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DETECTION OF SHALLOW SEDIMENTARY STRUCTURE IN SIYANG,JIANGSU PROVINCE BY MICROTREMOR H/V SPECTRAL RATIO METHOD

PENG Fei;WANG Wei-jun;XIONG Ren-wei;Lü Xiao-jian;YAN Kun;SUN Xin-zhe;GENG Shuang;KOU Hua-dong;Key Laboratory of Earthquake Prediction, Institute of Earthquake Forecasting,China Earthquake Administration;Institute of Geophysics,China Earthquake Administration;  
Earthquake sources, wave propagation effects and site effects directly affect the structural damage during earthquakes. Among these factors, site effects amplify and prolong the strong vibrations, playing a very important role in many great earthquakes such as the 1985 M8.1 Mexican earthquake, the 2015 M_W7.8 Gorkha, Nepal, earthquake and the 2016 M_W7.8 Kaikōura, New Zealand, earthquake. Microtremor is a random, natural and permanent complex vibration composed of body waves and surface waves, in which the energy of surface waves accounts for more than 70% of the total energy. Due to the multiple reflection and refraction of the wave, microtremor accumulates information reflecting the inherent characteristics of the soil layer of the site during the propagation process. Microtremor H/V spectral ratio method is an effective way to assess the site effects. Compared to the traditional seismic surveys, the low-cost convenient observation and rapid surface detection are the advantages of this method. Its results can be used as basic data for future earthquake hazard evaluation and urban construction planning.Siyang in Jiangsu Province is located in Tanlu seismic zone. In the history, there were some large earthquakes on the Tanlu earthquake zone. Among them, the Tancheng M8.5 earthquake is about 110 km from our study area, so there is a certain risk of earthquake disaster in this area. It is necessary to analyze the regional site effect and the distribution characteristics of the shallow sedimentary interfaces in detail. Site amplification effect is an important factor to aggravate earthquake hazard, which is closely related to the shallow structure. Based on 217 microtremor observations, we use H/V spectral ratio method to study the seismic site effect and the shallow sedimentary structure of Siyang. The results of H/V peak frequency distribution show that the resonance frequency of seismic site in Siyang study area is between 0.6~1.8 Hz with obvious fluctuations. The corresponding shallow sedimentary thickness is between 30 m and 200 m, which gradually deepens on the east and west sides with a shallow central region. In particular, the central urban area is 30~70 m thick and the southeast corner is the thickest. The shallow deposits show an obvious deep and shallow alternating band distribution in the NNE direction, consistent with the location and strike of the Haisi fault zone. The sedimentary structure of the soil layer obtained in this paper is basically the same as the geological structure, which can be verified with the results of the reflection seismic exploration profile. The comparison with two seismic exploration profiles for shallow reflection in the area shows that the bedrock shape obtained by the microtremor H/V spectral ratio method is reliable. Therefore, the sedimentary structure and site effect characteristics obtained by this method can provide useful reference for the microzoning of seismic risk in Siyang.
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