Magnetic fabric and strain in Miocene volcanics in the southern margin of the Bükk Mountains, Hungary

The magnetic anisotropy of two Miocene volcanic horizons south of the Bükk Mountains, northeast Hungary, was investigated (28 geographically distributed sites, over 400 samples). The horizons contain rocks from ignimbrites to porous tuffs. Microscopic and rock magnetic investigations showed that the main carrier of anisotropy is multidomain maghemite with varying composition. The complex magnetic fabric was separated into deformational and flow fabric in the upper horizon. This was carried out by monitoring the distribution of minimum susceptibility directions supposed to be elongated towards intermediates as a response to compressional strain. Jelínek's tensor statistics were used in the computations. Filtering out the effects of volcanic flow and compaction, north-south striking horizontal compressional strain was determined for the upper horizon in the geographic system. Owing to the lack of elongated distribution of minima, the characteristic intermediate susceptibility directions were taken as markers of strain directions in the lower horizon (WNW-ESE and NNE-SSW in the geographic system). The strain directions showed correlation with compressional stress orientations observed by microtectonics, thus a homogeneous stress-strain field was supposed within the area investigated. The tectonic component of the strain can be determined by taking the respective paleomagnetic declination rotations as the rotational components of the deformation.     

岩石磁组构可以揭示应变吗？——以华南地块早三叠世灰岩为例

一般认为磁组构能有效地反映岩石所经历的应变特征.为了研究不同类型的磁组构和不同期次应变之间的关系,对来自华南地块两个地区的早三叠世灰岩样品进行了岩石磁学、磁组构以及应变特征的对比分析.来自湖北通山县的样品经历了三期构造变形,这为解析磁组构和多期次应变提供了理想的机会.岩石磁学结果显示携磁矿物主要为磁铁矿.磁化率各向异性（AMS）和非磁滞剩磁各向异性（AAR）结果显示其最小轴与层面垂直,最大轴和中间轴分布于层面内,反映了沉积和压实作用产生的应变,而后期构造应变在磁组构中没有体现.来自广东连县的样品发育有渗透性压溶缝面理和方解石脉,说明经历了构造应变.AMS结果没有显示占优势的组构方向.AAR结果显示三轴组构,其最大轴分布于最大应力方位,与构造应变特征吻合,最初的压实组构被构造应变组构所代替.上述结果表明：（1）AAR可以很好地反映渗透性应变的特征,而AMS有时会失效;（2）应变的尺度要小于样品的尺度,磁组构才能有效地反映应变.     

A preliminary study of thermally enhanced magnetic fabric in the Tertiary sediments from the Qaidam basin,NW China

Summary A preliminary observation has been made of the magnetic fabric in 14 specimens of undeformed sediment which exhibit repeatable principal orientation in undemagnetized state as well as after some steps of thermal demagnetization. They were chosen from 2000 specimens of the Tertiary formations of inland fluvio-lacustrine facies in the Qaidam basin, NW China. It was found that no large changes in magnetic fabric occur after stepwise thermal demagnetization up to 300°C, but some significant changes take place after treatment at higher temperature (500° up to 750°C). It is shown that the orientations of the 3 principal axes remain unchanged after each demagnetization step, whereas the differences between the principal values (L=K1–K2 and F=K2–K3) are greatly enhanced after high temperatures have been applied. The enhancement of L and F with no change in principal orientations facilitates the determination of the principal orientations. In our case one may easily measure the thermally enhanced magnetic lineation to estimate the palaeocurrent direction.Presented at 2nd conference on New Trends in Geomagnetism, Bechyn Castle, Czechoslovakia, September 24–29, 1990.     

利用漳州深井4分量钻孔应变求测区应变场

利用漳州深井形变综合观测中的4分量钻孔应变观测数据,计算了测区的主应变大小和方向。计算得到的上下两个端面的主应变方位角都在170°附近变化,近东西向性质明显。与利用多个地震求解综合节面解推断出的构造应力场一致。     

Crustal Stress Map of Iran: Insight From Seismic and Geodetic Computations

We used the focal mechanisms of crustal earthquakes (depth <40 km) in the period 1909–2012 and the available GPS velocities, derived from the data collected between 1999 to 2011, to estimate the magnitude and directions of maximum principal stress and strain rates in Iran. The Pearson product moment correlation was used to find the correlation between the stress field obtained from the focal mechanism stress inversion and that obtained using the seismic and geodetic strain rates. Our assumption is that stresses in a continuum are produced by tectonic forces and the consequent deformation on the crustal scale. Therefore, the direction of the stress and strain (or strain rate) are ideally to be the same. Our results show a strong correlation between the directions of the principal components of stress and strain (rate) obtained using the different data/methods. Using weighted average analysis, we present a new stress map for Iran.     

