Width Of Damage Zone Research Articles

Fault damage zones width: Implications for the tectonic evolution of the northern border of the Araripe Basin, Brazil, NE Brazil

Fault zones commonly have spatially variable fault rocks and brittle structures. Differing fault-rock attributes may retard or increase cross- and along-fault permeability. Our work analyzed brittle deformation of a segment of the Patos shear zone, Triunfo Fault, which is located in the northern border of the Araripe Basin, NE Brazil. We investigated the structural evolution and tectonic implications of the Triunfo Fault by mapping damage zone width patterns and the types and distributions of breccia and brittle structure arrays within the fault zone using scanline fracture analysis techniques and topographic and aeromagnetometric data. Fractured rocks of the damage zone mainly comprise: mylonitic orthogneiss (footwall); and sandstone and carbonate rocks (hangingwall). The fault core is composed of tectonic breccia and cataclasites occurring as lenses, with calcite-filled veins. Due to fault sinuosity and rheology heterogeneities, the width of the damage zone ranges from 240 to 290m (footwall), and 372–610m (hangingwall). Our analysis obtained from the topographic and gravimetric data combined with field data indicate the following sequential deformation phases: a) Dn, Brasiliano Orogeny, dextral shear zone; b) Dn+1, Paleozoic time, brittle-ductile deformation; and c) Dn+2, Lower Cretaceous time, brittle deformation.

Normal fault networks and their spatial relationships in Plio-Quaternary sedimentary series: A case study in the Zanjan Depression, NW Iran

Well-exposed normal fault networks in the Plio-Quaternary sedimentary series of the Zanjan Depression, Iran, were studied in order to analyse the spatial relationships and evolution of fault and damage zone structures in low-burial (~200 m) conditions. Despite the poorly-consolidated, geologically young nature of sediment, fault networks show several characteristics more typical of faulting in lithified strata. Key controls on the evolution of fault networks are found to be fault interactions, segment linkage, and the mechanical stratigraphy resulting in dip refraction, fault segmentation, the development of fault bends and steps, vertical fault overlap zone breaching, and bed-parallel slip. The data show a decrease of fracture frequency away from the fault cores. The obtained data show that damage zones width increases with fault displacement in a non-linear manner. Furthermore, damage zones appear more deformed and wider in the hanging-wall than in the footwall. A plot of damage zone width versus total fault throw reveals a positive but non-linear correlation, and the best-fit function corresponds better to a power-law relationship. The relationship between displacement and length shows a sub-vertical growth path with large displacement gradients typical of faulting in partially-lithified strata, due to shale ductility accommodating fault tip-related strains. A model for the evolution of the normal fault system is proposed based on spatial distribution of deformation, particularly fault segmentation, segment linkage, and damage zone development, as influenced by initial fault spacing, mechanical stratigraphy and the low burial conditions.

Damage zone heterogeneity on seismogenic faults in crystalline rock; a field study of the Borrego Fault, Baja California

Complex fracture damage around large faults is often simplified to fit exponential or power law decay in fracture density with distance from the fault. Noise in these datasets is attributed to large subsidiary faults or random natural variation. Through a field study of the Borrego Fault (Baja California) damage zone, combining mm-resolution structural mapping and point sampling, we show that such variations are the expression of systematic damage heterogeneity. The oblique-slip Borrego Fault comprises NW and SE segments that ruptured during the Mw7.2 2010 El Mayor-Cucapah earthquake. Measurements of fracture density along eight linear fault-perpendicular transects and from a high-resolution 68 m2 structural map display a power law decay and define footwall damage zone widths of ~85 m and ~120 m for the NW and SE segments respectively. Variance in fracture density decays with distance following an inverse exponential relationship, to background variance at ~16 m. Spatial analysis of the high-resolution fracture map reveals a patchy distribution of high- and low-intensity clusters at metre- and decimetre-scales. We attribute high-intensity clusters at these scales to local complexity caused by interactions between minor subsidiary faults (101 m length and 10−2-10−1 m displacement). Fracture density differences between high- and low-intensity clusters decrease with distance from the fault, demonstrating a systematic change in outcrop-scale damage heterogeneity. Based on these observations we present a revised model for damage zone growth including growth of heterogeneity.

