Spatial zonations for regional assessment of seismic site effects in the Seoul metropolitan area

Earthquake-induced hazards are profoundly affected by site effects related to the amplification of ground motions, which are strongly influenced by local geologic conditions such as soil thickness or bedrock depth and soil stiffness. In this study, an integrated geographic information system (GIS)-based system for geotechnical data, called the geotechnical information system (GTIS), was developed to establish a regional counterplan against earthquake ground motions in the Seoul metropolitan area. In particular, to reliably predict spatial geotechnical information, a procedural methodology for building the GTIS within a GIS framework was developed and applied to the Seoul area in Korea. To build the GTIS, pre-existing geotechnical data were collected in and around the study area, and then a walk-over site survey was conducted to acquire surface geo-knowledge data. In addition, the representative shear wave velocities for geotechnical layers were derived by statistically analyzing many seismic test data in Korea. The GTIS was used in a practical application to estimate site effects in the study area; seismic zoning maps of geotechnical earthquake parameters, such as the depth to bedrock and the site period, were created and presented as a regional synthetic strategy for earthquake risk assessment. Furthermore, seismic zonation of site classification was also performed to determine the site amplification coefficients for seismic design and seismic performance evaluation at any site and administrative sub-unit in the study area. The methodology and results of the case study of seismic zonations in the Seoul area verified that the GIS-based GTIS can be very useful for the regional estimation of seismic risk and also to support decisions regarding seismic hazard mitigation, particularly in the metropolitan area.     

Local amplification of deep mining induced vibrations Part 1: Experimental evidence for site effects in a coal basin

The work presented in this paper lies under the scope of a research program aiming to assess the impact of deep coal mining induced vibrations on the surface constructions. The concerned section of the program is dedicated to the study of geological site effects and their influence on the mining induced vibrations for which the experimental investigations have been carried out and developed in this paper (Part 1). The empirical methods based on H/V spectral ratios have been applied on data sets provided from mining induced vibrations recorded within private residences above the deep coalmine as well as complementary measurements of ambient noise. The results evidence an amplified zone in the southern part of the Gardanne basin where drilling data confirmed the presence of particularly fractured and soft stratigraphic units. This joint analysis of induced seismicity and ambient noise enabled to validate the method based on H/V ratios applied to the mining context.     

The earthquake of 6 September 2002 and the seismic history of Palermo (Northern Sicily,Italy): Implications for the seismic hazard assessment of the city

On September 6, 2002, a M_L = 5.6 earthquake, occurring some tens of kilometres offshore from the Northern Sicilian coast (Southern Tyrrhenian Sea), slightly damaged the city of Palermo and surroundings (degree 6 in the European Macroseismic Scale 1998). The macroseismic investigation of the shock and a detailed study of effects of the main earthquakes which affected Palermo in the past have been performed in order to evaluate the seismic response of the city. Moreover, the comparison of the recent event, which is instrumentally constrained, with historical earthquakes allows us to infer new insights on the seismogenic sources of the area, that seem located offshore in the Tyrrhenian sea.In the last 500 years, Palermo has never been completely destroyed but has suffered effects estimated between intensities 6 and 8 EMS-98 many times (1693, 1726, 1751, 1823, 1940, 1968, 2002). The damage scenarios of the analysed events have shown that damage distribution is strongly conditioned by soil response in the different parts of the city and by a high building vulnerability, mainly in the historical centre and in the south-eastern zone of the modern city. As a matter of fact, Palermo has always suffered greater effects than those reported for other nearby localities. The hazard assessment obtained using observed site intensities has shown that the probability of occurrence for intensity 8 (the strongest intensity observed in Palermo) exceeds 99% for 550 years, while the estimated mean return period is 152 ± 40 years. These results, in connection with building vulnerability due to the urban expansion before the introduction of seismic code, suggest that the city is exposed to a relatively high seismic risk.This paper has not been submitted elsewhere in identical or similar form, nor will it be during the first 3 months after its submission to Journal of Seismology.     

