Evolution of Stress Fields and Faulting in Seismic Zones

—Measurements indicate that stress magnitudes in the crust are normally limited by the frictional equilibrium on pre-existing, optimally oriented faults. Fault zones where these limitations are frequently reached are referred to as seismic zones. Fault zones in the crust concentrate stresses because their material properties are different from those of the host rock. Most fault zones are spatially relatively stable structures, however the associated seismicity in these zones is quite variable in space and time. Here we propose that this variability is attributable to stress-concentration zones that migrate and expand through the fault zone. We suggest that following a large earthquake and the associated stress relaxation, shear stresses of a magnitude sufficient to produce earthquakes occur only in those small parts of the seismic zone that, because of material properties and boundary conditions, encourage concentration of shear stress. During the earthquake cycle, the conditions for seismogenic fault slip migrate from these stress-concentration regions throughout the entire seismic zone. Thus, while the stress-concentration regions continue to produce small slips and small earthquakes throughout the seismic cycle, the conditions for slip and earthquakes are gradually reached in larger parts of, and eventually the whole, seismogenic layer of the seismic zone. Prior to the propagation of an earthquake fracture that gives rise to a large earthquake, the stress conditions in the zone along the whole potential rupture plane must be essentially similar. This follows because if they were not, then, on entering crustal parts where the state of stress was unfavourable to this type of faulting, the fault propagation would be arrested. The proposed necessary homogenisation of the stress field in a seismic zone as a precursor to large earthquakes implies that by monitoring the state of stress in a seismic zone, its large earthquakes may possibly be forecasted. We test the model on data from Iceland and demonstrate that it broadly explains the historical, as well as the current, patterns of seismogenic faulting in the South Iceland Seismic Zone.     

Seismicity and Casing Failures Due to Steam Stimulation in Oil Sands

—This paper describes observations of seismicity and casing failures associated with steam stimulation operations at Imperial Oil Ltd.’s Cold Lake oil field in Alberta, Canada. A total of 11 oil-producing pads were monitored over a 1–2 year period using 3-component geophones cemented at depths ranging from 160 m to 400 m and data acquisition systems with a flat frequency response up to 1.5 kHz. Most of the seismicity was detected during the steaming operations and was located in the formation overlying the oil-bearing layer. Some activity was observed in the shales above, however, the reservoir itself showed almost no evidence of seismicity. The estimated seismic moment of the observed events was in the range 10⁵–10⁷ N·m (?2.7 < M < ?1.3). According to a theoretical model (Talebi et al., 1998) and in situ observations, the seismic source corresponding to casing failure events should be well described by a dipole registering seismic moment in the order of 2 · 10⁶ N·m. Seismic signals of a total of four observed casing failures were analyzed. The partial failures produced seismic moments slightly lower than this value while total failures were stronger by about one order of magnitude. The use of the SV/SH amplitude ratio, in conjunction with accurate source locations, provided a robust technique for the detection of casing failures.     

垂直向地震作用对节理岩体失稳破坏的影响

基于线弹性断裂力学理论分析了垂直向地震作用对节理岩体地震动力破坏的影响。在仅考虑峰值时,最不利的单向地震动加速度方向是水平倾向坡外,双向则依据破裂机制是拉剪或压剪,加速度分别是水平倾向坡外与向下或向上的组合。地震动的幅值、作用方向及双向地震动的组合都可使岩体的破坏机制发生转化,并且是突变的、不可逆的。较低峰值的双向地震动产生的应力强度因子可能大于较高峰值的单向地震动所产生的应力强度应子。在岩体节理分布特征和静态应力场一定的初始条件下,第一个导致岩体中产生破裂的地震动加速度幅值及其方向的组合唯一地决定了岩体不可逆破坏发展的方向、机制及最终的破坏特征,其复杂性远大于静力作用时的情况。对岩体地震动力破坏问题的认识应充分考虑垂直向地震动的重要影响。     

Review of Electric and Magnetic Fields Accompanying Seismic and Volcanic Activity

