Earthquake Rupturing in Fluid-Overpressured Crust: How Common?

Whether or not ruptures nucleate in fluid-overpressured crust (λ _v = P _f/σ _v > 0.4) is important because pore-fluids overpressured above hydrostatic lower fault frictional strength and may also vary through the earthquake cycle, acting as an independent variable affecting fault failure. Containment of fluid overpressure is precarious because pressure-dependent activation of faults and fractures allows drainage from overpressured portions of the crust. Discharge of fluids through activated fault-fracture permeability (fault-valve action) decreases overpressure so that subsequent failure depends on the cycling of both overpressure and frictional strength as well as tectonic stress. Geometric and mechanical considerations suggest that fluid overpressures are more likely to develop and be sustained in compressional/transpressional regimes as opposed to extensional/transtensional tectonic settings. On the basis of geophysical observations and force-balance analyses, subduction interface shear zones appear to be strongly but variably overpressured to near-lithostatic levels (λ _v > 0.9) over the full depth range of seismogenic megathrusts. Strong overpressuring at seismogenic depths is also documented in active fold-thrust belts and in areas of ongoing compressional inversion (e.g., northern Honshu) where inherited normal faults are reactivated as steep reverse faults, requiring near-lithostatic overpressures (λ _v → 1.0) at depths of rupture initiation. Evidence for overpressuring around strike-slip faults is less clear but tends to be strongest in areas of transpression. In areas of extensional tectonics coincident with particularly high fluid discharge, there is some evidence of overpressuring concentrated towards the base of the seismogenic zone. In general, because of the limited resolution of geophysical techniques, it is easier to make the case for rupture propagation through overpressured crust than to make a definitive case for the direct involvement of overpressured fluids in rupture nucleation, though in some instances the circumstantial evidence is compelling. An unresolved related issue is the heterogeneity of overpressuring. Do the active fault zones themselves serve as fluid conduits that are locally overpressured with respect to the surrounding crust?     

Structural analysis of the Brigham City-Weber segment boundary zone,Wasatch normal fault,Utah: Implications for fault growth and structure

We investigate the history, kinematics, principal stress orientations and geometry of deformation at the end of a bent normal fault segment of the Wasatch fault zone, Utah. Three fault types, developed in Archean crystalline rocks, reflect progressive uplift of fault-related footwall rocks. Chlorite-breccias and phyllonites reflect deep-level, reaction-assisted plastic deformation along the north-striking part of the segment. Planar, fretted faults which formed by cataclasis cut the phyllonites and breccias and are developed throughout the footwall of the segment. Youngest faults are hematitecoated, extremely narrow polished surfaces. Slip vectors and kinematic analyses of small faults developed in the footwall indicate oblique normal slip along the north-striking portion of the segment. Slip vectors and fault orientation along the northwest-striking portion of the segment reflect complexly oriented slip on faults which strike subparallel and at high angles to the main fault trace, yet slip is confined to a broad fault-parallel zone. Small faults at the southernmost tip of the segment indicate a strong influence of the north-striking Weber segment to the south. Inversion of fault data for principal stress orientations document complexly oriented principal stresses through the segment boundary zone and suggest that ₃ may have reoriented approximately 60° over the life of the segment. Subsurface structure combined with small fault data indicate the segment boundary is comprised of a southwest-plunging bedrock high which is reflected by a sharp bend in the Brigham City segment. The southern end of the Brigham City segment may have started, as a straight, north-striking fault which has bent due to changes in stress orientations and/or interaction with the adjacent Weber segment.     

