Principal Slip Surface Research Articles

Rupture direction of paleoearthquakes on the Alpine Fault, New Zealand, as recorded by curved slickenlines

We observed and further exhumed curved slickenlines on fault planes associated with paleo-surface rupture of the Alpine Fault, New Zealand’s ∼30 mm/yr continental transform plate boundary. Dynamic rupture modeling indicates that the geometry of such curvature provides a record of past earthquake rupture directions. We focused our efforts on three sites that span a region known to variably halt or allow passage of past earthquakes (an “earthquake gate”) to contribute rupture direction constraints to the fault’s spatiotemporally rich paleoseismic record. At Hokuri Creek and Martyr River, we observed both convex-up and convex-down curved slickenlines on and adjacent to principal slip surfaces, indicating past ruptures from both the northeast and southwest of these locations. At Martyr River, relationships suggest that the most recent event (inferred to correlate to 1717 CE) ruptured from the southwest. Our results demonstrate the utility of curved slickenlines as a valuable new paleoseismological tool for determining past rupture directions, applicable to surface-rupturing faults globally.

Synthesis of Current Seismicity and Tectonics Along the 1857 Mw7.9 Fort Tejon Earthquake Rupture and the Southernmost San Andreas Fault, California, USA

AbstractWe evaluate seismicity and tectonics along the San Andreas Fault (SAF) in southern California to elucidate ongoing near‐field crustal deformation processes. The principal slip surfaces (PSSs) or the fault core that accommodate major earthquakes, form the boundary between the tectonic plates. We analyze seismicity catalogs extending back to 1857, 1932, and 1981 with progressively improved magnitude of completeness and spatial resolution. The 1857 to present statewide catalog that is complete at M5.5+ documents minimal aftershock activity for the Mw7.9 1857 and 1906 Mw7.8 San Francisco earthquakes. The higher quality 1932 and 1981 catalogs show that the PSSs (the rupture zone) of the 1857 Mw7.9 Fort Tejon earthquake exhibits remarkable seismic quiescence both in the core and in the adjacent extended‐damage zone. Further south, the fault core is still aseismic but the shape of the SAF is more complex, and the rate of adjacent seismicity is much higher. This fault complexity and the seismicity rate are larger the more the strike of the SAF deviates from the Pacific plate velocity‐vector direction. The focal mechanisms of the SAF adjacent earthquakes are also heterogeneous and rarely have strikes and dips that are consistent with slip on the nearby PSSs. We infer that the southern SAF is locked, and a lack of seismicity at the core of the fault may be a standard feature of faults that almost exclusively accommodate high‐slip rates by producing major earthquakes. Correspondingly future aftershock sequences of major earthquakes on the southern SAF will likely have below average aftershock productivity.

Injection-induced fault slip and associated seismicity in the lab: Insights from source mechanisms, local stress states and fault geometry

Probing source mechanisms of natural and induced earthquakes is a powerful tool to unveil associated rupture kinematics. The source processes of failure and slip instability driven by stress loading are affected by fault geometry, but the source ruptures of injection-induced seismicity in relation to fault structures and local stress states remain poorly understood. We have conducted a series of fault reactivation and slip experiments on sandstone samples containing faults with different surface roughness (smooth saw-cut fault and fractured rough fault). We impose progressive fluid injection to induce fault slip, and simultaneously monitor the associated acoustic emission (AE) activity. Using high-resolution AE recordings, we perform full moment tensor inversion of all located AE sources, and investigate the changes of AE source characteristics associated with induced fault slip and their relation to fault roughness. For the complex and rough fault, we observe significant non-double-couple components of AE sources and a high degree of focal mechanism heterogeneity. The temporal changes of AE mechanisms associated with injection-induced fault slip on the smooth fault reveal increasing proportions of double-couple components and decreasing variability of AE focal mechanisms when approaching the onset of slip events. The observed inconsistency between the nodal planes of AE sources and the macroscopic fault plane orientation is attributed to the development of secondary fracture networks surrounding the principal slip surface. We analyze changes in the magnitude-frequency characteristics and source mechanisms of AEs with fault-normal distance, showing that for the smooth (mature) fault, Gutenberg–Richter b-value of on-fault seismicity is lower and focal mechanisms are less heterogeneous, compared to off-fault seismicity. Our results emphasize the important role of roughness-related changes in local fault geometry and associated stress heterogeneity for source mechanisms and rupture kinematics of injection-induced seismicity.