Three-dimensional finite strain analysis in the high-grade part of the Sanbagawa Belt using deformed meta-conglomerate

Abstract   Regional ductile deformation of the Sanbagawa belt is generally thought to be characterized by constrictional strain, based on strain analysis using deformed radiolarians in the low-grade regions. Similar strain analysis could not be carried out in the medium- to high-grade zones, because it is very difficult to identify individual radiolarians after strong recrystallization. However, discovery of the first known meta-conglomerate in the high-grade region of the Sanbagawa Belt allows quantitative 3-D strain to be estimated in this region. Using a development of the Rf-φ method, an evaluation of appropriate errors for this estimate can be determined. The principal strain ratios and estimated errors are X/Y = 5.4–6.6 and Y/Z = 3.8–3.9 implying deformation in the flattening field and refuting the idea of uniform constrictional strain. Semi-quantitative markers of the shape of the strain ellipse throughout the high-grade regions suggest that the deformation of the Sanbagawa Belt is dominantly in the flattening field. The difference with the earlier results may be due to late-stage overprinting by upright folding of the main ductile fabric in the low-grade region of western Shikoku.     

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.     

对中国大陆地壳水平变形的初步探索

根据全国ＧＰＳ网１９９４和１９９６年两期观测资料的处理结果,讨论了中国大陆地区现阶段应变场和应力场。青藏亚板块的西部和东部张应变起主导作用,中部压应变占优势,主压应变方向为北北东向;青藏亚板块东南部东南段云南地区的主压应变方向为北西向,压应变和张应变量级相当。新疆亚板块的主压应役北北东向至北东向为主,应变量存在差别。华北亚板块的主压应变方向是北东至近东西向为主导,局部地段存在差别,大部分地区压应变     

Study on the Horizontal Deformation Strain Field in the Central and Northern Parts of Yunnan Province

Based on the horizontal deformation field and the strain field derived from the GPS data over the period of 1999～2001 in the Yunnan area, the characteristics of deformation and strain in the northern part of Yunnan Province have been studied. The results indicate that the central part of the studied area is rather stable with little crustal displacement, while the western and eastern parts are active with larger displacement. The strain field reveals that the orientations of the principal compressive strain axis of the crust and the sub-blocks in the area are NW-SE, while the orientations of the principal tensile strain axis is NE-SW. In the studied area, the tensile strain is predominatly in the northern part and the compressive strain is predominatly in the central and southern parts. The stretching direction of the shear-strain contour is basically consistent with the strike of the active fault. The strain and stress fields of the fault activity are related to the structure where the fault is located, while the activity properties of the faults are different.     

First estimates of the principal directions of strains compared to those of stresses in seismic friuli,NE Italy

Three Cambridge wire strainmeters and a pair of horizontal-pendulum tiltmeters operate since 1978, in a cave in the northern Friuli seismic area. Straightforward considerations, based on the principles of continuum mechanics, permit a first estimate of the secular and seasonal variations of the cave strain rate tensor, under the assumption of homogeneous, constant-volume strain, together with an analysis of the effects on the tensor components of particular tiltmeter installation sites with respect to the orientation of both the strainmeters plane and the principal axes.The area lies at the complex intersection of NW-SE Dinaric rock structures and E-W Alpine ones. The latest period of strong seismic activity there started in mid-1976. The fault-plane solutions of the main shock revealed a direction of maximum compression perpendicular to the Alpine trend, but later in 1976 and 1977 such a direction seemed to have changed, having become perpendicular to the Dinaric trend.A plain analysis of the strain gauge data show that, from 1978 to 1981, the maximum shortening secular strain changed progressively from a SW bearing to a SSE one or even SE one. Hence, it would appear that by 1981 the direction of maximum compression had become Alpine again.In the same time interval, the principal axes of the five observed yearly seasonal strain rates maintained a Dinaric orientation similar to that of the geological structures around the cave.Finally the orientations of the secular strain acceleration principal axes appear to be nearly coaxial with those of the stress obtained from various 1977 fault-plane solutions, the compressional stress corresponding to extensional accelerations andvice-versa. Such observation suggests that the modifications of secular strain rates are related to the stress release connected with the seismic activity.     

