation curve of the amplification factor of the Arias intensity,and the envelope curve of the Xi'an site.We can accurately describe the dynamic response of ground fissures.We got the attenuation curve of amplification factor of response spectrum in the Chanba River alluvial plain,the attenuation curve of amplification factor of response spectrum in Loess Tableland,and the former amplification factor is bigger than the latter.?3?The influence range of hanging-wall amplification effect is 15 m,the influence range of footwall amplification effect is 15 m.The amplification factor is about 1.0.?4?Through numerical simulation,it can be found that the seismic acceleration response of the ground fissure site has amplification effect which of the hanging wall is larger than that of the footwall.The amplification factor range of Chanba River alluvial plain model is 1.0².04,the influence range of hanging-wall amplification effect is 35 m,the influence range of footwall amplification effect is 25m;The variation range of the amplification factor of the former Honghu platform model is 1.0⁴.06,the influence range of hanging-wall amplification effect is 35 m,the influence range of footwall amplification effect is 30 m.?5?Based on analysis results of the microtremor and the numerical simulation above,the paper puts forward the suggestion of designing the seismic acceleration of the ground fissures in Xi'an.