Coupled Models of Brittle-plastic Deformation and Fluid Flow: Approaches,Methods, and Application to Mesoproterozoic Mineralisation at Mount Isa,Australia

Rock deformation has an important effect on the spatial distribution and temporal evolution of permeability in the Earth’s crust. Hydromechanical coupling is of fundamental significance to natural fluid–rock interaction in porous and fractured hydrothermal systems, and in the assessment and production of hydrocarbon resources and geothermal energy. Shearing and fracturing of rocks can lead to the creation or destruction of permeability when fractures or faults form, or when existing structures are reactivated. Changes in stress orientation or fluid pressure can drive rock failure and create dilating fault zones that have the potential to focus fluid flow, or to breach seals above overpressured fluid compartments. Here, numerical models of deformation and fluid flow related to Mesoproterozoic copper mineralisation at Mount Isa, Australia, are presented that show how changes in deformation geometry in multiply deformed geological architectures relate to changes in dilation patterns, fluid pathways and flow geometry. Coupled numerical simulations of deformation and fluid flow can be useful tools to better understand structural control on fluid flow in hydrothermal mineral systems.     

The mechanism of formation of the Baikal basin

The Baikal is a deep long and narrow basin in East Siberia which follows a huge fault zone adjoining the Siberian Platform. The basin was formed by rapid subsidence of continental crust during the pas 3–4 Ma. It is bounded by normal faults which indicate extension of the crust during the subsidence. According to seismic reflection profiling data, the intensity of extension is not large (3–7%). It is much smaller than the thinning of the crystalline crust under the basin (up to 38%). The thinning and crustal subsidence can be explained by the transformation of gabbro in the lower crust into dense garnet granulites. The latter rocks (with V_p 7.7−7.8 km/sec) are still located under the remnant part of the crust. Rapid transformation took place due to an inflow of catalyzing fluid along the fault zone from the asthenospheric upwelling. This upwelling, which is at a depth of 80–90 km, caused a general uplift of a broad area in the south of East Siberia.     

Comprehensive study on stability of deep crust and unstable behavior of earthquake source by both failure mechanism and frictional sliding mechanism

In this paper the relation between fault movement and stress state in deep crust is discussed, based on synthetic analysis of the crustal stresses measured over the world and the concerned data of focal mechanism. Using Coulomb criterion for shear failure and frictional slip, analytical expressions for estimating stabilities of intact rock and existing fault in the crust and for identifying the type of faulting (normal, strike-slip or thrust fault) are derived. By defining the Failure FunctionF _m and the Fraction FunctionF _f, which may describe steadiness of crustal rock and existing fault, respectively, a synthetic model is set up to consider both fracturing mechanism and the sliding mechanism. By this model, a method to study stability and unstable behavior of crustal rock and fault at different depths is given. According to the above model, quantitative study on the crustal stability in the North China plain is made in terms of the measured data of hydraulic fracturing stress, pore-fluid pressure, terrestrical heat flow in this region. The functionsF _m andF _f and the shear stresses on faults with different strike angle and dip angle at various depths in this region are calculated. In the calculation the constraint condition of fault movement obeys Byerlee’s Law, and the depth-dependent nonlinear change in the vertical stress due to inhomogeneity of crustal density and the high anomalous pore-fluid pressure in deep crust of this region are considered. The conclusions are: the unstable behavior of the crust in the North China plain is not failure of crustal rock but slip on existing fault; the depth range where stick-slip of fault may happen is about from 8 to 20 km or more; stability of steep fault is lower than that of gentle sloping fault; the shear stresses in the range where may occur stick-slip are nearly horizontal; the steep faults trending from NNE to NE in this region are liable to produce strong earthquakes, whose co-seismic faultings are, for the most part, right lateral slip; the change in pore-fluid pressure in depth remarkably affects the stability of the crust and the increase in pore-fluid pressure, therefore, would be an important factor exciting strong earthquake in this region. The above theoretical inferences are consistent with the data measured in this region. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologia Sinica,13, 450–461, 1991. This work is supported by Chinese Joint Seismological Science Foundation.     

