Stress field and seismicity in the Indian shield: Effects of the collision between India and Eurasia

Intraplate stresses and intraplate seismicity in the Indian subcontinent are strongly affected by the continued convergence between India and Eurasia. The mean orientation of the maximum horizontal compression in the Indian subcontinent is subparallel to the direction of the ridge push at the plate boundary as well as to the direction of compression expected to arise from the net resistive forces at the Himalayan collision zone, indicating that the intraplate stresses in the subcontinent, including the shield area, are caused by plate tectonic processes. Spatial distribution of historic and instrumentally recorded earthquakes indicate that the seismic activity is mostly confined to linear belts while the remaining large area of the shield is stable. The available conventional heat flow data and other indicators of heat flow suggest hotter geotherms in the linear belts, leading to amplification of stresses in the upper brittle crust. Many of the faults in these linear belts, which happen to be 200–80 m.y. old, are being reactivated either in a strike-slip or thrust-faulting mode. The reactivation mechanisms have been analyzed by taking into consideration the amplification of stresses, pore pressures, geological history of the faults and their orientation with respect to the contemporaneous stress field. The seismicity of the Indian shield is explained in terms of these reactivation mechanisms.     

Application of high-resolution seismic techniques in the evaluation of earthquake site response, Ottawa Valley, Canada

High-resolution seismic surveys, including P- and S-wave studies, have been conducted in an area of the Ottawa River valley located 80 km east of Ottawa (Canada). Based on dating of paleolandslides, the existence of paleoearthquake activity has been postulated in this area. The target zone for the seismic survey is characterized by surface disturbance and sediment deformation. P-wave seismic imaging was used to map the overburden–bedrock interface as well as to indicate reflecting boundaries within the overburden. The area of surface disturbance was found to overlie a buried bedrock basin, 8 km in diameter, infilled with a maximum thickness of 180 m of unconsolidated Quaternary sediments. Preliminary results of core logging show the presence of sand overlain by deformed fine sediments within the disturbed area. Shear-refraction studies reveal differences in the velocity–depth profiles between the disturbed area and the surrounding undisturbed areas. The shear-wave reflection method was used to produce a fundamental resonant period map for the area. Surface sediment disturbance was probably due to a combination of ground-motion amplification due to the basin (thick soft sediments) and the presence of water-saturated sand at depth.     

Nonlinear surface ground analysis via statistical approach

A statistical approach is proposed for nonlinear surface ground analysis. In contrast to the conventional method which deals with only a single ground motion for equivalent linearization of soil properties, a design response spectrum defined at the upper level (bottom of the surface ground) of an engineering bedrock can be handled as the target design earthquake in the present paper. The effective shear strain in each soil layer is evaluated by means of a statistical procedure in which the mean peak shear strain is computed in terms of its standard deviation and the corresponding peak factor. The stiffness and damping ratio of each soil layer are obtained iteratively from the nonlinear relation of stiffness reduction factors and damping ratios with respect to the strain level. After the evaluation of the equivalent stiffness and damping ratio of every soil layer, the ground surface response spectrum is transformed from the design response spectrum defined at the upper level of the engineering bedrock via the one-dimensional wave propagation theory. The reliability and accuracy of the proposed analysis method is examined through the comparison with the results by the conventional method (represented by the program) for many simulated spectrum-compatible ground motions.     

Two-dimensional scattering of SH waves due to a discontinuity in bedrock

The direct boundary integral equation technique is applied to determine the impact on surface amplification caused by an inhomogeneity in a bedrock half-space. The particular soil-rock configuration studied is one in which a soil layer rests on a rock half-space which includes a rock inclusion. The particular rock inclusion considered for this study is a semi-infinite rock layer with its upper boundary bordering the soil layer. Materials are considered viscoelastic except for the section of the rock half-space below the level of the rock inclusion which is considered elastic. A parametric study is performed to determine controlling factors for surface displacement due to a vertically incident shear wave. The study includes varying the stiffness and the thickness of the inclusion for a range of frequencies. Solutions from a one-dimensional analysis are compared with the results of a two-dimensional analysis in order to establish limits inside of which a two-dimensional analysis is required. Copyright © 1999 John Wiley & Sons, Ltd.     

Adaptive base-isolation of civil structures using variable amplification

Semi-active dampers are used in base-isolation to reduce the seismic response of civil engineering structures. In the present study, a new semi-active damping system using variable amplification will be investigated for adaptive baseisolation. It uses a novel variable amplification device （VAD） connected in series with a passive damper. The VAD is capable of producing multiple amplification factors, each corresponding to a different amplification state. Forces from the damper are amplified to the structure according to the current amplification state, which is selected via a semi-active control algorithm specifically tailored to the system＇s tmique damping characteristics. To demonstrate the effectiveness of the VAD-damper system for adaptive base-isolation, numerical simulations are conducted for three and seven-story base-isolated buildings subject to both far and near-field ground motions. The results indicate that the system can achieve significant reductions in response compared to the base-isolated buildings with no damper. The proposed system is also found to perform well compared to a typical semi-active damper.     