Coseismic Frictional Heating With Concomitant Hydrothermal Fluid Circulation Revealed by Rock Magnetic Properties of Fault Rocks From the Rupture of the 2008 Wenchuan Earthquake, China

AbstractCoseismic frictional heating and associated hydrothermal fluid circulation play an essential role in the dynamic weakening of seismic faults. However, temperature rise induced by frictional heating during an earthquake is still difficult to constrain. Magnetic properties of fault rocks convey abundant information on faulting processes. In this study, detailed rock magnetic measurements in combination with electron microscopic observations are conducted on fault rocks and protoliths from the Shaba outcrop (Beichuan County) on the Yingxiu‐Beichuan Fault ruptured during the 2008 Wenchuan Mw 7.9 earthquake. Results show that protoliths and the majority of fault rocks are dominated by paramagnetic pyrite and/or Fe‐bearing clay minerals; in contrast, the presence of pyrrhotite and goethite is confined to the fault gouges just next to the principal slip surface. Pyrrhotite is a product of pyrite alteration at high temperatures (>500°C) induced by seismic frictional heating during earthquake slip. Meanwhile, the goethite implies the presence of coseismic hot fluids within the fault zone. All these observations strongly suggest the occurrence of thermal pressurization as a plausible mechanism of coseismic fault weakening during the Wenchuan earthquake.

A closer look into slickensides: Deformation on and under fault surfaces

Accurate descriptions of natural fault surfaces and associated fault rocks are important for understanding fault zone processes and properties. Slickensides--grooved polished surfaces that record displacement and wear along faults-- develop measurable roughness and characteristic microstructures during fault slip. We quantify the roughness of natural slickensides from three different fault surfaces by calculating the surfaces power spectra and height distributions and analyze the microstructures formed below the slickensides. Slickenside surfaces exhibit anisotropic self-affine roughness with corresponding mean Hurst exponents in directions parallel-- 0.53 ± 0.07-- and perpendicular --0.6 ± 0.1-- to slip, consistent with reports from other fault surfaces. Additionally, surfaces exhibit non-Gaussian height distributions, with their skewness and kurtosis roughness parameters having noticeable dependence on the scale of observation. Below the surface, microstructural analyses reveal that S–C–C′ fabrics develop adjacent to a C-plane-parallel principal slip zone characterized by a sharp decrease in clast size and a thin (≤100 μm) nanoparticulate-rich principal slip surface (PSS). These microstructures are present in most analyzed samples suggesting they commonly form during slickenside development regardless of lithology or tectonic setting. Our results suggests that 1) PSS likely arise by progressive localization along weaker oriented fabrics 2) deformation along PSS's is energetic enough to comminute the rocks into nanometric grains, and 3) fault geometry can be further characterized by studying the height distributions of fault surfaces, which are likely to impact stress distributions and frictional responses along faults.

Architecture and permeability structure of the Sibillini Mts. Thrust and influence upon recent, extension-related seismicity in the central Apennines (Italy) through fault-valve behavior

The central Apennines are a fold-thrust belt currently affected by post-orogenic extensional seismicity. To constrain the influence that the inherited thrust-related structures exert on the present seismic behavior of the belt, we provide the high-resolution structural and hydraulic characterization of one of the most external exposed thrust fault systems of the central Apennines, the Sibillini Mts. Thrust Front (STF). We integrate structural mapping, multiscale structural analysis, and in situ air permeability on the brittle structural facies of the thrust zone. We also performed K-Ar dating of selected fault rocks to better constrain structural inheritance. The STF is defined by a complex, ∼300-m-thick deformation zone involving Meso-Cenozoic marl and limestone that results from the accommodation of both seismic and aseismic slip during shortening. Permeability measurements indicate that the low permeability (10−2 ÷ 10−3 D) of the marly rich host rock diminishes within the thrust zone, where the principal slip surfaces and associated S-C structures represent efficient hydraulic barriers (permeability down to ∼3 × 10−10 D) to sub-vertical fluid flow. Field data and K-Ar dating indicate that the STF began its evolution ca. 7 Ma (early Messinian). We suggest that the studied thrust zone may represent a barrier for the upward migration of deep fluids at the hypocentral depth of present-day extensional earthquakes. We also speculate on the influence that similar deformation zones may have at depth on the overall regional seismotectonic pattern by causing transient fluid overpressures and, possibly, triggering cyclic extensional earthquakes on normal faults prone to slip while crosscutting the earlier thrust zones (as per a classic fault valve behavior). This mechanism may have controlled the origin of the 2016−2017 central Apennines devastating earthquakes.