Effect of fault jogs on frictional behavior: An experimental study

Studying the effect of geometrically irregular bodies on the mechanical behavior of fault activity is of significance in understanding the seismic activity along a fault zone. By using rock mechanics ex- periment with medium-scale samples, we have studied the effect of fault jogs, the most common irregularity along fault zones, on frictional behavior. The research indicates that extensional fault jog can be easily fractured because of its low strength and the fractured jog has no obvious resistance to fault sliding, and the micro-fractures occurring in the jog are indicative of stick-slip along the faults. The fault zone containing extensional jogs is characterized by velocity weakening and can be described by rate and state friction law. Compressional fault jog makes fault sliding more difficult because of its high fracturing strength, but the micro-fractures occurring in the tensile areas around fault ends at higher stress level can provide necessary condition for occurrence of stick-slip along the faults before the jog is fractured and thus act as precursors of fault instability. Compression jog can be taken as a stable indicator of fault segmentation until the jog is completely fractured and two faults are linked.     

地壳深部稳定性——震源体破裂机制和摩擦滑动机制的综合研究

本文通过地壳应力测量结果和地震资料的综合分析,对深部应力状态与断层运动的关系作了讨论:进而应用库仑准则,推导了三维应力作用下完整岩体和已有的任意空间方向断层面的失稳条件及其滑动方式的解析表达式.通过建立描述岩体和已有断层稳定性的两个函数————破裂函数 Fm 和摩擦函数 Ff,给出了应用破裂机制和摩擦滑动机制综合分析地壳稳定性和失稳性态的方法:根据这种分析方法并结合华北平原区的水压致裂应力测量资料,以及孔隙压力、大地热流等观测结果,定量研究了本区地壳的稳定问题,计算并图示了地壳内破裂函数沿深度的分布,以及各种走向和倾向断层面上的摩擦函数和剪应力分布;计算中以 Byerlee 定律作为断层运动的约束条件,并考虑了地壳密度纵向非均匀性导致的垂直应力沿深度的非线性增长以及深部超静水压力的异常孔隙压力作用.结果表明,华北平原区地壳失稳性态主要表现为已有断层的滑动;伴有高剪应力降的断层运动的深度范围在8至2.0多公里之间:陡断层稳定性低于缓断层,其运动方式以走滑为主;本区 NNE-NE 走向的陡断层是一组易震断层,其震时错动为右旋走滑;孔隙压力的增长对地壳稳定性有显著的影响;华北平原区深部高异常孔隙压力是地震活动的一种重要背景.      

An assessment of field evidence for ‘Byerlee’ friction

Structural analyses of the angles of frictional lock-up for fault sets that have become progressively misoriented, together with field observations from seismology, geomorphology, and borehole stress measurements, suggest that Byerlee friction coefficients (0.6<<0.85) are widely applicable to natural sliding surfaces with displacements of up to a few kilometres in the upper crust, from the surface of the earth to seismogenic depths. Extensional normal faults operating under presumed vertical trajectories of extreme compressive stress provide some of the best evidence for frictional lock-up followed by the initiation of new favorably oriented faults, but similar lock-up phenomena also occur in thrust and strike-slip fault systems.However, extensional detachments which appear to have formed and remained active at very low dips (<15°) lie well outside the dip range of currently active normal faults, requiring stress trajectories that deviate significantly from the vertical and horizontal during their initiation and perhaps also during their continued reactivation. Other conspicuous exceptions to the pattern of frictional lock-up expected from Byerlee friction are major transform structures, such as the San Andreas fault in California, which remain active though oriented at high angles to the maximum principal compression. On the basis of the evidence afforded by the lower displacement faults, the apparent weakness of such structures seems most likely to arise from locally elevated fluid pressure, rather than from the presence of anomalously low-friction material within the fault zones.     

