Induced seismicity in the Khibiny Massif (Kola Peninsula)

The topic of this paper is to review recent processes of increasing seismic activity in the Khibiny Massif in the Kcla Peninsula. It is a typical example of induced seismicity caused by rock deformation due to the extraction of more than 2·10⁹ tons of rock mass since the mid-1960s. The dependence of seismic activity on the amount of extracted ore is demonstrated. Some of the induced earthquakes coincide with large mining explosions, thus indicating a trigger mechanism. The largest earthquake, which occurred on 16 April 1989 (M _L= 4.1) could be traced along the surface for 1200 m and observed to a depth of at least 220 m. The maximum measured displacement was 15–20 cm.     

Study the elastic properties and the anisotropy of rocks using different machine learning methods

This paper aims to demonstrate that the elastic stiffnesses and the anisotropic parameters of rocks can be accurately predicted from geophysical features such as the porosity, the density, the compression stress, the pore pressure and the burial depth using relevant machine learning methods. It also suggests that the extreme gradient boosting method is the best method for this purpose. It is more accurate, extremely faster to train and more robust than the artificial neural networks and the support vector machine methods. Very high R-squared scores was obtained for the predicted elastic stiffnesses of a relevant dataset that is available in the literature. This dataset contains different types of rocks, and the values of the features are in large ranges. An optimal set of parameters was obtained by considering an appropriate sensitivity analysis. The optimized model is very easy to implement in Python for practical applications.     

郯庐断裂带中南段及邻区Pn波速度结构与各向异性

郯庐断裂带是一条纵贯我国大陆东部NNE走向的巨型深断裂,其中南段及邻区(115°E-122°E,29°N-38°N)跨越了华北断块区、扬子断块区和华南褶皱系三大一级构造单元,由于其重要性和复杂性,长期以来一直是地学家们研究的热点.本文从国际地震中心(ISC)、中国地震台网及区域地震台网的地震观测报告中精心挑选出6381个Pn震相数据,用Pn波时间项层析成像法反演得到了郯庐断裂带中南段及邻区上地幔顶部Pn波速度结构和各向异性.结果显示,研究区上地幔顶部具有显著的横向非均匀性,相对于7.95 km·s-1的平均速度而言,Pn波速度值在7.68~8.24 km·s-1范围内变化.Pn波速度分布在郯庐断裂带中段和南段具有分段性:沿中段及周边存在一NE向低速异常带,低速可能是由于岩石圈的减薄和软流圈的高温物质沿郯庐带上涌导致;沿南段表现为一NNE向弱高波速异常带,作为高低速的边界带清晰地勾勒出了华北与扬子这两个不同块体,该边界在江苏域向华北地块NW方向凹进.Pn波速度各向异性的强弱与速度分布存在一定的相关性.总体上,如鲁西隆起及以南等低速区、茅山断裂附近的高低速过渡带,其速度各向异性较为强烈;而在具有高速异常的苏北盆地、合肥盆地等稳定区域下方其各向异性较弱.本文通过Pn波震相基本未能探测到郯庐断裂带中段的方位各向异性,推测是上地幔顶部被"冻结"下来的各向异性痕迹被软流圈热物质上涌这一强烈构造运动削弱所导致.南段具有与断裂伸展方向近乎平行的快波速方向.Pn波速度横向变化和强震活动存在一定关联.强震主要发生在Pn波低速异常区或高低速过渡带上.郯城8.5级地震震中位于中段和南段高低速过渡带,该区域也是速度横向变化最大的地方,最容易集中应力和产生应力差.     

