A refined classification of 3D shape preferred orientation patterns in fault gouges: Insights from the Yangsan and Ulsan Fault Zones, South Korea

Solid-like to liquid-like transition of stick-slips in sheared fault gouge

The southeastern Iberian Peninsula, particularly the Águilas Arc within the Neogene Volcanic Province (NVP), represents a promising geothermal domain with complex tectonics and geology. The Palomares Fault Zone (PFZ), a key shear structure initiated during the Late Miocene, acts as a conduit for fluid migration, promoting mineralization and potential anomalies of rare and critical metals through fluid–rock interaction. This study investigates such interactions in the southernmost Águilas Arc, focusing on the El Arteal fault segment within the eastern PFZ strand. Mineralogical, geochemical, and hydrogeological analyses were performed using XRD, SEM, and ICP-MS techniques. Results reveal six mineral assemblages (MA) within the fault segment where the fault gouge samples were characterized by cataclastic textures and the occurrence of authigenic minerals, including halite, kaolinite, illite, paragonite, goethite, hematite, gypsum, barite, celestine, and quartz. Geochemical data indicate enrichment signatures in large-ion lithophile elements (LILE) and minor chalcophile and light rare-earth elements (LREE). Two thermal hydrofacies with alkaline metals enrichment were identified in wells and mine shafts: (1) Na+SO42− and (2) Na+Cl−, where the latter exhibits high Na+ and Cl− concentrations toward deeper sectors. These findings suggest multiple stages of fluid–rock interaction controlled by temperature: an early phase dominated by epithermal mineralization, followed by late-stage circulation of hypersaline fluids. This evolution provides an abnormal geochemical signature that is unique in the Aguilas Arc Geothermal System.

Abstract The Yingxiu‐Beichuan fault zone (YBFZ) has long been active and experienced repeated large earthquakes. The physicochemical properties of the deep fault zone (>1000 m) are the key to understanding the deformation mechanism of large earthquakes. This study uses rock magnetic, microstructural, and geochemical analyses of representative samples exposed in FZ1681 within the Wenchuan Earthquake Fault Scientific Drilling borehole 2 (WFSD‐2) cores. Fault gouge and fault breccia have higher magnetic susceptibility values than wall rocks, and they contain abundant paramagnetic minerals and small quantities of magnetite and monoclinic pyrrhotite. The magnetite and monoclinic pyrrhotite in the fault gouge were mainly formed by coseismic frictional heating, indicating that large earthquakes with frictional heating temperatures of ~500‐900°C once occurred in the YBFZ. The seismogenic and coseismic environment was reducing with a relatively high sulfur content. The monoclinic pyrrhotite in the fault breccia was formed mainly by low‐temperature hydrothermal fluid. This indicates that the fault zone experienced reducing and low‐temperature (<400°C) hydrothermal fluid with a relatively high sulfur content after the earthquake. The YBFZ, which experiences frequent large earthquakes, is weakly oxidizing environment at different depths, but the effect of the low‐temperature hydrothermal fluid is weaker at depth.

Abstract. Flash temperatures induced by flash heating can lead to thermal softening or decomposition of fault-zone materials at microscopic grain contacts and, consequently, cause a rapid reduction in fault strength during seismic slip. To quantify the efficiency of short-term frictional heating at the contact scales and its impact on the mechanical fault strength, we conducted rotary-shear friction experiments on Ottawa quartz sand “gouges” with variable grain sizes of 250–710 µm at a range of normal stresses of 1–7.5 MPa and slip velocities of 1–50 mm s−1 under room-dry and wet conditions. We employed a high-speed infrared camera to monitor temperature fluctuations along the outer circumference of the ring-shaped gouge layer during sliding, utilizing a frame rate of up to 1200 Hz with a spatial resolution of 15 µm to capture flash temperature occurring at asperity contacts. We show that flash temperature can be captured within the gouge layer in both room-dry and wet conditions with a peak value up to ∼ 220 and ∼ 100 °C, respectively. In addition, the flash temperature increases with increasing slip velocity and grain size, while decreasing at higher normal stress, which is likely associated with enhanced grain size reduction. In our study, we showed that flash temperatures in shearing fault gouges can be constrained using a fast thermal camera. Although difficulties remain in the experimental set-up related to the need to confine the gouge layer and to the evolution of contact size due grain size reductions, the trends in maximum temperatures we observed agree with those predicted from theory.

