Fault Zone Structure Research Articles

Fault zone structure and fluid–rock interaction of a high angle normal fault in Carrara marble (NW Tuscany, Italy)

We studied the geometry, intensity of deformation and fluid–rock interaction of a high angle normal fault within Carrara marble in the Alpi Apuane NW Tuscany, Italy. The fault is comprised of a core bounded by two major, non-parallel slip surfaces. The fault core, marked by crush breccia and cataclasites, asymmetrically grades to the host protolith through a damage zone, which is well developed only in the footwall block. On the contrary, the transition from the fault core to the hangingwall protolith is sharply defined by the upper main slip surface. Faulting was associated with fluid–rock interaction, as evidenced by kinematically related veins observable in the damage zone and fluid channelling within the fault core, where an orange–brownish cataclasite matrix can be observed. A chemical and isotopic study of veins and different structural elements of the fault zone (protolith, damage zone and fault core), including a mathematical model, was performed to document type, role, and activity of fluid–rock interactions during deformation. The results of our studies suggested that deformation pattern was mainly controlled by processes associated with a linking-damage zone at a fault tip, development of a fault core, localization and channelling of fluids within the fault zone. Syn-kinematic microstructural modification of calcite microfabric possibly played a role in confining fluid percolation.

Can compliant fault zones be used to measure absolute stresses in the upper crust?

Geodetic and seismic observations reveal long‐lived zones with reduced elastic moduli along active crustal faults. These fault zones localize strain from nearby earthquakes, consistent with the response of a compliant, elastic layer. Fault zone trapped wave studies documented a small reduction in P and S wave velocities along the Johnson Valley Fault caused by the 1999 Hector Mine earthquake. This reduction presumably perturbed a permanent compliant structure associated with the fault. The inferred changes in the fault zone compliance may produce a measurable deformation in response to background (tectonic) stresses. This deformation should have the same sense as the background stress, rather than the coseismic stress change. Here we investigate how the observed deformation of compliant zones in the Mojave Desert can be used to constrain the fault zone structure and stresses in the upper crust. We find that gravitational contraction of the coseismically softened zones should cause centimeters of coseismic subsidence of both the compliant zones and the surrounding region, unless the compliant fault zones are shallow and narrow, or essentially incompressible. We prefer the latter interpretation because profiles of line of sight displacements across compliant zones cannot be fit by a narrow, shallow compliant zone. Strain of the Camp Rock and Pinto Mountain fault zones during the Hector Mine and Landers earthquakes suggests that background deviatoric stresses are broadly consistent with Mohr‐Coulomb theory in the Mojave upper crust (with μ ≥ 0.7). Large uncertainties in Mojave compliant zone properties and geometry preclude more precise estimates of crustal stresses in this region. With improved imaging of the geometry and elastic properties of compliant zones, and with precise measurements of their strain in response to future earthquakes, the modeling approach we describe here may eventually provide robust estimates of absolute crustal stress.

Geometric and rheological asperities in an exposed fault zone

Earthquake dynamics are strongly affected by fault zone structure and fault surface geometry. Here we investigate the interplay of bulk deformation and surface topography using detailed structural analysis of a fault zone near Klamath Falls, Oregon, combined with LiDAR measurements of the fault surface. We find that the fault zone has a layered damage architecture. Slip primarily occurs inside a 1–20 mm wide band that contains principal slip surfaces with individual widths of ∼100 μm. The slip band sits atop a cohesive layer which deforms by granular flow. Several fault strands with total slips of 0.5–150 m also have cohesive layers with thicknesses increasing monotonically with slip. The thickness added to the cohesive layer per unit slip decreases with increasing displacement indicating that slip progressively localizes. The main fault is a continuous surface with 10–40 m long quasi‐elliptical geometrical asperities, i.e., bumps. The bumps reflect variations of the thickness of the granular cohesive layer and can be generated by a pinch‐and‐swell instability. As the granular layer is rheological distinct from its surroundings, the asperities are both geometrical and rheological inhomogenities. Modeling slip along wavy faults shows that slip on a surface with a realistic geometry requires internal yielding of the host rock. Our observations suggest that the internal deformation processes in the fault zone include ongoing fracture, slip along secondary faults, and particle rotation. Granular flow is an important part of faulting in this locale. Slip surfaces localize on the border of the granular cohesive layer. The ongoing slip smoothes the surfaces and thus the structural and geometrical evolution of the granular layer creates a preference for continued of slip on the same surface. There is a feedback cycle between slip on the surface and the generation of the granular layer that then deforms and controls the locus of future slip.

