Evolution of Wear and Friction Along Experimental Faults

We investigate the evolution of wear and friction along experimental faults composed of solid rock blocks. This evolution is analyzed through shear experiments along five rock types, and the experiments were conducted in a rotary apparatus at slip velocities of 0.002–0.97 m/s, slip distances from a few millimeters to tens of meters, and normal stress of 0.25–6.9 MPa. The wear and friction measurements and fault surface observations revealed three evolution phases: A) An initial stage (slip distances <50 mm) of wear by failure of isolated asperities associated with roughening of the fault surface; B) a running-in stage of slip distances of 1–3 m with intense wear-rate, failure of many asperities, and simultaneous reduction of the friction coefficient and wear-rate; and C) a steady-state stage that initiates when the fault surface is covered by a gouge layer, and during which both wear-rate and friction coefficient maintain quasi-constant, low levels. While these evolution stages are clearly recognizable for experimental faults made from bare rock blocks, our analysis suggests that natural faults “bypass” the first two stages and slip at gouge-controlled steady-state conditions.     

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.     

Characteristics of faults and shear zones in deep mines

The characteristics of fault and shear zones to depths of 2.5 km are well documented in deep mines in North America. The characteristics may be summarized as follows. (a) Fault zones usually are irregular, branched, anastomosed, and curved rather than simple and planar. (b) Faults are generally composed of one or more clay or clay-like gouge zones in a matrix of sheared and foliated rock bordered by highly fractured rock. (c) The widths of fault zones appear to be greater when faults have greater displacement, probably as a result of a long history of repeated minor movements. Fault zones with kilometers of displacement tend to be 100 m or more wide, whereas those with only a few hundred meters of displacement commonly are only 1 m or less wide. (d) Some zones represent shear distributed across hundreds of meters without local concentration in a narrow gouge zone. (e) Many fault zones are wet even above the water table, and water moves along them at various rates, but some also serve as subsurface dams, ponding ground water as much as several hundred meters higher on one side than on the other. No striking differences in the characteristics of faults over the vertical range of 2.5 km are documented.     

断层泥力学性质与含水量关系初探

断层泥是影响断层运动的重要地质因素。研究它的力学性质和影响其力学性质的各种因素具有重要的意义。本文利用最近完成的我国五大断裂断层泥基本力学性质的实验结果,分析了含水量对断层泥力学性质的影响。发现断层泥的粘滞系数、杨氏模量、抗压强度和残余体应变均随含水量的增加而明显地减小。断层泥的韧度随含水量的增大而增大。断层泥的体积模量、弹性波速和抗剪强度与含水量的关系不明显。天然断层泥是粘土、断层碎屑和水的混合物,水在其中的作用比水对岩屑或粘土本身的作用更复杂。强调了对天然断层泥进行实验研究的重要性     

Control on seafloor spreading geometries by stress- and strain-induced lithospheric weakening

Seafloor spreading typically occurs along ridge segments oriented at right angles to plate motion and offset by orthogonal transform faults. Few regions exhibit different patterns, such as the East Pacific Rise (EPR), which additionally displays overlapping spreading centres (OSCs) and microplates. We introduce a dynamical model using two independent, scalar types of damage in an elastic plate that generates most observed spreading geometries as natural failure modes, suggesting that the dynamics of the underlying mantle have only a minor influence on accretionary plate margins. The elastic modulus that is affected by the damage determines the type of localized deformation. Damage reducing the bulk modulus tends to result in tensile fractures, while a reduction in shear modulus leads to shear fractures. The damage source determines the fracture orientation. Material weakening in tension results in fractures perpendicular to the most tensile principal stress, while shear weakening results in two conjugate fractures at 45° relative to the applied stress. Strain or energy-dependent damage results in propagating, localized fractures. Stress-dependent damage tends to evolve into diffuse regions that may eventually focus into narrow zones. Starting from small perturbations with reduced elastic moduli as nucleation points, all ridge geometries start with ridge propagation caused by tensile energy reducing both elastic moduli by a model of damage caused by tensile energy reducing both moduli. Orthogonal transform faults develop in regions between offset segments if there is an additional reduction in shear modulus due to shear stress. The orthogonality of the transform faults does not derive from the local stress orientation but from the dynamics of damage focusing which causes the fault to converge towards an optimal geometry that concentrates nearly all deformation into damaged zones. OSCs form when the shear damage leading to transform faults is suppressed, while microplate formation requires additional reduction of the shear modulus by tensile energy. Oblique spreading at 45°, recently discovered along ultraslow spreading ridges, occurs when both moduli are weakened by shear energy. A parameter regime exists in which ridge-transform patterns develop at low applied tension, while microplates form at higher stresses. These results indicate that at least three different micromechanical processes operate with different evolution rates. OSCs and oblique spreading require different material properties.     

