Principal horizontal stresses in Southern Africa

A review ofin situ stress measurements at eight regional localities in Africa south of the 15°S parallel shows that average directions of the horizontal pricipal stresses are N-S and E-W. These directions agree with principal stress orientations deduced from earthquake fault plane solutions. However, the maximum and minimum principal horizontal stresses are not consistently oriented parallel to either the N-S or E-W direction; they may vary within an individual region because of local geological structures and from region to region. At the sites within the Witwatersrand sediments (all at depths greater than 500 m) the maximum stress tends to lie NW-SE but at three of the four sites outside the Witwatersrand sediments (all at depths less than 500 m) this stress is oriented approximately N-S.The data reported here are compared with horizontal stresses predicted for Southern Africa bySolomon, Sleep andRichardson (1975) from various plate tectonic driving force models. The agreement between orientations is fair for all sites but only the deép sites in the Witwatersrand sediments have comparable stress magnitudes.     

Reflections on measurement of residual stress in rock

Consideration of the behaviour of elastic bodies shows that it is not possible for a set of overcoring measurements that are made within isolated blocks to show residual strains or stresses that have a non-zero average, unless the size of the equilibrium volume over which the residual stresses balance is both considerably larger than the volume of the overcore and smaller than the size of the isolated block. Since some results have been reported that do not match these constraints, non-elastic behaviour must have occurred during overcoring. A possible explanation, is that oriented microcracks are somehow opened by the overcoring. In some cases, stress fields induced by microcracking near the overcoring cut may explain measured strain changes far from the cut. Consideration of various reported measurements in terms of relative sizes of overcoring and equilibrium volumes shows that care is necessary when interpreting residual stress measurements.     

Anomalous seismicity prior to rock bursts: Implications for earthquake prediction

Anomalous seismicity changes (increase followed by a decrease) were recorded prior to three moderate rock bursts in the Star mine, Burke, Idaho. In each case, based upon the anomalous seismicity behavior, miners were evacuated or were prohibited from entering active mine stopes that were located in the immediate vicinity of the seismicity buildup prior to the bursts. Analyses of pre- and post-seismic activity are interpreted in terms of, and shown to be consistent with, the inclusion theory of failure. Implications of these observational results for the problem of rock bursts and earthquake prediction are discussed.     

A mechanism for strain relaxation of barre granite: Opening of microfractures

Summary Strain relaxation in the Barre Granite and surrounding metasediments in Vermont, was measured by overcoring strain gauge rosettes bonded to outcrop surfaces. The average maximum expansion upon relieving 15.2 cm diameter cores trends N55°W, while the average maximum expansion of 7.6 cm diameter cores coaxial with 15.2 cm cores trends N70°W. The maximum strain relief of the internal overcores is normal to the microfracture fabric. Therefore, the mechanism of strain relaxation is attributed to the opening of microfractures either parallel to the rift direction of the Barre Granite or parallel to the foliation of the metasediment. The lack of parallelism between the normal to the rift plane and the maximum expansion of the initial overcore suggests an extenrally applied strain superimeposed on the strain caused by opening of microfractures.Lamont-Doherty Geological Observatory Contribution No. 2455.     

Geokinematics of Central Europe: New insights from the CERGOP-2/Environment Project

The Central European Geodynamics Project CERGOP-2, funded by the European Union from 2003 to 2006 under the 5th Framework Programme, benefited from repeated measurements of the coordinates of epoch and permanent GPS stations of the Central European GPS Reference Network (CEGRN), starting in 1994. Here we report on the results of the systematic processing of available data up to 2005. The analysis has yielded velocities for some 60 sites, covering a variety of Central European tectonic provinces, from the Adria Indenter to the Tauern Window, the Dinarides, the Pannonian Basin, the Vrancea Seismic Zone and the Carpathian Mountains. The estimated velocities define kinematical patterns which outline, with varying spatial resolution depending on the station density and history, the present-day surface kinematics in Central Europe. Horizontal velocities are analyzed after removal from the ITRF2000 estimated velocities of a rigid rotation accounting for the mean motion of Europe: a 2.3 mm/year north–south oriented convergence rate between Adria and the Southern Alps that can be considered to be the present-day velocity of the Adria Indenter relative to the European Foreland. An eastward extrusion zone initiates at the Tauern Window. The lateral eastward flow towards the Pannonian Basin exhibits a gentle gradient from 1 to 1.5 mm/year immediately east of the Tauern Window to zero in the Pannonian Basin. This kinematic continuity implies that the Pannonian plate fragment recently suggested by seismic data does not require a specific Eulerian pole. On the southeastern boundary of the Adria microplate, we report a velocity drop from 4 to 4.5 mm/year motion near Matera to 1 mm/year north of the Dinarides, in the southwestern part of the Pannonian Basin. A positive velocity gradient as one moves south from West Ukraine across Rumania and Bulgaria is estimated to be 2 mm/year on a scale of 600–800 km, as if the crust were dragged by the counterclockwise rotation along the North Anatolian Fault Zone. This regime apparently does not interfere with the Vrancea Seismic Zone: earthquakes there are sufficiently deep (>100 km) that the brittle deformation at depth can be considered as decoupled from the creep at the surface. We conclude that models of the Quaternary tectonics of Central and Eastern Europe should not neglect the long wavelength, nearly aseismic deformation affecting the upper crust in the Romanian and Bulgarian regions.     

