Fluid ascent through the solid lithosphere and its relation to earthquakes

The Earth is continuously expelling gases and liquids from great depths—juvenile volatiles from the mantle and recycled metamorphic products. Some of these fluids ascend through liquid rock in volcanic processes, but others utilize fractures and faults as conduits through the solid lithosphere. The latter process may have a major influence on earthquakes, since fluids at near lithostatic pressures appear to be required to activate deep faults that would otherwise remain locked.Fluids can be driven upward through solid rock by buoyancy, but only if present in sufficient concentration to form large-scale domains occupying interconnected fracture porosity. A growing fluid domain becomes so mobilized only when it attains the critical vertical dimension required for hydrostatic instability. This dimension, depending on the ultimate compressive yield strength of the rock, may be as much as several kilometers.Any column of fluid ascending through fractures in the solid lithosphere from a prolific deep source must become organized into a vertical sequence of discrete domains, separated by fluid-pressure discontinuities. This is required because a continuous hydrostatic-fluid-pressure profile extending from an arbitrarily deep source to the surface cannot be permitted by the finite strength of rock. A vertically stacked sequence of domains allows the internal fluid-pressure profile to approximate the external rock-stress profile in a stepwise fashion. The pressure discontinuity below the base of the uppermost hydrostatic domain may be responsible for some occurrences of so-called anomalous geopressures. An ascending stream of fluid that percolates upward from a deep source through a column of domains must encounter a sequence of abrupt pressure decreases at the transitions between successive domains. If supercritical gases act as solvents, the dissolved substances may drop out of solution at such pressure discontinuities, resulting in a local concentration of minerals and other substances.At great depths, brittle fracture would normally be prevented by high pressure and temperature, with all excessive stress discharged by ductile flow. Rock strata invaded by an ascending fluid domain are weakened, however, because cracks generated or reactivated by the high-pressure fluid can support the overburden, greatly reducing internal friction. This reduction of strength may cause a previously stressed rock to fail, resulting in hydraulic shear fracture. Thus, earthquakes may be triggered by the buoyant migration of deep-source fluids.The actual timing of the failure that leads to such an earthquake may be determined by the relatively rapid inflation of a fluid domain and not by any significant increase in the probably much slower rate of regional tectonic strain. Many earthquake precursory phenomena may be secondary symptoms of an increase in pore-fluid pressure, and certain coseismic phenomena may result from the venting of high-pressure fluids when faults break the surface. Instabilities in the migration of such fluid domains may also contribute to or cause the eruption of mud volcanoes, magma volcanoes, and kimberlite pipes.     

活断层上覆盖层中土壤氡浓度分布的数值模拟

本文利用实测地质剖面建立了活断层上覆盖层中氡迁移的地质-物理模型,根据氡的迁移机理建立了活断层上氡迁移的数学模型,采用有限差分方法求解氡迁移的二维微分方程的数值解,得到了直立活断层,倾斜活断层,地堑活断层,活动断裂带或破碎带上覆盖层中氡浓度的二维断面等值图和横切断层的剖面曲线,分析了氡浓度分布的二维断面特征和剖面曲线特征,它们与氡的迁移机制及活断层调查的实测剖面曲线相符,这些为活断层的调查和进一步反演奠定了理论基础.     

Imaging Pathways in Fractured Rock Using Three‐Dimensional Electrical Resistivity Tomography