Structural characterization and K–Ar illite dating of reactivated, complex and heterogeneous fault zones: lessons from the Zuccale Fault, Northern Apennines

Abstract. We studied the Zuccale Fault (ZF) on Elba, part of the Northern Apennines, to unravel the complex deformation history that is responsible for the remarkable architectural complexity of the fault. The ZF is characterized by a patchwork of at least six distinct, now tightly juxtaposed brittle structural facies (BSF), i.e. volumes of deformed rock characterized by a given fault rock type, texture, colour, composition, and age of formation. ZF fault rocks vary from massive cataclasite to foliated ultracataclasite, from clay-rich gouge to highly sheared talc phyllonite. Understanding the current spatial juxtaposition of these BSFs requires tight constraints on their age of formation during the ZF lifespan to integrate current fault geometries and characteristics over the time dimension of faulting. We present new K–Ar gouge dates obtained from three samples from two different BSFs. Two top-to-the-east foliated gouge and talc phyllonite samples document faulting in the Aquitanian (ca. 22 Ma), constraining east-vergent shearing along the ZF already in the earliest Miocene. A third sample constrains later faulting along the exclusively brittle, flat-lying principal slip surface to < ca. 5 Ma. The new structural and geochronological results reveal an unexpectedly long faulting history spanning a ca. 20 Myr time interval in the framework of the evolution of the Northern Apennines. The current fault architecture is highly heterogeneous as it formed at very different times under different conditions during this prolonged history. We propose that the ZF started as an Aquitanian thrust that then became selectively reactivated by early Pliocene out-of-sequence thrusting during the progressive structuring of the Northern Apennine wedge. These results require the critical analysis of existing geodynamic models and call for alternative scenarios of continuous convergence between the late Oligocene and the early Pliocene with a major intervening phase of extension in the middle Miocene allowing for the isostatic re-equilibration of the Northern Apennine wedge. Extension started again in the Pliocene and is still active in the innermost portion of the Northern Apennines. In general terms, long-lived, mature faults can be very architecturally complex. Their unravelling, including understanding the dynamic evolution of their mechanical properties, requires a multidisciplinary approach combining detailed structural analyses with dating the deformation events recorded by the complex internal architecture, which is a phenomenal archive of faulting and faulting conditions through time and space.

Seismicity in a weak crust: the transtensional tectonics of the Brawley Seismic Zone section of the Pacific–North America Plate Boundary in Southern California, USA