Dynamics, Radiation, and Overall Energy Budget of Earthquake Rupture With Coseismic Off‐Fault Damage

AbstractEarthquake ruptures dynamically activate coseismic off‐fault damage around fault cores. Systematic field observation efforts have shown the distribution of off‐fault damage around main faults, while numerical modeling using elastic‐plastic off‐fault material models have demonstrated the evolution of coseismic off‐fault damage during earthquake ruptures. Laboratory‐scale microearthquake experiments have pointed out the enhanced high‐frequency radiation due to the coseismic off‐fault damage. However, the detailed off‐fault fracturing mechanisms, subsequent radiation, and its contribution to the overall energy budget remain to be fully understood because of limitations of current observational techniques and model formulations. Here, we constructed a new physics‐based dynamic earthquake rupture modeling framework, based on the combined finite‐discrete element method, to investigate the fundamental mechanisms of coseismic off‐fault damage, and its effect on the rupture dynamics, the radiation and the overall energy budget. We conducted a 2‐D systematic case study with depth and showed the mechanisms of dynamic activation of the coseismic off‐fault damage. We found the decrease in rupture velocity and the enhanced high‐frequency radiation in near field due to the coseismic off‐fault damage. We then evaluated the overall energy budget, which shows a significant contribution of the coseismic off‐fault damage to the overall energy budget even at depth, where the damage zone width becomes narrower. The present numerical framework for the dynamic earthquake rupture modeling thus provides new insights into earthquake rupture dynamics with the coseismic off‐fault damage.

Seismic damage zone and width\u2013throw scaling along the strike-slip faults in the Ordovician carbonates in the Tarim Basin

Understanding the scaling relation of damage zone width with displacement of faults is important for predicting subsurface faulting mechanisms and fluid flow processes. The understanding of this scaling relationship is influenced by the accuracy of the methods and types of data utilized to investigate faults. In this study, seismic reflection data are used to investigate the throw and damage zone width of five strike-slip faults affecting Ordovician carbonates of the Tarim intracraton basin, NW China. The results indicate that fault slips with a throw less than 200 m had formed wide damage zones up to 3000 m in width. Also, damage zone width is found to have both a positive correlation and a power-law relation with throw of two orders of magnitude, with a ratio of these values varying in a range of 2–15. The relationship between throw and damage zone width is not a simple power-law and changes its slope from small to larger size faults. The results indicate that throw scales well with damage zone width for the studied faults, and hence these can be used to predict fault geometries in the Tarim Basin. The study of the wide carbonate damage zones presented here provides new insights into scaling of large-size faults, which involve multiple faulting stages.

Evolution of gold-bearing splays of crustal-scale fault zones of the south-central Archean Abitibi subprovince: Constraints from the Kirana deformation zone, Kirkland Lake, Ontario, Canada

We explored the effects of brittle-ductile deformation processes on the localization of strain and hydrothermal fluid flow throughout the formation and evolution of a fault zone. This was accomplished through detailed structural analysis of the Kirana deformation zone (Kdz), a splay of the Archean crustal-scale, orogenic gold-hosting Larder Lake-Cadillac deformation zone (LLCdz) in Ontario, Canada. Investigations included mapping, quantification of fault-related breccia by particle size distribution analysis, microstructural and petrographic characterization, and kinematic analysis of brittle-ductile structures. The Kdz records three principal evolutionary stages. Stage 1 involved seismogenic processes that resulted in the formation of a >50 m wide fault core and damage zone. Stage 2 is characterized by transpressional ductile shear localized along texturally favorable, mechanically weakened zones that formed during stage 1 and/or along zones where coeval fluid flow facilitated reaction weakening. Native gold is observed within the mylonitized fault core that is interpreted to have formed syn-to late-ductile shear. All structures are crosscut by small-scale faults and shear veins with disseminated alteration halos, defining stage 3. The Kdz demonstrates how early fault processes may control the spatial pattern and temporal progression of strain and hydrothermal fluid localization in orogenic gold-bearing fault splays.

Field-based investigation of fault architecture: A case study from the Lenghu fold-and-thrust belt, Qaidam Basin, NE Tibetan Plateau