Major challenges exist in delineating bedrock fracture zones because these cause abrupt changes in geological and hydrogeological properties over small distances. Borehole observations cannot sufficiently capture heterogeneity in these systems. Geophysical techniques offer the potential to image properties and processes in between boreholes. We used three‐dimensional cross borehole electrical resistivity tomography (ERT) in a 9 m (diameter) × 15 m well field to capture high‐resolution flow and transport processes in a fractured mudstone contaminated by chlorinated solvents, primarily trichloroethylene. Conductive (sodium bromide) and resistive (deionized water) injections were monitored in seven boreholes. Electrode arrays with isolation packers and fluid sampling ports were designed to enable acquisition of ERT measurements during pulsed tracer injections. Fracture zone locations and hydraulic pathways inferred from hydraulic head drawdown data were compared with electrical conductivity distributions from ERT measurements. Static ERT imaging has limited resolution to decipher individual fractures; however, these images showed alternating conductive and resistive zones, consistent with alternating laminated and massive mudstone units at the site. Tracer evolution and migration was clearly revealed in time‐lapse ERT images and supported by in situ borehole vertical apparent conductivity profiles collected during the pulsed tracer test. While water samples provided important local information at the extraction borehole, ERT delineated tracer migration over spatial scales capturing the primary hydrogeological heterogeneity controlling flow and transport. The fate of these tracer injections at this scale could not have been quantified using borehole logging and/or borehole sampling methods alone.     

Field-based description of near-surface crustal deformation in a high-strain shear zone: A case study in southern Kyushu,Japan

This study investigated geological evidence for near-surface crustal deformation in a high-strain shear zone that has been geodetically identified but which is not associated with obvious tectonic landforms. Fieldwork was conducted in the east–west-trending southern Kyushu high-strain shear zone (SKHZ), Japan, focusing mainly on occurrences of fracture zones, which are defined by a visible fracture density of >1 per 10 cm² and are commonly associated with cataclasite, fault breccia, and gouge. The area in which east–west-trending fracture zones are dominant is restricted to the east–west-trending, ~2-km-wide aftershock area of the 1997 Northwestern Kagoshima Earthquakes. Analysis of slip data from minor faults using the multiple inverse method, irrespective of whether the faults are in fracture zones, reveals that the area where the calculated main stress field is consistent with the current stress field estimated from focal-mechanism solutions of microearthquakes is restricted to the east–west-trending aftershock area. This finding for the SKHZ contrasts with the case of the Niigata–Kobe Tectonic Zone, which is a major strain-concentration zone with many exposed active faults in central Japan and for which the stress field estimated using fault-slip data is considered to be uniform and coincides with the current stress field. The cumulative amount of displacement estimated from the areal density of fracture zones in the SKHZ study area is smaller than that estimated from geodetically measured strain rates. Investigations based on slip data from minor faults and fracture-zone occurrence could help to identify concealed faults that are too small to generate tectonic landforms but which are sufficiently large to trigger major earthquakes.     

Estimating depth‐averaged velocities in rough ­channels

Profiles of streamwise velocity obtained from North Boulder Creek, Colorado, typically are non‐logarithmic in form and exhibit the strong influence of form drag associated with coarse bed roughness. The spatially averaged profile is consistent with recent theoretical profile forms suggested for rough channels that are based on a partitioning of the total stress between a fluid part and a part associated with form drag on bed particles. Estimates of local depth‐averaged velocity using algorithms that are based on several measurements in the flow column improve with explicit Riemann averaging, versus simple averaging, of the measurements. Estimates based on a single‐point measurement at 0·6 of the flow depth, assuming a logarithmic or approximately logarithmic velocity profile, are the least reliable. Copyright © 2000 John Wiley & Sons, Ltd.     

Structural Properties and Deformation Patterns of Evolving Strike-slip Faults: Numerical Simulations Incorporating Damage Rheology

We present results on evolving geometrical and material properties of large strike-slip fault zones and associated deformation fields, using 3-D numerical simulations in a rheologically-layered model with a seismogenic upper crust governed by a continuum brittle damage framework over a viscoelastic substrate. The damage healing parameters we employ are constrained using results of test models and geophysical observations of healing along active faults. The model simulations exhibit several results that are likely to have general applicability. The fault zones form initially as complex segmented structures and evolve overall with continuing deformation toward contiguous, simpler structures. Along relatively-straight mature segments, the models produce flower structures with depth consisting of a broad damage zone in the top few kilometers of the crust and highly localized damage at depth. The flower structures form during an early evolutionary stage of the fault system (before a total offset of about 0.05 to 0.1 km has accumulated), and persist as continued deformation localizes further along narrow slip zones. The tectonic strain at seismogenic depths is concentrated along the highly damaged cores of the main fault zones, although at shallow depths a small portion of the strain is accommodated over a broader region. This broader domain corresponds to shallow damage (or compliant) zones which have been identified in several seismic and geodetic studies of active faults. The models produce releasing stepovers between fault zone segments that are locations of ongoing interseismic deformation. Material within the fault stepovers remains damaged during the entire earthquake cycle (with significantly reduced rigidity and shear-wave velocity) to depths of 10 to 15 km. These persistent damage zones should be detectable by geophysical imaging studies and could have important implications for earthquake dynamics and seismic hazard.     