Characterization of Damage in Sandstones along the Mojave Section of the San Andreas Fault: Implications for the Shallow Extent of Damage Generation

Following theoretical calculations that suggest shallow generation of rock damage during an earthquake rupture, we measure the degree of fracture damage in young sedimentary rocks from the Juniper Hills Formation (JHF) that were displaced 21 km along the Mojave section of San Andreas Fault (SAF) and were not exhumed significantly during their displacement. In exposures adjacent to the fault, the JHF typically displays original sedimentary fabrics and little evidence of bulk shear strain at the mesoscopic scale. The formation is, however, pervasively fractured at the microscopic scale over a zone that is about a 100 m wide on the southwest side of the SAF near Little Rock. The abundance of open fractures, the poor consolidation, and the shallow inferred burial depth imply that the damage was generated close to the surface of the Earth. The spatial correlation of this damage with a seismically active trace of the SAF suggests that it was generated by SAF slip events that by assumption were of a seismic nature throughout the displacement history of the JHF. Thus the JHF provides a very shallow upper bound for the generation of brittle damage in a seismic fault zone. The fracture fabric is characterized by preferred orientations of fractures that split grains between contact points and is consistent with overall deformation under directed compression. However, the available results cannot be used to distinguish between proposed off-fault damage mechanisms. Fracture orientations are compatible with a maximum compressive stress oriented at a high angle to the fault at about 10 m, and at a lower, more variable angle farther away from the fault. The fracture distribution and fabric are consistent with observations made of the microscale damage characteristics of the Hungry Valley Formation in the northwestern section of the SAF in the Mojave, and with previous observations of exhumed, ancestral strands of the SAF.     

青海门源MS6.9地震前后甘青地区应力变化特征研究

选取甘—青地区地球物理台网钻孔应变2021年1月至2022年1月数据计算其面应变相关系数、主应变、主方位角并进行投影,以面应变相关系数和数据精度进行质量控制,以主应变、主方位角及其地图投影分析甘—青区域钻孔应变的空间分布特征和时间变化特征。分析发现2021年主应变、主方位角及其地图投影变化能清晰地映证该区域构造运动,2022年1月8日门源6.9级地震时钻孔应变主方位角、最大主应变投影同震变化与此次地震地表同震位移场、应变场变化一致。上述发现为研究区域地质构造变化,对该地区地震危险性跟踪及短临跟踪提供参考。     

对当前四分量钻孔应变观测的审视——以应变不变量为标尺

本文采用摩尔圆模型推导出中国和美国3种不同类型的四分量不同布设的两套钻孔应变的5组观测组合公式,随机选取了25个台站多年的观测资料计算5组最大、最小主应变及主方向,并采用两两相关和回归分析的方法对其一致性进行了检核.计算结果显示:经过大致1—2年时间后,应变不变量变化稳定,主应变及主方向越来越接近;大多数台站5组最大、最小主应变数值之间的相关系数和回归系数接近于1,表明这些仪器记录的应变数据具有客观真实性.5组主方向数值相差较大,本文对此进行了分析,并给出了5组观测组合公式的误差估计,发现主应变计算值的最大均方根误差是有上限的,主方向没有上限.最后对影响钻孔应变5组不变量一致性的一些因素进行了初步探讨.研究结果表明钻孔应变观测技术有其特殊的困难,在经历了半个世纪实践后,取得了重要进展,但仍有一些技术问题亟待解决.     