Physical property anisotropy of foliated fault rocks: Study from the Nobeoka Thrust,Shimanto Belt,southwest Japan

To investigate the physical property anisotropies of foliated fault rocks in subduction zones, the hanging wall phyllites and footwall cataclasites exhumed along the Nobeoka Thrust, a fossilized out‐of‐sequence‐thrust in the Shimanto Belt, Japan, was focused. Discrete physical property (electric resistivity, P‐ and S‐wave velocities, and porosity) measurements were conducted employing geologic coordinates (depth‐parallel direction, strike direction, and maximum dip direction of foliation), using the core samples obtained from the Nobeoka Thrust Drilling Project and compared the data to borehole geophysical logs. A higher sample P‐wave velocity (Vp), lower S‐wave velocity (Vs), higher Vp/Vs, and lower sample porosity and resistivity compared to the logs, are inferred to have been caused by the larger sampling scale of the logs and lower fluid saturation of the borehole. The phyllites and cataclasites exhibited substantial vertical and horizontal anisotropy of Vp (0.4–17.3 % and 2.7–13.8 %, respectively), Vs (0.5–56 % and 7.7–43 %, respectively), and resistivity (0.9–119 % and 2.0–65.9 %, respectively). The physical property anisotropies are primarily affected by the dip angles of foliation. The fault rocks that have gentler dip angles exhibit a higher Vp in the strike and maximum dip direction and a lower Vp in the depth‐parallel direction. In contrast, the fault rocks that have steeply dipping structures show a higher Vp in the strike and depth‐parallel directions with a lower velocity in the maximum dip direction. Resistivity anisotropy show a trend opposite to that of the Vp in relation to the dip angles. Our results show lower Vp anisotropy than those obtained in previous studies, which measured wave speeds perpendicular or parallel to foliation under confining pressure. This study highlights the significance of dip angles on vertical properties in geophysical surveys across foliated fault rocks.     

Characteristics of fault rocks and paleo-earthquake source along the Koktokay-Ertai fault zone,Xinjiang,China

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海原断裂带老虎山段断层岩磁学研究及其构造意义

断层岩,尤其是断层泥的磁性异常近年来被研究人员广泛关注,但关于其磁性异常的形成原因尚没有统一的解释.海原断裂是青藏高原东北缘一条重要的走滑断裂,前期研究发现海原断裂带景泰段出露有数十米至上百米的断层岩,是理想的研究材料.本研究选取海原断裂带景泰段老虎山山前一个断层岩剖面作为研究对象,拟通过测量断层岩的磁化率（χ）、非磁滞剩磁（ARM）、饱和等温剩磁（SIRM）、等温剩磁（IRM）以及磁化率随温度变化曲线（χ-T曲线）等磁学参数并结合粒度、碳含量、X射线衍射（XRD）等分析方法来探究海原断裂带老虎山段不同颜色断层岩的磁性特征及其形成机制.磁学研究显示黑色、红色及杂色断层泥相较于围岩和破碎带显示了低磁性,尤其是黑色断层泥,其磁化率值均小于10×10^-8m³·kg^-1.碳含量及矿物相分析结果指示黑色断层泥与断裂带附近石炭系煤层具有相似的矿物相组成,结合相似的χ-T曲线推断石炭系煤层为黑色断层泥的母岩.石炭系煤层经断层活动卷入断层,在断层强烈剪切摩擦作用下不断细化,形成伊利石等黏土矿物,并促使一部分顺磁性含铁硅酸盐矿物或其他含铁矿物发生化学变化形成亚铁磁性矿物,使得黑色断层泥的磁化率较其母岩石炭系煤层有一定升高.通过黑色断层泥的铁磁性磁化率结合χ-T曲线计算获得断层泥所经历的最高温度约为420℃,不超过450℃.老虎山段厚层碳质断层泥的存在为该地区发现的浅层蠕滑现象提供了一种解释.

     

Changes in the hydrochemical composition and diatomic flora of bottom sediments in the zone of influence of metal mining production (Kola Peninsula)

Paleoecological studies of small water bodies were carried out in a model subarctic lake-river system. The degree of transformations of aquatic ecosystems under a set of anthropogenic factors is shown to vary depending on the load and individual features of water bodies. Data on the state of aquatic ecosystems before the industrial development of the region are collected. The species composition of diatom algae and their ecological characteristics are examined.     

