Gouge Zone Research Articles

Characterization of slip zone associated with the 1999 Taiwan Chi-Chi earthquake: X-ray CT image analyses and microstructural observations of the Taiwan Chelungpu fault

To characterize the fault-related rocks within the Chelungpu fault, we performed X-ray computed tomography (CT) image analyses and microstructural observations of Hole B core samples from the Taiwan Chelungpu-fault Drilling Project. We identified the slip zone associated with the 1999 Chi-Chi earthquake, within the black gouge zone in the shallowest major fault zone, by comparison with previous reports. The slip zone was characterized by low CT number, cataclastic (or ultracataclastic) texture, and high possibility to have experienced a mechanically fluidized state. Taking these characteristics and previous reports of frictional heating in the slip zone into consideration, we suggested that thermal pressurization was the most likely dynamic weakening mechanism during the earthquake.

Subsurface structure, physical properties, fault-zone characteristics and stress state in scientific drill holes of Taiwan Chelungpu Fault Drilling Project

Continuous cores and a suit of geophysical measurements were collected in two scientific drill holes to understand physical mechanisms involved in the large displacements during the 1999 Chi-Chi earthquake. Physical properties obtained from wire-line logs including P- and S-wave sonic velocity, gamma ray, electrical resistivity, density and temperature, are primarily dependent on parameters such as lithology, depth and fault zones. The average dip of bedding, identified from both cores and FMI (or FMS) logs, is about 30° towards SE. Nevertheless, local azimuthal variations and increasing or decreasing bedding dips appear across fault zones. A prominent increase of structural dip to 60°–80° below 1856 m could be due to deformation associated with propagation of the Sanyi fault. A total of 12 fault zones identified in hole-A are located in the Plio-Pleistocene Cholan Formation, Pliocene Chinshui Shale and Miocene Kueichulin Formation. The shallowest fault zone occurs at 1111 m depth (FZ1111). It is a 1 m gouge zone including 12 cm of thick indurate black material. We interpreted this zone as the slip zone during Chi-Chi earthquake. FZ1111 is characterized by: 1) bedding-parallel thrust fault with 30-degree dip; 2) the lowest resistivity; 3) low density, V p and V s, 4) high V p/ V s ratio and Poisson's ratio; 5) low energy and velocity anisotropy, and low permeability within the homogeneous 1 m gouge zone; 6) increasing gas (CO 2 and CH 4) emissions, and 7) appearance of smectite within the primary slip zone. In situ stresses at the drill site were inferred from leak-off tests, borehole breakouts and drilling-induced tensile fractures from borehole FMS/FMI logs, and shear seismic wave anisotropy from DSI logs. The dominant fast shear-wave polarization direction is in good agreement with regional maximum horizontal stress axis, particularly within the strongly anisotropic Kueichulin Formation. A conjugate set of secondary directions are parallel to microcrack orientations. A drastic change of orientation of fast shear-wave polarization across the Sanyi thrust fault at the depth of 1712 m reflects the change of stratigraphy, physical properties and structural geometry.

Fault gouge evolution and its dependence on normal stress and rock strength—Results of discrete element simulations: Gouge zone properties

In order to study the process of gouge zone evolution, and its dependence on normal stress, σn, and uniaxial compressive strength, σucs, we simulate the breakdown of fault blocks and growth of fault gouge zones using the distinct element method (DEM) in two dimensions. Breakable elastic bonds were added between adjacent, closely packed particles to generate the fault blocks with a given σucs ranging from 100 to 260 MPa. DEM experiments were conducted by shearing the fault blocks along an initially flat surface for a range of σn from 10 to 100 MPa. The simulated fault gouge zones experience two distinct stages of evolution, i.e., fast growth and slow growth, distinguished by a switch in deformation mechanism from dominantly wear of the fault blocks to dominantly shearing of existing fault gouge. The rate of the gouge zone thickening decreases exponentially during the fast growth stage (to about 20% shear strain, i.e., 7.4 mm shear displacement) and then reaches a relatively constant value, marking the beginning of the slow growth stage. The thickening rate increases with increasing σn and decreasing σucs during the fast growth stage, but the dependency reverses during the slow growth stage. The mean grain size of the gouge shows a first‐order dependence on shear displacement, and varies less significantly with σn and σucs. In our simulated shear zones that undergo both surface wear and grain comminution, gouge grains develop power law size distributions characterized by a two‐dimensional fractal dimension D ranging from 0.6 to 2.4.