Composition, microstructures, and petrophysics of the Mozumi fault, Japan: In situ analyses of fault zone properties and structure in sedimentary rocks from shallow crustal levels

We characterize the chemical, microstructural, and geophysical properties of fault‐related rock samples from the 80–100 m wide Mozumi fault zone, north central Honshu, Japan. The fault is exposed in a research tunnel 300–400 m below the ground, and we combine geological data with borehole geophysical logs to determine the elastic and seismic properties of the fault zone. Detailed mapping within the tunnel reveals that the fault zone consists of two zones of breccia to foliated cataclasites 20 and 50 m thick. Two narrow (tens of centimeters wide) principal slip zones on which most of the slip occurred bound the central fault zone. The dominant deformation mechanisms within the fault zone were brittle fracture, brecciation, slip localization, plastic deformation, and vein formation in a sericite‐calcite rich matrix. Clay alteration patterns are complex within the fault zone, with clay‐rich fault breccia enriched in smectite, illite, and kaolinite relative to the kaolinite and illite dominant in the host rock. Whole rock geochemical analyses show that the fault‐related rocks are depleted in Fe, Na, K, Al, Mg, and Si relative to the host rock. The fault zone exhibits depressed electrical resistivity values by 10–100 ohm m relative to the wall rock, values of Vp and Vs values that are ∼0.30 and ∼0.40 km/s (10–20%) less than protolith values. The spontaneous potential logs indicate that the fault zone has increased freshwater content relative to formation waters. Wellbore‐based measurements of Vp and Vs in fault‐related rocks to enable us to calculate values of Young's modulus from 16.2 to 44.9 GPa and Poisson's ratio for the fault zone of 0.263 to 0.393. The protolith has Young's modulus of 55.4 GPa and a Poisson's ratio of 0.242. Lowest calculated values of Young's modulus and highest calculated values of Poisson's ratio correspond to fault breccia with increased porosity, high fluid content, and low resistivity values. Taken together, these data show that the shallow portion of the Mozumi fault consists of a complex zone of anastomosing narrow slip zones that bound broad zones of damage. Fluid‐rock alteration and deformation created altered fault‐related rocks, which have resulted in overall reduced interval velocities of the fault zone. These data indicate that seismic waves traveling along the interface or internally reflected in the fault zone would encounter rocks of differing and reduced elastic properties relative to the host rocks but that in detail, material properties within the fault may vary.

Characterization of the shallow structures of active fault zones using 3‐D ground‐penetrating radar data

Where they can be correlated with geological exposures and trenches, 3‐D ground‐penetrating radar (GPR) data can contribute critical subsurface information to paleoseismic investigations. Because active faults are typically characterized by complicated near‐surface structures that vary with the styles of faulting and the types of rock that are ruptured, GPR data can be difficult to interpret. We have acquired 3‐D GPR data sets across three active fault zones within New Zealand that have different deformation styles: the strike‐slip Wellington fault zone, reverse faults of the Ostler fault zone, and normal faults of the Maleme fault zone. To improve our interpretation of the processed GPR volumes, we employed two suites of geometric attributes. The first suite was computed using a coherence‐based algorithm. It provided estimates of the coherency, azimuth, and dip of reflections. The second suite quantified the volumetric textures of reflections, which allowed different reflection facies to be defined objectively. We have demonstrated how some attributes were more successful at visualizing certain structural or depositional characteristics than others. For example, the coherency attribute was an excellent tool for highlighting normal faults within volcanic deposits of the Maleme fault zone, whereas the texture‐based attributes were most useful for discriminating between the gravel and metasediment units juxtaposed by the Wellington fault zone. Our GPR data sets and associated attribute volumes showed details of near‐surface fault geometry that were not obvious from surface mapping, and they revealed evidence of off‐fault deformation, gravitational collapse, and topple structures.