Applicability of the Dual‐Domain Model to Nonaggregated Porous Media

More theoretical analysis is needed to investigate why a dual‐domain model often works better than the classical advection‐dispersion (AD) model in reproducing observed breakthrough curves for relatively homogeneous porous media, which do not contain distinct dual domains. Pore‐scale numerical experiments presented here reveal that hydrodynamics create preferential flow paths that occupy a small part of the domain but where most of the flow takes place. This creates a flow‐dependent configuration, where the total domain consists of a mobile and an immobile domain. Mass transfer limitations may result in nonequilibrium, or significant differences in concentration, between the apparent mobile and immobile zones. When the advection timescale is smaller than the diffusion timescale, the dual‐domain mass transfer (DDMT) model better captures the tailing in the breakthrough curve. Moreover, the model parameters (mobile porosity, mean solute velocity, dispersivity, and mass transfer coefficient) demonstrate nonlinear dependency on mean fluid velocity. The studied case also shows that when the Peclet number, Pe, is large enough, the mobile porosity approaches a constant, and the mass transfer coefficient can be approximated as proportional to mean fluid velocity. Based on detailed analysis at the pore scale, this paper provides a physical explanation why these model parameters vary in certain ways with Pe. In addition, to improve prediction in practical applications, we recommend conducting experiments for parameterization of the DDMT model at a velocity close to that of the relevant field sites, or over a range of velocities that may allow a better parameterization.     

Surface stresses associated with arrested dykes in rift zones

Many theoretical models predict that arrested dykes may generate major grabens at rift-zone surfaces. Arrested dyke tips in eroded rift zones, however, are normally not associated with major grabens or normal faults that could be generated by dyke-induced stresses ahead of the tips, and normal faults and grabens tend to be less common in those parts of eroded rift zones where dykes are comparatively abundant. Similarly, there are feeder dykes, as well as dykes arrested a few metres below the surface, that do not generate faults or grabens at the surface. Here I propose that this discrepancy between theoretical models and field observations may be explained by the mechanical layering of the crust. Numerical models presented here show that abrupt changes in Young's moduli, layers with high dyke-normal compressive stresses (stress barriers), and weak, horizontal contacts have large effects on the dyke-induced stress fields. For the models considered, the surface tensile stresses induced by arrested dykes are normally too small to lead to significant fault or graben formation at the rift-zone surface. The only significant dyke-induced surface tensile stresses (2 MPa) in these models are for a dyke tip arrested at 1 km depth below the surface of a rift zone with a weak contact at 400 m depth and subject to extension. That tensile stress, however, peaks above the ends of the weak horizontal contact, which, in the model considered, occur at distances of 4 km to either side of the dyke, and shows no simple relation to the depth to the dyke tip. Thus, for a layered crust with weak contacts, straightforward inversion of surface geodetic data to infer dyke geometries may result in unreliable results.Editorial responsibility: A. Woods     

鲜水河断裂带多断层相互作用的流变断裂力学分析

本文研究了流变介质内多断层的相互影响,通过对鲜水河断裂带的分析,对该断裂带提出一个不共线三断层的断裂力学模型.把介质考虑成流变的,用有限元法结合解析方法求解了流变断裂力学问题的应力、应变及能量场的时空变化.结果表明,多断层的力学场与单一断层的力学场相差甚远,在断层间形成了影响区.在影响区内应力集中,梯度大,分布复杂,应变能在影响区内形成较大范围集中.应变能等值线在孕震期间从影响区向外扩张,能量从外围流向断层区,流动在时空上都是不均匀的,孕育初期增加速率大,后期渐渐平缓.流向影响区的能量比其周围区域大,形成很大梯度.并讨论了地震前兆的某些特征.根据本文的结果,认为在道孚-乾宁-带发生地震的可能性较大.     