SUMMARYThe Brawley Seismic Zone (BSZ) is a ∼58-km-long section of the Pacific–North America Plate boundary that connects the southernmost San Andreas Fault (SAF) and the Imperial Fault in southern California. We analyse the BSZ as two segments: a north segment, dominated by SAF tectonics, and a south segment that accommodates a higher level of seismicity. The south segment includes a ∼6 km wide right lateral step-over that includes the Salton Sea geothermal field (SSGF) and Holocene subaerial lava domes at the south end of the Salton Sea, called Salton Buttes. In general, the 40 yr (1981–2021) of BSZ seismicity is characterized by transitory sequences that often are accommodated simultaneously or in quick succession on intricate ladder-like faults. These sequences rarely reactivate faults associated with previous seismicity alignments but rather activate adjacent faults, sometimes located within less than 1 km. They can include several events of similar size as the mainshock, followed by bursts of aftershocks sometimes located away from the mainshock rupture. The seismicity rate and the spatial geometrical complexity varies between the BSZ-north and BSZ-south segments. The 24-km-long BSZ-north accommodates a ∼12-km-long linear trend of seismicity that extends from the SAF terminus into the Salton Sea, where moderate-sized northeast striking sequences form the rungs in a ladder-structure in a weak crust. The seismicity in this area is most likely influenced by the stress state of the SAF. In contrast, the 34 km long BSZ-south segment, which also has a weak crust, has accommodated larger sequences that illuminate irregular ladder-type faulting, with aftershocks defining linear distributions striking either north or northeast. The focal mechanisms exhibit a mostly strike-slip style of faulting with minor dip-slip faulting in the south Salton Sea area as well as the Mesquite basin to the south. The state of stress, as determined from focal mechanisms, consists of almost horizontal σ1, and σ3 with vertical σ2. The activation angle between the trend of σ1 and the preferred nodal plane of the largest nearby event decreases systematically from north to south along the long-axis of the BSZ. In the step-over zone, the Holocene volcanism and the frequent seismicity sequences suggest crustal extension as well as associated reduced crustal strength as compared to other parts of the BSZ. The presence of weak, thin, hot crust and distributed ladder-like or conjugate predominantly strike-slip faulting suggests that the whole BSZ acts as a several kilometre wide fault zone where no prominent principal slip surfaces are required to accommodate moderate-sized (M6+) earthquakes.

The Role of Quartz Cementation in the Seismic Cycle: A Critical Review

AbstractBecause quartz veins are common in fault zones exhumed from earthquake nucleation temperatures (150°C–350°C), quartz cementation may be an important mechanism of strength recovery between earthquakes. This interpretation requires that cementation occurs within a single interseismic period. We review slip‐related processes that have been argued to allow rapid quartz precipitation in faults, including: advection of silica‐saturated fluids, coseismic pore‐fluid pressure drops, frictional heating, dissolution‐precipitation creep, precipitation of amorphous phases, and variations in fluid and mineral‐surface chemistry. We assess the rate and magnitude of quartz growth that may result from each of the examined mechanisms. We find limitations to the kinetics and mass balance of silica precipitation that emphasize two end‐member regimes. First, the mechanisms we explore, given current kinetic constraints, cannot explain mesoscale fault‐fracture vein networks developing, even incrementally, on interseismic timescales. On the other hand, some mechanisms appear capable, isolated or in combination, of cementing micrometer‐to‐millimeter thick principal slip surfaces in days to years. This does not explain extensive vein networks in fault damage zones, but allows the involvement of quartz cements in fault healing. These end‐members lead us to hypothesize that high flux scenarios, although more important for voluminous hydrothermal mineralization, may be of subsidiary importance to local, diffusive mass transport in low fluid‐flux faults when discussing the mechanical implications of quartz cements. A renewed emphasis on the controls on quartz cementation rates in fault zones will, however, be integral to developing a more complete understanding of strength recovery following earthquake rupture.

Crystal‐Plastic Deformation in Seismically Active Carbonate Fault Rocks

AbstractThe spatial separation of macroscopic rheological behaviors has led to independent conceptual treatments of frictional failure, often referred to as brittle, and viscous deformation. Detailed microstructural investigations of naturally deformed carbonate rocks indicate that both frictional failure and viscous mechanisms might operate during seismic deformation of carbonates. Here, we investigate the deformation mechanisms that were active in two carbonate fault zones in Greece by performing detailed slip‐system analyses on data from automated crystal‐orientation mapping transmission electron microscopy and electron‐backscatter diffraction. We combine the slip‐system analyses with interpretations of nanostructures and predictions from deformation mechanism maps for calcite. The nanometric grains at the principal slip surface should deform by diffusion creep but the activation of the (0001)<20> slip system is evidence for a contribution of crystal plasticity. A similar crystallographic preferred orientation appears in the cataclastic parts of the fault rocks despite exhibiting a larger grain size and a different fractal dimension, compared to the principal slip surface. The cataclastic region exhibits microstructures consistent with activation of the (0001)<20> and {104}<021> slip systems. Postdeformational, static recrystallization, and annealing produce an equilibrium microstructure with triple junctions and equant grain size. We propose that repeated introduction of plastic strain and recrystallization reduces the grain size and offers a mechanism to form a cohesive nanogranular material. This formation mechanism leads to a grain‐boundary strengthening effect resulting in slip delocalization which is observed over 6 orders of magnitude (μm‐m) and is expressed by multiple faults planes, suggesting cyclic repetition of deformation and annealing.

