江苏省泗阳浅层沉积结构的微动H/V谱比法探测

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

江苏省泗阳地处郯庐地震带内,存在一定的地震灾害风险。地震场地放大效应是加重地震灾害的重要因素,与浅层结构密切相关。文中基于217个微动观测的H/V谱比法结果,研究江苏省泗阳县城的地震场地效应和浅层沉积结构。H/V峰值频率分布结果表明,泗阳研究区地震场地的共振频率介于0.6~1.8Hz之间,有明显的起伏变化;对应的浅层沉积厚度为30~200m,整体上表现为中部浅、向东西两侧逐渐加深的特征,其中中部城区浅层沉积厚度为30~70m,东南角最厚;浅层沉积在NNE走向有显著的深浅交替条带,与海泗断裂带的位置和走向基本一致。与区内2条浅层反射地震勘探剖面进行对比发现,微动方法获得的基岩起伏形态较为可靠。通过微动H/V谱比法得到的沉积结构和场地响应特征,能够为泗阳地区地震危险性的小区划提供有益参考。

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 M 8.1 Mexican earthquake,the 2015 M_(W)7.8 Gorkha,Nepal,earthquake and the 2016 M_(W)7.8 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 M 8.5 earthquake is about 110km 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 0.6~1.8Hz with obvious fluctuations.The corresponding shallow sedimentary thickness is between 30m and 200m,which gradually deepens on the east and west sides with a shallow central region.In particular,the central urban area is 30~70m 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.