Research On Dynamic Response Characteristics Of Microtremor Of F₁ And F₂ Ground Fissure Sites In Xi’an And Its Numerical Analysis

Since 1959, due to the influence of natural factors and artificial factors, there has been so many ground fissures in Xi’an that it has become the most serious geological disaster, and it restricts urban construction and economic development. So far, there has been a lot of related research of ground fissures in Xi’an, such as its distribution, activity characteristics, formation mechanism, prevention and control measures and so on, researchers have achieved quite fruitful results. But the field on research on the site of ground fissures dynamic response characteristics is weak, and the results are less nad can not form a system. For some areas, Such as Xi’an, there is the distribution of dense ground fissures, and high seismic hazard. It is an urgent and important subject to study the dynamic effect of the regional mass ground fissures in the field.In this paper, taking the research object to ground fissure of F1 and ground fissure of F2 in Xi’an, on the basis of the information collected on the geological tectonic setting, topography, and stratum structure data of the study area, and field investigation, the microtremor test of F1 and F2 ground fissure sites is carried out, and obtaining predominant frequencies of dynamic response characteristics of microtremor of the ground fissure sites(or predominant periods), spheres of influence and amplification factor(amplification multiple); combining stratum structure of F1 and F2 ground fissure sites, formation difference between hanging wall and footwall and dip of the ground fissure and so on, generalizing structure profile model of ground fissure of F1 and ground fissure of F2; on the basis of the tructure profile model of ground fissurem, choosing OpenSEES finite element software as the numerical simulation platform, and studying on the dynamic response characteristics of ground fissure sites under ground motion. At last, comparing and analyzing the dynamic response characteristics of ground fissure sites under under ground microtremor and ground motion. Through the research, this paper has achieved the following results.1. Based on the microtremor test and Fourier analysis finding, predominant frequencies of F1 and F2 ground fissure sites is in the range of 2.79 ~ 3.16 Hz, the average predominant frequency 2.96 Hz, the spectral peak value of hanging wall and footwall of a ground fissure decreases with the increase of the distance to the ground fissures, when the distance to about 15 m, spectral peak tends to be stable, this shows that the ground fissure site has amplification effect, and the influence range is about 15m; the magnification factor is in the range of 1.64 to 2.38, among them, the average amplification factor of hanging wall is 2.01, and the average amplification factor of footwall is 1.81, the average amplification factor of hanging wall is significantly greater than that of footwall.2. Based on the microtremor test and energy flux analysis finding, accumulated energy(10s) of hanging wall and footwall of the ground fissure decreases with the increase of the distance to the ground fissures, when the distance to about 15 m, accumulated energy tends to be stable, this shows that the ground fissure site has amplification effect, and the amplification effect has a certain range of influence, the influence range is about 15m; This conclusion is consistent with Fourier analysis; moreover, by energy flux analysis obtaining that the magnification factor is in the range of 1.75 to 2.13, the average amplification factor of hanging wall is 2.09, and the average amplification factor of footwall is 1.87, the average amplification factor of hanging wall is significantly greater than that of footwall.3. Based on the microtremor test and response spectrum analysis finding, predominant periods of F1 and F2 ground fissure sites is in the range of 0.24 ~ 0.30 s, the average predominant frequency 0.26 s, the spectral peak value of hanging wall and footwall of a ground fissure decreases with the increase of the distance to the ground fissures, when the distance to about 15 m, spectral peak tends to be stable, this shows that the ground fissure site has an amplification effect, and the amplification effect has a certain range of influence, too, the influence range is about 15 m, this is consistent with the conclusion of Fourier analysis and energy flux analysis; by response spectrum analysis analysis obtaining that the magnification factor is in the range of 1.23 to 1.45, the average amplification factor of hanging wall is 1.43, and the average amplification factor of footwall is 1.33, like Fourier analysis and energy flux analysis, the average amplification factor of hanging wall is greater than that of footwall, but there is a big difference between the average amplification factor of hanging wall and footwall obtained by response spectrum analysis and that obtained by Fourier analysis and energy flux analysis.4. By numerical simulation obtaining, peak response of ground acceleration of hanging wall and footwall of the ground fissure decreases with the increase of the distance to the ground fissures, when the distance to about 30 m, peak response of ground acceleration tends to be stable, this shows that the ground fissure site under ground motion has amplification effect, too, the influence range is about 30 m, it is significantly greater than the scope of the ground fissure sites under microtremor, and it is about 2 times the scope of the ground fissure sites under microtremor; moreover, according to peak response of ground acceleration, the whole ground fissure field can be graded.