Seismic microzonation of Bangalore,India

In the present study, an attempt has been made to evaluate the seismic hazard considering local site effects by carrying out detailed geotechnical and geophysical site characterization in Bangalore, India to develop microzonation maps. An area of 220 km², encompassing Bangalore Mahanagara Palike (BMP) has been chosen as the study area. Seismic hazard analysis and microzonation of Bangalore are addressed in three parts: in the first part, estimation of seismic hazard is done using seismotectonic and geological information. Second part deals with site characterization using geotechnical and shallow geophysical techniques. In the last part, local site effects are assessed by carrying out one-dimensional (1-D) ground response analysis (using the program SHAKE2000) using both standard penetration test (SPT) data and shear wave velocity data from multichannel analysis of surface wave (MASW) survey. Further, field experiments using microtremor studies have also been carried out for evaluation of predominant frequency of the soil columns. The same has been assessed using 1-D ground response analysis and compared with microtremor results. Further, the Seed and Idriss simplified approach has been adopted to evaluate the soil liquefaction susceptibility and liquefaction resistance assessment. Microzonation maps have been prepared with a scale of 1:20,000. The detailed methodology, along with experimental details, collated data, results and maps are presented in this paper.     

Efficiency of horizontal-to-vertical spectral ratio (HVSR) in defining the fundamental frequency in Muscat Region,Sultanate of Oman: a comparative study

Muscat region is the most important political, economic, and densely populated region in the Sultanate of Oman. The proximity of Muscat region to the Oman Mountains and Makran subduction zones controls the earthquake hazard for Muscat. Evidences indicate the occurrence of a nearby historical earthquake with moderate magnitude M _S?=?5.5 in 1883. This event led to the damage of some villages near Nizwa City. The main objective of the current study is to compare the site characteristics of the region of interest in terms of the fundamental frequency using microtremors measurements with the numerical analysis results using one-dimensional (1-D) shear wave profiles. The microtremor measurements were performed at 99 sites distributed over the study region in order to calculate the horizontal-to-vertical spectral ratio (HVSR). The numerical modeling of horizontal shear (SH) waves in soil at the selected 99 sites are assessed by carrying out 1-D ground response analysis using the program SHAKE91. The required shear wave velocity profiles for the numerical modeling of SH-waves were derived using multichannel analysis of surface waves profiles. The amplification spectra have been evaluated for the soil column at each site location and the fundamental frequency obtained using SHAKE91 and HVSR are compared. Results were found to be compatible with the general surface geology of the region of interest and in most cases the HVSR is proved to be suitable for calculating the fundamental frequency in Muscat region.     

Seismic microzonation study in Doon valley, northwest Himalaya, India

Fundamental frequency map of site amplification at different sites in Doon valley, Uttarakhand, India is prepared from microtremor (ground ambient noise) using Horizontal to Vertical Spectral Ratio (HVSR) technique. The fan deposited alluvium filled synclinal valley of Doon lies between Main Boundary Thrust (MFT) and Himalayan Frontal Thrust (HFT) in the Himalayan active seismic belt and experienced many earthquakes in the past. The HVSR at different sites in the Doon valley ranges between the predominant frequencies 0.13 and 12.77 Hz. The HVSR in lower frequencies indicates that the site has either thick sediment covers or less compact rocks with fractures. Based on information on fundamental frequency and soft soil thickness, site classification map is generated. Results indicate that degree of compactness of rock types and presences of sediments vary significantly, which may play a major role in seismic hazard. The use of microtremor, therefore, constitutes an effective and inexpensive approach to site response and soft soil thickness estimation for preliminary microzonation.     

Characterization of site effects in Montreal,Canada

Recent destructive earthquakes have clearly shown that near-surface geological conditions play a major role in the level of ground shaking in urban areas. In Canada, Montreal is ranked second for seismic risk after Vancouver considering its population and regional seismic hazard. The city is largely built on recent unconsolidated marine and river deposits and most of its infrastructure is old and deteriorated. A seismic risk project that includes a combined methodology for site effects zoning in large cities, using microtremor measurements (H/V method) coupled with 1D numerical modelling (SHAKE91), has been initiated. The experimental approach gives good estimates of the fundamental frequency of soft deposits, while the numerical approach provides good estimates of the soil response in terms of amplification factor related to frequency. Main mechanical properties of soft soils were compiled from various data available, and a sample of input rock motions from real and synthetic earthquakes was used to compute soil response. The influence of marine clays on soil response is significant and is well correlated with thickness of these deposits. PGA amplification factors range from 2 to 4 at frequencies from 2 to 7 Hz, with some occasional larger values. The results demonstrate that the methodology used for our study is both fast and efficient to determine the influence of soft soils in urban environments. Such studies are essential for the effective deployment of seismic instrumentation, land-use planning and seismic mitigation.     