The model of the depth variations in the properties and state of the rocks in the upper,middle, and lower continental crust of the Kola-Norwegian block,Kola Peninsula

Based on the experimental and calculated data, the model of depth variations in density, as well as the velocities of longitudinal (P) and transverse (S) waves, within the upper, middle, and lower crusts of the Kola-Norwegian block down to a depth of about 40 km is suggested. The variations in density and the velocities of the P- and S-waves are primarily caused by the changes in the mineral composition of the rocks. The relative reduction in the velocities of the P- and S-waves under the action of the increasing pressure and temperature in the depth interval from 5 to 37 km is estimated at ～2%.     

Paleomagnetic studies of late pleistocene rocks from the Malyi Kut section (Taman Peninsula, Krasnodar krai)

Paleomagnetic studies of Quaternary deposits from the Malyi Kut rock sequence (Krasnodar krai, western part of the Taman Peninsula) in two time intervals of Late Pleistocene are carried out. The Malyi Kut sequence is a marine terrace of the Baku age, which is embedded in the disturbed marine Sarmatian deposits. The terrace of the Bakinian age nests the Karangat marine terrace. The presence of the marine molluscan fauna in the both terraces enables reliable dating of the studied deposits. The composition, grain size, and concentration of ferromagnetic fraction present in the studied rocks are investigated by a set of rock magnetic methods. The directions of natural remanent magnetization (NRM) are studied, and the reliability of their isolation is estimated. The results are compared with the paleomagnetic records of NRM in the rocks of the parallel coeval sections of the Tuzla (Taman Peninsula) and Roxolany (Ukraine). This correlation suggests that the studied rock sequences recorded the Mono Lake geomagnetic excursion.     

Characterizing the effect of elastic interactions on the effective elastic properties of porous,cracked rocks

Elastic interactions between pores and cracks reflect how they are organized or spatially distributed in porous rocks. The principle goal of this paper is to understand and characterize the effect of elastic interactions on the effective elastic properties. We perform finite element modelling to quantitatively study how the spatial arrangement of inclusions affects stress distribution and the resulting overall elasticity. It is found that the stress field can be significantly altered by elastic interactions. Compared with a non‐interacting situation, stress shielding considerably stiffens the effective media, while stress amplification appreciably reduces the effective elasticity. We also demonstrate that the T‐matrix approach, which takes into account the ellipsoid distribution of pores or cracks, can successfully characterize the competing effects between stress shielding and stress amplification. Numerical results suggest that, when the concentrations of cracks increase beyond the dilute limit, the single parameter crack density is not sufficient to characterize the contribution of the cracks to the effective elasticity. In order to obtain more reliable and accurate predictions for the effective elastic responses and seismic anisotropies, the spatial distribution of pores and cracks should be included. Additionally, such elastic interaction effects are also dependent on both the pore shapes and the fluid infill.     

Interpretation of MHD-sounding data from the Kola Peninsula by the electromagnetic migration method

The electromagnetic migration method, based on principles analogous to those of seismic migration, is developed.The concept of migrated fields is based on the Stratton-Chu type integrals, written in reverse time for the observed electromagnetic fields. Four types of migrated fields, which form a system of migration transformations of the transient electromagnetic field, are introduced.Study of the properties of the migrated fields by means of theoretical and model examples makes it possible to determine the optimal parameters of the migration procedure in which the anomalous field sources are localized by means of migration transformation.Information about the experiment using the MHD-generator (experiment ‘Chibini’) carried out on the Kola Peninsula to study the geoelectrical structure of the Earth's crust and upper mantle is included. The electromagnetic migration method for the interpretation of MHD-sounding data recorded on the Kola Peninsula, along a profile crossing the mineral-rich region of the Imandra-Varzuga structure, permits us to determine the location of a conducting zone at the depth of 10 km in the Earth's crust.     