The origin of accretionary lapilli

Experimental investigations in a recirculating wind tunnel of the mechanisms of formation of accretionary lapilli have demonstrated that growth is controlled by collision of liquid-coated particles, due to differences in fall velocities, and binding as a result of surface tension forces and secondary mineral growth. The liquids present on particle surfaces in eruption plumes are acid solutions stable at 100% relative humidity, from which secondary minerals, e.g. calcium sulphate and sodium chloride, precipitate prior to impact of accretionary lapilli with the ground. Concentric grain-size zones within accretionary lapilli build up due to differences in the supply of particular particle sizes during aggregate growth. Accretionary lapilli do not evolve by scavenging of particles by liquid drops followed by evaporation — a process which, in wind tunnel experiments, generates horizontally layered hemispherical aggregates. Size analysis of particles in the wind tunnel air stream and particles adhering to growing aggregates demonstrate that the aggregation coefficient is highly grain-size dependent. Theoretical simulation of accretionary lapilli growth in eruption plumes predicts maximum sizes in the range 0.7–20 mm for ash cloud thicknesses of 0.5–10 km respectively.     

Influence of a cyclic and dynamic loading history on dynamic properties of dry sand, part II: cyclic axial preloading

Initial fabric of a soil induced by its cyclic strain history is an important parameter among others like void ratio, state of stress and amplitude concerning the further accumulation of deformations under cyclic loading. It is of high importance to determine methods in order to estimate the initial fabric of the grain skeleton or the cyclic loading history of the soil. An attempt is made in this paper to correlate small strain stiffness of non-cohesive soil with its cyclic loading history. For this purpose several cyclic triaxial tests with specimens subjected to cyclic axial loading were performed and changes of small strain soil properties due to this cyclic loading were studied. All these tests showed only moderate changes of small strain stiffnesses independently of the different boundary conditions. Thus, a correlation between fabric or strain history and small strain stiffnesses seems not to be possible.     

Internal structure and occurrence of accretionary lapilli — a case study at Laacher See Volcano

Accretionary lapilli are common in fine-grained pyroclastic flow and surge deposits and related co-ignimbrite/co-surge ash layers of Laacher See volcano. Two morphologically different types are distin-guished: (1) Rim-type lapilli are composed of a coarse-grained core surrounded by a fine-grained rim. Rims are internally graded or made up of several layers of alternating fine and very-fine grained ash. (2) Core-type lapilli lack fine-grained rims. Field relationships, internal, and grain-size characteristics are specific to accretionary lapilli from different types of tephra deposits. Accretionary lapilli may therefore be a helpful tool to infer the origin of tephra of different origin. In co-ignimbrite ashfall, accretionary lapilli are generally concentrated at the base, whereas pyroclastic flow and surge deposits contain lapilli in the upper parts of individual, thin-bedded layers. Rim-type lapilli are found in pyroclastic flow and surge deposits up to 4 km from the source. Core-type lapilli occur at greater distances or are associated with vesiculated tuffs where they are within 1 km from the vent. Accretionary lapilli from co-ignimbrite/co-surge ash show open framework textures and edge-to-face contacts of individual ash particles. Vesicularity is generally low but the overall porosity of 40% to 50% results in an average density of 1200 kg/m³. Accretionary lapilli in pyroclastic flow and surge deposits are more densely packed and platy particles are often in face-to-face contacts. Vesicularity of those from pyroclastic flow deposits is significantly higher; the overall porosity is about 30% to 40% and the average density 1600 kg/m³. Grain-size analyses show that the accretionary lapilli in co-ignimbrite/co-surge ashfall deposits are the most fine-grained with a median (Md) of 20 to 30 m and a maximum grain size of 250 to 350 m. Accretionary lapilli from pyroclastic flow deposits have intermediate Md-values of 30 to 50 m and a maximum grain size of 350 to 500 m. Those of surge deposits are the coarsest grained with Md-values of 30 to >63 m and a maximum grain size up to 2 mm.     

Magnetic Anisotropy as an Aid to Identifying CRM and DRM in Red Sedimentary Rocks