Deformation-promoted defects and retrograde chloritization of biotite in slates from a shear zone, Southern Iberian Massif, SE Spain

AbstractNaturally deformed biotite in contact-metamorphosed slates affected by a shear zone of the Southern Iberian Massif near Jaén (SE Spain) were studied by X-ray diffraction, optical microscopy, scanning electron microscopy, electron probe microanalysis and high-resolution transmission and analytical electron microscopy. Biotite is found in the contact metamorphism aureole produced by the intrusion of a granodioritic stock, but shear strain caused its deformation. The southern part of the shear band is strongly deformed, containing thick clay gouge zones. The northern part is less deformed and develops weaker planar-linear fabrics. X-ray diffraction data reveal the predominance of the 2M1 biotite in the undeformed samples whereas the 1M polytype is predominant in the sheared samples. Chemical data and electron images of the biotite from unsheared slates do not show the presence of intercalated phases. This biotite is almost defect-free and electron diffraction patterns have sharp reflections indicating a two-layer polytype (probably 2M1). Back-scattered electron images from the deformed biotite in the moderate deformation part of the shear zone do not reveal intergrown minerals, but the electron microprobe analyses show some Fe- and Mg-enriched compositions. Transmission electron microscopy indicates that disordered polytype packets are predominant (probably 1Md). Their electron diffraction patterns have diffuse streaking along c*. These packets have high dislocation densities, microcavities with ∼5 Å latticefringe regions (probably brucite-like sheets) and interlayering of chlorite-berthierine. Kaolinized biotite can be observed in the clay gouges from the strongly deformed south part of the shear zone. The degree of streaking, as an indication of the intensity of deformation, revealed that the disordered polytype packets are more deformed than the two-layer polytype packets. The microcavities of the disordered polytype packets could act as potential channels for transport of fluids during the shearing stage and serve as sites for chloritization of biotite, producing chlorite-berthierine domains within biotite. Berthierine is an intermediate metastable phase replaced by chlorite with along-layer transitions.

Strain and shear types of the Louzidian ductile shear zone in southern Chifeng, Inner Mongolia, China

The Louzidian ductile shear zone at the south of Chifeng strikes NE-SW and dips SE at low-medium-angles. This ductile shear zone is mainly composed of granitic mylonite, which grades structurally upward into a chloritized zone, a microbreccia zone, a brittle fault and a gouge zone. All these zones share similar planar attitudes, but contain different linear attitudes and kinematic indicators. Finite strain measurements were performed on feldspar porphyroclasts using the Fry method. These measurements yield Fulin indexes of 1.25–3.30, Lode’s parameters of −0.535–−0.112 and strain parameters of 0.41–0.75 for the protomylonite, respectively. These data are plotted within the apparent constrictional field in Fulin and Hossack diagrams. In contrast, for the mylonite, corresponding parameters are 0.99–1.43, −0.176–−0.004 and 0.63–0.82, respectively, and located in the apparent constrictional field close to the plane strain. The mean kinematic vorticity numbers of the protomylonite and mylonite by using three methods of polar Mohr circle, porphyroclast hyperbolic and oblique foliation, are in the range of 0.67–0.95, suggesting that the ductile shearing is accommodated by general shearing that is dominated by simple shear. Combination of the finite strain and kinematic vorticity indicates that shear type was lengthening shear and resulted in L-tectonite at the initial stage of deformation and the shear type gradually changed into lengthening-thinning shear and produced L-S-tectonite with the uplifting of the shear zone and accumulating of strain. These kinds of shear types only produce a/ab strain facies, so the lineation in the ductile shear zone could not deflect 90° in the progressively deformation.