Fluid involvement in normal faulting

Evidence of fluid interaction with normal faults comes from their varied role as flow barriers or conduits in hydrocarbon basins and as hosting structures for hydrothermal mineralisation, and from fault-rock assemblages in exhumed footwalls of steep active normal faults and metamorphic core complexes. These last suggest involvement of predominantly aqueous fluids over a broad depth range, with implications for fault shear resistance and the mechanics of normal fault reactivation. A general downwards progression in fault rock assemblages (high-level breccia-gouge (often clay-rich) → cataclasites → phyllonites → mylonite → mylonitic gneiss with the onset of greenschist phyllonites occurring near the base of the seismogenic crust) is inferred for normal fault zones developed in quartzo-feldspathic continental crust. Fluid inclusion studies in hydrothermal veining from some footwall assemblages suggest a transition from hydrostatic to suprahydrostatic fluid pressures over the depth range 3–5 km, with some evidence for near-lithostatic to hydrostatic pressure cycling towards the base of the seismogenic zone in the phyllonitic assemblages. Development of fault-fracture meshes through mixed-mode brittle failure in rock-masses with strong competence layering is promoted by low effective stress in the absence of thoroughgoing cohesionless faults that are favourably oriented for reactivation. Meshes may develop around normal faults in the near-surface under hydrostatic fluid pressures to depths determined by rock tensile strength, and at greater depths in overpressured portions of normal fault zones and at stress heterogeneities, especially dilational jogs. Overpressures localised within developing normal fault zones also determine the extent to which they may reutilise existing discontinuities (for example, low-angle thrust faults). Brittle failure mode plots demonstrate that reactivation of existing low-angle faults under vertical σ₁ trajectories is only likely if fluid overpressures are localised within the fault zone and the surrounding rock retains significant tensile strength. Migrating pore fluids interact both statically and dynamically with normal faults. Static effects include consideration of the relative permeability of the faults with respect to the country rock, and juxtaposition effects which determine whether a fault is transmissive to flow or acts as an impermeable barrier. Strong directional permeability is expected in the subhorizontal σ₂ direction parallel to intersections between minor faults, extension fractures, and stylolites. Three dynamic mechanisms tied to the seismic stress cycle may contribute to fluid redistribution: (i) cycling of mean stress coupled to shear stress, sometimes leading to postfailure expulsion of fluid from vertical fractures; (ii) suction pump action at dilational fault jogs; and, (iii) fault-valve action when a normal fault transects a seal capping either uniformly overpressured crust or overpressures localised to the immediate vicinity of the fault zone at depth. The combination of σ₂ directional permeability with fluid redistribution from mean stress cycling may lead to hydraulic communication along strike, contributing to the protracted earthquake sequences that characterise normal fault systems.     

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.     

青藏高原东缘川滇构造区深部电性结构特征

本文对位于青藏高原东缘川滇构造区的贡山一绥江大地电磁测深(MT)剖面数据进行反演,获得沿剖面的深部电性结构,为研究喜马拉雅东构造结、川滇菱形地块与华南地块的构造变形特征、壳幔耦合关系、地块间接触关系以及相互作用等问题,提供电性结构的依据.研究发现:(1)电性结构揭示澜沧江断裂带和小金河断裂带为深大断裂带,控制着研究区的深部结构特征和形变机制;(2)澜沧江断裂带和金沙江断裂带之间的高阻体,可能是扬子古地块的残留部分;小金河断裂带和安宁河断裂带之间的高阻体,则是峨眉山大火山省喷发形成的冕宁一越西杂岩带;(3)在滇西地块、川滇地块和大凉山地块均存在低阻层,它们的介质属性有所不同,滇西地块下的低阻层"疑似"高热状态的岩浆囊,主要由缅甸弧向东俯冲运动引起的,中上地壳的高热状态使地块的活动性增强;川滇地块内部的壳内低阻层的成因为:理塘断裂带和小金河断裂带之间的地表低阻层由破碎带充水所致,而金沙江断裂带和理塘断裂带之间的中地壳低阻层可能是由局部熔融物质或含盐流体导致的,其为壳内物质运移的通道.从而在地下物质发生大规模走滑运动的过程中起到引导作用;川滇地块东部和大凉山地块西部的壳内低阻层可能与地慢物质的上涌有关;马边断裂带附近的低阻体可能与破碎带变宽和破碎带内的流体有关.     