A viscous layer at the crust-mantle boundary in the Far East of Russia

Geophysical indications, spatial parameters, and localization of the crustal and subcrustal viscous layers in the Far East of Russia are considered. The low-viscosity zones and layers are correlated equally distinctly with the Jurassic-Cretaceous plutonic and Cretaceous, Early Cenozoic, and Recent volcanic belts in tectonic domains of different age and geographic position, testifying to the long-term (up to 100–120 Ma) existence of viscous layers permanently fed by mantle-derived energy and mass flows through a deep fault zone and central-type structures of plume nature. The widespread low-viscosity zones in the lower crust and the uppermost mantle provided in the past and are providing now favorable conditions for lateral displacements of overlying tectonic sheets and fast relaxation of elastic stress in the Earth’s crust, provoking earthquakes.

Insights on the geometry and mechanics of the Umbria–Marche earthquakes (Central Italy) from the integration of field and laboratory data

The integration of seismic reflection profiles with well-located earthquakes shows that the mainshocks of the 1997–1998 Umbria–Marche seismic sequence nucleated at a depth of ∼ 6 km on normal faults within the Triassic Evaporites. In the same lithology, made of interbeds of dolomites and anhydrites, two deep boreholes (∼ 4–5 km), drilled northwest of the epicentral area, encountered CO 2 at near lithostatic pressure. In order to investigate the deformation processes operating at depth in the source region, we have combined field studies on a major evaporite-bearing normal fault (the Roccastrada fault) with mechanical and permeability data from rock deformation experiments on anhydrite rocks. The Roccastrada fault is a mature normal fault (inferred displacement > 100 m and exhumation from depths > 1 km) that dips in the range 40°–45°. The fault zone structure is characterized by a 5–6 m thick fault core, which appears to be zoned. The inner fault core is made of fine-grained fault rocks (∼ 1 m thick), with deformation localized along continuous and straight slip surfaces associated with a dolomite-rich cataclasite (brittle deformation). The outer fault core is mainly characterized by distributed deformation accommodated by a fault parallel fabric consisting of interbeds of cataclastic dolostones and foliated Ca-sulphate rocks. The damage zone consists of foliated Ca-sulphate rocks (foliation almost perpendicular to the fault zone) and heavily fractured and boudinaged dolostones. Mechanical data obtained from triaxial loading tests on borehole-recovered anhydrites with different grain size and mesoscopic fabric, show that the transition from localized to distributed deformation occurs at effective pressures of about 20 MPa. The permeability measured under hydrostatic stress conditions, before loading, is generally low and ranges between 10 − 21 ≤ k ≤ 10 − 19 m 2. During sample loading, the permeability increases up to 3 (prior to brittle localized failure) and 2 (prior to distributed ductile failure) orders of magnitude, with measured k values of 10 − 17 m 2 and 10 − 18 m 2, respectively. The integration of field observations with mechanical data allows us to propose a fault evolution model where fault initiation occurs as distributed deformation within the anhydrites, along fault zones dipping at ∼ 45°. With increasing displacement, dolostones are concentrated into the inner fault core. The ultimate result is a fault zone structure dipping at ∼ 45° , made of an inner fault core affected by brittle processes surrounded by a macroscopically ductile outer fault core. This fault zone evolution can explain the low rupture dip (38°–48°) of the Umbria–Marche 1997–1998 mainshocks. The combination of field observations and permeability measurements suggests a fault zone permeability low enough for fluid overpressures to develop. Therefore in the active area of the Umbria–Marche Apennines deep-seated CO 2-rich fluids can be trapped at seismogenic depths within evaporite-bearing faults and can potentially promote earthquake nucleation.

Scale dependence in the dynamics of earthquake propagation: Evidence from seismological and geological observations

We attempt to reconcile current understanding of the earthquake energy balance with recent estimates of fracture energy from seismological investigations and surface energy from geological observations. The complex structure of real fault zones suggests that earthquakes in such fault structures are dominated by scale-dependent processes. We present a model for an inelastic fault zone of finite thickness embedded in an elastic crust represented at a macroscopic scale by a mathematical plane of zero thickness. The constitutive properties of the fault zone are governed by physical processes controlling gouge and damage evolution at meso- and micro-scale. However, in order to model and interpret seismological observations, we represent dynamic fault weakening at the macroscopic scale in terms of traction evolution as a function of slip and other internal variables defining a phenomenological friction or contact law on the virtual mathematical plane. This contact law is designed to capture the main features of dynamic fault weakening during earthquake rupture. In this study we assume that total shear traction is friction and corresponds to shear resistance of the whole fault zone. We show that seismological observations, depending on finite and limited wavelength and frequency bandwidth, can only provide an estimate of breakdown stress drop and breakdown work (a more general definition of seismological fracture energy) representing a lower bound of the total intrinsic power of dissipation on the fault zone. We emphasize that geological estimates of surface energy can be compared with seismological estimates of breakdown work only if they are representative of the same macroscopic scale. In this case, it emerges that, contrary to surface energy, seismological breakdown work represents a non-negligible contribution to the earthquake energy budget.