Magnetic anisotropy of granitic rocks of the Bergell massif (Switzerland)

Magnetite as well as ilmenohematite are contained by the Bergell granitic rocks, but the fabric of low field susceptibility is due to the magnetite only and is equivalent to the macroscopic biotite fabric. Microscopic observations and high field anisotropy measurements show that the ilmenohematite trigonal axes are aligned parallel to the directions of minimum low field susceptibility anisotropy. The deflection of the stable direction of natural remanent magnetization caused by the alignment of ilmenohematite is corrected using the susceptibility anisotropy data. The resulting virtual palaeopole position deviates from other Oligocene to Miocene pole positions of stable Europe suggesting that since the time of intrusion some 25 my ago the Bergell massif has been rotated anti-clockwise by an angle of ～30° about a vertical axis.     

Calculation of elastic properties in lower part of the Kola borehole from bulk chemical compositions of core samples

In-situ elastic properties in deep boreholes are controlled by several factors, mainly by lithology, petrofabric, fluid-filled cracks and pores. In order to separate the effects of different factors it is useful to extract lithology-controlled part from observedin-situ velocities. For that purpose we calculated mineralogical composition and isotropic crack-free elastic properties in the lower part of the Kola borehole from bulk chemical compositions of core samples. We use a new technique of petrophysical modeling based on thermodynamic approach. The reasonable accuracy of the modeling is confirmed by comparison with the observations of mineralogical composition and laboratory measurements of density and elastic wave velocities in upper crustal crystalline rocks at high confining pressure. Calculations were carried out for 896 core samples from the depth segment of 6840–10535m. Using these results we estimate density and crack-free isotropic elastic properties of 554 lithology-defined layers composing this depth segment. Average synthetic P- wave velocity appears to be 2.7% higher than the velocity from Vertical Seismic Profiling (VSP), and 5% higher than sonic log velocity. Average synthetic S-wave velocity is 1.4 % higher than that from VSP. These differences can be explained by superposition of effects of fabric-related anisotropy, cracks aligned parallel to the foliation plain, and randomly oriented cracks, with the effect of cracks being the predominant control. Low sonic log velocities are likely caused by drilling-induced cracking (hydrofractures) in the borehole walls. The calculated synthetic density and velocity cross-sections can be used for much more detailed interpretations, for which, however, new, more detailed and reliable seismic data are required.     

The characteristics of the structure,strain and kinematic vorticity of the Wulian detachment fault zone,Shandong Peninsula,China

As one of the vital tectonic units of the Wulian Metamorphic Core Complex(MCC), the Wulian detachment fault zone(WDFZ), which developed in the Jiaodong peninsula, separates the lower plate of the ultrahigh pressure(UHP) metamorphic rocks in the Sulu orogenic belt from the upper plate of the early Cretaceous Zhucheng basin and the basin basement. The fault zone generally strikes NNE with a dip in the west along the southern portion of the MCC and strikes NE with a dip in the WNWalong the northern portion. The fault zone displays a wavy-tile-shaped extension on the plane, principally composed of the fault breccias and mylonite and transits downward to the mylonitic gneiss. As a whole, the detachment fault zone shows a top-towest or a WNW extension. By calculating the harmonic mean, we obtain a Flynn index K of 0.98–2.0, and the mean value is approximately 1.35 in the fault zone. According to the polar Mohr construction, the extensional crenulation cleavage, the RS/θ,and the quartz C-axial fabric methods, we acquire mean kinematic vorticity values of 0.64–0.97, 0.76–0.93, 0.6–0.92, and 0.63–0.98 with mean values of 0.83, 0.80, 0.78 and 0.86, respectively, for mylonite and promylonite. The strain measurement results and the kinematic vorticity values indicate that the WDFZ is a normal ductile shear zone developed in the extensional setting.The kinematic track shows that the kinematic vorticity value decreases gradually from the NW to the SE as a whole. A simple shear dominates in the middle and upper parts of the shear zone, which is reflected by a higher vorticity value(0.75, up to 0.98),a low thinning rate and a lower K value. In contrast, toward the footwall, the pure shear is increased significantly, showing a lower vorticity value(0.70, low to 0.64), a relatively high thinning rate and a higher K value. Combined with the geotectonic background, the development and evolution of the WDFZ should respond to lithospheric thinning and the destruction of the North China Craton(NCC). As a result, the WDFZ can be defined as a thinning normal shear zone developed in the extension tectonic setting and the combined result of the simple shear caused by the crust extension and pure shear led by the rapid uplift of the footwall and magmatic upwelling.     

