Apparent stress and stress drop for intraplate earthquakes and tectonic stress in the plates

The magnitude of shear stress in the lithosphere is bounded from below by the apparent stress and stress drop during intraplate earthquakes. Apparent stresses and stress drops for a number of mid-plate earthquakes are calculated from the earthquake magnitude, SH wave amplitude spectra, and estimates of the length of the fault zone. Apparent stresses vary between 0.1 and 2 bars, ifm _b is used as a measure of seismic energy, and stress drops lie between 2 and 70 bars. There is no systematic difference in either apparent stress or stress drop between these intraplate events and typical plate boundary earthquakes. These bounds on intraplate shear stresses are consistent with the inference from current models of plate tectonic driving forces that regional stress differences in the plates are typically on the order of 100 bars. The highest stress drops measured for midplate earthquakes under this model represent nearly total release of local tectonic stress.     

Wave radiation stress

There are differences in the literature concerning the vertically dependent equations that couple currents and waves. In this paper, currents are purposely omitted until the end. Isolating waves from currents allows one to focus on two main topics: an explanation of Stokes drift with apparent mean vorticity obtained from an otherwise irrotational flow and the determination of vertically dependent wave radiation stress which, when vertically integrated, conforms to that obtained by Longuet-Higgins and Stewart (1964) and Phillips (1977) nearly 50 years ago and, more recently, by Smith (2006). Discussion begins with the simple case of nonlinear flow beneath a stationary wavy wall.     

Study on determination of stress level by seismic stress drops and the stress axis deflections before and after large earthquakes

Introduction The study of tectonic stress field, a major branch of the Earth science, plays an important role in geodynamics. The world stress map plan started from 1980s and was leaded by Zoback. Lots of scientists from many countries participated this plan. The plan collected global tectonic stress measurements and research results and established global stress database. The world stress map was edited based on the global stress database. The world stress map reflected feature of global li…     

华北地区库仑应力演化与强震活动关系

采用更加符合长期变形和震后短期变形的Burgers流变模型,模拟了华北地区1303年以来6.5级以上地震引起的同震和震后库仑应力演化.结合华北地区历史地震期幕活动特征,分析了不同活跃期之间、同一活跃期内不同地震之间的库仑应力加卸载效应.结果表明：华北地区不同活跃期的强震主体活动区受控于历史强震的库仑应力加载作用;每个活跃期内强震活动主体区在空间上的迁移与该活跃期内首个7.5级以上地震的触发作用有关;华北地区每个活跃期内强震活动在时间上表现出的"平静期—活跃警告期—高潮期—剩余释放期"应该是区域动力加载过程的一种表现.岩石圈流变松弛效应引起的库仑应力变化对华北地区强震活动时空演化有显著的促进作用.本研究讨论了库仑应力变化在华北地区历史地震活动时空演化过程中可能扮演的角色,为探索华北地区强震活动空间迁移和韵律特征蕴含的构造动力学过程,以及与之相关的地震危险性判定提供参考依据.     

完全库仑破裂应力变化与云南龙陵震群序列的应力触发

计算和研究了1976年云南龙陵Ms73、Ms74双主震产生的完全库仑破裂应力变化及其对后续震群序列的动态和静态应力触发作用.结果显示,龙陵第2主震受到了第1主震的动态和静态库仑破裂应力的触发作用.龙陵双主震的13个后续强余震中,发生在龙陵三角形块体内部的强余震,90%都受到了第1或第2主震的动态库仑破裂应力触发或静态库仑破裂应力触发作用,发生在三角形块体外部的强余震,2/3受到了第2主震的动态库仑破裂应力触发作用.从触发作用的强度和范围上看,第2主震的动态和静态库仑破裂应力触发作用都大于第1主震.就近场而言,受到龙陵双主震的动态和静态库仑破裂应力触发作用的余震数目相当,但动态库仑破裂应力的触发作用范围大于静态库仑破裂应力.     