水平成层场地动剪应变与震动速度关系解析

以SHAKE2000为代表的频域等效线性化是目前主流的土层地震动计算方法。由于该方法在软土场地计算结果严重不合理,其改进方法成为目前研究热点,主要采用频率相关法,但一直没有取得实效。对频率相关法中土体动剪应变和震动速度成恒定比例关系的基本假定进行研究,从波动方程推导出了均匀全空间单向行波、单一均匀半无限场地、成层场地等3种典型场地中土体剪应变与震动速度关系的精确解答,通过数值试验研究了这一假定的合理性与偏差程度。结果表明：动剪应变和震动速度成恒定比例关系这一假定只有在无边界均匀介质单向行波情形下才成立,对于实际成层土场地,动剪应变与震动速度关系强烈依赖于波的频率和观察点的位置,如果在土层地震反应分析中忽略反射波而采用单向行波恒定比例假定将会使计算结果产生显著偏差;动剪应变与震动速度成恒定比例关系的假定导致的偏差达4个量级,即使对于单一均匀半无限空间模型偏差也十分显著;对实际土层地震动的求解,该假定在理论上定性错误,定量上偏差不可接受,必须放弃而另寻其他途径。     

Seismic hazard assessment for Mymensingh,Bangladesh

Mymensingh municipality lies in one of the most earthquake-prone areas of Bangladesh. The town was completely destroyed during the Great Indian Earthquake of 12 June 1897, for which the surface-wave magnitude was 8.1. In this study the 1897 Great Indian Earthquake was used as a scenario event for developing seismic microzonation maps for Mymensingh. For microzonation purposes SPT data from 87 boreholes were collected from different relevant organizations. To verify those data ten boreholes of depth up to 30 m were drilled. Intensity values obtained for different events were calibrated against attenuation laws to check applicability to the study area. Vibration characteristics at diverse points of the study area were estimated by employing the one-dimensional wave-propagation software SHAKE. SHAKE discretizes the soil profile into several layers and uses an iterative technique to represent the non-linear behavior of the soil by adjusting the material properties at each iteration step. The required input information includes depth, shear wave velocity, damping factor, and unit weight of each soil layer. The liquefaction resistance factor and the resulting liquefaction potential were estimated to quantify the severity of liquefaction. Quantification of secondary site effects and the weighting scheme for combining the various seismic hazards were heuristic, based on judgment and expert opinion.     

关于抗震设计场地反应分析客观性的一些探讨

在工程抗震设计领域,以SHAKE为工具的场地反应分析已得到广泛应用。场地反应分析简单、易行,然而要得到准确、可靠的分析成果仍需要长期的经验积累和相当的技巧。其中需特别注意的是：场地反应分析所需要的各土层参数均具有相当大的不确定性。这些不确定因素主要来自于：初始剪切模量G0（或最大剪切模量Gmax）、剪切模量衰减曲线、阻尼曲线以及可能存在的软弱土层。对于大型基建工程,场地反应分析的成果对于整个工程的安全、经济性都有重大影响;因此,针对其不确定性应该进行严谨地论证,并在工程设计中充分考虑。建议对各主要参数进行敏感度分析,并将结果综合起来以形成一套理性、可靠的场地反应分析方法。     

Dangerous ice phenomena on the lowland rivers of European Russia

The site amplification functions at 48 sites of NCR have been estimated in this study using the waveforms of locally recorded 23 earthquakes. Due to the absence of a suitable reference site in the region, the widely used horizontal-to-vertical spectral ratio (HVSR) technique has been used for this purpose. The maps showing the spatial distribution of predominant frequencies and the site amplifications at different frequencies corresponding to the natural frequencies of the different-storey buildings have been presented. The predominant frequencies in general are found to be in the range 2.5–7.5 Hz with an average of 4.4 Hz for the region having older alluvium sediments and in the range 1.1–6.4 Hz with an average of 3.3 Hz for the region with the younger alluvium deposits. The average value of the site amplifications for the frequency band 3.0–10.0 Hz is in the range 2.0–5.3 for the sites with significant soil cover, while the spectral amplification corresponding to the predominant frequency varies from 2.5 to 7.5 at most of the sites. The spectral amplification level lies in the range 2.0–3.0 for the sites with less or no sediment cover. The spectral amplification levels presented for the different-storey buildings may be used for the mitigation of seismic hazard in the region. The estimated site amplification functions may be used in the simulation of the site-specific strong ground motions and therefore useful for the evaluation of seismic hazard of a region.     