Investigation of excess argon in ultramafic rocks from the Kola Peninsula by the40Ar/39Ar method

In several xenolithic ultramafic rocks from the Kola Peninsula, including a magnetic separate, abnormally high⁴⁰Ar/³⁹Ar ratios persisted at low and high temperatures. The lowest⁴⁰Ar/³⁹Ar ratio was consistently observed at intermediate temperatures (900–1100°C), indicating an apparent age of 2.8–3.1 b.y.; however, this may not indicate the formation age.The quantity of excess⁴⁰Ar was estimated at each temperature fraction, adopting ages inferred from published Rb-Sr ages or the minimum⁴⁰Ar/³⁹Ar age. Excess⁴⁰Ar is abundantly trapped both in mineral lattices and nonretentive trapping sites, but the trapping sites are different from those of in-situ radiogenic⁴⁰Ar. The high temperature component of excess⁴⁰Ar is considered to represent Ar dissolved during mineral formation in the upper mantle or the lower crust.A correlation between the amount of high temperature excess⁴⁰Ar and³⁶Ar exists for some samples. The⁴⁰Ar_excess/³⁶Ar ratios of the rocks of probable upper mantle or lower crust origin vary from about 10 000 to 35 000, which may suggest large fluctuations of this ratio in the deep interior of the earth. The high value implies that most³⁶Ar was already degassed from the earth's interior at least 2 or 3 b.y. ago.     

红河断裂带研究区深部结构特点的地球物理研究

红河断裂带是认识板块相互运动、深部过程和地质演化的窗口，一直受到重视与关注，关于红河断裂带入海后的去向问题特一直是讨论和研究的热点.本文对15°N~25°N，102°E~112°E地区的重力、磁力数据进行了相关研究，结合哀牢山—红河断裂带地区的地震层析成像结果，对该断裂带入海之后的去向进行了讨论.并依据重力数据对莺歌海中的两条剖面重建了密度结构，认为该区浅部与深部构造存在明显的不协调性.同时认为莺歌海地区的中地壳中有一低密度层存在，推测红河断裂带在莺歌海地区的走滑，很有可能就是沿着这一层发生的.     

五莲拆离断层带的结构、应变及运动学涡度特征

五莲拆离断层带发育于胶东半岛,作为五莲变质核杂岩的重要构造单元,分隔了作为下盘的苏鲁造山带超高压变质岩与上盘的早白垩世诸城盆地与盆地基底.断层带走向由南向北总体由NNE转为NE向,倾向W或WNW,平面上具有"波瓦状"延伸特征,主要由断层角砾岩、糜棱岩组成,向下过渡为糜棱岩化片麻岩.拆离断层带总体具有上盘向W或WNW向伸展运动.调和平均值法计算获得断层带中弗林指数K取值范围0.98～2.0,平均值约为1.35.极摩尔圆法、伸展褶劈理法、Rs/θ法、石英光轴法等多种方法获得糜棱岩、初糜棱岩的平均运动学涡度值分别为0.64～0.97、0.76～0.93、0.6～0.92、0.63～0.98,均值分别为0.83、0.80、0.78和0.86;应变测量与运动学涡度估算的结果共同说明五莲拆离断层带是在伸展拉伸条件下形成的一般型韧性剪切带.运动学轨迹显示,从NW向SE方向运动学涡度值总体存在递减的趋势,在剪切带的中上部,简单剪切作用更为明显,表现为较高的涡度值(涡度值>0.75,最大可达0.98)、低的减薄率和较低的K值;向下盘方向,纯剪切应变的成份明显增加,表现为较低的涡度值(涡度值<0.70,最小可达0.64)、相对高的减薄率和较高的K值.结合大地构造背景,五莲拆离断层带的形成与演化是华北克拉通岩石圈减薄与破坏作用在其东部边缘的一种反映.五莲断层带可以被厘定为形成于地壳伸展环境的减薄型一般剪切带,是地壳伸展引起断层带简单剪切、下盘快速抬升及岩浆上涌引起垂向施加在断层带的纯剪切联合作用的结果.     

