| Inner Mongolia Autonomous Region is one of provinces which frequently happen seismic activity. Hohhot city, metropolis of Inner Mongolia Autonomous Region, locates at the northeast margin of the Hohhot-Baotou Fault Basin of the east section of the Hetao Fault Depression Zone of northwest North China Fault Block. There would be large earthquakes in Hohhot city in future. However, it is still difficult to make reliable earthquake prediction at present. And it is important to pay more needed attention to strengthening seismic damage prevention in Hohhot city. Site condition is one of importance influencing factors in earthquake disaster. With different site foundational soil, seismic damage of building is variable. It is significant to master the underground structure. However, traditional conjecture method of the underground structure, which costs long time, money and is complex, affects the prevention work of seismic damage in urban area. In this paper, observing the foundation soil in Hohhot city by using microtremor apparatus and mastering vibration characteristics of foundation soil in this area. Based on microtremor observation data and literatures about the underground exploration material speculate the average shear wave velocity and establish the estimation formula of the depth of foundation soil. At the same time, verify the reliability of formula. The main conclusions are as follows:(1) By using microtremor observation obtained the predominant period of foundation soil in research area. And it found that the predominant period of foundation soil is between 0.21 and 1.25 seconds, from the south to the north in the city, the figure become smaller, at the foot of Daqing Mountain reached the minimum value, from the west to the east in the city gradually become small, the predominant period of partial southwest of the city center reaches the maximum value. In addition, according to the observation predominant period and association for engineering construction standardization standard “site micro vibration measurement technology rules†CEC74:95, classify the foundation soil types in urban area, the foundation soil in Hohhot city give priority to the Ⅳth class of very soft and thick, to north the closer the foot of Daqing Mountain, the harder and the thinner of the foundation soil, to the foot of Daqing Mountain occurred the Ⅲth class and theⅡnd class.(2) Speculated surface shape of the average shear wave velocity of the foundation soil in Hohhot city. first of all, taking collected each the drill point as the reference point, the thickness of each layer and the shear wave velocity of shallow layer of this point directly extracted in collected the drilling data; Referring to “target zone near fault strong motion evaluation †report of geophysical institute, China seismological Bureau, extracted each layer depth and the average shear wave velocity of deep layer of the point. At last, on the basis of the foundation soil’s the average shear wave velocity and the drill point distribution location, by using the method of interpolation distribution, Speculated surface shape of the average shear wave velocity of the foundation soil in Hohhot city.(3) Established speculation formula of depth of the foundation soil in Hohhot city based on microtremor measurements. At the same time, verified the reliability of this formula. According to the 1/4 wavelength criterion, using surface shape of the average shear wave velocity of the foundation soil in Hohhot city and getting the predominant period by microtremor observation, established empirical formula, obtained formula H=T×Vsm÷3.6, T is the predominant period, H is depth of the foundation soil, Vsm is the average shear wave velocity of the foundation soil. At the same time, using verified the reliability of this formula. In the future, anywhere in Hohhot city has been observed by microtermor, obtained the predominant period T, T is input to established empirical formula, can calculate directly depth of foundation soil in the area. |
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