Fault oneoff versus coseismic fluids reaction

The fault activation (fault on) interrupts the enduring fault locking (fault off) and marks the end of a seismic cycle in which the brittle-ductile transition (BDT) acts as a sort of switch. We suggest...     

http://www.sciencedirect.com/science/article/pii/S1674987114000024

We propose that the brittle-ductile transition(BDT) controls the seismic cycle.In particular,the movements detected by space geodesy record the steady state deformation in the ductile lower crust,whereas the stick-slip behavior of the brittle upper crust is constrained by its larger friction.GPS data allow analyzing the strain rate along active plate boundaries.In all tectonic settings,we propose that earthquakes primarily occur along active fault segments characterized by relative minima of strain rate,segments which are locked or slowly creeping.We discuss regional examples where large earthquakes happened in areas of relative low strain rate.Regardless the tectonic style,the interseismic stress and strain pattern inverts during the coseismic stage.Where a dilated band formed during the interseismic stage,this will be shortened at the coseismic stage,and vice-versa what was previously shortened,it will be dilated.The interseismic energy accumulation and the coseismic expenditure rather depend on the tectonic setting(extensional,contractional,or strike-slip).The gravitational potential energy dominates along normal faults,whereas the elastic energy prevails for thrust earthquakes and performs work against the gravity force.The energy budget in strike-slip tectonic setting is also primarily due elastic energy.Therefore,precursors may be different as a function of the tectonic setting.In this model,with a given displacement,the magnitude of an earthquake results from the coseismic slip of the deformed volume above the BDT rather than only on the fault length,and it also depends on the fault kinematics.     

全球地震震源深度统计分析与地震层分布

地震数据可以反演地球内部的构造信息,能够查明地球内部层圈结构及地震发生的机制,对揭示地球结构,进行地震预测及防震减灾具有重要意义。与此同时,对地震数据的统计分析可以获得地壳变形及其深度层次的相关信息。本文利用美国地质调查局（USGS）记录的1900～2018年近3 000 000条全球地震数据进行分析,立足地震震源计算方法,排除不确定性数据,使用震源深度频次分析及高斯分解得到地震层和地震集中区深度的推定。以国际地震中心（ISC）的1970～2016年近2 000 000条数据及中国国家地震科学数据共享中心2009～2018年近300 000条数据进行比较。结果显示,地球标准椭球体深度10 km左右普遍存在一个全球性的地震集中区,与地壳中脆韧性过渡带一致。其上的地壳是全球绝大多数地震发震的深度范围,推定其为地震层;与此同时,局限在洋壳俯冲带中,约35 km处出现了另一个地震集中区,认为是地球岩石圈深度内不容忽视的界面。研究表明,在地球表层（40 km以内）,层圈结构对地震有较大的控制作用;而对有洋壳俯冲区域的地震三维结构图成图显示有“贝尼奥夫带”形态,表明俯冲机制能将具有弹性力学性质的刚性块体带至地球较深部,并孕育中、深源地震。因此,地震震源分布可以指示地震层,即刚性块体,在岩石圈中的分布,地震震源深度集中区是岩石圈和中、上地幔最主要的变形和能量释放区。     

甘肃西部昌马地区金矿成矿条件及找矿标志

通过对区内金矿特征及地质背景的分析，认为鹰嘴山、寒山、车路沟一带的金矿分别为受脆韧性剪切带改造的蚀变岩型金矿、受脆韧性剪切带控制的蚀变岩型金矿和受断裂破碎带控制的金矿，它们分别形成于加里东晚期、华力西中期和华力西晚期--印支期．指出：发育于寒武纪火山岩和加里东期基性超基性岩中的韧性剪切带，是寻找受脆韧性剪切带改造的蚀变岩型金矿的有利地段：寒山一带的脆韧性剪切带被近EW向断裂叠加的部位，是受脆韧性剪切带控制的蚀变岩型金矿成生的有利地段；发育于车路沟花岗闪长岩体外接触带的NNW向断裂，是寻找受断裂破碎带控制的金矿的有利地段．     

1995年日本兵库县南部地震时淡路岛上出现的地震断层的形态特征

1995年日本兵库县南部地震时在淡路岛上出现的地表地震断层主要由三条地表破裂带组成、野岛地震断层、松帆地震层和楠本地震断层。野岛地震断层从淡路岛北端的淡路町住西南延伸到一宫町的尾崎,长达18km,其北段沿着早期存在的野岛断层分布,而南段则作为新断层出现,野岛地震断层的断层的北段主要由一些相互平行或次平行的右列剪切断层和许多左列和性裂隙组成,其南段则是由集中在十多米宽的大量不连续的地表破裂带所组成。野岛地震断层一般走向N30°～60°E倾向SE,地貌错位和断层擦痕均显示出此断层为一具有逆断层性质的右旋走滑断层,沿一些主要露头测定的北段水平位移量一般为100～200cm,垂直位移量为5O～100cm;而南段的水平、垂直位移量均只有几厘米至20cm。最大位移量在平林断层崖测得;水平180cm、垂直l30cm、实际位移量2l5cm。松帆地震断层走向N40°～60°W,沿着淡路岛北端部的海岸线分布,长达约1km。楠本地震断层沿早期存在的楠本断层出现,分布于淡路岛东北部的海岸边上,走向N35°～6O°W倾向NW 。根据地表地震断层的形态及地貌错位特征,野岛地震断层可被分为四条断层段,并在形态上呈现右列。地质和地貌证据以及余震分布的特征清楚地表明这4条断层段的几何形态和分布特征是受早期存在的地质构造所控制的,同时也说明了地震断层的破裂过程具有拉分(pulling-apart)和推隆(pushing－up)的过程,这两个过程分别产生了张性裂隙、拉分盆地、逆断层和挤压隆起等构造。     