Seismic site effects by an optimized 2D BE/FE method I. Theory, numerical optimization and application to topographical irregularities

This paper deals with the evaluation of seismic site effects due to the local topographical and geotechnical characteristics. The amplification of surface motions is calculated by a numerical method combining finite elements in the near field and boundary elements in the far field (FEM/BEM). The numerical technique is improved by time truncation. In the first part of this article, the accuracy and the relevance of this optimized method are presented. Moreover, parametric studies are done on slopes, ridges and canyons to characterize topographical site effects. The second part deals with sedimentary valleys. The complexity of the combination of geometrical and sedimentary effects is underlined. Extensive parametrical studies are done to discriminate the topographical and geotechnical effects on seismic ground movement amplifications in two-dimensional irregular configurations. Characteristic coefficients are defined to predict the amplifications of horizontal displacements. The accuracy of this quantitative evaluation technique is tested and discussed.     

Site effects of the 2002 Molise earthquake, Italy: analysis of strong motion, ambient noise, and synthetic data from 2D modelling in San Giuliano di Puglia

On 31 October and 1 November 2002, the Basso Molise area (Southern Italy) was struck by two earthquakes of moderate magnitude (M _L = 5.4 and 5.3). The epicentral area showed a high level of damage, attributable both to the high vulnerability of existing buildings and to site effects caused by the geological and geomorphological settings. Specifically, the intensity inside the town of San Giuliano di Puglia was two degrees higher than in neighbouring towns. Also, within San Giuliano di Puglia, the damage varied notably. The site response in the city was initially evaluated from horizontal-to-vertical spectral ratios (HVSR) from a limited number of strong motion recordings of the most severe aftershocks. Several microtremor measurements were also available. Both data sets indicated the simultaneous presence of two amplification peaks: one around 6 Hz, attributed in previous studies to the strong, shallow impedance contrast among landfill/clay and calcarenites, and one at 2 Hz related to the first S-wave arrivals and predominantly seen only on one receiver component. Further studies performed on weak-motion recordings also showed strong amplification on the vertical receiver component, thus indicating an underestimation of the amplification by the HVSR technique. Additionally, a 2D-model of the geology of the sub-surface was developed, reproducing the flower-shaped structure generated during the late orogenic transpressive regime. The numerical (finite-difference hybrid) simulation reproduced the two peaks of the observed data at slightly higher frequencies. The model also confirmed that the borders of the flower structure define a boundary between amplification levels, with higher amplification inside.     

Evidence for fault-related directionality and localized site effects from strong motion recordings of the 2003 Boumerdes (Algeria) earthquake: Consequences on damage distribution and the Algerian seismic code

The Algiers–Boumerdes region has been struck by a destructive magnitude 6.8 (M_w) earthquake on May 21, 2003. The study presented in this paper is based on main shock strong motions from 13 stations of the Algerian accelerograph network. A maximum 0.58g peak ground acceleration (PGA) has been recorded at 20 km from the epicenter, only about 150 m away from a PGA of 0.34g, with both a central frequency around 5 Hz, explained by a strong very localized site effect, confirmed by receiver function technique results showing peaks at 5 Hz with amplitudes changing by a factor of 2. Soil amplifications are also evidenced at stations located in the quaternary Mitidja basin, explaining the higher PGA values recorded at these stations than at stations located on firm soil at similar distances from the epicenter. A fault-related directionality effect observed on the strong motion records and confirmed by the study of the seismic movement anisotropy, in agreement with the N65 fault plan direction, explains the SW–NE orientation of the main damage zone. In the near field, strong motions present a high-frequency content starting at 3 Hz with a central frequency around 8 Hz, while in the far field their central frequency is around 3 Hz, explaining the high level of damage in the 3- to 4-story buildings in the epicentral zone. The design spectra overestimate the recorded mean response spectra, and its high corner frequency is less than the recorded one, leading to a re-examination of the seismic design code that should definitively integrate site-related coefficient, to account for the up to now neglected site amplification, as well as a re-modeling of the actual design spectra. Finally, both the proposed Algerian attenuation law and the worldwide laws usually used in Algeria underestimate the recorded accelerations of the 6.8 (M_w) Boumerdes earthquake, clearly showing that it is not possible to extrapolate the proposed Algerian law to major earthquakes.     