Crustal stress in Iceland

Hydrofracturing stress measurements have been carried out to about 0.4 km in two boreholes in Quaternary volcanic rocks in Reykjavik, Iceland, on the flank of the Reykjanes-Langjökull continuation of the Mid-Atlantic Ridge. The measurements indicate a dominant orientation of _{H max} approximately perpendicular to the axial rift zone, in contrast to earthquake focal mechanism solutions from within the axial rift zone of the Reykjanes Peninsula. In one hole (H32) a depth-dependent change in stress orientation is indicated, with ₁ horizontal above a depth of about 0.25 km, and vertical below it; however the orientation of _{H max} remains unchanged. The data thus suggest reconciliation of an apparent conflict between the dominantly compressive indications of shallow overcoring stress measurements and dominant extension as required by focal mechanism solutions. The measured stresses are supported by the more reliable of overcoring measurements from southeast Iceland, and by recent focal mechanism solutions for the intraplate Borgarfjördur area. A fundamental change in crustal stresses appears therefore to occur as a function of distance from the axis of the axial rift zone. The data seem reasonably explicable in terms of a combination of thermoelastic mechanisms associated with accretion and cooling of spreading lithosphere plates. Stresses directly associated with the driving mechanisms of plate tectonics apparently do not dominate the observed stress pattern.     

On stress-forecasting strategy of earthquakes from stress buildup,stress shadow and stress transfer (SSS) based on numerical approach

Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR), used for monitoring crust deformation, are found to be very promising in earthquake prediction subject to stress-forecasting. However, it is recognized that unless we can give reasonable explanations of these curious precursory phenomena that continue to be serendipitously observed from time to time, such high technology of GPS or InSAR is difficult to be efficiently used. Therefore, a proper model revealing the relation between earthquake evolution and stress variation, such as the phenomena of stress buildup, stress shadow and stress transfer (SSS), is crucial to the GPS or InSAR based earthquake prediction. Here we address this question through a numerical approach of earthquake development using an intuitive physical model with a map-like configuration of discontinuous fault system. The simulation provides a physical basis for the principle of stress-forecasting of earthquakes based on SSS and for the application of GPS or InSAR in earthquake prediction. The observed SSS associated phenomena with images of stress distribution during the failure process can be continuously simulated. It is shown that the SSS are better indicators of earthquake precursors than that of seismic foreshocks, suggesting a predictability of earthquakes based on stress-forecasting strategy.     

On stress-forecasting strategy of earthquakes from stress buildup, stress shadow and stress transfer （SSS） based on numerical approach

Global Positioning System(GPS) and Interferometric Synthetic Aperture Radar(InSAR),used for monitoring crust deformation,are found to be very promising in earthquake prediction subject to stress-forecasting.However,it is recognized that unless we can give reasonable explanations of these curious precursory phenomena that continue to be seren-dipitously observed from time to time,such high technology of GPS or InSAR is difficult to be efficiently used.Therefore,a proper model revealing the relation between e...     

兰州至玛曲地区地应力测量与现今构造应力场特征研究

兰州—玛曲地区是印度板块北东向推挤引起青藏块体强烈变形的前缘区,该区的现今构造应力场研究对研究大陆动力学问题具有重要意义.本文给出了兰州—玛曲地区不同地点的现今地应力实测值的大小和方向.测量方法采用压磁应力解除法,测点分别布置在阿姨山、大水、尕海、玛艾以及清水.为系统研究本区及邻近地区现今构造应力场特征,对已有应力实测数据进行了整理分析.研究结果表明,本区及邻区几十米浅表部应力与其他地区相比,属于中等大小量值;应力随深度增加而加大,但在不同构造单元,应力增加梯度有所不同;最大水平主应力方向总体上为北东向,不同构造单元上方向有所不同,鄂尔多斯地块最大水平主应力方向为近东西向,河西走廊带最大水平主应力方向在北北西—北东方向内变化,祁连山东南端最大水平主应力方向变化较大,西秦岭地块是现今地应力的一个过渡带,最大水平主应力方向由北侧的NE向逐渐转变为中部的EW向和南侧的SEE向.本文给出的结果与由GPS观测给出的该区域应变场分布具有一致性.     

根据走滑大地震前后应力轴偏转和应力降求取偏应力量值的研究

运用走滑地震造成的地震前后应力方向偏转和地震应力降Δτ推导得到地震震源处偏应力量值τ的解析表达式为τ=［Δτcos2(φ′[KG-*2]P-E)］/sin2(φ′[KG-*2]P-φP)(其中,φ\-P和φ′[KG-*2]P分别为地震前后的统计P轴走向,E为地震断层走向.).当震前P轴与震后P轴与断层走向夹角为45°时,该公式失效.对偏应力值与应力降比值随应力场主压应力轴与断层走向夹角及应力场主压应力轴偏转的变化进行分析表明,相同应力降造成的应力轴偏转越大,地下偏应力越小; 断层走向越接近主压应力轴方向,地震应力降场对偏应力场的贡献越小.将该方法运用于Landers地震震源区,求得了该地震Homestead Valley段的偏应力量值为10MPa.     