Frictional properties of natural fault gouge from a low‐angle normal fault, Panamint Valley, California

We investigate the relationship between frictional strength and clay mineralogy of natural fault gouge from a low‐angle normal fault in Panamint Valley, California. Gouge samples were collected from the fault zone at five locations along a north–south transect of the range‐bounding fault system, spanning a variety of bedrock lithologies. Samples were powdered and sheared in the double‐direct shear configuration at room temperature and humidity. The coefficient of friction, μ, was measured at a range of normal stresses from 5 to 150 MPa for all samples. Our results reinforce the intuitive understanding that natural fault gouge zones are inherently heterogeneous. Samples from a single location exhibit dramatic differences in behavior, yet all three were collected within a meter of the fault core. For most of the samples, friction varies from μ = 0.6 to μ = 0.7, consistent with Byerlee's law. However, samples with greater than 50 wt % total clay content were much weaker (μ = 0.2–0.4). Expandable clay content of the samples ranged from 10 to 40 wt %. Frictional weakness did not correlate with expandable clays. Our results indicate that friction decreases with increasing total clay content, rather than with the abundance of expandable clays. The combination of field relations, analytical results, and friction measurements indicates a positive correlation between clay content, fabric intensity, and localization of strain in the fault core. A mechanism based upon foliations enveloping angular elements to reduce friction is suggested for weakening of fault gouge composed of mixed clay and granular material. We provide broad constraints of 1–5 km on the depth of gouge generation and the depth at which fault weakness initiates. We show that slip on the Panamint Valley fault and similar low‐angle normal faults is mechanically feasible in the mid‐upper crust if the strength of the fault is limited by weak, clay‐rich fault gouge.

Different styles of faulting deformation along the Dead Sea Transform and possible consequences for the recurrence of major earthquakes

Abstract We compare fault-related deformation of three segments of the major transform plate boundary between Africa and the Arabian plate, the Dead Sea Transform (DST), namely the Arava/Araba fault (Jordan/Israel), the Serghaya fault and the Ghab fault (both Syria). These segments show both similarities and marked differences in faulting deformation and fluid–rock interactions. In the case of the Arava fault, fault damage occurs across a zone up to 300 m wide. A fault core/gouge zone is not exhibited. Along the Serghaya fault, the typical fault zone architecture with a main gouge zone and a damage zone of up to 100 m thickness is exposed. Effects of faulting in the Ghab segment are shown by subsidiary faults and the formation of fault breccias. As in the Arava fault segment, a fault core is not exposed. Fluid–rock interactions are not equally distributed. At the Arava fault segment, the small amount of veins and the lack of alteration and dissolution processes in limestones suggest reduced fluid–rock interactions and limited fluid flow. The fault likely did not act as an important fluid conduit and hydrothermal reactions (cementation, dissolution) did not affect the strength of the fault zone. Contrary to the Arava fault, fluid-assisted fault zone healing processes (i.e. veining and cementation) were active along Serghaya fault and Ghab fault, where fault rock cementation led to porosity reduction and lower permeability. Such low permeability could create domains of higher pore fluid pressure, which reduce the effective shear stress required for slip on the fault. These differences in fluid–rock interactions may have played an important role with respect to the occurrence of earthquakes. We suggest that the recurrence interval on a fault segment that recovers after an earthquake without fluid assisted healing (e.g. Arava fault) should be longer than on segments with strong fluid-assisted healing (cementation; e.g. Serghaya fault, Ghab fault), given that the regional stress field is the same.