Internal structure of active fault zones based on deep seismic sounding data: Evidence from the Cyprian and Kuril-Kamchatka arcs

Internal structure of active fault zones based on deep seismic sounding data: Evidence from the Cyprian and Kuril-Kamchatka arcs

Predictive permeability model of extensional faults in crystalline and metamorphic rocks; verification by pre-grouting in two sub-sea tunnels, Norway

This paper link quantitative fault zone descriptions, qualitative fracture and fault rock properties, and engineering data in the study of the permeability structure of fault zones. Datasets include scan-lines, drill cores and cement pre-grouting from two sub-sea tunnels in gneissic and granitic rocks, from which systematic pre-grouting volumes can be used to analyse the in-site relative permeability both in host rocks and fault zones. Major extensional faults intersected by the tunnels reveal common fault rocks surrounding intensively fractured rock lenses in the core. Fracture frequencies in these lenses can reach 100 fractures/metre (f/m). In the bounding damage zones, networks of fracture sets make up an inner zone of fairly high frequency (20–30 f/m) of fault-parallel, long fractures connected by shorter fractures. An outer zone has lower frequencies (<20 f/m) and more diverse fracture orientations and lengths. There is a general increase in fracture frequency from the background level of the protolith towards the fault core. Tunnel-scale injection of cement reveals patterns that can be ascribed to the impact of faulting; there is an increase in cement injection in fault zones compared to areas with background fracturing away from faults. In detail, there is an innate division of the rock volume into sub-zones characterized by distinct structural style and permeability, with a background level and three fault related sub-zones (fault core, inner damage zone, and outer damage zone). Injection data shows that the background sub-zone commonly can be injected with less than 0.05 m 3 cement per metre tunnel (commonly not injected), whereas the fault core has permeability characteristics nearly as low as the outer damage zone, represented by 0.1–0.2 m 3 cement per metre tunnel, with occasional peaks towards 0.5 m 3. The maximum of cement injection lies in the inner damage zone, marginal to the fault core, with 0.3–0.7 m 3 cement per metre tunnel, locally exceeding 1 m 3. This gives a relative relationship for cement injection of approximately 1:2:1 between fault core, inner damage zone, and outer damage zone of extensional fault zones in crystalline and metamorphic rocks.

Spatial Variations in Fault-Zone Structure along the Nojima Fault, Central Japan, as Inferred from Borehole Observations of Fault-Zone Trapped Waves

Abstract We investigated the fault-zone structures (fault-zone width, shear-wave velocity, and Q s) of the Nojima fault, Japan, using the Love-wave-type fault-zone trapped waves (LTWs) recorded at two borehole stations—TOS2 and HRB—located along the fault. TOS2 (depth: 1673 m) is located at an end of the fault, while HRB (depth: 600 or 720 m) is located in the middle section of the fault where the greatest surface displacement of 2 m was recorded during the 1995 Kobe earthquake. The distance between the two stations is about 4 km. We found 22 records that exhibit typical LTW. We assumed the fault-zone structure to be a 2D uniform low-velocity wave guide and estimated the averaged fault-zone structure from the hypocenter to receiver by modeling the LTWs. Because we can infer the low-velocity fault zone to a depth of 8 km by observing the duration of LTWs, we obtain the average fault-zone structure from the borehole stations to a depth of approximately 8 km. The width, shear-wave velocity, and Q s of the fault zone beneath HRB are 100 m, 2.9 km/sec, and 60, respectively. On the other hand, the average width of the fault zone beneath TOS2 is 220 m, which is larger than that beneath HRB. We also conducted 3D finite-difference modeling of LTWs to confirm the spatial variations in the fault-zone structure to a depth of 8 km. The numerical simulation suggests that the Nojima fault has uniform elastic and attenuation properties along the fault zone to a depth of 8 km, while the width of the fault zone increases with the distance from the HRB to the south.