Eclogitization of lower crustal granulites by fluid migration through shear zones

The granulite facies assemblages of the anorthositic rocks of the Bergen Arcs (stable at 800–900°C and 10 kbar) have been transformed to eclogite facies assemblages (stable at 700–750°C and 16–19 kbar) in the vicinity of Caledonian shear zones. This section of the root zone of the Caledonian mountain chain reveals a deep polymetamorphic crust where Precambrian granulites (mean density 3.02 g/cm³) and Caledonian eclogites (mean density 3.19 g/cm³) alternate on a scale of meters over a minimum area of 3 × 12 km. Detailed mapping of three localities shows that eclogites account for up to 30–45% of the rock volume. The stabilitization of the eclogite mineralogy is controlled by fluids penetrating these deep crustal shear zones. The eclogitization is independent of preexisting compositional variation in this anorthosite-norite complex. The Bergen Arcs example suggests that the amount of eclogite versus granulites in the lowermost crust is a function of deformation and fluid access, rather than being controlled byT, P and rock composition alone. These relationships may explain the gradual increase in seismic velocity observed in some deep crustal sections and also the complex reflection pattern obtained from the lowermost crust in many areas.     

Fracturing and hydrothermal alteration in normal fault zones

Large normal fault zones are characterized by intense fracturing and hydrothermal alteration. Displacement is localized in a slip zone of cataclasite, breccia and phyllonite surrounding corrugated and striated fault surfaces. Slip zone rock grades into fractured, but less comminuted and hydrothermally altered rock in the transition zone, which in turn grades abruptly into the wall rock. Fracturing and fluid flow is episodic, because permeability generated during earthquakes is destroyed by hydrothermal processes during the time between earthquakes.Fracture networks are described by a fracture fabric tensor (F). The permeability tensor (k) is used to estimate fluid transport properties if the trace of F is sufficiently large. Variations in elastic moduli and seismic velocities between fault zone and wall rock are estimated as a function of fracture density (). Fracturing decreases elastic moduli in the transition zone by 50–100% relative to the country rock, and similar or even greater changes presumably occur in the slip zone.P-andS-wave velocity decrease, andV _p /V _s increases in the fault zone relative to the wall rock. Fracture permeability is highly variable, ranging between 10^–13 m² and 10^–19 m² at depths near 10 km. Changes in permeability arise from variations in effective stress and fracture sealing and healing.Hydrothermal alteration of quartzo-feldspathic rock atT>300°C creates mica, chlorite, epidote and alters the quartz content. Alteration changes elastic moduli, but the changes are much less than those caused by fracturing.P-andS-wave velocities also decrease in the hydrothermally altered fault rock relative to the country rock, and there is a slight decrease inV _p /V _s , which partially offsets the increase inV _p /V _s caused by fracturing.Fracturing and hydrothermal alteration affect fault mechanics. Low modulus rock surrounding fault surfaces increases the probability of exceeding the critical slip distance required for the onset of unstable slip during rupture initiation. Boundaries between low modulus fault rock and higher modulus wall rock also act as rupture guides and enhance rupture acceleration to dynamic velocity. Hydrothermal alteration at temperatures in excess of 300°C weakens the deeper parts of the fault zone by producingphyllitic mineral assemblages. Sealing of fracture in time periods between large earthquakes generates pods of abnormally pressured fluid which may play a fundamental role in the initiation of large earthquakes.     

A structural model for active extension in Central Italy

This work presents a structural model for earthquake faulting in the Umbria-Marche Apennines (Central Italy). The model is derived by an integrated analysis of geological, geophysical and seismological data. At regional scale, the distribution and character of the seismicity appear to be mainly controlled by a low-angle east-dipping normal fault (Altotiberina fault, AF). The latter is the lower boundary of an active, continuously deforming hangingwall block moving toward NE. Moderate magnitude earthquakes (4 < M < 6), such as the Norcia 1979 (M = 5.9), the Gubbio 1984 (M = 5.2) and the Colfiorito 1997 (M_max = 5.9), occur within the active hangingwall block and are related to the activity of major west-dipping normal faults detaching on the AF. The geometry of the deep seismogenic structures is listric (as in the case of Colfiorito) or more complex, because of local reactivation of pre-existing low-angle thrust (e.g. Gubbio) or high-angle strike-slip faults (e.g. Norcia). For all the analysed earthquakes the rupture nucleation is located at the base of the aftershock volumes, near the line of intersection between the SW-dipping normal faults and the east-dipping AF basal detachment. The progressive increase in depth of the earthquake foci from the north–west (e.g. Gubbio, 6–7 km) to the south–east (e.g. Norcia, 11–12 km) appears to be related to the eastward deepening of the basal detachment. These seismotectonic features are relevant for determining the seismogenic potential of the Apennine active faults, which depends not only on the length of the faults, but also on the depth of the detachment zone as well.     