Element enrichment/depletion during faulting in shale-rich surface exposures of the 2008 Wenchuan earthquake (Mw 7.9) and implications for coseismic temperature

The coseismic frictional temperature is increasingly recognised as an important factor for the transformation mechanism in shallow crustal faulting. We report changes in the mineralogical and geochemical compositions across the fault zone in the Shaba (SB) exposure of the 2008 Wenchuan earthquake (Mw 7.9), which contained the maximum value of vertical displacement and cut through shale-rich strata by the principal slip surface. Assuming Yb to be an immobile trace element, with respect to those in the adjacent damaged zone and wall rocks, Li, Rb, Cs and most light rare earth elements (LREE) exhibited marked enrichments of 143%, 247%, 475% and >60%, respectively; and a 40% depletion of Sr and <5% depletion of most heavy rare earth elements (HREE) was found, responding to mineralogical transformation such as alteration and dissolution of feldspar and formation of abundant clay minerals. In addition, heterogeneous adsorption of clay minerals could result in fractionation between and within LREE and HREE as well as negative Eu anomalies and positive Ce anomalies. The element enrichment/depletion model of the northern segment of the Yingxiu–Beichuan (YX–BC) fault zone was quite different from the middle/southern segments characterising faults in carbonate and granitic rocks. Through model calculation, the calculated trace element compositions (Li, Rb, Sr, Cs, Ba, La, Sm and Pb) of the fault gouge were obtained at 250 °C, 300 °C and 350 °C, and were produced by frictional heating. By comparing with the observed values, we suggest that high temperatures derived from frictional heating or due to buffering of the slip zone temperature by evaporation of pore water during the earthquake within the principal slip surface in the northern segment of the YX–BC fault zone may not have existed. Mechanical crushing and chemical alteration (not under high-temperature conditions) within the fault zone during the coseismic stage may result in mass removal and element enrichment/depletion. KEY POINTS Mineralogical and geochemical compositions were reported in the Shaba exposure of the 2008 Wenchuan earthquake. Li, Rb, Cs and most LREE exhibited marked enrichments in the fault gouge. Fractionation between and within LREE and HREE, negative Eu anomalies and positive Ce anomalies occur in the fault gouge. High temperatures may not have existed within the principal slip surface of the northern segment of the YX–BC fault zone.

Multistage strain localisation and fluid-assisted cataclasis in carbonate rocks during the seismic cycle: Insights from the Belluno Thrust (eastern Southern Alps, Italy)

Strain localisation in carbonate rocks impacts upon the nucleation and development of structures within the shallow seismogenic crust. Studying deformed carbonate rocks is thus a key to the refined understanding of the mechanical behaviour of rock successions with high potential to fail coseismically in response to stress accumulation. We studied deformation structures and mechanisms of the Miocene Belluno Thrust from the seismically active eastern Southern Alps, Italy. The thrust contains several structural facies and features (pressure-soluted and veined lithons, cataclasites, S–C fabrics, principal slip surface), which make it possible to define a sequence of deformation events in space and through time during thrusting. Image analysis of cataclasite particle size distribution and morphology, supported by cathodoluminescence imaging, indicates corrosive wearing as the main comminuting mechanism during cataclasis and repeated ingress of overpressured aqueous fluids promoting carbonate clast dissolution. We propose an evolutionary scenario of multiple episodes of fabric formation and rejuvenation that localised and accommodated increasing shear strain into a progressively narrowing fault core. We propose that strain localisation in carbonate rocks can be greatly facilitated by cyclic fluctuations between strain hardening and softening conditions punctuated by transient spikes of coseismic failure accommodated by cataclasis during the ingress of overpressured, chemically reactive fluids that control carbonate precipitation and dissolution.