Dissipative structures in fault zones and their diagnostic criteria (from physical modeling data)

A fault zone is considered an open nonequilibrium dynamic system in which its constituent subsystems evolve by self-organization in a nonlinear synergetic process of successive changes of structural levels (stages). A new term fault dissipative structure is suggested for an ordered 3D space-time structure produced in a fault zone by self-organization of fracturess of different scales. The approach is used in physical modeling to simulate the evolution of a strike-slip fault zone in an elastic-viscoplastic lithosphere and to recognize space and time criteria diagnostic of fault dissipative structures through fractal and entropy analyses.

Automatic reconstruction of fault networks from seismicity catalogs: Three‐dimensional optimal anisotropic dynamic clustering

We propose a new pattern recognition method that is able to reconstruct the three‐dimensional structure of the active part of a fault network using the spatial location of earthquakes. The method is a generalization of the so‐called dynamic clustering (or k means) method, that partitions a set of data points into clusters, using a global minimization criterion of the variance of the hypocenters locations about their center of mass. The new method improves on the original k means method by taking into account the full spatial covariance tensor of each cluster in order to partition the data set into fault‐like, anisotropic clusters. Given a catalog of seismic events, the output is the optimal set of plane segments that fits the spatial structure of the data. Each plane segment is fully characterized by its location, size, and orientation. The main tunable parameter is the accuracy of the earthquake locations, which fixes the resolution, i.e., the residual variance of the fit. The resolution determines the number of fault segments needed to describe the earthquake catalog: the better the resolution, the finer the structure of the reconstructed fault segments. The algorithm successfully reconstructs the fault segments of synthetic earthquake catalogs. Applied to the real catalog constituted of a subset of the aftershock sequence of the 28 June 1992 Landers earthquake in southern California, the reconstructed plane segments fully agree with faults already known on geological maps or with blind faults that appear quite obvious in longer‐term catalogs. Future improvements of the method are discussed, as well as its potential use in the multiscale study of the inner structure of fault zones.

Recent advances in the understanding of fault zone internal structure: a review

Abstract It is increasingly apparent that faults are typically not discrete planes but zones of deformed rock with a complex internal structure and three-dimensional geometry. In the last decade this has led to renewed interest in the consequences of this complexity for modelling the impact of fault zones on fluid flow and mechanical behaviour of the Earth's crust. A number of processes operate during the development of fault zones, both internally and in the surrounding host rock, which may encourage or inhibit continuing fault zone growth. The complexity of the evolution of a faulted system requires changes in the rheological properties of both the fault zone and the surrounding host rock volume, both of which impact on how the fault zone evolves with increasing displacement. Models of the permeability structure of fault zones emphasize the presence of two types of fault rock components: fractured conduits parallel to the fault and granular core zone barriers to flow. New data presented in this paper on porosity–permeability relationships of fault rocks during laboratory deformation tests support recently advancing concepts which have extended these models to show that poro-mechanical approaches (e.g., critical state soil mechanics, fracture dilatancy) may be applied to predict the fluid flow behaviour of complex fault zones during the active life of the fault. Predicting the three-dimensional heterogeneity of fault zone internal structure is important in the hydrocarbon industry for evaluating the retention capacity of faults in exploration contexts and the hydraulic behaviour in production contexts. Across-fault reservoir juxtaposition or non-juxtaposition, a key property in predicting retention or across-fault leakage, is strongly controlled by the three-dimensional complexity of the fault zone. Although algorithms such as shale gouge ratio greatly help predict capillary threshold pressures, quantification of the statistical variation in fault zone composition will allow estimations of uncertainty in fault retention capacity and hence prospect reserve estimations. Permeability structure in the fault zone is an important issue because bulk fluid flow rates through or along a fault zone are dependent on permeability variations, anisotropy and tortuosity of flow paths. A possible way forward is to compare numerical flow models using statistical variations of permeability in a complex fault zone in a given sandstone/shale context with field-scale estimates of fault zone permeability. Fault zone internal structure is equally important in understanding the seismogenic behaviour of faults. Both geometric and compositional complexities can control the nucleation, propagation and arrest of earthquakes. The presence and complex distribution of different fault zone materials of contrasting velocity-weakening and velocity-strengthening properties is an important factor in controlling earthquake nucleation and whether a fault slips seismogenically or creeps steadily, as illustrated by recent studies of the San Andreas Fault. A synthesis of laboratory experiments presented in this paper shows that fault zone materials which become stronger with increasing slip rate, typically then get weaker as slip rate continues to increase to seismogenic slip rates. Thus the probability that a nucleating rupture can propagate sufficiently to generate a large earthquake depends upon its success in propagating fast enough through these materials in order to give them the required velocity kick. This propagation success is hence controlled by the relative and absolute size distributions of velocity-weakening and velocity-strengthening rocks within the fault zone. Statistical characterisation of the distribution of such contrasting properties within complex fault zones may allow for better predictive models of rupture propagation in the future and provide an additional approach to earthquake size forecasting and early warnings.