Fluids in the lithosphere, 1. Experimentally-determined wetting characteristics of CO2H2O fluids and their implications for fluid transport, host-rock physical properties, and fluid inclusion formation

The equilibrium distribution of CO₂H₂O fluids in synthetic rock samples (principally dunite and quartzite) has been characterized by measurements of the dihedral wetting angle, θ, resulting from 5-day annealing periods at 950–1150°C and 1 GPa. For fluids in equilibrium with polycrystalline quartz, θ varies systematically from 57° for pure H₂O to 90° at X_CO2 0.9. Similarly, for San Carlos olivine, θ varies from 65° for pure H₂O to 90° at X_CO2 0.9. The addition of solutes (NaCl, KCl, CaF₂, Na₂CO₃) to H₂O causes a major decrease in θ in the quartz/fluid system (to values as low as 40°), but has no effect on fluid wetting in dunite. Reconnaissance experiments on other mono- and polymineralic aggregates indicate universally high wetting angles (θ 60°) in upper mantle assemblages and for CO₂ in felsic compositions. For diopside + H₂O, θ 80°, with large variation due to crystalline anisotropy. In no case does θ approach 0°, the condition necessary for fluid to be present along all grain boundaries.Because a value of θ greater than 60° precludes the existence of an interconnected fluid phase in a rock, our results have important implications not only for fluid transport but also for the physical properties of the bulk fluid/rock system. Any static fluid present in the upper mantle must exist as isolated pores located primarily at grain corners, and transport can occur only by hydrofracture. In the continental crust, aqueous fluids (especially saline ones) are likely to form an interconnected network along grain edges, thus contributing to high electrical conductivity and allowing the possibility of fluid transport by porous flow or surface energy-driven infiltration.     

Fluid transport by solitary waves along growing faults: A field example from the South Eugene Island Basin, Gulf of Mexico

The Red Fault system is one of the main growth faults found in the South Eugene Island Basin, a salt withdrawal minibasin located offshore Louisiana, in the Gulf of Mexico. This fault system corresponds to a lateral boundary between fluid overpressured compartments. In addition, there is a set of observations indicating that the Red Fault system exhibits rapid episodic migration of fluids. This fault represents an example of preferential pathway for the upward episodic migration of overpressured hydrocarbons from deep, heavily pressured, compartments on time scales of years. The migrations of fluids into active growing faults could take the form of propagating surges (solitary waves) that propagate upward along the fault planes in a wave-like manner at km/yr. Solitary waves represent a very efficient mechanism for the upward transport of fluids along growth faults in sedimentary basins generating its own permeability. In addition, this mechanism is compatible with the fact that the fault plane is observed to sustain a static pore fluid pressure difference between its two sides. The propagation of solitary waves in active growth faults appears as a fundamental mechanism to understand the nature of upward fast migration of fluids along active growth faults in compartimentalized sedimentary basins.     

