Preliminary seismic microzonation of Sivas city (Turkey) using microtremor and refraction microtremor (ReMi) measurements

Sivas city, located in the inner east part of Anatolia (Turkey), is far from seismic sources. However, the city is under risk owing to strong earthquakes occurring around the area, and different soil conditions that can produce variation in the ground motion amplification. Microzonation of cities provides a basis for site-specific hazard analysis in urban settlements. In particular, seismic microzonation can be achieved by means of detailed seismic assessment of the area, including earthquake recordings and geological studies. In this paper, we propose a preliminary microzonation map for the city of Sivas, based on the variation in the dominant periods of the sediments covering the area. The periods are retrieved from microtremor measurements conducted at 114 sites, using the horizontal-to-vertical spectral ratio technique. The results of microtremor analysis were compared with those obtained from refraction microtremor measurements at two profiles crossing the studied area. According to the classification of dominant periods, Sivas area can be divided into four zones, probably prone to different levels of seismic hazard. However, specific studies including analysis of weak earthquakes are required in the future to validate our microzonation map.     

Application of the microtremor measurements to a site effect study

Earthquake has left much life and property damages. The occurrence of such events necessitates the execution of plans for combating the earthquakes. One of the most important methods for combating earthquakes includes assessing dynamic characteristics of soil and site effect. One of the methods by which one can state dynamic characteristics of the soil of an area is the measurement of microtremors. Microtremors are small-scale vibrations that occur in the ground and have an amplitude range of about0.1–1 microns. Microtremor measurement is fast, applicable, cost-effective. Microtremor measurements were taken at 15 stations in the Babol, north of Iran. Regarding H/V spectral ratio method, peak frequency and amplification factor were calculated for all microtremor stations.According to the analysis, the peak frequency varies from0.67 to 8.10 Hz within the study area. Also, the authors investigated the validity of the results by comparing them with SESAME guidelines and geotechnical conditions of study area. The microtremor analysis results are consistent with SESAME guidelines and geotechnical condition of study area. The results show that the microtremor observations are acceptable methods for assessing dynamic characteristics of soil and site effect in the Babol City.     

应用微动H/V谱比法和台阵技术探测场地响应和浅层速度结构

为了快速而且廉价地获取北京市详细的场地响应和浅层速度结构,应用于地震动模拟和地震灾害预防,我们开展了微动观测技术和处理方法研究.本文利用2007年夏季北京五棵松地区进行的几个微动观测实验数据,使用单台H/V谱比法分析场地的卓越频率及其对应的放大系数,并对比了不同地震仪和观测时间对H/V曲线的影响;应用高分辨率F-K频谱分析方法从微动台阵数据中得到Rayleigh波的频散曲线并使用邻域算法反演出浅层速度结构.H/V结果表明该地区卓越频率在2.1～2.2 Hz之间,对应的放大系数下限约为3;利用微动H/V方法得到的场地卓越频率具有较高的稳定性.微动台阵反演结果给出了比较合理的波阻抗界面深度和层平均速度结构,认为地下80多米处的波阻抗界面是决定场地卓越频率和其场地放大系数的主要界面.本研究表明微动技术应用于评估城市地震场地响应和浅层速度结构是可行且易于实施的.     

Seismic Zonation of Barcelona Based on Numerical Simulation of Site Effects

—?Seismic responses of different sites of Barcelona have been investigated through numerical modelling. Geological maps and geotechnical data available from drillings for buildings and infrastructures have been used to determine the dynamical properties of the soils through different correlations between standard geotechnical data and dynamical parameters obtained in other regions. An estimation of the depth of the Palaeozoic basement has been obtained through an inversion of a detailed gravity survey. A 1-D equivalent linear method has been used to compute complete transfer functions and other spectral responses, such as PSA and PSV for various damping values, with the purpose of classifying zones with similar behaviour. Given the uncertainties associated with the input data, a Montecarlo's simulation process has been carried out. Four zones, characterized by their corresponding transfer function and by PGA amplifications, are proposed. The numerical results are compared with those previously obtained through microtremor measurements, showing that predominant periods derived from Nakamura's technique should be taken carefully.     