The internal structure of fault zones: fluid flow and mechanical properties

W. KURZ1, J. IMBER2, C. A. J. WIBBERLEY3, R. E. HOLDSWORTH4 & C. COLLETTINI5 Institut fuer Angewandte Geowissenschaften, Technische Geologie und Mineralogie, Technische Universitaet Graz, Rechbauerstrasse 12, A-8010 Graz, Austria Reactivation Research Group, Dept of Earth Sciences, University of Durham, South Road, Durham DH1 3LE, UK TOTAL, CSTJF, Avenue Larribau, 64018 PAU Cedex, France (e-mail: christopher.wibberley@total.com) Department of Earth Sciences, University of Durham, South Road, Durham, DH1 3LE, UK Dipartimento di Scienze della Terra, Universita di Perugia, Piazza dell’Universita 1, 06100, Perugia, Italy

On the structure and mechanical properties of large strike-slip faults

Abstract Elucidation of the internal structure of fault zones is paramount for understanding their mechanical, seismological and hydraulic properties. In order to observe representative brittle fault zone structures, it is preferable that the fault be passively exhumed from seismogenic depths and the exposure must be in arid or semi-arid environments where the fragile rocks are not subject to extensive weathering. Field observations of two such faults are used to constrain their likely mechanical properties. One fault is the Carboneras fault in southeastern Spain, where the predominant country rocks are phyllosilicate-rich lithologies, and the other is part of the Atacama fault system in northern Chile, where faults pass through crystalline rocks of acidic to intermediate composition. The Carboneras fault is a left lateral fault with several tens of kilometres offset exhumed from approximately 4 km depth, and displays multiple strands of clay-bearing fault gouge, each several metres wide, that contain variably fractured lenses of protolithic mica schists. The strain is evenly distributed across the gouge layers, in accordance with the measured laboratory mechanical behaviour which shows predominantly strain hardening characteristics. The overall width of the fault zone is several hundred metres. Additionally, there are blocks of dolomitic material that are contained within the fault zones that show extremely localized deformation in the form of faults several centimetres wide. These are typically arranged at an angle of c . 20° to the overall fault plane. These differing types of fault rock products allow for the possibility of ‘mixed mode’ seismicity, with fault creep occurring along the strands of velocity strengthening clay-rich gouge, punctuated by small seismic events that nucleate on the velocity weakening localized faults within the dolomite blocks. The Caleta Coloso fault in northern Chile has a left-lateral offset of at least 5 km and was exhumed from 5–10 km depth. The fault core is represented by a 200–300 m wide zone of hydrothermally altered protocataclasite and ultracataclasite. This is surrounded by a zone of micro and macro-fractures on the order of 150 m thick. The fault core shows a heterogeneous distribution of strain, with alternate layers of ultracataclasite and lower strain material. The strain-weakening behaviour of crystalline rocks might be expected to produce highly localized zones of deformation, and thus the wide core zone must be a result of additional process such as precipitation strengthening or geometric irregularities along the fault plane.

About this title ‐ The Internal Structure of Fault Zones: Implications for Mechanical and Fluid-Flow Properties

Faults are primary focuses of both fluid migration and deformation in the upper crust. The recognition that faults are typically heterogeneous zones of deformed material, not simple discrete fractures, has fundamental implications for the way geoscientists predict fluid migration in fault zones, as well as leading to new concepts in understanding seismic/aseismic strain accommodation. This book captures current research into understanding the complexities of fault-zone internal structure, and their control on mechanical and fluid-flow properties of the upper crust. A wide variety of approaches are presented, from geological field studies and laboratory analyses of fault-zone and fault-rock properties to numerical fluid-flow modelling, and from seismological data analyses to coupled hydraulic and rheological modelling. The publication aims to illustrate the importance of understanding fault-zone complexity by integrating such diverse approaches, and its impact on the rheological and fluid-flow behaviour of fault zones in different contexts.