Structural, Mineralogical, and Geochemical Characterization of the Chelungpu Thrust Fault, Taiwan

The Chelungpu fault, Taiwan, produced a northward propagating rupture on 21 September 1999 resulting in a M(subscript w) 7.6 earthquake with a~90 km long N-S trending fault scarp. The mineralogic and physical character of the fault-related rocks within the Chelungpu fault zone, as measured at 9 sites along 70 km of the 1999 rupture trace, changes significantly along strike and with depth. The northern section of the Chelungpu fault has a 10-30 m-wide primary damage zone that is characterized by increased fracture density and alteration, but little microstructural damage to within 1 m from the main fault. The southern section of the Chelungpu fault has a 25-70 m wide primary damage zone that is characterized by increased fracture density and alteration, the presence of intensely sheared rock, and numerous secondary faults and gouge zones as far as 240 m from the main fault. The complexity of the damage zone, geochemistry, and clay mineralogy of the southern fault zone may reflect its relative maturity (~1 Ma) compared to the northern fault zone (~46-100 Ka). The major down-dip mineralogic variation is a transition from a significant amount of smectite in exhumed fault cores to little or no smectite in the fault core at sampled depths of 200 to 1000 m. This transition may be influenced by weathering processes at the surface, however co-seismic fluid flow may have a role in illite-smectite reactions. The composition of clays has important seismologic implications as clays play a role in fault weakening.

Subsurface Structure, Physical Properties, and Fault Zone Characteristics in the Scientific Drill Holes of Taiwan Chelungpu-Fault Drilling Project

1 Department of Earth Sciences and Institute of Geophysics, National Central University, Chung-Li, Taiwan, ROC 2 Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, Kochi, Japan 3 Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC 4 Exploration and Development Research Institute, Chinese Petroleum Corporation, Miaoli, Taiwan, ROC 5 Department of Earth Science, National Taiwan Normal University, Taipei, Taiwan, ROC * Corresponding author address: Prof. Jih-Hao Hung, Department of Earth Sciences and Institute of Geophysics, National Central University, Chung-Li, Taiwan, ROC; E-mail: jhhung@ncu.edu.tw doi: 10.3319/TAO.2007.18.2.271(TCDP) Continuous coring and a suite of geophysical measurements were collected in two scientific holes to understand physical mechanisms involved in large displacements in the 1999 Chi-Chi earthquake. Physical properties of formations obtained from wire-line logs including Pand S-wave sonic velocity, gamma ray, electrical resistivity, density and temperature, are primarily dependent on parameters such as lithology, depth and fault zones. The average dip of bedding, identified from both cores and FMI (or FMS) logs, is about 30 degrees towards the SE. Nevertheless, local azimuthal variations and increasing or decreasing bedding dips appear across fault zones. A prominent increase in structural dip to 60° 80° below 1856 m could be due to deformation associated with propagation of the Sanyi fault. A total of 12 fault zones identified in Hole-A are located in the PlioPleistocene Cholan Formation, Pliocene Chinshui Shale and Miocene Kueichulin Formation. The shallowest fault zone at 1111 m in depth, FZ1111, Terr. Atmos. Ocean. Sci., Vol. 18, No. 2, June 2007 272 is a 1-m long gouge zone that includes 12-cm thick indurate black material and has been interpreted as the slip zone during Chi-Chi earthquake. FZ1111 is characterized by: 1) a bedding-parallel thrust fault with a 30-degree dip; 2) the lowest resistivity; 3) low density, Vp and Vs ; 4) high Vp /Vs ratio and Poisson’s ratio; 5) low energy and velocity anisotropy, and low permeability or fluid mobility within the homogeneous gouge zone; 6) increasing gas (CO2 and CH4) emissions; and 7) being rich in smectite within the primary slip zone. Formation physical properties hold consistent relationships with either depth or lithology. Anisotropy of shear-wave velocity shows that the dominant fast shear polarization direction is in good agreement with an overall azimuth of the maximum horizontal stress axis, particularly within the strong anisotropic Kueichulin Formation. A drastic change in orientation of fast shear polarization across the Sanyi thrust fault at a depth of 1710 m reflects changes in stratigraphy, physical properties and structural geometry. (