北天山中东段活动断层滑动与现代构造应力场

利用北天山中段地区主要活动断裂关键构造部位测得的断层滑动数据,采用断层滑动资料反演构造应力张量的方法,确定了该区现代(最新构造变动时期)构造应力场的基本特征,结果是:应力方向表现为近南北向的挤压;应力结构以逆冲型为主,兼有走滑型。该结果与前人利用震源机制解资料反演得到的区域构造应力场的基本特征一致。两类不同应力资料反演得到的构造应力场特征具有较好的一致性,表明在一个较长的地质时期内研究区的构造应力作用比较稳定。     

Susceptibility of lakeside phragmites reeds to environmental stresses: Examples from Lake Constance-Untersee (SW-Germany)

The lakeside border of Phragmites reeds is under the control of many environmental factors, among which the flooding height and the degree of mechanical load by waves and washes are the most prominent. Examples from Lake Constance-Untersee show that the reed front retreats under enhanced stresses. Models are presented that describe the susceptibility of lakeside reeds to mechanical damage and to flooding events. A monitoring programme which ran from 1984/85 to 1992/93 showed a slow lakeward progression of the reeds in a decade of abnormally low water conditions.     

2019年6月17日四川长宁6.0级地震中心震源机制解及震源区构造应力场研究

不同资料和方法给出的2019年6月17日四川长宁6.0级地震震源机制解存在较大差异,为了找到1个合适的震源机制解来研究此次地震的发震方式,通过数学方法得到了与现有震源机制解差别最小的中心震源机制解,节面I的走向、倾角、滑动角分别为194.78°、52.68°和139.16°,节面Ⅱ的走向、倾角、滑动角分别为312.44°、58.67°和45.22°,根据本次地震余震分布拟合得到的断层面的走向为312.17°,与中心震源机制的节面Ⅱ走向一致,因而推断节面Ⅱ为本次地震的发震断层面。之后,利用此次地震之前震源区地震的震源机制解,反演了震源区的震前构造应力场。结果表明,长宁6.0级地震的中心震源机制解和震源区震前应力场均为逆冲型为主兼走滑分量的类型,震前应力场压轴为NWW—SEE向,中间轴为NNE—SSW向,两轴倾角接近水平,而张轴较陡,表现为逆冲型的应力场。将反演得到的应力场投影到中心震源机制解给出的与余震分布一致的节面上,发现中心震源机制解的滑动角和应力场预测的滑动角差别仅为13.45°,表明此次地震受背景应力场控制而发生在先存的薄弱面上。     

汶川Mw7.9和集集Mw7.6地震前应力场转换现象及其可能的前兆意义

2008年四川汶川Mw7.9地震和1999年台湾集集Mw7.6地震均为挤压推覆构造环境下发生的板内逆断层型地震.通过对比分析2次地震前的CMT解、震源区附近的中小地震震源机制解及其反演的应力场可知,集集地震主震震源机制解与用台湾内陆中西部的CMT解反演得到的逆断层类型构造应力场吻合,而在主震前震源区附近中小地震震源机制...     

晋冀豫交界构造应力场分析

筛选晋冀豫交界2008-2017年468个地震的1 143个P波初动符号,应用综合震源机制解方法,反演该区域构造应力场,结果表明：①晋冀豫交界精细应力场整体受NE向P轴和NW向T轴控制,且P轴和T轴倾角近水平;②薄壁断裂以东出现局部应力场紊乱,安阳南断裂东南部应力场P轴由NE转为NEE至EW向,T轴呈放射状,从NW向逐渐转为NWW向,且倾角较大;③晋冀豫交界除符合华北应力场整体特征外,还具有局部构造应力场独特性。     

Recent tectonic stress field and major earthquakes of the Bohai Sea basin

Introduction The present Bohai Sea is a half-closed shallow one in the continent, located at the northeast to North China, with an area about 7.3104 km2. Geologically, it is situated in the northern North China basin and of a short development history. Previous studies (WANG, LI, 1983; Institute of Oceanology, Chinese Academy of Sciences, 1985; HUANG, et al, 1993) show that the Fu-jian-Lingnan uplift in the East China Sea continental shelf sank gradually into the oceanic bottom, mak…     