Influence of tectonic stress on coalification:Stress degradation mechanism and stress polycondensation mechanism

The function of pressure for coalification is a long-term controversial issue, and the main cause is that the strata pressure and the tectonic stress were confused, which are two different actions of "pres-sure" . The former benefits the physical coalification but retards the chemical coalification, whereas the latter may not only affect the physical structure of coal but also promote its chemical composition changes. In accordance with the organic molecule evolution of coal, there are two kinds of basic mechanisms of the influence of the tectonic stress on the chemical coalification: the tectonic stress degradation and the tectonic stress polycondensation. The stress degradation mechanism is a process of that, when the tectonic stress acted on the large molecule of coal in the form of mechanical force or kinetic energy, some chemical bonds of low decomposed energy, such as aliphatic side-chain and oxygenic functional groups, were broken up and then were degraded into free radicals of less mo-lecular weight, and finally escaped from coal in the form of liquid organic matter (hydrocarbon). The stress polycondensation is considered that, under the control of the anisotropic tectonic stress, the condensed aromatic nucleus trend to be parallel arranged and to be enhanced through rotating or displacing of aromatic rings, the basic structural unit of coal (BSU) increases by directional develop-ment and preferential stack. X-ray diffraction (XRD), Fourier transformation infrared microspectroscopy (FTIR), and rock pyrolysis analysis (Rock-eval) were employed to study the deformed coal series and the non-deformed coal series. The results showed that, compared with the non-deformed coal, the de-formed coal exhibits particular characteristics: weaker aliphatic absorbance peak and stronger aro-matic absorbance peak, lower pyrolysed hydrocarbon yield, and more increscent BSU. The concepts of stress degradation mechanism and stress polycondensation mechanism presented here would not deny the dominant function of the temperature in coalification, but emphasize the "catalysis" of the tectonic stress in coalification.     

新加坡地应力测量

2008年和2009年, 通过和新加坡宜康地质技术有限责任公司合作, 在新加坡18个浅钻孔中进行了水压致裂地应力测量. 通过测量获得了新加坡地壳浅部的现今应力状态, 为新加坡拟建地下管缆隧道设计施工提供了地应力依据, 也为世界应力环境数据库提供了参考. 测试结果表明, 在38.00~56.95 m深度域内最大水平主应力σ_h值为3.45~9.64 MPa, 最小水平主应力σ_h为2.49~6.28 MPa. 水平主应力和用上覆岩层重量计算的垂直应力σv之间的关系为σ_h>σ_h>σv,最大、最小水平主应力平均值分别是垂直应力平均值的5.13倍和3.23倍, 表明新加坡应力场以水平应力为主导. 初步分析认为, 新加坡陆域浅部构造应力场的量值大致呈南、北两端偏低, 中间高的态势, 在北端最大与最小水平主应力值都小于它们各自的平均值, 而南端则与平均值相当. 最大水平主应力优势方向为NE—NEE向, 该方向与前人在其邻区获得的构造应力场方向和由震源机制解反映的主压应力方向较为一致. 新加坡现今构造应力场状态与印度板块、澳大利亚板块、菲律宾板块与欧亚板块的汇聚作用有着密切的联系. 根据库仑摩擦滑动准则, 断层面摩擦系数取0.6~1.0, 分析了研究区内断层产生摩擦滑动的可能性.     

Research on seismic stress triggering

Introduction What is called seismic stress triggering refers to that stress change tensor produced by previous earthquake projected on the fault plane and slip direction of subsequent earthquakes, taking into account normal stress, pore pressure and friction coefficient, and Coulomb failure stress change DCFS can be obtained. If the direction of Coulomb failure stress changes are consistent with slip direction of subsequent seismic mechanism, e.g., Coulomb failure stress changes are positive…     

Torsional wave under initial stress

Summary In this paper, the frequency equation for phase velocity of torsional wave in a rod of circular cross section under initial tension has been obtained. The graphs are plotted to compare the velocity with that in the absence of initial tension. Some analytical informations have been obtained for instability of the body under initial compression.     