Assessment of predominant frequencies using ambient vibration in the Kachchh region of western India: implications for earthquake hazards

The Kachchh region is the second most seismically active region in India after the Himalaya. One of the disastrous Indian earthquakes of the millennium was the Bhuj earthquake of January 26, 2001, which caused about 14,000 casualties and huge property damage. The main reason for such devastation is due to lack of earthquake awareness and poor construction practices. Hence, an increase in the knowledge and awareness, based on improved seismic hazard assessment, is required to mitigate damage due to an earthquake. Natural predominant ground frequencies have been investigated in the Kachchh region of western India using ambient vibrations. The horizontal-to-vertical spectral ratio technique has been applied to estimate the predominant frequency at 126 sites. The ambient vibration measurements were conducted for about 1 h at each site in the continuous mode recording at 100 samples/s. We have validated the estimated predominant frequency with earthquake data recorded at six broadband stations in the region. It has been observed that geological time period has a significant effect on predominant frequency of the ground. The estimated predominant frequencies vary from 0.24 to 2.25 Hz for the Quaternary, 0.41–2.34 Hz for the Tertiary, 0.32–4.91 Hz for the Cretaceous, and 0.39–8.0 Hz for the Jurassic/Mesozoic. In the Deccan trap, it varies from 1.30 to 3.80 Hz. We found distinct variation of predominant frequencies of sites associated with hard rock and soft soil. The predominant frequencies were related to the thickness of the sediments, which are deduced by other geophysical and geological methods in the region. Our results suggest that frequencies of the region reveals the site characteristics that can be considered for studying the seismic risks to evolve a plan for disaster risk mitigation for the region.     

西安典型地裂缝场地地脉动测试及地震响应特征分析

西安地裂缝广泛发育,严重的制约了城市的建设和规划。地裂缝的存在使得建筑场地在地震作用下表现出不同的地震响应特征,因此,研究西安地裂缝场地地震响应特征对地裂缝场地的建筑抗震设防具有重要意义。以西安地区典型地裂缝为研究对象,通过野外调查获得了西安地裂缝场地特征;在此基础上选取典型地裂缝场地,通过地脉动现场测试方法,获取了地裂缝场地卓越频率这一反映地层动力特性的重要参数,通过不同测点傅里叶谱分析得出了场地动力响应规律抗震设防距离。研究表明：西安地裂缝场地的卓越频率在2.79～3.16 Hz,平均卓越频率为2.96 Hz;地裂缝处场地地震响应明显,随着距地裂缝距离的增加场地地震响应逐渐减小,影响范围为15 m左右;地裂缝场地地震响应放大倍数上盘大于下盘,表现出“上盘效应”,放大倍数在1.64～2.38。研究结果对西安地裂缝场地工程抗震设防具有重要意义。     

微动信号的数据处理与岩土频谱特性分析

频谱是微动信号的一个重要动力学参数,微动信号的频谱特性与场地覆盖层厚度和地基土刚度的变化密切相关,既可反映岩土层的类型特征,又能反映工程场地的动力特性。详细探讨了目前较为常用的利用微动信号进行场地动力特性分析的方法,并对这些方法进行了研究和比较。简要介绍了功率谱估计方法,同时,通过实例分析了微动信号的变化规律和场地微动信号的频率响应特性。     

Seismic hazard assessment of Chennai city considering local site effects

Chennai city suffered moderate tremors during the 2001 Bhuj and Pondicherry earthquakes and the 2004 Sumatra earthquake. After the Bhuj earthquake, Indian Standard IS: 1893 was revised and Chennai city was upgraded from zone II to zone III which leads to a substantial increase of the design ground motion parameters. Therefore, a comprehensive study is carried out to assess the seismic hazard of Chennai city based on a deterministic approach. The seismicity and seismotectonic details within a 100 km radius of the study area have been considered. The one-dimensional ground response analysis was carried out for 38 representative sites by the equivalent linear method using the SHAKE91 program to estimate the ground motion parameters considering the local site effects. The shear wave velocity profile was inferred from the corrected blow counts and it was verified with the Multichannel Analysis of Surface Wave (MASW) test performed for a representative site. The seismic hazard is represented in terms of characteristic site period and Spectral Acceleration Ratio (SAR) contours for the entire city. It is found that structures with low natural period undergo significant amplification mostly in the central and southern parts of Chennai city due to the presence of deep soil sites with clayey or sandy deposits and the remaining parts undergo marginal amplification.     