拉分盆地形成机制三维数值模拟——以海原断裂带老龙湾盆地为例

拉分盆地是走滑断层系中受拉伸作用形成的断陷盆地.一般在两条平行断层控制下发育.盆地形似菱形,几何形态主要受两条主控走滑断层错距和叠接长度影响.本文以青藏高原东北缘海原断裂带老龙湾拉分盆地第四纪所处的构造环境为基础,参考盆地周围断层几何分布,建立了三维有限元数值模型,模拟该拉分盆地的演化过程;进一步分析了断层力学性质、地壳分层结构等各因素对盆地形成和演化的影响.模拟结果显示,盆地地表沉降伴随有下地壳物质的上涌,此上涌对盆地地表沉降存在阻碍作用.各因素的影响具体表现为:(1)断层力学性质(弹性模量和黏滞系数)越弱,其对构造应力较低的传递效率导致盆地两端差异性运动越明显,从而形成较大的盆地地表沉降和明显的上地壳减薄.(2)平行主控断层的叠接长度反映盆地形成的拉伸作用范围,叠接长度越大,相同的差异性运动在单位面积形成的拉伸应力越小,盆地地表沉降较小.(3)下地壳流变性影响其物质的上涌量,下地壳黏滞系数越小,其对上部拉伸作用的响应越明显,上涌量越大,此上涌对上地壳沉降形成的阻碍作用也越明显.根据老龙湾拉分盆地所处的构造格局,将平行断层的叠接长度取20km,当断层黏滞系数取值为周围基岩的1/10,参考该盆地第四纪构造演化历史,模拟得到的盆地第四纪下沉量与盆地内第四系沉积层厚度在规模上近似,下地壳黏滞系数取值在(2.5~5.0)×1021 Pa·s范围内时,盆地下沉量模拟结果与老龙湾拉分盆地第四系地层厚度吻合较好.     

A numerical method for determining the state of stress using focal mechanisms of earthquake populations: application to Tibetan teleseisms and microseismicity of Southern Peru

The numerical method described in this paper enables the study in terms of stress of the kinematics of seismic faults provided by focal mechanisms. This method assumes a mean state of stress in the source region and is based on the simple mechanical model used for fault population analysis which supposes slip in the direction of the resolved shear stress acting on the fault plane. The proposed algorithm first defines compressional and tensional zones resulting from superimposition of the compressional and tensional quadrants limited by the nodal planes. This enables one to test the data homogeneity. Furthermore, this restricts the space where the principal stress axes have to be searched. Then, for each principal stress reference whose location is constrained by above confined zones, the R value (chosen equal to (σ₂′ − σ₁′)/(σ₃′ − σ₁′)) is computed which fits the slip vector on each nodal plane. This permits one to select a set of preferred seismic fault planes from a set of auxiliary planes. Finally, a state of stress is computed from the preferred seismic fault plane set using the non-linear simplex method already applied to fault populations. This algorithm is constructed so as to avoid two major difficulties: misleading estimation of the deviatoric stress tensor which may result from excessive emphasis by minimizing the residuals by a least squares method if some of the data are wrong and a lengthy prospection of the stress references over all the space of directions. This calculation does not take into account possible changes of the strike and dip of the nodal planes. However, we use rotations of nodal planes in agreement with the first arrival data to test the compatibility of these data with the computed state of stress. Tibetan teleseisms and southern Peruvian microseisms are analyzed and results are compared with kinematics of superficial recent and active faults measured in field in the same regions. This suggests that major seismic events may give a good evaluation of the regional (mean) state of stress. Focal mechanisms of the small events are also modelled by a mean stress deviator, but this differs significantly from the regional state of stress in southern Peru deduced from Quaternary faulting. This may be due to imprecise location of the small events. However, some movements are strongly different from those predicted by the computed mean stress deviator. This suggests that small fault kinematics surely depend on the regional state of stress but that local effects may control the small deformations.     

Late Cenozoic faulting in SW Bulgaria: Fault geometry,kinematics and driving stress regimes. Implications for late orogenic processes in the Hellenic hinterland