AbstractThe fault zone architecture of a thrust fault zone is critical for understanding the strain accommodation and structural evolution in contractional systems. The fault architecture is also important for understanding fluid-flow behavior both along and/or across thrust fault zones and for evaluating potential fault-related compartmentalization. Because mesoscale (1–100 m) structural features are normally beyond seismic resolution, high-resolution outcrop in situ mapping (5–10 cm resolution) was employed to study the deformation features of a thrust fault zone located in the Qaidam Basin, northeastern Tibetan Plateau. The excellent exposure of outcrops enables the detailed investigation of the Lenghu thrust fault zone and its architecture. The Lenghu thrust fault, a seismically resolvable fault with up to ∼800 m of throw, exhibits a large variation of fault architecture and strain distribution along the fault zone. Multiple structural domains with different levels of strain were observed and are associated with the fault throw distribution across the fault. Based on previously proposed models and high-resolution outcrop mapping, an updated fault zone model was constructed to characterize the structural features and evolution of the Lenghu thrust. The possible parameters that impact fault architecture and strain distribution, including fault throw, bed thickness, lithology, and mechanical heterogeneity, were evaluated. Fault throw distributions and linkages control the strain distribution across a thrust fault zone, with local folding processes contributing important elements in Lenghu, especially where more incompetent beds dominate the stratigraphy. Mechanical heterogeneity, induced by different layer stacking patterns, controls the details of the fault architecture in the thrust zone. The variations in bed thicknesses and mechanical property contrasts are likely to control the initial fault dips and fault/fracture density. Large fault throws are associated with wide strain accommodation and damage zones, although the relationship between the development and width of the fault zone and the throw accumulation remains to be assessed. By presenting the high-resolution mapping of fault architecture, this study provides an insight into the subseismic fault zone geometry and strain distributions possible in thrust faults and reviews their application to assessments of fault zone behavior.

Numerical and Experimental Study on the Cracking Behavior of Marble with En-Echelon Flaws

En-echelon fractures have been commonly observed in nature. To understand the interaction of en-echelon flaws, numerical simulation with flat-jointed model and laboratory experiment has been conducted on specimens containing en-echelon flaws of various configurations. With reliable micro-parameters of the flat-joint contact through a comprehensive calibration, numerical results reveal a strong interaction in flaw configurations of extensional bridges but a mild interaction in flaw configurations of contractional bridges. The patterns of linking cracks are found to be dependent on the flaw configurations. Furthermore, two typical linking cracks, tensile and shear cracks, are identified in terms of contact force evolution, crack development, micro-components and damage zone width. Experimental results not only verify the above conclusions given by simulation, but also provide more distinctions between tensile and shear cracks in grain scale. Based on these findings, an empirical correlation is suggested between debonding behavior in PFC models and microcracking behavior in laboratory experiments. Finally, the tendency of crack initiation stress and peak stress in simulation and experiment is compared and explained with respect to the flaw-array angle. A good agreement between simulation and experiment indicates the capability of the flat-jointed model of PFC2D in handling complex crack coalescence problems in rocks.

Repeated Seismic Slipping Events Recorded in a Fault Gouge Zone: Evidence From the Nojima Fault Drill Holes, SW Japan

AbstractDrilling investigations and structural analysis of drill cores reveal that a fault gouge zone of 10–30 cm in width was observed at depths of ~260 to 900 m in nine drill holes that intersected the Nojima Fault (NF), on which the 1995 Mw 6.9 Kobe (Japan) earthquake occurred. Logging data and an analysis of mesostructures and microstructures in drill cores show that (i) a ~60‐m wide fault damage zone containing a 10‐ to 30‐cm‐thick fault gouge zone developed in the NF, (ii) the fault gouge zone can be divided into 11–20 thin layers of different color, and (iii) the individually colored layers contain different color breccias of fault gouge that are offset and/or cut by cracks and crack‐filled calcite and quartz veinlets. Our results reveal that the fault gouge zone probably records more than 11–20 paleoseismic faulting events along the NF during the late Pleistocene‐Holocene.

Regional‐Scale Detection of Fault Scarps and Other Tectonic Landforms: Examples From Northern California

AbstractFault scarps and fault‐related landforms provide important information about fault zone activity over timescales that are not captured by instrumental measurements or historic records. Semiautomated methods for delineating these landforms using topographic data from light detection and ranging (lidar) and spaceborne imaging systems offer the opportunity to characterize fault zones on a global scale. We present a computationally efficient method for extracting scarp‐like landforms from high‐resolution (≤2 m), regional‐scale (≥ 100‐km‐long) digital topographic data sets. We identify fault‐related landforms using a curvature template based on the diffusion model for scarp degradation and extract scarp heights and morphologic ages at each pixel. The method was applied to the GeoEarthScope Northern California data set, an airborne lidar acquisition imaging nearly 2,500 km2 of the northern San Andreas Fault system, by adapting the algorithm to use cloud computing resources. Template results and fault trace mapping show spatial agreement in active fault zones with clear topographic expression, including detection of fault scarps, shutter ridges, and elongated drainages. Comparison of the method against field‐based morphologic dating of scarps along the southern San Andreas reveals a trade‐off between template window size and morphologic age contrasts resolved between strike‐slip fault scarps of different relative ages. Detection performance suggests that window size and orientation constraints may play a key role in improving the accuracy of methods for semiautomated fault zone mapping. As data availability grows, these methods could constrain key earthquake simulation parameters such as damage zone width or rupture length and improve fault maps worldwide.