We determine the feedback between fault dynamics and fault gouge structures by examining gouge structures that form during rupture and slip of initially intact granite under upper crustal conditions. Experiments were conducted under quasi-static (3 × 10−5 mm/s), weakly dynamic (0.27 mm/s) and fully dynamic (≫1.5 mm/s) slip conditions, with or without fluids, and limited slip displacement (max. 4 mm). The extent in gouge amorphization positively correlates with deformation rate, and we detect evidence of melting, e.g., magnetite nanograins, associated with the highest deformation rates. Gouge nanostructure is directly correlated to power dissipation rather than total energy input. The presence of amorphous material has no detectable impact on the strength evolution during rupture. We highlight that gouge textures, generally associated with large displacements and/or elevated pressure and temperature conditions, can form during small slip events (Mw < 2) in the upper crust from initially intact materials.

Abstract Geothermal reservoirs commonly record seismic and creep events, constituting earthquake hazards in enhanced geothermal systems. To understand the geological controls on mixed‐mode frictional behaviors in these settings, we performed friction experiments on fault gouges from the polyphase‐altered carbonate Massenkalk Formation in Germany, a regional priority target for geothermal energy production. The experiments, performed at conditions consistent with ∼4 km deep geothermal reservoir, yield evolved friction coefficients of ∼0.48 for limestone and ∼0.61 for dolostone, whereas the silicate‐carbonate fault gouges exhibit ∼0.55. Dolostone gouges exhibit unstable sliding at low displacement rates, whereas the limestone and silicate‐carbonate gouges are stable at all investigated conditions, with the dolostone and silicate‐carbonate samples exhibiting the most significant microstructural strain localization. These results suggest that the geochemical alteration history of geothermal systems may facilitate hybrid frictional behaviors of seismic and aseismic slip within the same reservoir, and that cataclastic strain localization style is independent of frictional behavior.

Abstract The hydraulic and rheological properties of shear zones that develop in dome‐building eruptions govern the potential for explosive and seismic behavior at those volcanoes. Previous isostatic hot‐pressing experiments demonstrated that crystalline fault gouge undergoes solid‐state sintering at high temperature and pressure, and sintering causes lithification and permeability loss within volcanic shear zones over years. We present results of torsion experiments in which we document, for the first time, gouge rheological behavior at the temperature‐normal stress‐strain rate conditions expected in volcanic shear zones, and determine the effect of shear on solid‐state sintering rate. Gouge sheared at 500°C, 50 or 100 MPa normal stress and 10 −4 to 10 −3 s −1 does not sinter, exhibits strain‐independent behavior, and deforms by distributed granular flow. In contrast, gouge sheared at 900°C, 50 or 100 MPa normal stress and 10 −5 to 10 −3 s −1 sinters, strain‐weakens, and exhibits microstructural evidence of strain localization and significant lithification. Samples sheared at 900°C for &lt;1 hr have final porosities and permeabilities comparable to gouge samples hot‐pressed for 60 hr, indicating sintering is enhanced by a shear stress acting on grain boundaries in addition to a normal stress. These results are used to develop a model for time‐dependent densification by solid‐state sintering that is applicable when gouge is static or shearing. Finally, we propose that sintering‐driven lithification can cause deforming shear zones to transition from aseismic stable sliding to seismogenic sliding, potentially resulting in the drumbeat seismicity frequently observed at dome‐building volcanoes.

Elevated temperatures promote frictional instability in kaolinite-quartz fault gouges

Abstract The Xianshuihe fault zone (XSHF) in southwestern China accommodates both aseismic creep and seismic slip, yet geological evidence constraining these processes remains limited. Hence, we conducted microstructural, geochemical, and rock magnetic analyses of fault‐zone materials from the Cuoniulongba River outcrop. Fault breccias and relatively undeformed sandstones adjacent to the principal slip zone (PSZ) exhibit elevated magnetic susceptibility (MS), likely reflecting monoclinic pyrrhotite precipitated from post‐seismic low‐temperature hydrothermal fluids. Creep‐related fault gouge within the PSZ is characterized by low MS. Notably, the gouge sample S4 within the PSZ shows high MS, reflecting magnetite neoformation induced by coseismic frictional heating (&gt;500°C), consistent with the observation of thermal decomposed kaolinite due to fluid drainage. This study presents the geological evidence associated with the XSHF linked to aseismic creep, coseismic frictional heating, and post‐seismic hydrothermal alteration, and provides new insights into the seismic behavior of continental strike‐slip faults.