Variations in thickness of fault‐controlled dikes and its implications: A case study of Gosung area,South‐East Korea

Dikes are natural records that can be used to understand the way magma flows in the crust. Coastal platform outcrops in Gosung, South Korea, show clear evidences that their intrusion took place along pre‐existing fractures. We analyzed outcropping dikes, measuring variations in dike thickness as well as fracture density (cumulative number of fractures along strike) and geometry around the dikes. The geometry and thickness variations of dikes intruded along pre‐existing fractures can be interpreted to understand the effect of pre‐existing fractures to evolution on magma flow, especially related with fault damage zones. This helps us to gain a better understanding of magma and fluid flow along pre‐existing fractures. Magma flow is greater along planes that strike perpendicular to the direction of least compressive horizontal stress, and along well connected fractures that show a high degree of connectivity. At the fault tip and linkage damage zone, there is a concentration of extensional fractures; in these areas injected dikelets can form. As faults become linked, the fracture density increases, until they become fully linked and act as one through‐going fault plane. As faults evolve, the boundary conditions of the faults vary and this has an impact on dike characteristics. Fracture geometry around dikes that intruded pre‐existing faults can be used as a record of fault evolution and this can give insights into how the maturity of a fault system can affect to the related magma or fluid flow characteristics.     

Time-series temperature measurements at high-temperature hydrothermal vents, East Pacific Rise 9°49′–51′N: evidence for monitoring a crustal cracking event

Temperature measurements of hydrothermal vent fluids provide an important indicator of the physical and chemical state of mid-ocean ridge crest hydrothermal and magmatic systems. Changes in vent fluid temperature and chemistry can have dramatic effects on biological communities that inhabit these unique ecosystems. In an attempt to understand temporal variability of ridge crest hydrothermal activity as it relates to geological processes at the ridge axis, six high-temperature hydrothermal vents on the East Pacific Rise crest between 9°49′N and 9°51′N were instrumented and sampled repeatedly during five years following a submarine volcanic eruption in 1991. Bio9 vent, located on the floor of the axial trough near 9°50.2′N, has the most complete record of fluid temperatures from 1991 to 1997, including a continuous temperature record of nearly three years (1994–1997). Bio9 vent fluids were 368°C in 1991, increased to an estimated temperature ≥388°C after a second volcanic event in 1992, and thereafter declined over the next 2 years reaching a temperature of 365°C in December 1993. Continuous temperature records and point measurements made by Alvin's thermocouple probe show Bio9 vent fluids were stable for 15 months at 365±1°C, until March 26, 1995. On March 26, an abrupt 7°C increase occurred over a period of eight days at this vent, and a maximum temperature of 372±1°C persisted for 14 days. The vent fluid cooled gradually over 3.5 months to 366±1°C, and for several months at the end of the recording period the temperature increased a few degrees. A continuous record of fluid temperature at this vent between November 1995 and November 1997 shows a 5±1°C increase for the two-year period. The abrupt temperature increase at Bio9 vent, and coincident changes in faunal community structure, and geochemistry of vent fluids from this area suggest that a crustal event occurred, either in the form of a cracking front in the crust or intrusion of a small dike. Based on the results of a microseismicity experiment conducted around the Bio9 vent in 1995 [Sohn et al., Trans. Am. Geophys. Union 78 (1997) F647; Sohn et al., Nature (in press)], and the identification of a small earthquake swarm which occurred on March 22, 1995 we conclude that the temperature anomaly measured at Bio9 four days following the swarm was caused by a cracking front penetrating into hot crustal rocks beneath the vent.     