Elastic Properties and Seismic Anisotropy Across the Alpine Fault, New Zealand

AbstractSeismically detected low‐velocity zones are commonly associated with major crustal faults. In order to accurately interpret these low‐velocity zones and understand processes that produce them, direct measurements of seismic wave speed through fault zone rocks are needed. The Alpine Fault dominates the active transpressional plate boundary of the South Island, New Zealand. We examine heterogeneity by determining elastic properties of the Alpine Fault using Deep Fault Drilling Project (DFDP)‐1 drill core and borehole logs, and outcrop samples from central Alpine Fault field localities. We measured P and S wave velocities on saturated rock samples, in three orthogonal directions, over a range of effective pressures. P and S wave velocities in fault rock range from 2.5 to 5.0 km/s and 1.3 to 2.7 km/s, respectively, consistent with the borehole sonic log. These velocities are slower than those in the surrounding host rock corresponding to a 20% to 40% decrease in velocity, caused by mechanical and chemical alteration processes, extending at least 30 m from the principal slip surface. Hanging wall host rocks are strongly anisotropic, while brittle fault rock and footwall host rock are mostly isotropic. Fault zone trapped wave observations are geometrically and quantitatively consistent with field observations and laboratory measurements of the damage zone. Scale independence of the velocity measurement in this zone across core, log, and field scales shows that drill core and outcrop samples are good proxies for in situ rocks, suggesting that the low velocity is due to microscale features, such as microfractures and clay content.

“Brittle structural facies” analysis: A diagnostic method to unravel and date multiple slip events of long-lived faults

Regional-scale faults typically experience complex, long-lasting histories, commonly recording evidence of multiple reactivation events. Therefore, they contain multiscalar structural domains characterised by varying microstructures, mineralogical compositions and kinematics. These domains result from differential strain partitioning during the recorded faulting stages, and, as a result, can preserve the isotopic and kinematic signature of the different slip periods. Their detailed structural analysis integrated with K-Ar dating of the fault rock assemblage can help to identify these commonly tightly juxtaposed, although not coeval, domains, which we refer herein to as “Brittle Structural Facies” (BSF). BSF analysis is pivotal (i) to understand the structural heterogeneity of fault zones, (ii) the diachronic formation of geometrically and kinematically complex fault cores and (iii) to reconstruct faults' evolution in time and through space. Following this approach, this study relies on meso- and microstructural analysis, chemical characterisation and K-Ar dating to unravel the evolution of the Lærdal-Gjende Fault (LGF, southwestern Norway). The LGF is a multiply reactivated top-to-the-NW extensional fault with a 1 m thick poorly consolidated core. We recognised, sampled and characterised five BSF: I) Indurated dark reddish gouge, (II) Poorly consolidated cataclasite, (III) Weakly foliated greenish gouge, (IV) Clay-rich gouge and (V) A few mm-thick clay smear decorating the principal slip surface. Samples were separated into five grain size fractions (from <0.1 to 6-10 μm) and analysed by X-Ray Diffraction, Transmission Electron Microscopy and K-Ar geochronology. A c. 180 Ma age cluster defined by 10 ages of the coarsest grain size fractions (2-10 μm) likely documents fault nucleation during Jurassic rifting in the North Sea. The ages of the finest fractions, enriched in synkinematic K-bearing minerals (illite, smectite and K-feldspar), constrain four periods of faulting at c. 121 ± 3, 87 ± 2, 78 ± 2 and 57 ± 1 Ma. Ages indicate that the LGF accommodated strain due to hyperextension of the Mid-Norwegian margin down to the Late Cretaceous and finally slipped again during the Paleogene. The alternating widening and narrowing of the active fault zone in response to varying deformation mechanisms, including coseismic rupturing, formed the present complex fault architecture. This study highlights the importance of BSF characterisation as part of a multidisciplinary workflow to derive structural and temporal datasets of complex fault zones. BSF analysis, moreover, is demonstrated to be key for investigating the diachronic evolution of fault cores and to resolve multiple slip events of faults.