应用地震数据体结构特征法预测塔河油田八区油层分布规律

地震数据体结构特征,指对二维或三维地震数据体中每一地震道离散数据点按时间顺序排列所显示的波形特征。应用地震数据体结构特征法对储层进行油气预测,是近年来新兴的一项储层预测技术。塔河油田八区的主产层为碳酸盐岩裂缝一缝洞型储层,具有很强的非均质性,给储层的油气预测带来了一定的困难。针对塔河油田八区特殊的地质条件,采用了地震数据体结构特征法对该区奥陶系储层进行了油气预测,并对油区内、外的区域进行了有利区块的划分。在有利区块中设计的18口钻探井位,经钻井证实均获得了较高的油气产量,取得了很好的经济效益。     

Parametric analysis of the seismic response of a 2D sedimentary valley: implications for code implementations of complex site effects

A detailed 2D model has been constructed and validated for Euroseistest valley, in northern Greece. We take advantage of this model to investigate what parameters, in addition to surface soil conditions (obviously the most important parameter), can be used to correctly characterize site response in a 2D structure. Through a parametric analysis using 2D numerical simulations for SH waves, we explore the differences between the computed ground motion for different simplifications of the valley's structure. We consider variations in the velocity structure within the sediments, and variations of the shape between sediments and bedrock. We also compare the results from different 1D models reflecting current approaches to the determination of site response. Our results show clearly that, in the case of Euroseistest, site response owes fundamentally to its closed basin shape because it is largely controlled by locally generated surface waves. Thus, in terms of predicting site response, a rough idea of its shape ratio and of the average mechanical properties of the sediments are better than a very detailed 1D profile at the central site. Although the details of ground motion may vary significantly between the models, the relative amount of surface waves generated in the 2D models seems to be relatively constant. Moreover, if we quantify the additional amplification caused by the lateral heterogeneity in terms of the ‘aggravation factor’ introduced by Chávez-García & Faccioli [7], a roughly constant factor between 2 and 3 seems to appropriately take into account the effects of lateral heterogeneity. Of course, a correct estimate of the overall impedance contrast is necessary to correctly predict the maximum amplification, a caveat that also applies to 1D models. In this sense, Euroseistest rings an alarm bell. In this valley the more significant impedance contrast lies at about 200 m depth, and it is missed both by consideration of the average shear wave velocity of the first 30 m (the V_s30 criterion) or using the detailed velocity profile down to a depth where a shear wave velocity larger than 750 m/s is found. Our conclusions indicate that, in order to improve current schemes to take into account site effects in building codes, the more to be gained comes from consideration of lateral heterogeneity, at least in the case of shallow alluvial valleys, where locally generated surface waves are likely to be important.     

联合采用定性和定量断层资料的应力张量反演方法及在乌鲁木齐地区的应用

地壳应力场是地球动力学研究和地震孕育环境研究的重要基础资料.求解应力场的一种重要方法是根据断层擦痕资料反演,然而擦痕往往难以观测,但断层滑动性质也与应力场有紧密的关系,这部分资料十分丰富.本研究按照应力张量在断层面上的剪应力方向与擦痕滑动方向及断层滑动性质一致为准则提出了联合采用擦痕数据和定性断层滑动数据求解应力场的网格搜索法,并给出了反演参数的一定置信度下的置信区间.采用假定的走滑型、挤压型和拉张型应力张量产生由7个擦痕数据和80个定性断层滑动数据组成的三组数据集.采用这些人工合成数据求解应力张量的实验结果表明,该方法可以更为准确地反演应力张量参数,定性断层滑动资料的参与使得应力张量的主应力方向更加接近假定的主应力方向,并且,应力张量参数的不确定性大大减小.将该方法应用于乌鲁木齐附近的定量擦痕和定性断层滑动性质数据,得到该地区较为精确的应力张量.结果表明,乌鲁木齐主压应力方向为近南北向、主张应力方向近垂直向,应力形因子为0.03. 该地区占主导地位的近东西向的逆冲断层是近南北向挤压和近垂直向拉张形成的.较低的应力形因子表明乌鲁木齐地区在南北向挤压并辅以博罗科努山和博格达山的东、西向挤压的应力状态下处于隆升状态.     