Experimental studies on elastic and rheological properties of amphibolites at high pressure and high temperature

The experimental studies done at high temperature and high pressure find that increased temperature can lead to dramatic velocity and strength reductions of most of rocks at high confining pressure[1,2]. What causes this phenomenon? Is it due to dehydrati…     

滇西地区地壳浅部基底速度细结构的研究

本文论述滇西地区思茅－中甸剖面基底导速度细结构的研究结果。基底层项部Ｐｇ界面深度为０－３．５ｋｍ，而底部Ｐ１＾０界面深度为１１．０－１７．０ｋｍ。基底层速度在金河－洱海断裂以南为５．７０－６．３０ｋｍ／ｓ，断裂以北增至６．３０－６．５０ｋｍ／ｓ，其过渡带位于剑川附近，剖面上断裂附近，除界面深度变化外，Ｐｇ面速度横向变化也较明显。速度等值线较为稀疏的景云桥炮南侧、大仓炮附近与支梯炮南侧地区，估计发生     

THE GEOLOGICAL AND ROCK MECHANICAL DISTINCTION EVIDENCE BETWEEN STICK-SLIP AND CREEP IN HOST ROCK SEGMENTS OF FAULT

Paleo-seismic and fault activity are hard to distinguish in host rock areas compared with soft sedimentary segments of fault. However, fault frictional experiments could obtain the conditions of stable and unstable slide, as well as the microstructures of fault gouge, which offer some identification marks between stick-slip and creep of fault. We summarized geological and rock mechanical distinction evidence between stick-slip and creep in host rock segments of fault, and analyzed the physical mechanisms which controlled the behavior of stick-slip and creep. The chemical composition of fault gouge is most important to control stick-slip and creep. Gouge composed by weak minerals, such as clay mineral, has velocity weakening behavior, which causes stable slide of fault. Gouge with rock-forming minerals, such as calcite, quartz, feldspar, pyroxene, has stick-slip behavior under condition of focal depth. To the gouge with same chemical composition, the deformation mechanism controls the frictional slip. It is essential condition to stick slip for brittle fracture companied by dilatation, but creep is controlled by compaction and cataclasis as well as ductile shear with foliation and small fold. However, under fluid conditions, pressure solution which healed the fractures and caused strength recovery of fault, is the original reason of unstable slide, and also resulted in locking of fault with high pore pressure in core of fault zone. Contrast with that, rock-forming minerals altered to phyllosilicates in the gouges by fluid flow through degenerative reaction and hydrolysis reaction, which produced low friction fault and transformations to creep. The creep process progressively developed several wide shear zones including of R, Y, T, P shear plane that comprise gouge zones embedded into wide damage zones, which caused small earthquake distributed along wide fault zones with focal mechanism covered by normal fault, strike-slip fault and reverse fault. However, the stick-slip produced mirror-like slide surfaces with very narrow gouges along R shear plane and Y shear plane, which caused small earthquake distributed along narrow fault zones with single kind of focal mechanism.     

Influence of Outcrop Scale Fractures on the Effective Stiffness of Fault Damage Zone Rocks

We combine detailed mapping and microstructural analyses of small fault zones in granodiorite with numerical mechanical models to estimate the effect of mesoscopic (outcrop-scale) damage zone fractures on the effective stiffness of the fault zone rocks. The Bear Creek fault zones were active at depths between 4 and 15 km and localize mesoscopic off-fault damage into tabular zones between two subparallel boundary faults, producing a fracture-induced material contrast across the boundary faults with softer rocks between the boundary faults and intact granodiorite outside the boundary faults. Using digitized fault zone fracture maps as the modeled fault geometries, we conduct nonlinear uniaxial compression tests using a novel finite-element method code as the experimental “laboratory” apparatus. Map measurements show that the fault zones have high nondimensional facture densities (>1), and damage zone fractures anastamose and intersect, making existing analytical effective medium models inadequate for estimation of the effective elastic properties. Numerical experiments show that the damage zone is strongly anisotropic and the bulk response of the fault zone is strain-weakening. Normal strains as small as 2% can induce a reduction of the overall stiffness of up to 75%. Fracture-induced effective stiffness changes are large enough to locally be greater than intact modulus changes across the fault due to juxtaposition of rocks of different lithologies; therefore mesoscopic fracturing is as important as rock type when considering material or bimaterial effects on earthquake mechanics. These results have important implications for earthquake rupture mechanics models, because mesoscopic damage zone fractures can cause a material contrast across the faults as large as any lithology-based material contrast at seismogenic depths, and the effective moduli can be highly variable during a single rupture event.     