Matrix hydrodynamic properties of carbonate rocks from the Betic Cordillera (Spain)

Laboratory tests performed on 181 rock samples from boreholes drilled in different areas of the Betic Cordillera allow us to calculate their hydraulic conductivity and open porosity values. Higher values are generally associated with Miocene calcareous sandstones, although hydraulic conductivity reaches its highest values in some isolated limestone and dolostone samples. Lower values were found in marly limestones and marbles. Specific yield ranged from 0 to 0·0798, with a mean value of 0·00579. A total of 79 samples did not release water during the specific yield test, while another 11 samples released water for more than 30 min. Such wide ranges of variation show the great diversity of behaviour that the matrix of the carbonate rocks can have, referring to water storage and transfer, and its influence on pollutants spread, for example. A weak relation between interconnected porosity and hydraulic conductivity was found. The relation between interconnected porosity and specific yield is slightly stronger, except in the case of the dolomites, where a high correlation was found. No dependence on depth was found for hydraulic conductivity and interconnected porosity. Copyright © 2004 John Wiley & Sons, Ltd.     

鄂尔多斯块体西缘地壳介质各向异性:从银川地堑到海原断裂带

通过收集鄂尔多斯块体西缘固定地震台网2010年6月至2017年8月的近场地震资料,选择符合剪切波分裂分析的14个台站记录的共137个有效事件波形,得到了剪切波分裂参数,即快剪切波（简称快波）偏振方向和慢剪切波（简称慢波）时间延迟.结果表明,研究区的快波偏振方向和慢波时间延迟具有明显的分区特征,快波偏振方向主要与构造应力场方向或者断层走向大体一致.鄂尔多斯西缘紧邻块体边界的台站,快波偏振方向自北向南呈现NS、NNE、NE向的变化,与青藏高原东北缘主压应力方向变化基本一致.银川地堑东西两侧的快波偏振方向有差异,东侧区域主要受青藏高原NNE向挤压和黄河-灵武断裂共同影响,而西侧区域可能受到阿拉善块体与鄂尔多斯块体之间的NW方向的主张应力和阿拉善块体内部应力分布的影响;鄂尔多斯块体、阿拉善块体与青藏高原的交汇区快波优势偏振方向为NE向,与青藏高原东北缘主压应力方向一致;海原断裂带及以南区域快剪切波优势偏振方向为WNW向,与断裂走向基本一致,较好的说明了海原断裂带为活跃的活动断裂.构造与断裂分布都是控制快波偏振方向的主要因素,走滑断裂上的台站快波偏振方向与断裂走向一致,表明这些台站主要受到断裂的强烈影响;走滑断裂附近的个别台站快波偏振方向呈现与构造应力场一致的方向,表明几乎没有受到断裂的影响.鄂尔多斯、阿拉善与青藏高原的交汇区平均时间延迟高于其他地区,反映了青藏高原在NE向运动过程中,受到稳定的鄂尔多斯块体阻挡作用,导致了交汇区地壳介质各向异性程度增加.以海原断裂带到六盘山断裂带为界,其两侧区域的各向异性差异性明显,揭示了应力与介质特性的差异,暗示其邻近区域,特别在海原断裂带东端到六盘山断裂带与鄂尔多斯块体西缘交汇区域,可能有较高的强震危险背景.本研究还对该区域的地壳和上地幔的耦合问题进行了初步讨论.