Fracture attributes in reservoir-scale carbonate fault damage zones and implications for damage zone width and growth in the deep subsurface

The width of reservoir-scale fault damage zone governs fluid flow of the tight reservoirs in subsurface petroliferous basins. Together with cores, thin sections, geophysical well logs, and production data from more than 60 wells, we describe fracture attributes and fault damage zones associated with a conjugate strike-slip fault system in tight Ordovician carbonate rock at depths of between 6500 m and 7500 m in the Tarim intracratonic basin in NW China. Fracture frequency, aperture, and oil production in wells reveals a power-law relationship with the distance from fault cores, with a variation in two orders of magnitude. The cumulative data of fracture frequency, aperture and oil production show multiple gradients in slope with the distance. Except for sampling bias, the varied gradients are possibly related to the overlaying multiple fault zones or multiple sections along a large fault zone, which are useful to constrain damage zone width. Together with fracture frequency, cumulative fracture frequency, aperture and oil production, four major damage zone boundaries are identified at ∼200, ∼420 m, ∼780 m and ∼1100 m. Therefore, a wide fault damage zone with a width of more than 2 km in the carbonate rocks could be inferred, which mostly depends on the interaction and superposition of secondary faults. In addition, there is a relatively high fracture frequency decay exponent (>1) with distance, which is possibly related to fault size, fault splays, background fracture frequency and the lithology. In turn, the growth of wide damage zone also resulted in a large scattered distribution of the fracture attributes with the distance from fault cores. Attributes of fractures of various scales from the deep subsurface provide powerful tools for assessing damage zone width and growth.

Basement control on fault formation and deformation band damage zone evolution in the Rio do Peixe Basin, Brazil

The objective of this study is to analyze the influence of brittle reactivations of continental-scale Precambrian ductile shear zones on the evolution of deformation band damage zones. Our results indicate that the Rio do Peixe Basin was formed by the Cretaceous reactivation of its basement EW- and NE-SW-striking major fault segments. These fault segments are composed of a footwall with Precambrian crystalline mylonites and a hanging wall with Cretaceous fine to coarse sandstones. In the hanging wall, deformation bands occur in poorly sorted, fine to very coarse sandstones. These deformation bands occur as single bands or clusters mostly within damage zones ~84 m from the basement faults and are widespread in structural highs. Our data plot slightly above the medium fault displacement vs. the damage zone width solution but falls within the point cloud of global patterns. The slip surfaces strike EW and NE-SW and are parallel or slightly oblique to the basement rift faults. The kinematics of slip surfaces are consistent with those of the reactivated basement faults. In addition, a logarithmic decrease in deformation band frequency occurs away from the basement slip surface. The deformation band frequencies peak close to the fault core with values as high as 22–48 bands/m. The lithology grain size also influences deformation as clusters of deformation bands and slip surfaces are only observed in coarse sandstones. This new case of deformation bands in damage zones of basement faults is characterized by a strong attitude and geometric and kinematic inheritance but follows the deformation band frequency of global patterns.

Predicting the width and average fracture frequency of damage zones using a partial least squares statistical analysis: Implications for fault zone development

We introduce the partial least squares (PLS) statistical analysis that quantifies and predicts the observed relationships among normal fault slip, fracturing associated with the fault, and lithology. We describe the systematic process for constructing a multivariate PLS model that predicts the average fracture frequency and the width of fracture-dominated fault damage zones from fault, lithologic and fracture data. Conversely, the model can also predict normal fault net slip for a defined lithology given the average fracture frequency and width of a fracture-dominated fault damage zone, hereafter defined as a fracture intensification domain (FID). Fracture, fault and lithologic data were collected in the Mohawk Valley of New York State from outcrops in the Upper Ordovician Utica Group and Lorraine Group. Data collection was focused on faults with observable slip, associated FIDs, and no observable lateral restriction. Our statistical analysis used three variables to describe the geometry of the FID: FID width (FIDw), average fracture frequency within the FID (FIDƒ), and the power law regression exponent (FIDR) of the least squares trend line. We incorporated additional data from literature and tested multiple PLS models in order to refine the analysis using quality indicators provided by the PLS summary statistics output. Variables included in the final predictive model included FIDw, FIDƒ, fault slip, grain size and clay percent. Fault slip and grain size were found to have a positive covariance with FIDw while clay percent had a negative covariance. Fault slip, grain size and clay percent all showed a negative covariance with FIDƒ. Results from this research indicate that increasing fault slip leads to wider FIDs and lower average fracture frequency within the FID. The lower average fracture frequency in wider FIDs is primarily attributed to an increase in the length of the low-frequency FID tail away from the associated fault. A possible secondary influence reducing fracture frequency is due to the progressive development of a fault core at the expense of the adjacent damage zone and the consumption of the highest-frequency fractures adjacent to the fault surface.