用微振动勘探方法探测隐伏断裂构造

介绍了运用微振动勘探方法探测隐伏断裂构造的方法和思路,并对郯庐断裂带两条分支断裂进行了试验性勘探．通过与人工地震勘探结果对比分析,发现微振动勘探方法对隐伏断裂构造具有较好的空间定位能力,其对岩性差异面形成的断裂定位误差与人工地震勘探结果相当; 又因其具有较大的勘探深度,因而可以有效揭示勘探区域下方地层及断裂的空间分布特征及其接触关系,这对于识别和确定断裂构造的主破裂面具有重要意义. 但微振动勘探方法对地层或断裂构造细节特征的揭示能力较差．因此,在实际地层或断裂勘探中,宜采用人工地震勘探方法与微振动勘探方法相结合的方式确定断裂的深浅构造关系,可以用微振动勘探方法进行普查,在确定断裂初步位置及空间展布特征后,再选用人工地震勘探方法进行断裂精确定位．      

Microtremor Measurements for the Microzonation of Dinar

v--vThe geotechnical site conditions in Dinar town located in western Turkey were investigated after the 1995 Dinar earthquake based on borings, in situ penetration tests, seismic wave velocity measurements, and microtremor records. The variation of damage distribution within the town was evaluated with respect to 23 district damage ratios calculated, based on the detailed damage survey conducted by the General Directorate of Disaster Affairs. Site amplifications were estimated from microtremor spectral ratios and microzonation was performed using a GIS methodology. The results of in situ penetration tests and seismic wave velocity measurements as well as the damage distribution were compared with the amplification zonation obtained from microtremor records. The results indicate the applicability of microtremor spectral ratios for assessing the local site conditions and site amplifications.     

海原地区黄土的微观结构及地脉动特征分析

1920年8.5级地震诱发了大量的黄土滑坡,对研究黄土地区的滑坡灾害、土层地脉动特征分析具有重要意义。基于高精度电子扫描显微镜（SEM）分析发现海原地震现场表层黄土为架空孔隙弱胶结微观结构,且该黄土是以粉粒为主的低黏性粉土,有震陷和地震滑坡的高风险。基于地脉动测试得到滑坡场地15个地脉动数据,采用傅里叶变换法对苏堡村滑坡土层的脉动特征进行分析,获得地脉动频谱特征曲线。研究表明场地卓越周期建议值为0.126 s,将其划分为坚硬场地,可为该滑坡场地类别划分作为参考,同时发现滑坡体与未滑体的地脉动特征有较大差异,可为海原地震滑坡的脉动特性分析提供依据。     

张家口地震台和兴隆地震台地脉动信号初步分析

选取河北省张家口和兴隆2个台站的数字化地震仪连续1年,每日02～03时的地脉动记录,计算功率谱和自相关函数。结果发现兴隆台脉动谱中有4个比较突出的峰值,分别为0．3Hz、3Hz、8Hz、15Hz,张家口台只有3个峰值0．3Hz、3Hz、15Hz。兴隆台UD向的自相关函数呈现出一种比较典型的“红噪声”形态,EW向则呈现出负相关特征,认为可能是只作用于水平向的倾斜脉动的影响。小幅度的仪器干扰信号无论从脉冲标定还是地震波形中都很难发现的,而对脉动进行谱分析则很容易分辨。兴隆台和张家口台地脉动卓越周期时间曲线,基本集中在0．3秒,比较稳定。     

利用微动勘察方法探测煤矿陷落柱

陷落柱是危害煤矿安全生产的主要因素之一,如何有效探测陷落柱是实现煤矿安全、高效生产的当务之急.本文首次将微动勘察方法应用于煤矿采区勘探,结果表明,该方法对陷落柱异常区反映敏感.在山西潞安漳村煤矿2002工作面测区已知陷落柱上,无论是单点反演获得的S波速度结构,还是微动视S波速度剖面,均能清晰显示陷落柱.微动剖面确定的陷落柱位置与巷道揭露位置一致,边界误差在10 m左右.微动勘察方法是探测陷落柱的一种行之有效的物探方法,由于其分辨率高、无需人工源、野外施工方法灵活便捷、不受施工场地限制等特点,对探测村庄覆盖区之下的煤层构造、圈定陷落柱等,具有得天独厚的技术优势和应用前景.     