Geometry of the Nojima Fault at Nojima-Hirabayashi, Japan – II. Microstructures and their Implications for Permeability and Strength

Samples of damage-zone granodiorite and fault core from two drillholes into the active, strike-slip Nojima fault zone display microstructures and alteration features that explain their measured present-day strengths and permeabilities and provide insight on the evolution of these properties in the fault zone. The least deformed damage-zone rocks contain two sets of nearly perpendicular (60–90° angles), roughly vertical fractures that are concentrated in quartz-rich areas, with one set typically dominating over the other. With increasing intensity of deformation, which corresponds generally to increasing proximity to the core, zones of heavily fragmented rock, termed microbreccia zones, develop between prominent fractures of both sets. Granodiorite adjoining intersecting microbreccia zones in the active fault strands has been repeatedly fractured and locally brecciated, accompanied by the generation of millimeter-scale voids that are partly filled with secondary minerals. Minor shear bands overprint some of the heavily deformed areas, and small-scale shear zones form from the pairing of closely spaced shear bands. Strength and permeability measurements were made on core collected from the fault within a year after a major (Kobe) earthquake. Measured strengths of the samples decrease regularly with increasing fracturing and fragmentation, such that the gouge of the fault core and completely brecciated samples from the damage zone are the weakest. Permeability increases with increasing disruption, generally reaching a peak in heavily fractured but still more or less cohesive rock at the scale of the laboratory samples. Complete loss of cohesion, as in the gouge or the interiors of large microbreccia zones, is accompanied by a reduction of permeability by 1-2 orders of magnitude below the peak values. The core samples show abundant evidence of hydrothermal alteration and mineral precipitation. Permeability is thus expected to decrease and strength to increase somewhat in active fault strands between earthquakes, as mineral deposits progressively seal fractures and fill pore spaces.     

Reconstructing the evolution of fault zone architecture: Field‐based study of the core region of the Atera Fault,Central Japan

In order to reconstruct the architectural evolution of a fault zone with heterogeneous structures, we studied the Atera Fault in Central Japan, and described the detailed mesoscopic and microscopic features of the zone. The fault zone studied consists of a 1.2‐m wide fault core of fault breccia mixed with fragments derived from welded tuff, granite, and mafic volcanic rocks. The 1.2‐m wide fault core is bordered by a western damage zone characterized by a welded tuff fault breccia and an eastern damage zone characterized by a granite cataclasite. A secondary fault core, a 30‐cm wide granite‐derived fault gouge, cross‐cuts the granite cataclasite. Although welded tuff fault breccia and granite cataclasite are also pervasively fractured and fragmented, the fault cores are significantly affected by fragment size reduction due to intense abrasive wear and comminution. The 1.2‐m wide fault core includes fragments and a sharp dark layer composed of mafic volcanic rocks, which can be correlated with neighboring 1.6 Ma volcanic rocks. This observation places a younger constraint on the age of the fault core formation. Carbonate coating on basalt fragments in the 1.2‐m wide fault core has also been fractured indicating the repetition of intense fragmentation. Bifurcated, black and gray veins near the 1.2‐m wide fault core are likely injection veins, formed by the rapid injection of fine material within fault zones during seismic events. The granite‐derived fault gouge, characterized by hard granite fragments without intense brecciation and microfracturing, in a kaolinite‐rich clay matrix, is interpreted as the most recent slip zone within the exposed fault zone. A preview of published geological and hydrological studies of several fault zones shows that clay‐rich fault cores can exhibit much lower permeability than the adjacent damage zones represented in this present case by the welded tuff fault breccia and granite cataclasite.     