To further evaluate the potential of magnetic anisotropy techniques for determining the origin of the natural remanent magnetization (NRM) in sedimentary rocks, several new remanence anisotropy measurement techniques were explored. An accurate separation of the remanence anisotropy of magnetite and hematite in the same sedimentary rock sample was the goal.In one technique, Tertiary red and grey sedimentary rock samples from the Orera section (Spain) were exposed to 13 T fields in 9 different orientations. In each orientation, alternating field (af) demagnetization was used to separate the magnetite and hematite contributions of the high field isothermal remanent magnetization (IRM). Tensor subtraction was used to calculate the magnetite and hematite anisotropy tensors. Geologically interpretable fabrics did not result, probably because of the presence of goethite which contributes to the IRM. In the second technique, also applied to samples from Orera, an anisotropy of anhysteretic remanence (AAR) was applied in af fields up to 240 mT to directly measure the fabric of the magnetite in the sample. IRMs applied in 2 T fields followed by 240 mT af demagnetization, and thermal demagnetization at 90°C to remove the goethite contribution, were used to independently measure the hematite fabric in the same samples. This approach gave geologically interpretable results with minimum principal axes perpendicular to bedding, suggesting that the hematite and magnetite grains in the Orera samples both carry a depositional remanent magnetization (DRM). In a third experiment, IRMs applied in 13 T fields were used to measure the magnetic fabric of samples from the Dome de Barrot area (France). These samples had been demonstrated to have hematite as their only magnetic mineral. The fabrics that resulted were geologically interpretable, showing a strong NW-SE horizontal lineation consistent with AMS fabrics measured in the same samples. These fabrics suggest that the rock's remanence may have been affected by strain and could have originated as a DRM or a CRM.Our work shows that it is important to account for the presence of goethite when using high field IRMs to measure the remanence anisotropy of hematite-bearing sedimentary rocks. It also shows that very high magnetic fields (>10 T) may be used to measure the magnetic fabric of sedimentary rocks with highly coercive magnetic minerals without complete demagnetization between each position, provided that the field magnetically saturates the rock.     

Rating damage potential of ground motion records

In this study, a total of 115,246 ground motions recorded during earthquakes of Moment magnitudes ranging from M_w 5.0 to M_w 9.0 are analyzed statistically. A total of 21 ground motion parameters characterising the recorded acceleration time histories are used in the analysis. Classification of these parameters through statistical correlation is reported and a parameter called "distance from zero-amplitude axis," or d_Z-A, is formulated in the principal component space. The ability for d_Z-A to rate the damage potentials of strong motion records is evaluated through correlation of d_Z-A with Japan Meteorological Agency(JMA) instrumental seismic intensities. This parameter can be used to rate damage potential of any strong motion record irrespective of the magnitude and location of the earthquake. It can also be used in selecting ground motion records of appropriate damage potential in seismic design and probabilistic analysis.     

3-D consistency dynamic constitutive model of concrete

Based on the consistency-viscoplastic constitutive model,the static William-Warnke model with three-parameters is modified and a consistency-viscoplastic William-Warnke model with three-parameters is developed that considers the effect of strain rates. Then,the tangent modulus of the consistency viscoplastic model is introduced and an implicit backward Elure iterative algorithm is developed. Comparisons between the numerical simulations and experimental data show that the consistency model properly provides the uniaxial and biaxial dynamic behaviors of concrete. To study the effect of strain rates on the dynamic response of concrete structures,the proposed model is used in the analysis of the dynamic response of a simply-supported beam and the results show that the strain rate has a significant effect on the displacement and stress magnitudes and distributions. Finally,the seismic responses of a 278 m high arch dam are obtained and compared by using the linear elastic model,as well as rate-independent and rate-dependent William-Warnke three-parameter models. The results indicate that the strain rate affects the first principal stresses,and the maximal equivalent viscoplastic strain rate of the arch dam. Numerical calculations and analyses reveal that considering the strain rate is important in the safety assessments of arch dams located in seismically active areas.     

Microshear zones in a Miocene submarine dacite dome of southwest Japan

Microscopic shear zones have been found in the groundmass of glassy rocks of a Miocene submarine dacite dome in southwest Japan. Similar textures have been reported previously but only in dykes. These textures give a valuable insight into the deformation of the dome during its emplacement by recording the orientations of the principal strains. Detailed textural analysis indicates that the microshear zones formed as a result of flattening and stretching of the magma simultaneously with quenching. Measured stretching directions are near-parallel suggesting the magma flowed in one dominant direction rather than in a radial pattern. The strain is believed to also be influenced by high magma pressure inside the dome being imposed on the high viscosity outer part of the dome. Present address: Faculty of Resource Science and Management, University of New England-Northern Rivers. P. O.Box 157 Lismore NSW 2480 Australia