Fault zone structures and strain localization in clinoptilolite-tuff (Nižný Hrabovec, Slovak Republic)

Clinoptilolite is a natural micro-porous zeolite composed of silica and alumina tetrahedra that commonly forms as a devitrification product of glass in volcanic tuffs. Besides applications in the agriculture, building and manufacturing industries, clinoptilolite has been widely studied in veterinary and human medicine because of its positive effects on health. Based on field observations and microstructural investigations, we show, for the first time, strain localization features along faults in natural zeolite tuffs in a clinoptilolite-tuff open-pit mine at Nižný Hrabovec (Slovak Republic). The faults are localized along pre-existing joints with plumose structures that acted as a pathway for the infiltration of mainly Mn- and K-rich fluids. Displacement along the faults created structures that are indicative of both velocity hardening (accommodated by dissolution precipitation creep on SC/SCC′ foliation) and velocity weakening (accommodated by multi-generations of ultracataclasites along principal slip surfaces) behaviors. Rock-fluid interaction was characterized by a high-mobility of K-rich fluids, with K-feldspar decorating the SC/SCC′ foliation, infiltrating fractures in the damage zone of the faults and precipitating as idiomorphic crystals in open cavities and along the fault surface. Microstructures such as polished slickensides, injection of fluidized cataclasites, clast-cortex grains in cataclasites and truncated grains along principal slip surfaces probably suggest that at least some of the faults formed during seismic slip.

Identification of capable faults using fault rock geochemical signatures: A case study from offset granitic bedrock on the Tsuruga Peninsula, central Japan

The identification of capable faults is crucial for reliable seismic hazard assessments. To identify geochemical signatures characterizing recent, near-surface faulting, we analyzed the mineral and bulk-rock compositions of a range of fault rocks. We compare the compositions of two faults in central Japan that displace the same granitic unit, but record distinct geological histories: the capable Shiraki–Nyu Fault and a minor, incapable fault. Brownish gouge along the Shiraki–Nyu Fault (representing the principal slip surface) records MnO enrichment and FeO depletion and has a positive Ce anomaly, which was likely produced through oxidation reactions. Compositional mapping of outcrop and core samples from the Shiraki–Nyu Fault indicates that compositional changes (Mn- and Fe-oxide and/or oxyhydroxide precipitation along the principal slip surface) were driven by the fault-driven upward migration and oxidation of reduced fluids from depth to the near-surface. Mapping also indicates that Mn-rich gouge (and ultracataclasite) has undergone brecciation due to recent shallow-level faulting. In the Mn-rich gouge, Ce, Ba, and Y may have been enriched through co-precipitation and/or sorption onto the Mn oxide surfaces. Pale gray gouge from the principal slip surface of the minor incapable fault records no oxidation-related chemical changes. However, anastomosing/grid-like fractures filled with dark gray clay, which cut the pale gray gouge yield elevated MnO and Ba concentrations, low FeO concentrations and positive Ce anomalies, as well as high halloysite abundances. These chemical changes are attributed to post-faulting infiltration of oxidized surface water rather than repeated faulting. We show that geochemical analysis, combined with detailed geological observations, can be used to determine the spatial distributions of elements in heterogeneous fault zones, and contribute to the identification of recent, near-surface faulting events.

Flow pathway and evolution of water and oil along reverse faults in the northwestern Sichuan Basin, China

As a three-dimensional geological body, a fault zone has a complex internal structure. Disputes remain over flow pathways of fluids within fault zones. Well and seismic data cannot be used to effectively identify the internal structures of a fault zone. Furthermore, continuous core sampling in fault zones is commonly limited. Fewer studies of flow pathways along reverse faults are done in a sedimentary basin. Through extensive outcrop observations, sampling, and measurements in the northwestern Sichuan Basin of China, this study enhances our understanding of fluid evolution and the main pathway of vertical fluid flow along a reverse fault. In the studied carbonates, deep hot brine initially entered the fault zone and migrated upward along the fault core, then moved to shallow strata, mixed with meteoric water, and cooled in the fault zone. In the studied sandstone and shale, a paleo-oil pool formed in the fault damage zone. After that, forced by uplift and reactivation, oil migrated into the fault core along fractures and was cooled, washed, biodegraded, and oxidized by meteoric water. In the sandstone–sandstone juxtaposition faults, the oil shows are distinctly different between hanging wall and footwall. Fault rocks (sand and shale gouges) that developed along the principal slip surface seem to have prevented fluid flow across the fault. This evidence suggests that fault core and inner damaged zone are the main pathways of vertical fluid flow along the investigated reverse fault zone.