Permeability evolution in remolded fault gouge creates critical uncertainties in geotechnical parameterization for dam foundations. However, the underlying multi-scale mechanisms, including mineral migration and pore structure changes, remain insufficiently understood. This study investigates these mechanisms using remolded plastic-thrust fault gouge from the Yulong Kashi hydropower project in China. We developed an innovative sample preparation method that combines in situ mineral self-cementation and directional compaction. The study integrated multidisciplinary tests including field in situ permeability tests; seepage–stress coupling tests; and micro-scale NMR/XRD/SEM-EDS analyses. Results demonstrate that remolded samples exhibit 1–2 orders of magnitude lower permeability (10−7 cm/s) than in situ samples (10−5 cm/s). This significant reduction is primarily caused by the loss of cementing agents and the uniform compaction of remolded samples, which leads to degraded pore connectivity. SEM-EDS analysis highlighted the leaching of cementing materials (such as K+, Ca2+ ions), while XRD revealed changes in mineral composition, with chlorite dissolution being the primary mineral alteration associated with permeability decay. Additionally, artificially enhanced cohesion distorted the mechanical behavior of the samples. These findings provide an explanation for why conventional laboratory tests tend to underestimate in situ permeability and overestimate shear strength in fault zones. This study establishes microstructure-informed correction frameworks for hydraulic and mechanical parameters in fault-crossing hydraulic engineering applications

Abstract Fault zones, with their dense networks of fractures and microfractures, often serve as primary pathways for fluid migration. However, their role as conduits or barriers depends on their development stages. Previous studies on the fluid transport properties of fault zones have primarily relied on permeability measurements, microstructural analyses and numerical simulations. In this study, magnetic measurements were conducted on the fault rocks from the Middle Valley Fault (MVF) of the Red River Fault (RRF) system, supplemented by scanning electron microscope observations. Results show that the protoliths are dominated by paramagnetic pyrite, while weakly paramagnetic siderite and ferrimagnetic magnetite are unevenly distributed across the fault zone. Along with the widespread presence of barite, this suggests that hydrothermal fluids have circulated within the fault zone. The healing effects of these fluids, coupled with the enrichment of clay minerals, significantly reduce the permeability of the fault gouge. Consequently, the ∼90‐cm‐thick fault gouge zone acts as a barrier, inhibiting fluid migration across the fault zone. Fluid‐rock interactions varied across fault compartments, producing diverse magnetic assemblages in the fault rocks. Moreover, integrating outcrop‐scale and microstructural observations with relevant geological data, this reveals that the MVF has undergone at least three tectonic events since the Paleogene, with hydrothermal fluids transitioning from sulfate‐dominated during the Paleogene and early Neogene to CO 2 ‐rich since the Neogene. These findings provide unique insights into the evolutionary history and contemporary deformation of the RRF system and offer a novel magnetic perspective for studying fluid migration in fault zones.

Effects of normal stress and shear velocity on the frictional healing behavior of halite fault gouge

Frictional behavior of dolomite-rich fault gouges: Effects of mineralogical heterogeneity and implications for induced seismic risk in carbonate reservoirs

Atomistic-scale sliding friction of fault gouge: Insight from a quartz-kaolinite-quartz system

Physical Origins of Shear Strength Fluctuations in Fault Gouges

Abstract Laboratory acoustic emissions (AEs) represent microslip events analogous to small-scale earthquakes, providing valuable insights into the mechanics of frictional instabilities. With technological advancements in acoustic monitoring, thousands of AE waveforms can now be collected in minutes of experimental time, requiring efficient methods for their detection and analysis. In this study, we introduce a deep learning model for automatically detecting AEs in laboratory shear experiments. Our dataset consists of about 30,000 manually picked AE waveforms collected under different experimental boundary conditions using two fault gouge materials: Min-U-Sil quartz gouge and glass beads. By adapting the PhaseNet model, originally developed for natural earthquake phase detection, we train AEsNet, a robust AE picker that outperforms pre-existing picking methods for the tested materials. To investigate whether trained models can generalize across different boundary conditions and materials, and overcome the limitations of small, manually labeled datasets, we apply transfer learning to analyze performance relative to training size and material diversity. Our results indicate that model performance is largely independent of experimental conditions but strongly dependent on material type. This finding suggests that direct transfer of models trained on one material to another is often ineffective due to distinct frequency characteristics of AEs, which are closely linked to the microphysical processes driving emissions in the different granular materials. However, quick fine-tuning significantly enhances pretrained AEsNet performance, even surpassing that of a fine-tuned PhaseNet model pretrained on natural earthquakes. This underscores the importance of customizing models to the specific attributes of laboratory-generated AEs—a conclusion consistent with findings from transfer learning applications in natural seismicity. In conclusion, our approach provides an efficient tool for enhancing AE detection, even with limited data from diverse laboratory conditions, enabling the creation of reliable AE catalogs that can significantly advance our understanding of fault mechanics in controlled experimental settings.