We investigate the geometry and kinematics of the faults exposed in basement rocks along the Strouma River in SW Bulgaria as well as the sequence of faulting events in order to place constraints on the Cenozoic kinematic evolution of this structurally complex domain. In order to decipher the successive stress fields that prevailed during the tectonic history, we additionally carried out an analysis of mesoscale striated faults in terms of paleostress with a novel approach. This approach is based on the P–T axes distribution of the fault-slip data, and separates the fault-slip data into different groups which are characterized by kinematic compatibility, i.e., their P and T axes have similar orientations. From these fault groups, stress tensors are resolved and in case these stress tensors define similar stress regimes (i.e., the orientations of the stress axes and the stress shape ratios are similar) then the fault groups are further unified. The merged fault groups after being filled out with those fault-slip data that have not been incorporated into the above described grouping, but which present similar geometric and kinematic features are used for defining the final stress regimes. In addition, the sequence of faulting events was constrained by available tectonostratigraphic data.Five faulting events named D1, D2, D3, D4 and D5 are distinguished since the Late Oligocene. D1 is a pure compression stress regime with σ₁ stress axis trending NNE-SSW that mainly activated the WNW-ESE to ENE-WSW faults as reverse to oblique reverse and the NNW-SSE striking as right-lateral oblique contractional faults during the Latest Oligocene-Earliest Miocene. D2 is a strike-slip − transpression stress regime with σ₁ stress axis trending NNE-SSW that mainly activated the NNW-SSE to N-S striking as right-lateral strike-slip faults and the ENE-WSW striking faults as left-lateral strike-slip ones during the Early-Middle Miocene. D3 extensional event is associated with a NW-SE to WNW-ESE extension causing the activation of mainly low-angle normal faults of NE-SW strike and NNE-SSW to NNW-SSE striking high-angle normal faults. D4 is an extensional event dated from Late Miocene to Late Pliocene. It activated NNW-SSE to NW-SE faults as normal faults and E-W to WNW-ESE faults as right-lateral oblique extensional faults. The latest D5 event is an N-S extensional stress regime that dominates the wider area of SW Bulgaria in Quaternary times. It mainly activated faults that generally strike E-W (ENE-WSW and WNW-ESE) normal faults, along which fault-bounded basins developed. The D1 and D2 events are interpreted as two progressive stages of transpressional tectonics related to the late stages of collision between Apulia and Eurasia plates. These processes gave rise to the lateral extrusion of the Rhodope and Balkan regions toward the SE along the Strouma Lineament. The D3 event is attributed to the latest stage of this collision, and represents the relaxation of the overthickened crust along the direction of the lateral extrusion. The D4 and D5 events are interpreted as post-orogenic extensional events related to the retreat of the Hellenic subduction zone since the Late Miocene and to the widespread back-arc Aegean extension still prevailing today.     

Neotectonics of the Dinarides–Pannonian Basin transition and possible earthquake sources in the Banja Luka epicentral area

This study provides evidence for post-5 Ma shortening in the transition area between the Dinarides fold-and-thrust belt and the Pannonian Basin and reviews possible earthquake sources for the Banja Luka epicentral area (northern Bosnia and Herzegovina) where the strongest instrumentally recorded earthquake (M_L 6.4) occurred on 27 October 1969. Geological, geomorphological and reflection seismic data provide evidence for a contractional reactivation of Late Palaeogene to Middle Miocene normal faults at slip rates below 0.1 mm/a. This reactivation postdates deposition of the youngest sediments in the Pannonian Basin of Pontian age (c. 5 Ma). Fault plane solutions for the main 1969 Banja Luka earthquake (M_L 6.4) and its largest foreshock (M_L 6.0) indicate reverse faulting along ESE–WNW-striking nodal planes and generally N–S trending pressure axes. The spatial distribution of epicentres and focal depths, analyses of the macroseismic field and fault-plane solutions for several smaller events suggest on-going shortening in the internal Dinarides. Seismic deformation of the upper crust is also associated with strike-slip faults, likely related to the NE–SW trending, sinistral Banja Luka fault. Possibly, this fault transfers contraction between adjacent segments of the Dinarides thrust system. The study area represents the seismically most active region of the Dinarides apart from the Adriatic Sea coast and the bend zone around Zagreb. We propose that on-going thrusting in the internal Dinarides thrust system takes up a portion of the current Adria–Europe convergence.     