Review study on near-natural conditions of Central European mountain streams, with particular reference to debris and beaver dams: Results of the “REG meeting” 2000

This paper summarises the results of the “Rhithron Ecology Group” meeting in Essen (March 2000), supplemented by a literature evaluation.An extended view of small mountain streams in Central Europe under “potentially natural conditions” is presented. We focus on the potential impact of natural transverse structures (debris dams and beaver dams) on stream morphology, hydrology, habitat composition and communities. Furthermore, impact of other stream morphological features, which are presently lacking in the Central European landscape, is described.We suggest that Central European mountain streams are characterised by a higher proportion of lentic zones under “potentially natural conditions”. Morphological degradation leads to a loss of lentic zones and to an increase of current velocities and corresponding changes in faunal composition, particularly higher abundance and proportion of rheophilic species and a decrease of species preferring lentic zones. This should be regarded in future stream assessment.     

四川盆地荣县—威远—资中地区发震构造几何结构与构造变形特征:基于震源机制解的认识和启示

四川盆地荣县—威远—资中地区属于历史弱震区,然而2019年相继发生多次破坏性地震事件.本文基于四川区域地震台网宽频带地震仪记录波形资料,利用CAP （Cut and Paste）波形反演方法,获得了2016年以来发生在荣县—威远—资中地区的26个M_S≥3.0地震的震源机制解、震源矩心深度和矩震级,对该区域发震构造几何结构与变形特征及构造应力场特征进行了初步分析.主要获得如下认识：（1）26个M_S≥3.0地震的震源矩心深度在1.5~5 km之间,平均深度3.4 km,表明事件发生在上地壳浅部沉积层内;震源深度分布揭示发震断层面倾向SE、缓倾角.（2）26个地震的震源机制全部为逆冲型,表明发震构造整体为逆断层性质.节面优势方位NNE-NE,结合走向与倾角统计结果,本文推测发震构造可能为威远背斜南翼一系列倾向SE、走向NNE-NE的缓倾角盲冲断层.（3）P、T、B轴优势方位单一,表明研究区域处于相对简单的构造应力环境.区域应力场反演获得的最大主压应力轴σ₁方位NW-SE,近水平,与目前已知的该区域构造应力场水平主压应力方向一致,反映区内构造活动主要受区域构造应力场控制;其明显有别于四川盆地南缘2019年6月17日长宁M_S6.0地震余震区NE-SW向的最大主压应力轴方位也揭示出四川盆地构造应力场具有明显的分区特征.（4）26个地震整体的应变花表现为NW-SE向挤压白瓣形态,表明区内发震构造整体呈NW-SE向纯挤压变形模式,明显有别于2019年长宁M_S6.0地震序列NE-SW向挤压兼具小量NW-SE向拉张分量的构造变形模式,进一步表明四川盆地构造变形模式也具有明显的分区特征.     

Alpine tectonic evolution of a Jurassic subduction-accretionary complex: Deformation,kinematics and Ar/Ar age constraints on the Mesozoic low-grade schists of the Circum-Rhodope Belt in the eastern Rhodope-Thrace region,Bulgaria-Greece

Deformation of the Circum-Rhodope Belt Mesozoic (Middle Triassic to earliest Lower Cretaceous) low-grade schists underneath an arc-related ophiolitic magmatic suite and associated sedimentary successions in the eastern Rhodope-Thrace region occurred as a two-episode tectonic process: (i) Late Jurassic deformation of arc to margin units resulting from the eastern Rhodope-Evros arc–Rhodope terrane continental margin collision and accretion to that margin, and (ii) Middle Eocene deformation related to the Tertiary crustal extension and final collision resulting in the closure of the Vardar ocean south of the Rhodope terrane. The first deformational event D₁ is expressed by Late Jurassic NW-N vergent fold generations and the main and subsidiary planar-linear structures. Although overprinting, these structural elements depict uniform bulk north-directed thrust kinematics and are geometrically compatible with the increments of progressive deformation that develops in same greenschist-facies metamorphic grade. It followed the Early-Middle Jurassic magmatic evolution of the eastern Rhodope-Evros arc established on the upper plate of the southward subducting Maliac-Meliata oceanic lithosphere that established the Vardar Ocean in a supra-subduction back-arc setting. This first event resulted in the thrust-related tectonic emplacement of the Mesozoic schists in a supra-crustal level onto the Rhodope continental margin. This Late Jurassic-Early Cretaceous tectonic event related to N-vergent Balkan orogeny is well-constrained by geochronological data and traced at a regional-scale within distinct units of the Carpatho-Balkan Belt. Following subduction reversal towards the north whereby the Vardar Ocean was subducted beneath the Rhodope margin by latest Cretaceous times, the low-grade schists aquired a new position in the upper plate, and hence, the Mesozoic schists are lacking the Cretaceous S-directed tectono-metamorphic episode whose effects are widespread in the underlying high-grade basement. The subduction of the remnant Vardar Ocean located behind the colliding arc since the middle Cretaceous was responsible for its ultimate closure, Early Tertiary collision with the Pelagonian block and extension in the region caused the extensional collapse related to the second deformational event D₂. This extensional episode was experienced passively by the Mesozoic schists located in the hanging wall of the extensional detachments in Eocene times. It resulted in NE-SW oriented open folds representing corrugation antiforms of the extensional detachment surfaces, brittle faulting and burial history beneath thick Eocene sediments as indicated by 42.1–39.7 Ma ⁴⁰Ar/³⁹Ar mica plateau ages obtained in the study. The results provide structural constraints for the involvement components of Jurassic paleo-subduction zone in a Late Jurassic arc-continental margin collisional history that contributed to accretion-related crustal growth of the Rhodope terrane.     