Advanced seismic imaging techniques characterize the Alpine Fault at Whataroa (New Zealand)

AbstractThe plate‐bounding Alpine Fault in New Zealand is an 850 km long transpressive continental fault zone that is late in its earthquake cycle. We have acquired and processed reflection seismic data to image the subsurface around the main drill site of the Deep Fault Drilling Project (DFDP‐2). The resulting velocity models and seismic images of the upper 5 km show complex subsurface structures around the Alpine Fault zone. The most prominent feature is a strong reflector at depths of 1.5–2.2 km with an apparent dip of 48° to the southeast below the DFDP‐2 borehole, which we assume to be the main trace of the Alpine Fault. Above the main reflector, parallel reflectors suggest the presence of a ∼600 m wide damage zone. Additionally, subparallel reflectors are imaged that we interpret as secondary branches of the main fault zone. Conjugate faults imaged by the data show the complexity of the subsurface. The derived P wave velocity model reveals a 300–600 m thick sedimentary layer with velocities of ∼2.3 km/s above a schist basement with velocities of 4.5–5.5 km/s. A low‐velocity layer can be observed within the basement at 0.8–2 km depth. A small‐scale low‐velocity anomaly appears at the top of the basement that can be correlated to the fault zone. The results provide a reliable basis for a seismic characterization of the DFDP‐2 drill site that can be used for further structural and geological investigations of the architecture of the Alpine Fault in this area.

Fracture networks of normal faults in fine-grained sedimentary rocks: examples from Kilve Beach, SW England

Abstract Interbedded shale and limestone successions in the Kilve Beach area, Bristol Channel Basin, UK, provide insights on fracture networks around normal faults in fine-grained lithologies. Fracture sets with distinct orientations are characteristic of both shale and limestone beds. Shear fractures (mode II) predominate in the shaly units, and they have typically more gentle dips and a larger spread in orientations than extension veins and shear fractures in the limestones. Fracture intensities decrease away from the fault core, but maximum intensities, total number of fractures and widths of the damage zones appear to be independent of throw for normal faults with offsets of less than 20 m. Thus, there is no clear systematic relationship between fault throw and damage zone width in the shales studied by us. However, an asymmetry in the fracture distribution is evidenced by a wider hanging-wall damage zone and differences in fracture orientations in some cases. We interpret the asymmetry and spread in fracture orientations to be the result of propagating fault-tip process zones and the tempo-spatial impact of fault-slip events.

Evolution and progressive geomorphic manifestation of surface faulting: A comparison of the Wairau and Awatere faults, South Island, New Zealand: REPLY

Field mapping and lidar analysis of surface faulting patterns expressed in flights of geologically similar fluvial terraces at the well-known Branch River and Saxton River sites along the Wairau (Alpine) and Awatere strike-slip faults, South Island, New Zealand, reveal that fault-related deformation patterns expressed in the topography at these sites are markedly less structurally complex along the higher-displacement (hundreds of kilometers), structurally mature Wairau fault than along the Awatere fault (∼13–20 km total slip). These differences, which are generally representative of the surface traces of these faults, provide direct evidence that surface faulting becomes structurally simpler with increasing cumulative fault offset. We also examine the degree to which off-fault deformation (OFD) is expressed in the landscape at the Saxton River site along the less structurally mature Awatere fault. Significantly greater amounts of OFD are discernible as a wide damage zone (∼460 m fault-perpendicular width) in older (ca. 15 ka), more-displaced (64–74 m) fluvial terraces than in younger (ca. 1–7 ka), less-displaced (<55 m) terraces; no OFD is discernible in the lidar data on the least-displaced (<35 m) terraces. From this, we infer that OFD becomes progressively more geomorphically apparent with accumulating displacement. These observations imply that (1) the processes that accommodate OFD are active during each earthquake, but may not be evident in deposits that have experienced relatively small displacements; (2) structures accommodating OFD will become progressively geomorphically clearer with increasing displacement; (3) geomorphic measurements of overall fault zone width taken in deposits that have experienced small displacements will be underestimates; and (4) fault slip rates based on geomorphic surface offsets will be underestimates for immature faults if based solely on measurements along the high-strain fault core.