New observations of magnetic, electric and electromagnetic field variations, possibly related to recent volcanic and seismic events, have been obtained on Mt. Unzen in Japan, Reunion Island in Indian Ocean, the Long Valley volcanic caldera in California, and for faults in China and Russia, California and several other locations. For volcanic events, contributions from different physical processes can be identified during the various eruption stages. Slow processes (weeks to months) include near-surface thermal demagnetization effects, piezomagnetic effects, and effects from rotation/displacement of magnetized material. Rapid processes (seconds to days) include piezomagnetic effects from instantaneous stress redistribution with explosive eruptions and electrokinetic effects from rupture of high pressure fluid compartments commonly encountered in volcanic regions. For seismic events, the observed coseismic offsets are instantaneous, provided care has been taken to ensure sensors are insensitive to seismic shaking and are in regions of low magnetic field gradient. Simple piezomagnetic dislocation models based on geodetically and seismically determined fault parameters generally match the observed signals in size and sign. Electrokinetic effects resulting from rupture of fluid filled compartments at hydrostatic to lithostatic pore pressures can generate transient signals in the frequency band 100 Hz to 0.01 Hz. However, large-scale fluid driven processes are not evident in near-field measurements in the epicentral region minutes to weeks before large earthquakes. The subset of ionospheric disturbances generated by trapped atmospheric pressure waves (also termed gravity waves and/or acoustic waves, traveling ionospheric disturbances or TID's) that are excited by earthquakes and volcanic eruptions are common and propagate to great distances. These are known and expected consequences of earthquakes, volcanic explosions (and other atmospheric disturbances), that must be identified and their effects removed from VLF/ULF electromagnetic field records before associating new observations of ionospheric disturbances with earthquake activity.     

装配式双钢套管混凝土组合剪力墙抗震性能有限元分析

为了研究不同含圆钢率对新型可适用于高层建筑的装配式内置双钢套管混凝土组合剪力墙抗震性能的影响,采用有限元软件ABAQUS对该组合剪力墙进行拟合分析.依据组合剪力墙试验数据等参数建立SW1～SW4、SW6模型,进行水平单推加载并与试验结果骨架曲线进行对比.有限元模拟所得位移-荷载曲线和试验值具有较好的吻合度,极限承载力误...     

Production-induced Normal Faulting in the Valhall and Ekofisk Oil Fields

—?In situ stress and pore pressure data from the Valhall and Ekofisk oil reservoirs indicate that at the onset of production in both fields an incipient state of normal faulting existed in the crest of the anticlinal structures. In contrast, on the flanks of the structures the initial least principal stress values indicate an almost isotropic state of stress. Oil production from both fields caused marked pore pressure reductions as well as poroelastic reductions of the least principal stress in both the crest and flanks of the two structures. We demonstrate that as a result of production-induced pore pressure and stress changes, normal faulting appears to have spread out from the crests of the structures on to the flanks. Further evidence of a normal faulting stress state at Valhall has been found using data from a passive seismic monitoring experiment. Numerous microearthquakes were recorded during a six week monitoring period that are located at the very top of the reservoir or in the shale caprock immediately above it. An inverse/composite focal plane mechanism of these microearthquakes is consistent with a normal faulting stress regime.     

点支式玻璃建筑结构地震损伤评估模型研究

地震对建筑结构的损伤轻则影响建筑完整性,重则导致建筑崩塌。近几年,地震损伤评估问题得到地震工程研究领域的高度重视,但对点支式玻璃建筑结构的损伤评估研究较少。为此,构建一种点支式玻璃建筑结构地震损伤评估模型。采用基于HHT变换的结构损伤部位识别方法判断地震中点支式玻璃建筑损伤部位,建立点支式玻璃建筑结构地震损伤评估多元联系数模型,评估点支式玻璃建筑损伤部位的损伤情况。结果表明,该评估模型对某地区点支式玻璃建筑结构地震损伤评估情况与实际结果一致,且该模型可控性较强,评估范围全面,评估效率明显优于其他评估模型,应用价值较高。     