紫坪铺水库区域地壳Qs成像及其与渗透关系研究

利用衰减结构层析成像方法尝试对紫坪铺水库区域地壳Q_s进行成像,获得该区浅层地壳静态及蓄水前、后的Q_s变化特征,结合实验研究成果、岩体岩性、断裂构造、水文地质条件探讨库水的渗透作用及其对地壳介质的影响,对研究区内地震活动类型及发震成因的介质物性变化进行了讨论.研究结果表明:紫坪铺水库区域地壳Q_s横向不均匀变化显著,库区周边近似存在低Q_s值环形区域,主要包括水库东北、东南、西南、西北及库区中段五个区域.其中,水库东北、库区中段、西南低Q_s区域与库水渗透关系密切,初步认为库水可能沿着通济场断裂中段和两端的岩石破碎带及节理、裂隙发育地区向地下渗透,使岩石孔隙中充满流体,内摩擦增大,地震波大大衰减,从而导致Q_s值大幅下降.同时,在西南区诱发了水库小震群,在东北区和东南区诱发了部分水库小震群.另外,紫坪铺水库区域东北、东南、西南三个小震群基本位于高Q_s值(低衰减)和低Q_s值(高衰减)过渡区域,可能由于低Q_s区地震波衰减大,不易积累能量,而高、低Q_s值的过渡区域,介质介于"软"、"硬"之间,有可能积累应变能,孕育地震.当高、低Q_s过渡区域岩石裂纹饱含水或部分含水后,水对裂纹边界起潮湿和润滑作用,降低了发震断层的抗剪强度,使滑动容易产生,从而诱发地震.     

Imaging Faults and Shear Zones Using Receiver Functions

The geometry of faults at seismogenic depths and their continuation into the ductile zone is of interest for a number of applications ranging from earthquake hazard to modes of lithospheric deformation. Teleseismic passive source imaging of faults and shear zones can be useful particularly where faults are not outlined by local seismicity. Passive seismic signatures of faults may arise from abrupt changes in lithology or foliation orientation in the upper crust, and from mylonitic shear zones at greater depths. Faults and shear zones with less than near-vertical dip lend themselves to detection with teleseismic mode-converted waves (receiver functions) provided that they have either a contrast in isotropic shear velocity (V _s), or a contrast in orientation or strength of anisotropic compressional velocity (V _p). We introduce a detection method for faults and shear zones based on receiver functions. We use synthetic seismograms to demonstrate common features of dipping isotropic interfaces and contrasts in dipping foliation that allows determination of their strike and depth without making further assumptions about the model. We proceed with two applications. We first image a Laramide thrust fault in the western U.S. (the Wind River thrust fault) as a steeply dipping isotropic velocity contrast in the middle crust near the surface trace of the fault; further downdip and across the range, where basin geometry suggests the fault may sole into a subhorizontal shear zone, we identify a candidate shear zone signal from midcrustal depths. The second application is the use of microstructural data from exhumed ductile shear zones in Scotland and in the western Canadian Shield to predict the character of seismic signatures of present-day deep crustal shear zones. Realistic anisotropy in observed shear fabrics generates a signal in receiver functions that is comparable in amplitude to first-order features like the Moho. Observables that can be robustly constrained without significant tradeoffs are foliation strike and the depth of the foliation contrast. We find that an anisotropy of only a few percent in the shear zone is sufficient to generate a strong signal, but that the shear zone width is required to be >2 km for typical frequencies used in receiver function analysis to avoid destructive interference due to the signals from the boundaries of the shear zone.