Dilatant normal faulting in jointed cohesive rocks: a physical model study

Abstract. Dilatant faults often form in rocks containing pre-existing joints, but the effects of joints on fault segment linkage and fracture connectivity are not well understood. We present an analogue modeling study using cohesive powder with pre-formed joint sets in the upper layer, varying the angle between joints and a rigid basement fault. We analyze interpreted map-view photographs at maximum displacement for damage zone width, number of connected joints, number of secondary fractures, degree of segmentation and area fraction of massively dilatant fractures. Particle imaging velocimetry provides insight into the deformation history of the experiments and illustrates the localization pattern of fault segments. Results show that with increasing angle between joint-set and basement-fault strike the number of secondary fractures and the number of connected joints increase, while the area fraction of massively dilatant fractures shows only a minor increase. Models without pre-existing joints show far lower area fractions of massively dilatant fractures while forming distinctly more secondary fractures.

Architecture of fault damage zones of normal faults, Gebel Ataqa area, Gulf of Suez rift, Egypt

Well-exposed contemporaneous E-W en echelon normal faults and NW-SE oriented normal faults in Gebel Ataqa area (Gulf of Suez) allow comparison of their damage zones. Across all investigated damage zones, the majority of fractures are parallel and sub-parallel to their respective faults irrespective of the regional stress field of the area. Four fracture sets have been recorded across the E-W en echelon normal faults and three fracture sets across the NW-SE normal faults. These fractures form a network by their cross-cutting relationship to generate lithons and rock fragments of various sizes within the damage zones. The area around the fault surface exhibits high level of strain with high fracture density. Fracture spacing increases away from the fault. Fracture density is mirrored across the fault in hanging wall and footwall blocks. The average half width of the studied damage zones ranges between 12 and 14.5 m. Slight change in damage zone width along the fault strike is related to change in throw along the fault.

Myths about normal faulting

Abstract Analyses of normal faults in mechanically layered strata reveal that material properties of rock layers strongly influence fault nucleation points, fault extent (trace length), failure mode (shear v. hybrid), fault geometry (e.g. refraction through mechanical layers), displacement gradient (and potential for fault tip folding), displacement partitioning (e.g. synthetic dip, synthetic faulting, fault core displacement), fault core and damage zone width, and fault zone deformation processes. These detailed investigations are progressively dispelling some common myths about normal faulting held by industry geologists, for example: (i) that faults tend to be linear in dip profile; (ii) that imbricate normal faults initiate due to sliding on low-angle detachments; (iii) that friction causes fault-related folds (so-called normal drag); (iv) that self-similar fault zone widening is a direct function of fault displacement; and (v) that faults are not dilational features and/or important sources of permeability.

Low resistivity and permeability in actively deforming shear zones on the San Andreas Fault at SAFOD

AbstractThe San Andreas Fault Observatory at Depth (SAFOD) scientific drill hole near Parkfield, California, crosses the San Andreas Fault at a depth of 2.7 km. Downhole measurements and analysis of core retrieved from Phase 3 drilling reveal two narrow, actively deforming zones of smectite‐clay gouge within a roughly 200 m wide fault damage zone of sandstones, siltstones, and mudstones. Here we report electrical resistivity and permeability measurements on core samples from all of these structural units at effective confining pressures up to 120 MPa. Electrical resistivity (~10 Ω‐m) and permeability (10−21 to 10−22 m2) in the actively deforming zones were 1 to 2 orders of magnitude lower than the surrounding damage zone material, consistent with broader‐scale observations from the downhole resistivity and seismic velocity logs. The higher porosity of the clay gouge, 2 to 8 times greater than that in the damage zone rocks, along with surface conduction were the principal factors contributing to the observed low resistivities. The high percentage of fine‐grained clay in the deforming zones also greatly reduced permeability to values low enough to create a barrier to fluid flow across the fault. Together, resistivity and permeability data can be used to assess the hydrogeologic characteristics of the fault, key to understanding fault structure and strength. The low resistivities and strength measurements of the SAFOD core are consistent with observations of low resistivity clays that are often found in the principal slip zones of other active faults making resistivity logs a valuable tool for identifying these zones.