CyberShake: A Physics-Based Seismic Hazard Model for Southern California

CyberShake, as part of the Southern California Earthquake Center??s (SCEC) Community Modeling Environment, is developing a methodology that explicitly incorporates deterministic source and wave propagation effects within seismic hazard calculations through the use of physics-based 3D ground motion simulations. To calculate a waveform-based seismic hazard estimate for a site of interest, we begin with Uniform California Earthquake Rupture Forecast, Version 2.0 (UCERF2.0) and identify all ruptures within 200?km of the site of interest. We convert the UCERF2.0 rupture definition into multiple rupture variations with differing hypocenter locations and slip distributions, resulting in about 415,000 rupture variations per site. Strain Green Tensors are calculated for the site of interest using the SCEC Community Velocity Model, Version 4 (CVM4), and then, using reciprocity, we calculate synthetic seismograms for each rupture variation. Peak intensity measures are then extracted from these synthetics and combined with the original rupture probabilities to produce probabilistic seismic hazard curves for the site. Being explicitly site-based, CyberShake directly samples the ground motion variability at that site over many earthquake cycles (i.e., rupture scenarios) and alleviates the need for the ergodic assumption that is implicitly included in traditional empirically based calculations. Thus far, we have simulated ruptures at over 200 sites in the Los Angeles region for ground shaking periods of 2?s and longer, providing the basis for the first generation CyberShake hazard maps. Our results indicate that the combination of rupture directivity and basin response effects can lead to an increase in the hazard level for some sites, relative to that given by a conventional Ground Motion Prediction Equation (GMPE). Additionally, and perhaps more importantly, we find that the physics-based hazard results are much more sensitive to the assumed magnitude-area relations and magnitude uncertainty estimates used in the definition of the ruptures than is found in the traditional GMPE approach. This reinforces the need for continued development of a better understanding of earthquake source characterization and the constitutive relations that govern the earthquake rupture process.     

A Model for Deepwater Oil/Gas Blowouts

When gas is released in deepwater, the high pressure and low temperature can convert the gases into hydrates, which are buoyant. As these hydrates travel upwards they will encounter regions of lower pressure and can decompose into free gas. The presence or absence of hydrates has a significant impact on the behaviour of the jet/plume due to the alteration of the buoyancy. The free gas may dissolve in water. This paper describes a computer model developed to simulate the behaviour of oil and gas released from deepwater locations in the ocean. The model integrates the hydrodynamics and thermodynamics of the jet/plume with kinetics and thermodynamics of hydrate formation/decomposition. Model formulation and comparison of results with laboratory data for hydrates is presented. Scenario simulations show the behaviour of oil/gas under different deepwater conditions.     

A Computer Model for Cyclic Activity of Strong Earthquakes in Continental Seismic Zones

Based on the study of activity of earthquakes with M**********>7.0 in mainland China,we have found a dynamic pattern,i.e.,the cyclic characteristics in time and migration from one seismic zone to another in space.In order to understand the physical mechanism of this pattern,we use a nonlinear dynamic model to simulate the seismic activity in fault zones under a unified tectonic stress field.The basic elements in our model consist of a Maxwell body and a rigid sliding block.Basic elements in a column represent a fault,and coupling elements connecting different columns simulate the interaction among faults and fault segments.The results provide insights to the cyclic activity of strong earthquakes and to the feature of mutual influence between strong earthquakes in groups in the climax of seismic activity.     

多分量地震记录P和SV波场的τ-p域分离方法

多分量地震观测记录到来自地下介质的全波场,利用分离的P和SV波场联合反演可以提供地下介质岩性、孔隙度、孔隙流体、裂隙和各向异性的重要信息.     

中国大陆强震轮回活动的计算机模型研究

通过对中国大陆7级以上强震的研究发现,地震活动呈现为时间上轮回和空间上迁移的动态图像。为探索这种图像的物理本质,本文应用非线性动力学模型来模拟在统一构造应力场作用下含有多个活动地震带的地震构造块体的地震活动。文中用若干列并联排列的基本元件(由Maxwell体和刚性滑块组成)来模拟地震活动带,并用耦合元件来模拟地震活动带之间和带内各段之间的相互耦合作用。本文的研究结果有助于理解强震的轮回活动以及高潮中成组强震间的相互影响特征。     

Phase and Correlation in `Random' Seismic Fields and the Reconstruction of the Green Function