地铁交通引起的环境振动的实测与分析

本文根据现场实测数据,对某地铁交通1号线沿线典型区段引起的环境振动实况和振动特性及传播规律等进行了分析研究。结果表明,地面振动Z振级主要由测点到轨道中心的水平距离决定,并且在离开地铁隧道中心线一定距离范围内,地面振动Z振级存在振动放大区。此外,本文还根据实测资料和现有研究成果提出了该地铁1号线引起环境振动Z振级的统计回归公式。该式从数学表达式的角度反映了振动信号的放大效应,可为预测或估计地铁运营诱发的环境振动提供参考。     

Effect of site amplification on inelastic seismic response

The available models for eff ective periods of site and structure are reviewed in context of frequency tuning in the inelastic seismic response of soil-structure system. The eff ect of seismic intensity and ductility demand, on the eff ective periods, is investigated, and inelastic site amplifi cation is shown to be strongly correlated to the normalized eff ective period. Two non-dimensional parameters, analogous to the conventional site amplifi cation factors in codes, are defi ned to quantify the inelastic site amplifi cation. It is shown that the inelastic site amplifi cation factor (i.e. ratio of constant ductility spectral ordinates at soil site to those at rock outcrop) is able to represent the site eff ects more clearly, as compared to the inelastic site amplifi cation ratio (i.e. ratio of inelastic spectral ordinates at soil site to the corresponding elastic spectral ordinates at rock outcrop). Further, the peak in the amplifi cation factor corresponding to the eff ective site period diminishes rapidly with increasing ductility demand.     

Modelling non‐linear ground response of non‐liquefiable soils

A non‐linear finite element (FE) model is presented to account for soil column effects on strong ground motion. A three‐dimensional bounding surface plasticity model with a vanishing elastic region, appropriate for non‐liquefiable soils, is formulated to accommodate the effects of plastic deformation right at the onset of loading. The elasto‐plastic constitutive model is cast within the framework of a FE soil column model, and is used to re‐analyse the downhole motion recorded by an array at a Large‐Scale Seismic Test (LSST) site in Lotung, Taiwan, during the earthquake of 20 May 1986; as well as the ground motion recorded at Gilroy 2 reference site during the Loma Prieta earthquake of 17 October 1989. Results of the analysis show maximum permanent shearing strains experienced by the soil column in the order of 0.15 per cent for the Lotung event and 0.8 per cent for the Loma Prieta earthquake, which correspond to modulus reduction factors of about 30 and 10 per cent respectively, implying strong non‐linear response of the soil deposit at the two sites. Copyright © 2000 John Wiley & Sons, Ltd.     

Characteristics of response spectra for long-periods of main-shock recordings of the Chi-Chi earthquake

Current practice uses predictive models to extrapolate long-period response spectra based on far-field recordings in moderate and weak earthquakes. However, the spectra are not long enough and the data are often not reliable, which means that the seismic design code cannot accurately define seismic design requirements for long-period structures. The near-field recordings in the main-shock of the Chi-Chi earthquake have a large signal-to-noise ratio (SNR), which makes them suitable for studying the long-period acceleration response spectrum up to 20 sec. The acceleration response spectra from 246 stations within 120 km of the causative fault are statistically analyzed in this paper. The influence of distance and site conditions on long-period response spectrum is discussed, and the shapes of the amplification spectra are compared with the standard spectra specified in the seismic design code of China. Finally, suggestions for future revisions to the code are proposed.     

曼氏无针乌贼(Sepiella maindroni)-肌动蛋白基因的 cDNA 全长克隆与序列分析

采用RT-PCR和快速扩增cDNA末端(rapid amplification of cDNA ends, RACE)技术首次克隆了曼氏无针乌贼?-肌动蛋白基因的 cDNA 全序列, 该序列全长为 2000bp, 由长 197bp 的 5’非翻译区(untranslated region, UTR), 669bp 的 3’非翻译区, 和 1134bp 的开放阅读框(open reading frame, ORF)组成。阅读框共编码 377 个氨基酸, 推算的分子量约为 42.0kDa, 理论等电点为 5.16。曼氏无针乌贼-actin 氨基酸序列中 Ile12、Ser172、Ser174、Gln223、His227、Ile231、Gly232、Ser320、Glu328、 Thr360等10个氨基酸残基具有特异性, 以及2个特殊的氨基酸残基位点和2个软体动物特有的氨基酸残基。曼氏无针乌贼?-actin 氨基酸序列与软体动物、节肢动物、脊椎动物的相似性高达 97%。NJ法系统进化分析显示曼氏无针乌贼首先与软体动物聚在一起, 然后与节肢动物聚在一起, 再与鱼类、两栖类、哺乳类聚在一起。