剪切破裂与粘滑——浅源强震发震机制的研究

周口店花岗闪长岩的高温高压三轴实验和理论分析表明,剪切破裂和摩擦滑移具有类似的孕育过程和发生机制。剪切破裂贯通强度就是一种摩擦强度。剪切破裂和摩擦滑移各自都有渐进式和突发式之分。突发式摩擦滑移是已有断层的粘滑滑移。突发式剪切破裂则是完整岩石的初始粘滑滑移。考虑到地壳温度随深度增加,完整岩石剪裂强震要求较高的围压,因此,多数浅源强震的发震方式很可能是已有断层的粘滑     

Internal structures and high-velocity frictional properties of a bedding-parallel carbonate fault at Xiaojiaqiao outcrop activated by the 2008 Wenchuan earthquake

This paper reports internal structures of a bedding-parallel fault in Permian limestone at Xiaojiaqiao outcrop that was moved by about 0.5 m during the 2008 M_W7.9 Wenchuan earthquake. The fault is located about 3 km to the south from the middle part of Yingxiu-Beichuan fault, a major fault in the Longmenshan fault system that was moved during the earthquake. The outcrop is also located at Anxian transfer zone between the northern and central segments of Yingxiu-Beichuan fault where fault system is complex. Thus the fault is an example of subsidiary faults activated by Wenchuan earthquake. The fault has a strike of 243° or N63°E and a dip of 38°NW and is nearly optimally oriented for thrust motion, in contrast to high-angle coseismic faults at most places. Surface outcrop and two shallow drilling studies reveal that the fault zone is several centimeters wide at most and that the coseismic slip zone during Wenchuan earthquake is about 1 mm thick. Fault zone contains foliated cataclasite, fault breccia, black gouge and yellowish gouge. Many clasts of foliated cataclasite and black gouge contained in fault breccia indicate multiple slip events along this fault. But fossils on both sides of fault do not indicate clear age difference and overall displacement along this fault should not be large. We also report results from high-velocity friction experiments conducted on yellowish gouge from the fault zone using a rotary shear low to high-velocity frictional testing apparatus. Dry experiments at normal stresses of 0.4 to 1.8 MPa and at slip rates of 0.08 to 1.35 m/s reveal dramatic slip weakening from the peak friction coefficient of around 0.6 to very low steady-state friction coefficient of 0.1-0.2. Slip weakening parameters of this carbonate fault zone are similar to those of clayey fault gouge from Yingxiu-Beichuan fault at Hongkou outcrop and from Pingxi fault zone. Our experimental result will provide a condition for triggering movement of subsidiary faults or off-fault damage during a large earthquake.     

沂沭断裂带及其附近断裂的断层泥分形特征及其地震地质意义

利用岩石碎裂数目的分形理论，分析，计算了沂沭断裂带及其西侧北西向断裂的断层泥粒度成分的分维值，讨论了断层泥粒度成分分维的地震地质意义，研究结果表明，沂沭断裂带内的断裂活动的强度大于北西向断裂活动强度；Ｆ２是沂沭断裂带中活动最强的一条断裂，断层泥粒度成分分维值可作为表征断裂活动时代，破裂形式和断层泥形成斫代等的参量；分维值还与断层泥的母岩，厚度，粘土矿物含量和所处的断裂部位等相关。     

Structural and material records of paleoearthquakes in terrigenous rocks: Analysis and interpretation

The results of an instrumental and analytical investigation of the products of mineral and textural transformations in the tectonic slickensides and fault gouge in the near-surface terrigenous sedimentary rocks (clays, arkose sandstones, shungites) which have undergone localized deformations in fault zones of presumably seismogenic origin are presented. Based on this, several peculiarities in the behavior of a dynamic slip in the upper transition horizon from aseismic to seismogenic mode of faulting in the Earth’s crust are elucidated. The changes in the mineral phase compositions of the fault facies against the protolith composition are estimated; the parameters of the temperature regime and thermal energy balance of deformational metamorphic reactions are determined. The probable causes of instability in the faults, the mechanisms of the loss of strength, the weakening and strengthening during seismogenic dynamic slip are considered. The role of tribochemical phenomena in the course of a rock’s transformation into a fault gouge and the related energy effects are discussed. An inventory of the possible energy costs on the processes of transforming material in dynamic slip zones is compiled.     