青藏高原腹地现今地应力测量与应力状态研究

青藏高原腹地现今构造活动强烈，为了解强烈构造活动下的应力背景，我们在五道梁、风火山、雁石坪和安多不同构造部位的四个测点采用压磁应力解除法进行了现场应力实测工作，测量深度12～20m.测量结果表明, 最大主应力方向总体为北东向，与地质、地震以及大地测量等方面的研究成果基本吻合；最大主应力量值为36～68MPa（安多测点除外），与其他地区测量结果相比，属于中等偏高；安多测点最大主应力方向为北西西向，最大主应力量值为81MPa，与其他测点有较大差别，反映了板块缝合带附近现今应力状态的复杂性.     

应力时空演化揭示出的汶川地震前亚失稳过程

近年来从岩石实验研究针对应力水平进入峰值强度后至断层快速失稳前的短临阶段变形物理场的时空演化过程提出了亚失稳阶段的概念,该阶段的基本特征为应力由峰值前的积累为主转为峰值后稳定释放并加速至最终快速失稳,这一过程伴随着其他变形物理场的相应转变.把握该阶段应力场的转变特征对认识未来地震危险区及其变形阶段至关重要.以2008年汶川地震为例,运用基于区域多个地震震源机制反演定量应力张量的方法对强震前是否存在可观测的亚失稳应力状态进行了研究.获得的震源附近紫坪铺水库区域临震前应力状态时空演化过程显示,水库测震台网开始运行的2004年8月至2007年6月期间该区域归一化有效平均应力水平相对较低且均匀分布,随时间无明显变化;而2007年6月后至汶川主震发生前应力水平局部显著增强.基于不同时段的定量应力进一步获得了随时间演化的增量场,显示2007年6月后除了汶川附近和彭县附近为应力增强区,其他大部分区域表现为幅值相对较低的应力释放,表明研究区在2007年6月后应力水平明显弱化;而在2007年6月之前的应力增量场却显示区域应力随时间以积累为主.根据应力演化特征及分时段的应力增量场可推断孕震区在2007年中期达到强度极限,峰值前区域应力随时间平稳变化并以积累为主,而峰值后则转变为明显的应力释放.该现象显示汶川地震前区域应力存在由积累为主转为释放为主的可观测阶段,符合实验研究中断层由稳态进入亚失稳状态的应力演化特征.结合其他研究结果,推测汶川地震前震源断层及附近区域存在联动加速变形过程,与震前亚失稳应力状态相关.     

基于应力偏量的横波纯应力逆时偏移成像方法

基于弹性波解耦延拓方程的波场分离方法不仅可以得到解耦的纵、横波质点振动速度场,还可以获得纵、横波应力场.针对横波纯应力场在利用单一分量成像时不具有明确物理意义的问题,本文将应力偏张量引入到横波应力场中,基于应力偏量第二不变量构建得到横波应力不变量并将其用于应力场的逆时偏移成像中,获得了可完整表征横波应力场的成像结果.模型试算表明,本文构建的横波应力不变量可以有效利用横波应力张量中的波场信息,并得到准确的弹性逆时偏移成像结果.     

青岛崂山地区环境应力与视应力变化分析

利用青岛市地震监测中心数字台网的地震波形记录,选取震级下限为ML1.0,计算了2003~2004年崂山地震序列过程中青岛地区地震视应力aσpp和环境剪应力τ0值的变化,结果表明,崂山地震序列的中等地震发生前相近震级小震所反映出来的应力增强变化并不明显,2003年以来青岛地区背景应力场也不存在明显的趋势性变化。对青岛崂山地震序列中261个小震的数字化波形记录计算分析后发现,环境剪应力0τ值和视应力aσpp值与震级明显相关,并且相同震级的地震事件所计算出二者的变化与地震序列的活动状态密切相关。     

鲜水河断裂带库仑应力演化与强震间关系

以鲜水河断裂带为研究区,首先验证了该断裂带上1893年以来M6.7以上地震的相互触发作用,然后采用更符合实际的分层黏弹介质模型研究强震震后黏滞松弛引起的库仑应力变化对后续地震的影响,并基于负位错理论计算鲜水河断裂带10个断层段的震间长期构造加载作用引起的断层上的应力积累.在此基础上,讨论同震、震后、震间效应引起的累积库仑应力变化与区域强震活动的关系,给出断层上库仑应力随时间的演化.结果表明,鲜水河断裂带上1893年以来发生的7次强震均在其前面一系列强震及构造应力加载的驱使下发生,同震、震后、震间三方面效应均引起了鲜水河断裂带不可忽略的库仑应力变化.对断层上的库仑应力状态的研究,可进一步为揭示地震的发生规律、找寻危险断层段提供线索.