Three‐dimensional VS profiling using microtremors in Kushiro,Japan

A practical method is presented for determining three‐dimensional S‐wave velocity (V_S) profile from microtremor measurements. Frequency–wave number (f–k) spectral analyses of microtremor array records are combined, for this purpose, with microtremor horizontal‐to‐vertical (H/V) spectral ratio techniques. To demonstrate the effectiveness of the proposed method, microtremor measurements using arrays of sensors were conducted at six sites in the city of Kushiro, Japan. The spectral analyses of the array records yield dispersion characteristics of Rayleigh waves and H/V spectra of surface waves, and joint inversion of these data results in V_S profiles down to bedrock at the sites. Conventional microtremor measurements were performed at 230 stations within Kushiro city, resulting in the H/V spectra within the city. Three‐dimensional V_S structure is then estimated from inversion of the H/V spectra with the V_S values determined from the microtremor array data. This reveals three‐dimensional V_S profile of Kushiro city, together with an unknown hidden valley that crosses the central part of the city. The estimated V_S profile is consistent with available velocity logs and results of subsequent borings, indicating the effectiveness of the proposed method. Copyright © 2008 John Wiley & Sons, Ltd.     

High-resolution grid of H/V spectral ratios and spatial variability on microtremors at Port of Spain,Trinidad

We performed dense single mobile microtremor measurements at 1181 sites throughout the city of Port of Spain, Trinidad. The results yielded fundamental periods of the soil of 1.2–1.7 s in the coastal areas, 0.3–1.2 s in the downtown area, and 0.0–0.3 s in the surrounding areas of the city. We employed the variogram plot to retrieve the spatial variability of fundamental periods. The results suggest that the variability of the period yields about 0.010 s² at short distances of 100 m and it can be modeled as a Gaussian distribution which is directionally dependent. Finally, we propose an elastic design response spectra for different zones in the city in accordance with the observed periods of vibration and compatible with the International Building Code (IBC) provisions.     

测线方位对微动线形台阵探测效果影响研究

微动探测技术是一种经济环保、可靠实用并广泛应用于资源勘探及工程地质领域的新型物探技术。为了探讨测线方位对线形台阵探测结果的影响,研究利用武汉中心地震台实测的小尺度密集台阵数据,对比分析了多时段双重圆形、T形和不同方向线形台阵微动探测结果。研究发现不同方向的线形台阵探测结果具有明显差异,测线方位从北向东变化时实测相速度逐渐增加,且在NE45°角左右时与双重圆形结果最接近。数值模拟结果也显示当测线与主噪声源中心线夹角在30°～55°范围内时,线形台阵探测效果较好,且随着夹角增加实测相速度逐渐增加。在测区周边存在主噪声源时,实测和模拟结果均表明有限地增长观测时间难以弥补线形台阵方位分布的不足。因此,在微动探测实践前应考虑测区周边可能存在的噪声源分布,选择合适的台阵形式进行探测。     

SPAC法在江汉平原地层结构分层中的应用

微动探测可以确定岩土性质差异较大的地质界面。利用微动探测方法,获取S波速度剖面,对潜江市浩口中学地下介质结构分层,并与该场地地震监测井项目的钻孔对比。结果表明,微动探测方法能较好分辨地层结构,对岩土性质差异较大的地层探测效果良好,可为地层分层提供重要依据。     

泉州地震台数字地震仪台址的地脉动噪声分析

利用2008年泉州台甚宽频带数字地震仪连续48 h和全年每月连续48 h内白天与夜间各连续4 h的地脉动记录,计算5 Hz、5 s和100s的静态地脉动噪声功率谱密度、1/3倍频程1-20Hz带宽各段的平均静态地脉动噪声有效值、1/3倍频程100-30 s带宽各段的平均静态地脉动加速度噪声有效值,以及上述3个频点和2个...     