Modified three‐dimensional seismic assessment method for buildings based on ambient vibration tests: extrapolation to higher shaking levels and measuring the dynamic amplification portion of natural torsion

This paper presents applications of the modified 3D‐SAM approach, a three‐dimensional seismic assessment methodology for buildings directly based on in situ experimental modal tests to calculate global seismic demands and the dynamic amplification portion of natural torsion. Considering that the building modal properties change from weak to strong motion levels, appropriate modification factors are proposed to extend the application of the method to stronger earthquakes. The proposed approach is consistent with the performance‐based seismic assessment approach, which entails the prediction of seismic displacements and drift ratios that are related to the damage condition and therefore the functionality of the building. The modified 3D‐SAM is especially practical for structures that are expected to experience slight to moderate damage levels and in particular for post‐disaster buildings that are expected to remain functional after an earthquake. In the last section of this paper, 16 low to mid‐rise irregular buildings located in Montreal, Canada, and that have been tested under ambient vibrations are analyzed with the method, and the dynamic amplification portion of natural torsion of the dataset is reported and discussed. The proposed methodology is appropriate for large‐scale assessments of existing buildings and is applicable to any seismic region of the world. Copyright © 2016 John Wiley & Sons, Ltd.     

倾斜层转换波速度比分析和叠加——潘一煤层Ps波处理的实际应用

目前转换波处理主要局限在水平层假设的前提下,使用斜层共转换点叠加有助于缩短处理流程,提高剖面质量.基于射线定律和几何关系,推导出了单一斜层的转换点位置迭代公式和解析表达式,对不同的深度比,速度比和角度正弦对转换点位置的影响作出分析并与平层渐近转换点公式对比.应用斜层方法对单一煤层进行速度比扫描,提取该属性并对煤层叠加成像,比平层假设方法更能反映目的层的变化.进一步使用插值模型使连续层叠加不再使用拼接方式,而是采用逐个采样点归位叠加的方式形成完整意义上的剖面.对实际资料的三个煤层和新生界进行了方法实验,比单一斜层叠加和水平层假设的效果有所提高.     

基于Theta法改进的均衡滤波器在位场边界识别中的应用--以庐枞矿集区为例

利用观测数据确定地质体的边界位置是位场数据解译中的一项重要工作,传统的边界识别滤波器通常不能均衡深、浅部的地质体边界,近些年相关研究开始致力于发展均衡边界滤波器.本文基于Theta图法定义了新的边界识别滤波器,详述了滤波器波数域及空间域的主要计算公式,通过模型验证,该滤波器显著压制了Theta图法对深部地质体边界的放大作用,较好地平衡了深部和浅部边界.通过与传统的边界识别滤波器对比,本文定义的滤波器能够清晰且更加收敛地圈定出地质体的水平边界位置.以长江中下游成矿带庐枞矿集区为例,开展了1：5万重磁数据的处理分析,并结合物性资料进行了讨论,结果表明：重磁数据的检测结果精确刻画了郯庐断裂带的位置;庐枞盆地的磁力数据检测边界整体与盆地的地质边缘一致,明确了边界断裂在深部倾向盆地内部;识别出庐枞盆地外围一系列环形边界,这些边界封闭区域与最新勘探发现的深部岩体及铁铜矿化体相对应,对于指导区域深部找矿工作有着重要的意义.

     

Use of multi‐platform,multi‐temporal remote‐sensing data for calibration of a distributed hydrological model: an application in the Arno basin,Italy

Images from satellite platforms are a valid aid in order to obtain distributed information about hydrological surface states and parameters needed in calibration and validation of the water balance and flood forecasting. Remotely sensed data are easily available on large areas and with a frequency compatible with land cover changes. In this paper, remotely sensed images from different types of sensor have been utilized as a support to the calibration of the distributed hydrological model MOBIDIC, currently used in the experimental system of flood forecasting of the Arno River Basin Authority. Six radar images from ERS‐2 synthetic aperture radar (SAR) sensors (three for summer 2002 and three for spring–summer 2003) have been utilized and a relationship between soil saturation indexes and backscatter coefficient from SAR images has been investigated. Analysis has been performed only on pixels with meagre or no vegetation cover, in order to legitimize the assumption that water content of the soil is the main variable that influences the backscatter coefficient. Such pixels have been obtained by considering vegetation indexes (NDVI) and land cover maps produced by optical sensors (Landsat‐ETM). In order to calibrate the soil moisture model based on information provided by SAR images, an optimization algorithm has been utilized to minimize the regression error between saturation indexes from model and SAR data and error between measured and modelled discharge flows. Utilizing this procedure, model parameters that rule soil moisture fluxes have been calibrated, obtaining not only a good match with remotely sensed data, but also an enhancement of model performance in flow prediction with respect to a previous calibration with river discharge data only. Copyright © 2006 John Wiley & Sons, Ltd.