The progression from damage to localization of displacement observed in laboratory testing of porous rocks

Abstract Laboratory experiments suggest that fault zones form in porous rocks through the extension and coalescence of fractures of predictable geometries. These fractures form in an array of Riedel fractures in R 1 , R 2 , P and Y orientations. Displacement along closely spaced fractures leads to the formation of comminuted fault gouge. Localization of displacement within the fault gouge progresses from distributed shearing to comminution and compaction of the fault rock material culminating in fractures in the Riedel orientations. Colour boundaries within the simulated gouge zones show the change in accommodation of displacement to fractures in the Y orientation as shear strain progresses. Clay–quartz mixtures demonstrate that the clay inhibits localization and the achievement of steady-state sliding as well as stick-slip. A reduction in the coefficient of friction does not occur until about 30% of the clay is present and continues to decrease until about 70% is reached. Localization of slip appears as a necessary condition for steady-state sliding as well as unstable behaviour. Field studies show the implication of grain-size reduction in the localization process by porosity decrease inhibiting fluid flow normal to the fault zone. The pervasive Y fractures, however, facilitate fluid migration parallel to some faults.

Low total and inorganic carbon contents within the Taiwan Chelungpu fault system

We presented the total and inorganic carbon contents of core samples recovered from the Taiwan Chelungpu fault system, which slipped at the 1999 Chi-Chi earthquake, and reported lower contents of inorganic carbon within the black gouge zone in FZB1136 (fault zone at depth 1136 m in Hole B) and in the black-material disks in FZB1194 and FZB1243.

Thermal history estimation of the Taiwan Chelungpu fault using rock‐magnetic methods

To investigate the cause of high magnetic susceptibility in disk‐shaped black materials (BM disks) within core samples of Hole B, the Taiwan Chelungpu‐fault Drilling Project, we carried out magnetic analyses of samples from the BM disks and surrounding gouge zones. Frequency dependence of low‐temperature magnetic susceptibility and thermal demagnetization of low‐temperature remanent magnetization revealed lower amounts of superparamagnetic grains in the BM disks than in the surrounding gouges. Magnetic grain‐size reduction by shearing is therefore not a plausible explanation for the high magnetic susceptibility of the BM disks. Thermomagnetic analyses of the gouge samples showed that above 400°C magnetization could be increased by thermal decomposition of paramagnetic minerals, whereas analyses of the BM disks showed a much smaller increase in magnetization. Thus, formation of ferrimagnetic magnetite or maghemite by thermal decomposition of paramagnetic minerals was inferred to be the cause of the high magnetic susceptibility of the BM disks, indicating that the BM disks, but not the gouges, have experienced temperatures of at least 400°C.

Slip zone and energetics of a large earthquake from the Taiwan Chelungpu-fault Drilling Project

Determining the seismic fracture energy during an earthquake and understanding the associated creation and development of a fault zone requires a combination of both seismological and geological field data. The actual thickness of the zone that slips during the rupture of a large earthquake is not known and is a key seismological parameter in understanding energy dissipation, rupture processes and seismic efficiency. The 1999 magnitude-7.7 earthquake in Chi-Chi, Taiwan, produced large slip (8 to 10 metres) at or near the surface, which is accessible to borehole drilling and provides a rare opportunity to sample a fault that had large slip in a recent earthquake. Here we present the retrieved cores from the Taiwan Chelungpu-fault Drilling Project and identify the main slip zone associated with the Chi-Chi earthquake. The surface fracture energy estimated from grain sizes in the gouge zone of the fault sample was directly compared to the seismic fracture energy determined from near-field seismic data. From the comparison, the contribution of gouge surface energy to the earthquake breakdown work is quantified to be 6 per cent.