Detecting underground cavities using microtremor data: physical modelling and field experiment

In this paper, we study the possibilities of the use of microtremor records in the detection and delineation of near‐surface underground cavities. Three‐dimensional physical modelling data showed that the averaging amplitude spectra of a large number of microtremor records makes it possible to evaluate the frequencies and amplitudes of compressional standing waves generated by microtremor in the space between the ground surface and underground cavities. We illustrate how these parameters can be used to estimate the shape of the underground cavity horizontal projection. If the compressional wave velocity in the enclosing rock is known, it is possible to evaluate the depth to the cavity top using the frequencies of the standing waves. The results of the field experiment confirmed the possibility of underground cavities detection using microtremor data.     

微动多阶瑞雷波SPAC系数反演方法及应用研究

为充分利用微动信号中基阶和高阶模式瑞雷波,本文研究了基于多阶瑞雷波SPAC系数直接反演的方法.该方法首先基于地层介质响应计算多阶瑞雷波的能量占比,考虑实际观测台阵有限台站个数对SPAC系数影响,正演计算多阶瑞雷波SPAC系数,再采用快速模拟退火算法对其反演以获得地下介质横波速度结构.在此基础上,本文通过数值模拟验证该方法的可靠性,分别选取三种典型地质模型,基于模式叠加算法合成理论微动信号,采用本文方法计算其理论多阶瑞雷波SPAC系数并反演,给出反演结果与真实模型对比.我们将该方法应用于上海中心城区的地质调查中,通过与钻探结果对比,进一步验证该方法的有效性.本文理论与实际应用研究表明,基于多阶瑞雷波SPAC系数直接反演的微动探测方法有助于提高反演结果的可靠性,尤其对含软硬夹层的复杂地层介质,可提高探测精度.     

西北地区戈壁砂砾石场地的脉动特征

阐述了我国西北部戈壁砂砾石层的划分及其时代确定等问题，以近几年西北地区地震小区划和重大工程建设项目中的实际观测资料为依据，对西北戈壁滩砂砾石场地的脉动特征进行了分析和总结，在此基础上探讨了场地脉动频谱特性与场地有关参数的关系，同时还就地脉动测试在工程中的推广应用等问题提出了几点看法。     

用于微动探测的低成本自存储式数字地震检波器

微动探测是利用地球表面的天然震动信号进行探测的被动源探测方法.相对于传统的物探方法,微动探测野外施工灵活,阵列形状多样,受施工场地限制小,对施工环境影响小.目前进行微动探测所用的地震检波器大多成本较高、重量和体积大,不便于运输与大规模布设.本文由微动探测方法的特点出发,优化改造了自行研发的低成本自存储式数字地震检波器,使其性能满足微动探测的需要.通过对地震计系统噪声的详细分析、微动探测原理的讨论,结合现场试验数据,探讨了该检波器应用于大规模微动探测的可行性.     

Reliability of rayleigh wave dispersion curve obtained from f–k spectral analysis of microtremor array measurement

The paper utilizes previously developed microtremor simulation technique to evaluate the reliability of Rayleigh wave dispersion curve estimated by f–k spectral analysis of microtremor array measurement. The simulated microtremors are obtained for a fictitious (TEST) site. Attempt is also made to obtain the dispersion curve for two real sites (OHDATE and SKC) by inverse analysis of the microtremor array measurement using f–k spectra method. The estimated dispersion curve from simulated microtremors (TEST site) compares well with the theoretical dispersion curve, demonstrating the reliability of f–k spectra method and indicating that the estimated dispersion curve from microtremor measurements could be adequately used as the target for inverse analysis purposes. It is also demonstrated that the dispersion curve from microtremor measurements can be utilized to estimate the soil profile at OHDATE and SKC sites by inverse analysis. Results show that the theoretical dispersion curve of the fundamental mode of Rayleigh wave after the end of inverse analysis are in good agreement with the dispersion curve obtained by f–k spectral analysis of microtremor array measurement.