Gravity‐Independent Grain Size Segregation in Experimental Granular Shear Flows as a Mechanism of Layer Formation

AbstractFine grain bands in faults are normally interpreted as indicative of slip localization. However, grains can migrate systematically during shear resulting in nonintuitive effects on the final structure. Here we report the results of shear experiments performed on natural granular materials and discuss their possible implications for natural shear systems. We sheared dry grains in a rheometer over a range of velocities, followed the structural evolution with video during shear, and characterized the final structures. Fine particles comminuted during high‐velocity shear migrated from the highest shear rate regions to a layer where shear rate is near zero. This migration is an example of shear segregation that is driven by grain‐inertia effects. The segregation forms an apparent shear band displaced from the principal slip surface and has a stabilizing effect on subsequent flow. This shear segregation may result in fine‐particle layers and slip localization in natural shear zones.

A new interpretation for the nature and significance of mirror-like surfaces in experimental carbonate-hosted seismic faults

Highly reflective, continuous smooth surfaces, known as mirror-like (MSs), have been observed in experimental carbonate-hosted faults, which were sheared at both seismic and aseismic velocities. MSs produced during high-velocity friction experiments (>0.1 m s–1) are typically interpreted to be frictional principal slip surfaces, where weakening mechanisms are activated by shear heating. We re-examined this model by performing friction experiments in a rotary shear apparatus on calcite gouge, at seismic velocities up to v = 1.4 m s–1 and normal stress σn = 25 MPa, to analyze the evolution of microstructures as displacement increases. After the onset of dynamic weakening, when the friction coefficients are low (μ << 0.6), sheared gouges consistently develop a well-defined, porosity-free principal slip zone (PSZ) of constant finite thickness (a few tens of micrometers) composed of nanometric material, which displays polygonal grain shapes. MSs occur at both boundaries of the PSZ, where they mark a sharp contrast in grain size with the sintered, much coarser material on either side of the PSZ. Our observations suggest that, with the onset of dynamic weakening, MSs partition the deformation by separating strong, sintered wall rocks from a central weak, actively deforming viscous PSZ. Therefore, the MSs do not correspond to frictional slip surfaces in the classical sense, but constitute sharp rheological boundaries, while, in the PSZ, shear is enhanced by thermal and grain-size–dependent mechanisms.

Slow fault propagation in serpentinite under conditions of high pore fluid pressure

The rupture, localization, and slip of faults in serpentinite were studied under varying pore fluid pressure conditions to understand deformation mechanisms potentially responsible for slow slip in fault zones. Experiments were conducted at a constant effective confining pressure of 10 MPa and under pore fluid pressures from 0 to 120 MPa and at temperatures from 23 to 110 °C. With no fluid pressure, faulting occurs rapidly and audibly, and the duration of failure increases monotonically with increasing fluid pressure and temperature. Although non-dilatant during initial strain hardening, the serpentinite dilates during strain weakening concomitant with fault rupture and slip. Non-dilatant strain hardening occurs by microcracking along serpentine basal planes and grain boundaries and rarely in mode I orientations, consistent with previous studies. Dilatant fault rupture produces a network of transgranular shear fractures in conjugate orientations, generally with one dominant fracture. Structural observations show that as fluid pressure increases, the number of transgranular fractures increases. We propose that when faulting occurs over a distributed zone rather than a pre-existing principal slip surface, dilatant hardening causes deformation to migrate. This process causes an increase in slip weakening distance and fracture energy at elevated fluid pressures that can lead to more stable failure. Further, thermally-activated processes caused deformation at propagating crack tips, which also increases the slip weakening distance and the effective fracture energy with increasing temperature. Given the geologic settings for slow slip, our results indicate that high fluid pressure, distributed deformation, and thermally-activated processes may all contribute to slow fault rupture and slip.