First results on the LPO-derived seismic properties of iron ores from the Quadrilátero Ferrífero region, southeastern Brazil

The role of hydrothermal fluids in assisting the activity of strike-slip faults is investigated using a range of new geological, geophysical, and geochemical data obtained on the Argentat fault, Massif Central, France. This fault zone, 180-km-long and 6 to 8 km-width, has experienced coeval intense channeling of hydrothermal fluids and brittle deformation during a short time span (300–295 Ma). According to seismic data, the fault core is a 4-km-wide, vertical zone of high fracture density that rooted in the middle crust (~ 13 km) and that involved fluids in its deeper parts (9–13 km depth). If stress analyses in the fault core and strain analyses in the damage zone both support a left-lateral movement along the fault zone, it is inferred that hydrothermal fluids have strongly influenced fault development, and the resulting fault has influenced fluid flow. Fluid pressure made easier fracturing and faulting in zones of competent rocks units and along rheological boundaries. Repeated cycles of increase of fault-fracture permeability then overpressure of hydrothermal fluids at fault extremity favored strong and fast development of the crustal-scale strike-slip fault. The high permeability obtained along the fault zone permitted a decrease of coupling across the weak fault core. Connections between shallower and lower crustal fluids reservoirs precipitate the decrease of fault activity by quartz precipitation and sulfides deposition. The zones of intense hydrothermal alteration at shallows crustal levels and the zones of fluid overpressure at the base of the upper crust both controlled the final geometry of the crustal-scale fault zone.     

Orientation,composition, and entrapment conditions of fluid inclusions in the footwall of the northern Snake Range detachment,Nevada

Footwall rocks of the northern Snake Range detachment fault (Hampton and Hendry's Creeks) offer exposures of quartzite mylonites (sub-horizontal foliation) that were permeated by surface fluids. An S–C–C′ mylonitic fabric is defined by dynamically recrystallized quartz and mica. Electron backscatter diffraction analyses indicate a strong preferred orientation of quartz that is overprinted by two sets of sub-vertical, ESE and NNE striking fractures. Analyses of sets of three perpendicular thin sections indicate that fluid inclusions (FIs) are arranged according to macroscopic fracture patterns. FIs associated with NNE and ESE-striking fractures coevally trapped unmixed CO₂ and H₂O-rich fluids at conditions near the critical CO₂–H₂O solvus, giving minimum trapping conditions of T = 175–200 °C and ∼100 MPa H₂O-rich FIs trapped along ESE-trending microcracks in single crystals of quartz may have been trapped at conditions as low as 150 °C and 50 MPa indicating the latest microfracturing and annealing of quartz in an overall extensional system. Results suggest that the upper crust was thin (4–8 km) during FI trapping and had an elevated geotherm (>50 °C/km). Footwall rocks that have been exhumed through the brittle-ductile transition in such extensional systems experience both brittle and crystal-plastic deformation that may allow for circulation of meteoric fluids and grain-scale fluid–rock interactions.     

Fluids migration and dynamics of a blocks-and-faults system

We explore the impact of fluids migrating through a fault network on the dynamics of lithosphere, both on slow movements and seismicity. For that purpose fluids in the fault zones are incorporated into modelling of blocks-and-faults systems, which takes into account driving forces and the system's geometry. Simulations have been performed for two-dimensional models: an idealised “brick wall” structure, and a coarse image of Sinai Subplate. Migrating fluids originating in different locations are considered, as well as fluids trapped in closed pockets. Basic features of the modelled and observed seismicity are in good accord, as shown by comparison with the earthquake catalog compiled by Geophysical Institute of Israel.     

Detailed surface co-seismic displacement of the 1999 Chi-Chi earthquake in western Taiwan and implication of fault geometry in the shallow subsurface

The northern segment of the Chelungpu Fault shows an unusually large co-seismic displacement from the event of the Mw 7.6 Chi-Chi earthquake in western Taiwan. Part of the northern segment near the Fengyuan City provides an excellent opportunity for characterizing active thrust-related structures due to a dense geodetic-benchmark network. We reproduced co-seismic deformation patterns of a small segment of this Chelungpu Fault using 924 geodetic benchmarks. According to the estimated displacement vectors, we identified secondary deformations, such as local rigid-block rotation and significant shortening within the hanging wall. The data set also allows us to determine accurately a 3D model of the thrust fault geometry in the shallow subsurface by assuming simple relations between the fault slip, and the horizontal and vertical displacements at the surface. The predicted thrust geometry is in good agreement with borehole data derived from two drilling sites close to the study area. The successful prediction supports our assumptions of rigid displacement and control of displacement in the hanging wall by the fault geometry being useful first approximations.