Displacement-length scaling for single-event fault ruptures: insights from Newberry Springs Fault Zone and implications for fault zone structure

Abstract The Newberry Springs Fault Zone experienced slip associated with the 1992 Landers earthquake in the Mojave Desert of California, USA. Detailed analysis of scaling relationships from single-event ground ruptures in the Newberry Springs Fault Zone mapped in the field shows an average maximum displacement to length ( D max / L ) relationship for fault segments (rupture lengths in the range of 100–1000 m) of 8×10 −5 –consistent with previously published D max / L ratios for normal fault earthquake ground ruptures (rupture lengths in the range of 1–100 km) of 7×10 −5 . To explore the ability of remote sensing (interferometric synthetic aperture radar or InSAR) to map small-displacement single-event fault ruptures and add constraints on segment displacements, we applied established interferometry methods with phase unwrapping to produce maps of line-of-sight displacement and displacement gradient. These maps highlight fault traces that experienced displacement during the time between collection of the synthetic aperture radar images. Comparison of published 1992 single-event ground rupture maps with mapping based on photogeologic interpretation of 1950s vintage aerial photographs indicates that most of the 1992 ruptures occurred as reactivation of pre-existing slip surfaces. In general, D max / L for total fault displacement is approximately 100 times D max / L for single-event ruptures. Evidence from the Newberry Springs Fault Zone indicates that, since the Pleistocene, at least 10–20 Landers-like slip events have occurred, reactivating the Newberry Springs Fault Zone. Evidence of wide damage zones and reactivation of individual segments developed in alluvial floodplain deposits, at relatively small (order of metres) fault displacements, supports a conceptual model of fault damage zone width being established early, during fault propagation. With continued displacement by accumulation of additional slip events, fault zone damage intensifies. The fault zone width may remain relatively stable, although the active portion of the fault zone will likely narrow as faulting continues and a throughgoing slip surface develops and accumulates the bulk of displacement.

High-resolution Imaging of Fault Zone Structures with Seismic Fault Zone Waves

Large fault zone (FZ) structures with damaged rocks and material discontinuity interfaces can generate several indicative wave propagation signals. High crack density may produce prominent scattering and non-linear effects. A preferred crack orientation can lead to shear wave splitting. A lithology contrast can produce FZ head waves that propagate along the material interface with the velocity and motion polarity of the faster medium. A coherent low velocity layer may generate FZ trapped waves. These signals can be used to obtain high resolution imaging of the subsurface structure of fault zones, and to track possible temporal evolution of FZ material properties. Several results have emerged from recent systematic analyses of such signals. The trapped waves are generated typically by ~100-m-widem-wide-widewide layers that extend only to ~3–4 km depth and are characterized by 30�–�0� velocity reduction and strong attenu�–�0� velocity reduction and strong attenu–�0� velocity reduction and strong attenu

Fault Characteristics, Energy Estimates, and Earthquake Recurrence: What One Seismologist Wants from Fault Drilling

There are some significant differences between the way seismologists and geologists typically look at faulting from large earthquakes. Seismologists tend to analyze waveform data which can usually resolve structures on the order of tens of kilometers; possibly down to one kilometers there is very good near-field data. Also, the derived physically properties are averaged over this scale length. Although it is difficult to resolve small scale features, seismologic studies have the advantage of seeing all depths of the fault. In contrast fault zone geologists can make direct observations of the fault zone structures and properties, which are quite different from the remote sensing techniques of seismologists. Geologists tend to look at structures on the scale of microns to meters when examining the cores obtained from drilling into faults. These observations are spot measurements and it is often difficult to assess how representative they are of the entire fault. Also, fault zone sample studies are usually limited to a few kilometers depth, which may barely be in the range of the seismogenic zone that produces the seismic waves. These differences mean that seismologists and geologists are often looking at quite different aspects of the earthquake process. The approaches are different, but can also be complementary.