Geomechanical Characterization of Faulted Rock Materials in Korea

A faulted rock usually shows the swelling behavior because of clay minerals which consist of the fault gouges. It makes rock mass unstable and threatens the safety of structures built in rock mass. This study was aimed at clarifying characteristics of physical and mechanical properties of faulted rock materials. At first, microstructures and mineralogical composition associated with faulting in the fault gouge zones were analyzed by using X-ray diffractometry (XRD) and SEM microphotographs. Physical properties of the faulted rock materials from fields were measured in the laboratory. It is well known that the mechanical properties are sensitive to the mineralogical assemblage and are affected by the shapes, distribution and preferred crystallographic orientation of the components. Material and direct shear tests were also conducted on faulted rock materials under saturated and unsaturated conditions. The mechanical results were analyzed together with the analyzed result of XRD and SEM.

High magnetic susceptibility of fault gouge within Taiwan Chelungpu fault: Nondestructive continuous measurements of physical and chemical properties in fault rocks recovered from Hole B, TCDP

The Taiwan Chelungpu‐fault Drilling Project (TCDP) was undertaken in 2002 to investigate the faulting mechanism of the 1999 Taiwan Chi‐Chi earthquake. Hole B penetrated the Chelungpu fault, and recovered core samples from between 948.42 m and 1352.60 m depth. Three zones, marked 1136mFZ, 1194mFZ and 1243mFZ, were recognized in the core samples as active fault‐zones within the Chelungpu fault. Multi‐Sensor Core Logger measurements revealed lower densities and higher magnetic susceptibilities within the black gouge zones in all three fault zones. Even though the fault zone that slipped during the 1999 earthquake has not been identified, higher magnetic susceptibilities indicate that frictional heating has taken place in the Chelungpu fault.

Rock Control of Faulting: A Case Study from the Icelandic Rift Zone

Abstract The work examines relationships between faulting and the physical properties of the host rock based on a study from the active rift zone in Iceland. Although compositionally identical, hyaloclastites, pillow lavas, and columnar‐jointed lavas have different mechanical properties that exert strong controls on the distribution of faults and strain within fault zones and the mechanical processes associated with displacement. In hyaloclastite, a fault zone comprises a braided array of lower‐order shear fractures. Veins of silica within gouge zones imply periodic dilational shear. In pillow lavas, reactivation of the near‐random arrays of primary fractures results in wide fault zones with diffuse margins. Displacement is achieved by the accumulation of small increments of slip on primary joints and by the development of through‐going shear fractures. Columnar lavas have the highest bulk strength and the fewest primary weaknesses. The subvertical cooling joints are favorably oriented for dilational‐shea...

Exhumation of the Ailao Shan shear zone recorded by Cenozoic sedimentary rocks, Yunnan Province, China

The role of strike‐slip faults, such as the Ailao Shan shear zone in southwestern China, in accommodating the India‐Eurasia collision remains controversial. Cenozoic sedimentary rocks preserved along the northeast side of the Ailao Shan shear zone, however, record the nature and timing of exhumation of shear zone rocks. Field mapping shows that the lower strata consist of early Oligocene lacustrine red mudstone and gypsum and the upper of late Oligocene to Miocene fluvial conglomerate with rare, fine‐grained, fossil leaf‐bearing intervals. Sediment derived from shear zone gneiss is present only in the upper part of the section. Our observations indicate deformation began in Oligocene time, with the onset of rapid exhumation by early Miocene time, consistent with existing geochronologic results. Exhumation must have ended before the formation of a regional low‐relief erosion surface and the onset of brittle deformation along the Red River fault, probably in early Pliocene time. Rapid exhumation of the Ailao Shan shear zone has been interpreted to have occurred in transtension, accommodated by a normal fault on the northeast margin of the shear zone. However, we observe consistent and pervasive shortening structures within the Cenozoic strata, including overthrusting and folding of the basin along faults defined by thick gouge zones within weak Cenozoic lithologies. Deformation is likely syndepositional. These structures are distinct from subvertical deformational features associated with the active Red River fault. Our field data thus support mid‐Cenozoic transpressional exhumation of the Ailao Shan shear zone. This is consistent with regional evidence for widespread left‐lateral and compressional deformation in the region to the east of the eastern Himalayan syntaxis during this time. Focused erosion along the Red River, established by late Oligocene time, may have contributed to deeper exhumation of the northeast part of the shear zone.