红河断裂带断层泥基本力学性质的初步研究

本文对红河断裂带断层泥的某些基本力学性质进行了实验研究,得到红河断层泥的体积压缩模量为104巴的量级,杨氏模量为102—103巴的量级,泊松比为0.3—0.4,单轴抗压强度由数巴至数十巴,单轴抗剪强度为数巴,粘滞系数为1010—1011泊的量级,弹性波速为1.5—2.2公里/秒。对比红河断裂带南北两段断层泥的力学性质,看出南段断层泥的体积压缩模量,杨氏模量、抗压强度、粘滞系数和弹性波速比北段断层泥高。此外,南段断层泥弹性波速基本上各向同性而北段断层泥弹性波速显示出各向异性。      

The Fractal Feature of Granulometric Composition in Fault Gouges from the Yishu Fault Zone and Adjacent Northwest-Trending Faults and Its Seismo-geological Implications

In this work,the fractal dimension of granulometric composition in the fault gouge from the Yishu fault zone and northwest-trending faults on its west side is calculated and studied based on the fractal theory of rock fragmentation.The seismo-geological implications of the fractal dimension of granulometric composition in fault gouges are also discussed.The results show that the Yishu fault zone is more active than the northwest-trending faults and the Anqiu-Juxian fault is the most active in the Yishu fault zone.The fractal dimension of fault gouge is a parameter describing the relative active age and rupture mode of the fault and forming age of the fault gouge.The fractal dimension value is also related to the parent rock,thickness,structural position,and clay content of the fault gouge.     

Breaking Up: Comminution Mechanisms in Sheared Simulated Fault Gouge

The microstructural state and evolution of fault gouge has important implications for the mechanical behaviour, and hence the seismic slip potential of faults. We use 3D discrete element (DEM) simulations to investigate the fragmentation processes operating in fault gouge during shear. Our granular fault gouge models consist of aggregate grains, each composed of several thousand spherical particles stuck together with breakable elastic bonds. The aggregate grains are confined between two blocks of solid material and sheared under a given normal stress. During shear, the grains can fragment in a somewhat realistic way leading to an evolution of grain size, grain shape and overall texture. The ‘breaking up’ of the fault gouge is driven by two distinct comminution mechanisms: grain abrasion and grain splitting. The relative importance of the two mechanisms depends on applied normal stress, boundary wall roughness and accumulated shear strain. If normal stress is sufficiently high, grain splitting contributes significantly to comminution, particularly in the initial stages of the simulations. In contrast, grain abrasion is the dominant mechanism operating in simulations carried out at lower normal stress and is also the main fragmentation mechanism during the later stages of all simulations. Rough boundaries promote relatively more grain splitting whereas smooth boundaries favor grain abrasion. Grain splitting (plus accompanying abrasion) appears to be an efficient mechanism for reducing the mean grain size of the gouge debris and leads rapidly to a power law size distribution with an exponent that increases with strain. Grain abrasion (acting alone) is an effective way to generate excess fine grains and leads to a bimodal distribution of grain sizes. We suggest that these two distinct mechanisms would operate at different stages of a fault’s history. The resulting distributions in grain size and grain shape may significantly affect frictional strength and stability. Our results therefore have implications for the earthquake potential of seismically active faults with accumulations of gouge. They may also be relevant to the susceptibility of rockslides since non-cohesive basal shear zones will evolve in a similar way and potentially control the dynamics of the slide.     