The potential for earthquakes triggered by modulus softening in fault cores has been extensively documented, with a particular emphasis on calculating and characterizing the modulus within fault gouges. Traditionally, the modulus is treated as an average parameter of the entire assembly, and its anisotropic nature is often overlooked. This study derives and verifies formulae to calculate the anisotropy of the elastic modulus of fault material in ellipsoidal assemblies of different shapes using the discrete element method. It defines the anisotropy of the elastic modulus on an irreducible tensor basis in the normal direction of contact forces between particles. The findings indicate that shape-induced anisotropy significantly affects the elastic modulus. Given the consistency between the elastic modulus and wave velocity, the process of elastic wave propagation is simulated. The wave velocity is estimated using the time-of-flight method, which validates the accuracy of the anisotropic decomposition. The relationship between velocity and shape, ascertained by the time-of-flight method, is consistent with that derived from the anisotropic decomposition of the elastic modulus. In contrast, the global average modulus, which disregards anisotropy, fails to acknowledge this relationship. This study highlights the critical importance of considering modulus anisotropy in fault gouges. It evidences the efficacy and universality of this approach, which can be readily applied to other physical properties with orientation dependencies, such as polarization, magnetization, principal axes of stress or strain, and crystallographic axes, among others.

Abstract The Eastern Alps have been influenced by post‐collisional indentation tectonics since the Miocene. Currently, Adria‐Europe convergence, albeit slow, is accommodated and distributed across several faults. The seismogenic potential of some of these faults is unclear. We applied optically stimulated luminescence (OSL) and electron spin resonance (ESR) dating to fault gouges to constrain which portions of the Periadriatic Fault (PAF) System, Lavanttal Fault, and Šoštanj Fault experienced surface‐rupturing earthquakes during the Quaternary. The saturation level of the quartz ESR Al center signal was used as a metric of seismic activity to compare between the faults. Our results showed that the Lavanttal Fault experienced the least Quaternary seismic activity, followed by the PAF, while the Šoštanj Fault exhibits the most recent seismic events. The Lavanttal Fault samples showed ESR saturation levels of approximately 97%, and minimum ages ranging 863 ± 41–2,151 ± 323 ka, indicating that if earthquakes occurred, they did so before this period. The PAF was seismically active during the Pleistocene, with maximum ESR ages ranging 899 ± 67–305 ± 25 ka and minimum K‐feldspar OSL ages ranging 179 ± 13 to 62 ± 4 ka (pIRIR225). Despite a spread in ESR ages, the saturation level was consistent between samples, averaging 66 ± 4%. The Šoštanj Fault yielded an ESR age of 644 ± 30 ka and OSL of 30 ± 4 ka. The age difference lies in the dating range of both systems, but the results still suggest a shorter recurrence interval than the other two faults. Overall, our results show the utility of ESR and OSL dating in identifying periods of fault activity and complementing other approaches to fill temporal gaps.

Abstract The dynamics of fluid flow within faults plays a critical role in the evolution of fault strength through the seismic cycle. The key processes that control how fluids affect fault slip behavior are the evolution of fault porosity and fluid recharge and drainage during slip that, in turn, determine dilational strengthening or compaction weakening. Despite the significance of these processes, high‐fidelity lab measurements that include the evolution of porosity, fluid pressure and frictional properties are sparse. Here, we report such data for drained and undrained velocity‐stepping experiments from 3 to 300 µ/s on natural fault gouges from the seismogenic zone of injection well 16A (2,050–2,070 m) of the Utah FORGE enhanced geothermal site site. We conducted a suite of experiments under constant normal stresses (44 MPa) and pore fluid pressures (13, 20, 27 MPa) corresponding to pore fluid factors between 0.3 and 0.65. We carefully monitor the volumetric strain and show that the dilatancy coefficient of the material ranged from 5 to , and showed minor sensitivity to fluid boundary conditions. In some cases, we see that larger slip velocities cause a transition from dilatancy strengthening to compaction weakening via fluid pressurization. Two effective fault permeability terms were required to model that fault‐fluid response with slip. We posit that the spatial‐temporal pattern of pore‐throat size and connectivity creates a spectrum of fault drainage conditions, ultimately controlling the mode of fault slip.