Structural characteristics of shallowly buried accretionary prism: Rapidly uplifted Neogene accreted sediments on the Miura‐Boso Peninsula, central Japan

The upper Miocene Misaki and Nishizaki formations on the Miura and Boso peninsulas in central Japan preserve the deformation features of an off‐scraped accretionary prism. The spatial distribution, geometry, and style of accretion‐related deformation with paleotemperature and burial depth estimation are elucidated in this study. The deformation structures and textures are similar to those of modern accretionary prisms. The low maximum paleotemperature (<50°C) and high preserved porosity of the sediments (30–50%) imply a maximum burial depth of less than 1000 m. On the basis of the mode of deformation, this off‐scraped body is divided into an imbricate thrust, a thrust unit, and an upper coherent unit in ascending order. The imbricate thrust corresponds to a branch from the basal décollement zone and is subdivided into a brecciated zone, a main gouge zone, and a shear band zone. The thrust unit hosts a concentration of thrust systems that form various orders of duplex structures, while the upper coherent unit is characterized by gravitational instability‐ and earthquake‐induced deformation without thrust faulting. The duplex distribution, shear strain, and fluid migration associated with the off‐scraping processes are clearly localized within the imbricate thrust and thrust unit. This accretion‐related deformation occurred before lithification of the sediment and under high fluid pressure induced by shear deformation, thickening of the sedimentary sequence, and earthquake‐induced liquefaction. These processes are inferred to control the effect on the mode of deformation and the location of thrusting during early deformation of the accretionary prism.

Fossil fluid reservoir beneath a duplex fault structure within the Central Range of Taiwan: implications for fluid leakage and lubrication during earthquake rupturing process

AbstractIn order to understand the kinematics which likely facilitated the speedy rupturing process of the 1999 Mw 7.6 Taiwan Chi‐Chi earthquake, we examined exposed rocks in the Taiwan Slate belt, where the pressure and temperature conditions most resembled the hypocentre of the Chi‐Chi earthquake, i.e. sub‐greenschist facies. Field observations and composition analyses of the silicified vein‐rich zones beneath the duplex structure suggest that impermeable slate layers may serve as cap rocks for confining deep‐seated fluids. These fluids most likely come from the Taiwan metamorphic complex at deeper depths by the dehydration and decarbonation reactions (or partial melting). In addition, the gouge zone of a link fault above the detachment also indicates the presence of overpressured fluids during faulting. It is probable that episodic leakage of the confined fluid reservoirs may provide essential fluids for fault lubrication during earthquake ruptures.

Grain size distribution and thickness of breccia and gouge zones from thin (

Through field and laboratory analyses, the cross-sectional structure and the grain size distribution of 10 strike-slip fault cores less than 1 m thick were studied. The fault cores are exposed in Jurassic platform limestone within the Mattinata Fault zone located in the Adriatic–Apulian foreland of southern Italy. Each fault core consists of a breccia zone and a gouge zone, which differ in thickness and grain size distribution. Through the conventional sieving-and-weighting method, the grain size distribution of 20 samples of fault rocks was obtained. The distributions follow power-laws with fractal dimension ( D) in the 2.00977–3.04008 range. Gouge D-values are proportional to the normalised thickness of the corresponding gouge zones. For a gouge D-value≈2.2, the thickness of the corresponding gouge zone is only about 3% of the fault core thickness, whereas for a gouge D-value≈3.0, the thickness of the corresponding gouge zone is almost 90% of the fault core thickness. Results from this study suggest that, with progressing fault displacement: (i) grain comminution in fault cores occurred mostly by early bulk fragmentation of grains and late grain abrasion; (ii) breccia zones were progressively incorporated into the adjacent gouge zones.