Structure and deformational character of strike-slip fault zones

Strike-slip fault zones observed either in the field or in model experiments generally consist of several subparallel faults which make these zones complicated in geometry and kinematics. The geometry of a strike-slip fault or shear zone is dependent on arrangement (pinnate or en echelon), on step (left step or right step), and on the rank )smaller faults within larger faults) of the subparallel fault. The relations and interactions of these three factors create a variety of dynamic circumstances and tectonic settings within the strike-slip fault zones. These include pull-aparts in the release area between subparallel faults, push-ups in the jogs where the subparallel faults overlap, and pivotal movements, or rotation, of single faults along the whole fault zone. Each kind of tectonic setting is in itself characteristic, each setting consists of many subtypes, which are controlled chiefly by the geometric parameters of the subparallel faults. One of the most important phenomena revealed in the field work is two different kinds of evolution of strike-slip fault zones: one is the evolution of a zone with a tensile component, which is related to the growth of rock bridges, and the other, of one with a compressional component, which develops by the destruction of rock bridges. In this paper we discuss, on the basis of recent research on four strike-slip fault zones in China, the essential characteristics of strike-slip faults and the possible causes of the observed structural phenomena. Attention is focussed on the deformation, development, and distribution of horizontal displacements within strike-slip fault zones.     

Characteristics of Fault Gouge from the Northwest-Trending Faults in Western Shandong and Its Seismogeological Significance

A systematical study is made on the characteristics of fault gouge from NW-trending faults in western Shandong.The macroscopic features of the fault gouge zones were investigated.From the data on surface structure of quartz fragments from the fault gouge and its maturity,the relative age,the movement mode,the rupture pattern,and the activities of the NW-trending faults are determined.Moreover,their seismogeological significance and some problems in study of fault gouge are discussed.     

Short-and medium-term earthquake precursors as evidence of the sliding instability along faults

A theory of waves propagating along faults (trapped waves) is constructed. They have such a dispersion that their frequency decreases with approach to the catastrophic rupture threshold followed by a uniform slip on a fault (earthquake). At the same time, the group velocity of the trapped waves tends to infinity, which means the breaking of the wave pulse front. Together with the frequency decrease (spreading of the pulse), this phenomenon can serve as a short-term earthquake precursor. The sliding instability on faults dividing a real solid body into blocks leads to its unstable deformation, a significant decrease in the shear modulus, and development of plasticity. Because plasticity is accompanied by seismic anisotropy and precedes or is associated with earthquakes, seismic anisotropy can be regarded as a medium-term precursor of an earthquake.     

Seismic measurements of the internal properties of fault zones

The internal properties within and adjacent to fault zones are reviewed, principally on the basis of laboratory, borehole, and seismic refraction and reflection data. The deformation of rocks by faulting ranges from intragrain microcracking to severe alteration. Saturated microcracked and mildly fractured rocks do not exhibit a significant reduction in velocity, but, from borehole measurements, densely fractured rocks do show significantly reduced velocities, the amount of reduction generally proportional to the fracture density. Highly fractured rock and thick fault gouge along the creeping portion of the San Andreas fault are evidenced by a pronounced seismic low-velocity zone (LVZ), which is either very thin or absent along locked portions of the fault. Thus there is a correlation between fault slip behavior and seismic velocity structure within the fault zone; high pore pressure within the pronounced LVZ may be conductive to fault creep. Deep seismic reflection data indicate that crustal faults sometimes extend through the entire crust. Models of these data and geologic evidence are consistent with a composition of deep faults consisting of highly foliated, seismically anisotropic mylonites.     

安丘—莒县断裂小店—大店段断层活动证据及断层泥分布特征初步研究

郯庐断裂带最新活动的断层分布于潍坊至嘉山一带, 而安丘—莒县断裂是郯庐断裂带的重要活动断裂之一。 基于地质考察、 高密度电法探测和钻孔联合剖面, 对安丘—莒县断裂小店—大店段断层活动证据及断层泥分布特征进行研究。 地质考察发现紫红色砂砾岩逆冲到全新世耕植土上方, 两者之间发育断层泥带, 断层泥内含少量角砾岩, 为全新世活动逆断层。 高密度电法探测结果表明断层通过位置电阻率差异明显。 钻孔联合剖面揭露的地层: 全新世耕植土、 全风化砂质泥岩、 强风化砂质泥岩、 碎裂岩、 断层泥带及中风化砂质泥岩。 钻孔K1和K2揭露深灰色断层泥带, 倾向西, 倾角约为74°, 厚度约为13.9 m, 与南侧出露的断层泥带产状相协调。 工作区范围内, 发育多处褶皱、 破碎带和断层泥带, 断层表现为全新世逆断层性质。