We first present a summary of recent results on coda interpretation. We emphasize the observation of the stabilization of P to S energy ratio indicating the modal equipartition of the wavefield. This property clearly shows that the coda waves are in the regime of multiple scattering. Numerical solutions of the elastic radiative transfer equation are used to illustrate the evolution of the wave-field towards P-to-S energy stabilization, and asymptotically to complete isotropy. The energy properties of the coda have been widely studied but the phase properties have often been neglected. The recently observed coherent backscattering enhancement, an expression of the so-called `weak localization', demonstrates that interference effects still persist for multiple diffracted waves. Another manifestation of the persistence of the phase is the possibility to reconstruct the Green function between two stations by averaging the cross correlation of coda waves produced by distant earthquakes and recorded at those two stations. This reconstruction is directly related to the properties of reciprocity and time reversal of any wavefield. Using broadband seismic coda waves, we show that the dominant phases of the Green function in the band 2 s–10 s, namely fundamental mode Rayleigh and Love waves, are reconstructed. We analyze the time symmetry of the cross correlation and show how the level of symmetry evolves with the isotropization of the diffuse field with lapse time. Similarly we investigate the correlation in continuous ambient noise records. Whereas the randomness of the coda results from multiple scattering by randomly distributed scatterers, we assume that the seismic noise is random mostly because of the distribution of sources at the surface of the Earth. Surface waves can be extracted from long time series. The dispersion curves of Rayleigh waves are deduced from the correlations. On paths where measurements from earthquake data are also available, we show that they are in good agreement with those deduced from noise correlation. The measurement of velocities from correlation of noise along paths crossing different crustal structures opens the way for a `passive imaging' of the Earth's structure.     

Analysis of Geological Background of Seismic Activity in the Chengdu Plain from Data of Seismic Prospecting for Oil

Summarizing the existing data of seismological and geological investigations and of strong and intermediate-strong earthquakes in‘ the Sichuan basin and its adjacent areas accumulated by the seismological and petroleum organizations in Sichuan and of the results of seismic prospecting and detailed exploration in Chengdu depression during the last 20 years permitted us to studythe types and distribution of hidden structures in Chengdu depression and its adjacent areas, in particular, to identify in, detail the “hidden faults” in the Chenngdu-Deyang area on the one hand; The obtained data indicate that the NE-trending Xinjin fault ruus northward and dies out in the south of Penzhen town of Shuangliu County; Meanwhile, we studied genetic relations of seismic activity to active faults and their corresponding movernent charaeteristics on the other hand, Moreover, the Surface faults and deep-seated faults are clearly defined and outlined,and 5 types of seismogenic faults suggestd. The knowledge thus obtained enables us to delimit the focal zones for potential strong earthquakes in Chengdu depression. The study suggests that a zone of 40 km wide and more than 100 km long on sides along the Chengu Deyang lim bas a stable seismogeological background and good engineering-seismological conditions.     

Slope Failure Initiation by Seismic Loading from Different Sources

Izvestiya, Physics of the Solid Earth - Abstract—We present the results of laboratory studies of slope failure criteria by dynamic impulse loading in the range of peak ground velocities (PGV)...     

一种简化的混凝土结构地震损伤评估模型及方法

随着地震工程学的发展和结构抗震设计思想和理论的进步,探讨结构在地震作用下的地震损伤破坏机理和基于结构性能的抗震设计方法(PBSD)逐渐得到了各国专家、学者的重视。而地震损伤评估的研究就是其中的一个重要方向。鉴于现阶段混凝土结构地震损伤评估方法的局限性,本文采用推广的混凝土材料的Mazars损伤模型,进而提出了一种基于常规有限元分析荷载子步的简化的地震损伤评估方法,这种方法实现简单,便于实际工程应用,同时具有一定的精细性。最后,将本文的损伤评估方法应用到一个钢筋混凝土框架的地震损伤评估实例中,分析结果与实验和实际的震害比较吻合,表明本文提出的模型和方法是有效的。     

Numerical Modeling of Transient Seismic Fields in Viscoelastic Media Based on the Laguerre Spectral Method

--The wave propagation in real media can be described within the theory of linear viscoelasticity. The presence of convolutional integral in Boltzmann's superposition principle poses the main difficulties in implementing the direct numerical methods in time domain. The paper presents a new algorithm, based on the application of the spectral Laguerre method for the approximation of temporal derivatives and convolution as applied to the problem of seismic wave propagation in the heterogeneous viscoelastic medium. Examples of the calculation of synthetic seismograms for different models of viscoelastic media are presented.     

论行业抗震设防标准的确定原则和依据

以广播电影电视行业为例,探讨了确定行业抗震设防标准的原则和依据,并对其合理性进行了比较分析。文章不仅论证了为广播、电影、电视行业各类工程所确定的抗震设防标准是可行的,而且对确定其他行业的抗震设设标准也有所裨益。