Annealing algorithms in computing stress tensors: A set of numerical simulations for local stress tensor recovery

The determination of a local lithospheric stress pattern from low magnitude seismic activity recorded at a loca array is attempted by means of numerical simulations that include waveform inversion and annealing inversion techniques. A set of flat stratified layers are assumed for the propagating medium. The tests show that the annealing algorithm is a useful tool in this kind of study where collected seismic activity at local arrays could help us better understand local stress distributions in the lithosphere. Although real data recorded at local seismic arrays should be processed, assuming more realistic crust and upper mantle models, many characteristics of the annealing process described here could be used when studying real problems.     

In situ stress state in Mainland China

In situ stress measurements are one of the important methods for studying the recent tectonic stress field and stress state. In this paper, the stress state and variation of stresses with depth in Mainland China are presented and analyzed on the basis ofin situ stress data measured by both hydrofracturing and overcoring techniques.     

河北临城地震序列揭示的一条隐伏断层

2017年9月4日河北临城发生ML4.4地震,这是邢台地区自2003年以来发生的唯一一次ML4以上地震.震后大量余震沿条带分布,揭示了一条前人未发现的隐伏断层(根据其经过的地点称之为齐家庄-东双井断裂).为研究该隐伏断层的几何形状和滑动性质,首先基于河北数字地震台网资料对地震序列进行精定位,利用精定位地震数据拟合发震断...     

汞胁迫对菹草无菌苗生理代谢的影响

以菹草(Potamogeton crispus L.)无菌苗为实验材料,研究其在不同浓度汞(Hg²⁺)的胁迫下,光合色素含量、叶绿体自发荧光强度、ATP酶活性及相关矿质营养元素(Ca、Mg、Na、K、Fe)的含量变化以及可溶性蛋白含量的变化,以探讨Hg²⁺胁迫对菹草无菌苗光合系统及相关生理指标变化的影响.研究表明:随着Hg²⁺浓度的增加,各种光合色素的含量均呈现下降的趋势,叶绿体自发荧光强度逐渐减弱;Na/K逐渐增大,Na⁺ K⁺-ATP酶活性逐渐降低,Ca²⁺ Mg²⁺-ATP酶活性呈先升后降趋势;可溶性蛋白含量先升后降.结果表明,菹草无菌苗在Hg²⁺胁迫下,光合及其它正常的生理过程遭到破坏,功能紊乱.     

九寨沟M_S7.0地震静态库仑应力触发及对附近断层影响分析

计算和研究了1973~2013年巴颜喀拉块体东边界发生的6次M_S≥6.5地震引起的静态库仑应力变化图像,认为先前发生的6次地震对2017年8月8日九寨沟M_S7.0地震具有触发作用。九寨沟M_S7.0地震发生后,对震中附近主要断裂应力转移的计算结果显示,在巴颜喀拉块体东边界7次M_S≥6.5地震的共同作用下,东昆仑断裂带东段及龙日坝断裂带库仑应力水平升高,具有触发作用;岷江断裂带处于库仑应力影区内,具有减震效应。     

2018年宁强MS5.3地震前重力场变化特征分析

利用陕甘川地区2016—2018年5期流动重力观测资料,分析了陕西宁强MS5.3地震前半年和1年尺度的重力场变化特征。同时采用小波多尺度分解方法,对宁强MS5.3地震震中区的布格重力异常进行1～4阶小波分解,并对1～4阶小波细节图像进行分析。研究结果表明:①宁强MS5.3地震重力场呈现“正向变化—正向减弱—负向变化—负向减弱”演化过程,震前重力场呈现四象限分布特征,震中位于鞍部位置,符合走滑型地震特征;②布格重力异常2～3阶小波细节对宁强MS5.3地震孕震特征有较好的反映,对应的场源深度为10～15km,与本次地震震源深度11km有较好的对应关系。