Conduit system and formation mechanism of heat fluids in diapiric belt of Yinggehai basin, China

The conduit system of heat fluids in diapiric belt of Yinggehai basin is dominantly vertical faults and fractures. Detailed research on the formation mechanism and their occurrence features shows that the faults and fractures can be classified into three types: intrastratal dispersive hydrofracture, puncturing fault and upwarping-extensional fault. The development of the fault and fracture system not only resulted in the changes of the temperature and pressure fields in the basin, but also affected the hydrocarbon migration in the overpressured system. These faults and fractures constituted the main pathways for vertical hydrocarbon migration, and opening and closing intermittently led to episodic expulsion of overpressured fluid compartment. Thus there formed the pool-forming model of multi-source mixing and ploy-stage migration and accumulation for hydrocarbons in the Yinggehai basin. Project jointly supported by the Ministry of Science and Technology of China (Grant No.95-Pre-39) and the National Natural Science Foundation of China (Grant No. 49732005), and A Hundred Excellent Researchers Foundation from the Ministry of Land and Resources.     

Relationships between volcanic patterns and neotectonics in Eastern Anatolia from analysis of satellite images and DEM

The late Neogene to Quaternary volcanism in Eastern Anatolia is related to the Arabia–Eurasia convergence but a clear deformation pattern has not yet been established in this region. We have used the distribution and shape of volcanoes and fault geometry as indicators of the tectonic regime. Volcanic edifices and related faults were analyzed in vertical view using SAR–ERS, Spot images and a Digital Elevation Model (DEM). In several places, adjacent volcanoes that form linear clusters or elongated volcanoes are clearly rooted on vertical tension fractures. These are compatible with horizontal σ₃ striking 90°N, associated with σ₁ horizontal (strike-slip regime) or vertical (extensional regime). We mapped the recent faults that are directly associated to volcanoes. Volcanic vents are related to tail-crack, horsetail or releasing bend structures. In this work, it has been possible to define the ESE-striking, 270-km-long Tutak–Hamur–Çaldiran fault that forms a releasing bend testifying to right-lateral motion. Extension is well documented for few places but no recent fold has been observed. Since 8 Ma, the tectonic system is principally strike-slip. Most of the tension fractures being 2 to 10 km in length, so we infer that they affect only part of the crust. Most strike-slip fault zones are of several tens to a few hundred kilometers long and thus not of lithospheric scale. Therefore, the channels used by the magma to reach the surface are crustal structures.     

2016年阿克陶MS 6.7地震地表破裂特征和帕米尔现代构造应力场初步分析

帕米尔高原位于地中海－喜马拉雅地震带上,晚新生代以来随着印度板块向欧亚板块持续不断地挤压汇聚,其构造运动是欧亚大陆最强烈的地区。高原腹地发育一系列近SN向正断层,包括近SN向的塔什库尔干正断层所处的帕米尔中部现代区域的构造应力场以EW向水平拉张为主。2016年11月25日发生的阿克陶M_S 6.7级地震的发震构造为塔什库尔干断层分支的NWW向木吉盆地北缘断层,其具有右旋走滑兼正断性质。地震在震中附近产生同震地表形变带,全长约1km,呈近SN－NNE向水平拉伸,发育近EW—NWW向的张裂缝,为地震破裂的产物,张裂缝的最大水平拉伸位移量和最大垂直位移量分别为46cm和16cm。地表破裂带中的NE和NW向张剪裂缝只是连接贯通这些雁列的张裂缝,其水平相对位移量取决于张裂缝的水平拉伸量和张裂缝之间的几何关系。地表形变带表现的拉张性质与帕米尔高原腹地区域现代应力场最大主压应力为垂直向基本一致,可能与深部热物质上涌造成的上地壳拉伸有关。而地表形变带呈近SN向水平拉张,与区域近EW向拉张应力场之间存在显著差异,这可能是木吉盆地北缘右旋走滑正断层阶区局部应力场调整的结果。