Site amplification at Avcılar, Istanbul

Avcılar is the suburb of Istanbul that was most heavily damaged during the August 17, 1999 M_w 7.4 Izmit earthquake. Strong ground motion caused fatalities and damage in Avcılar despite being 90 km from the epicenter. We deployed five portable seismograph stations equipped with Reftek 24-bit recorders and L4C-3D seismometers for 2 months, in order to understand why the local site response was different from elsewhere in Istanbul. A reference station was placed on a hard rock site, and the remaining four stations were placed on other geological units, in areas that had experienced varying levels of damage. We calculated frequency-dependent ground amplification curves by taking the ratios of the spectra at soft and hard rock sites. We obtained similar site response curves for most earthquakes at each site in the frequency range of 0.3–1.6 Hz, and observed no significant site amplification beyond 2.0 Hz at any site. The overall characteristics of the recorded S-waveforms and our modeling of the calculated site amplification curves are consistent with amplification as a result of trapping of seismic energy within a 100–150 m thick, low-velocity subsurface layer. We also review the applicability of microtremor measurements to estimate local site effects at Avcılar. For these data, we used ratios of spectra of horizontal to vertical components to obtain each site response. These results are compared with standard spectral ratios. These microtremor measurements provide consistent estimates of the amplification at most sites at the higher end of the frequency band, namely above 1 Hz. The results from both methods indeed agree well in this part of the frequency band. However, the microtremor method fails to detect amplification at lower frequencies, namely <1.0 Hz.     

福建沿海地区场地土对地脉动动力响应的研究

以福建省泉州市区和福州市区场地土的动力特性和地脉动频谱结构为研究对象,通过对这两个地区的地脉动信号的频谱分析获得频谱结构特征,由此来深入研究该地区岩土的动力特性及场地土层的地脉动动力响应。通过场地脉动的测试,利用快速傅立叶变换(FFT)方法对观测到的地脉动信号进行频谱分析。根据实际观测资料研究了地脉动频谱结构特征及场地土对地脉动的频率响应的特性。研究不同地基土层构造与地脉动的频率依赖以及频率选择作用。结果表明,地脉动的频谱特性与场地覆盖层厚度和地基土刚度的变化密切相关;覆盖层厚度是影响地脉动卓越周期的重要原因,其中软土层对地脉动的卓越周期有一定的放大作用。     

Analysis of wave propagation in dry granular soils using DEM simulations

In this paper, a three-dimensional particle-based technique utilizing the discrete element method (DEM) is proposed to study wave propagation in a dry granular soil column. Computational simulations were conducted to investigate the soil response to sinusoidal motions with different amplitudes and frequencies. Three types of soil deposits with different void ratios were employed in these simulations. Different boundary conditions at the base such as rigid bedrock, elastic bedrock, and infinite medium were also considered. Analysis is done in time domain while taking into account the effects of soil nonlinear behavior. The computational approach is able to capture a number of essential characteristics of wave propagation including motion amplification and resonance. Dynamic soil properties were then extracted from conducted simulations and used to predict the response of the soil using the widely used equivalent linear method program SHAKE and compare its predictions to DEM results. Generally, there was a good agreement between SHAKE and DEM results except when the exciting frequency was close to the resonance frequency of the deposit where significant discrepancy in computed shear strains between SHAKE predictions and DEM results was observed.     

基于Hardin曲线的土体边界面本构模型在ADINA软件中的实现

为了提出一种适合于岩土地震数值模拟的土体本构模型,基于土体动应力-应变关系的Hardin曲线及其在非等幅往返荷载下的Pyke修正,采用von Mises准则在偏应力平面上构造边界面,以反向加载点和当前应力点的连线在边界面上投影的比例作为硬化参数,推导了塑性硬化模量并给出该边界面本构的具体增量表述。在有限元软件ADINA中通过自定义材料的二次开发实现了该本构模型,并利用动三轴试验对该本构模型进行了验证。数值模拟与试验结果的对比表明,本构模型能如实反映土体的应力-应变关系。针对实际工程场地的地震反应,应用边界面本构模型在ADINA中进行了二维数值模拟,与SHAKE91的计算结果进行了对比,说明了该本构模型应用于岩土地震工程问题的合理性。     

基于ANN和GIS耦合模型的场地地震液化势评价系统研究

采用组件式GIS (COMGIS)技术开发了结合BP神经网络分析模型的场地地震液化势评价系统,调用水平成层土地震反应分析程序SHAKE91实现设定地震下地震动影响场的模拟。在VB下调用Matlab神经网络工具箱来完成场地地震液化势评价模型在COMGIS系统中的模块化;利用GIS技术对评价结果,即液化势等级进行空间复合,给出场地潜在的地层液化势空间分布图。研究表明,SHAKE91应用程序在系统菜单下可直接调用,实现地震动影响场计算的模块化;BP神经网络技术应用于场地地震液化势评价中能达到较为理想的效果;系统的GIS空间分析功能可使评价结果与场地信息进行空间匹配,实现目标场地潜在地震液化势的快速评估。     

河北平原典型地裂缝场地动力响应特征研究

河北平原地裂缝分布广泛、致灾严重,给当地工程建设和人民生活造成了严重影响。目前对于河北平原地裂缝的研究主要是发育特征、成因机理以及分布等方面,对该区域场地动力特性的研究不足。为讨论河北平原地裂缝对场地动力响应的影响,分别选取该地区具有代表性的地裂缝(高丽营地裂缝和隆尧地裂缝(构造成因)、大名地裂缝(古河道成因)、唐山地裂缝(地震成因))进行野外地脉动测试,并通过傅里叶谱分析、反应谱分析和Arias烈度分析,揭示了不同类型地裂缝场地的动力响应特征。结果表明,地裂缝对场地卓越频率和卓越周期的影响较小,但对场地动力响应强度的影响较大。在地裂缝附近区域具有放大效应,上盘放大倍数达到2.2~2.7,下盘放大倍数达到1.7~2.4,并随着与地裂缝距离的增大逐渐衰减,一定距离后趋于平稳,且活动性更强的上盘的放大效应更明显。不同类型地裂缝对场地动力响应的影响趋势基本一致,但影响范围不同:构造地裂缝场地的放大效应影响范围最大,约24 m,地震地裂缝次之,约22 m,古河道地裂缝最小,约20 m。综合考虑河北平原地裂缝对场地影响范围:上盘约24 m,下盘约20 m。场地放大效应及其影响范围可以为河北平原地裂缝场地工程结构的合理抗震设防提供依据。      

Liquefaction-induced settlement,site effects and damage in the vicinity of Yalova City during the 1999 Izmit earthquake,Turkey

Yalova City (Turkey) is in a tectonically active location that is particularly affected by the northern branch of the North Anatolian Fault Zone. Magnitudes 7.4 and 7.2 earthquakes in 1999 caused great destruction in Yalova. The heavy damage to buildings and other civil engineering structures was mainly due to liquefaction-induced settlement and site effects such as resonance and amplification. In the first phase of this study, the soil liquefaction potential index (PL) and the induced soil settlement were estimated. In the second phase, the effects on sites in Yalova soil were investigated using microtremor and earthquake data. The fundamental periods and amplification in soft soil were compared with microtremor data and strong ground motion records obtained by a local array of eight accelerograph stations deployed in Yalova. Thirty-seven ‘single site’ ambient noise measurements were taken in a dense grid of points covering the centre of the city. A comparison between fundamental periods obtained from strong ground motion records and from microtremor measurements showed similarities, in the 0.1–5 Hz range. Finally, soil liquefaction and amplification (or resonance) were divided into regions according to the extent of damage and the geotechnical/geophysical results.     

Seismic vulnerability assessment in the new urban area of Diriyah Governorate,Riyadh, Saudi Arabia

Nine seismic refraction profiles were conducted and processed to study the near-surface sediments in the new urban area of Diriyah. The 2D geoseismic models illustrate two layers: a surface layer of soft sediments and weathered to hard limestone bedrock. Moreover, microtremor measurements were performed at 38 sites for 40 min using three-component seismographs and processed to assess the peak spectral amplitude and the corresponding fundamental resonance frequency. The seismic vulnerability index at each measurement site was estimated. These results correlate well with the geotechnical borehole data. The north-western zone is highly vulnerable due to the great thickness of the soft sediments.