Fault Behavior Research Articles

Does Subsurface Fault Geometry Affect Aleatory Variability in Modeled Strike-Slip Fault Behavior?

ABSTRACT The nonplanar geometry of faults influences their seismotectonic behavior, affecting the initiation, propagation, and termination of individual earthquakes as well as the stress–slip relationship and probability of multisegment rupture. Consequently, computer simulations that aim to resolve the earthquake rupture process and make predictions about a fault’s future behavior should incorporate nonplanar fault geometries. Although surface traces of faults can be mapped with high accuracy, a key challenge is to define a fault’s detailed subsurface geometry due to a general lack of data. This raises the question of which geometry to use. Does it matter which subsurface geometry is utilized in earthquake rupture simulations, as long as at least the fault trace is considered? How different is the simulated fault behavior for faults that share the same surface trace but different subsurface geometries? Using the physics-based earthquake-cycle simulator MCQsim, we generate seismic catalogs for 100 × 20 km strike-slip faults, assuming variations in fault surface trace, subsurface geometry, and strength distribution. We investigate how the long-term fault behavior—in the form of magnitude–frequency distribution, earthquake interevent time, and maximum earthquake size—is affected by fault geometry and strength distribution. We find that the simulated behavior of strike-slip faults with identical fault traces is interchangeable—even if their subsurface fault geometries differ. Implementing the fault trace constrains possible fault geometries to a level that makes the long-term behavior indistinguishable—at least for strike-slip faults with “known” strength distribution and length-to-width aspect ratios that are equal or larger than what we used here. The fault trace can provide a satisfactory representation of subsurface geometry for assessing long-term fault behavior.

Failure Modes and Fault Morphology

Abstract Fault failure modes determine the geometric characteristics of faults and fault zones during their formation and early development. These geometric properties, in turn, govern a wide range of fault processes and behaviors, such as reactivation potential, fault propagation, and growth, and the hydraulic properties of faults and fault zones. Here, we use field data and close-range digital photogrammetry to characterize, in detail, the surface morphology of three normal faults with cm-scale displacements in mechanically layered carbonates of the Cretaceous Glen Rose Formation at Canyon Lake Gorge, Comal County, Texas. Analyses demonstrate complex fault surface geometries, a broad spectrum of slip tendency (Ts) and dilation tendency (Td), and variable failure behavior. We show that (i) fault patches coated with coarse calcite cement tend to have moderate to high dips, low to high Ts, and high to very high Td; (ii) slickensided fault patches exhibit low to moderate dips, moderate to very high Ts, and moderate to high Td; and (iii) slickolite patches exhibit low dips, moderate Ts, and low to moderate Td. Calcite-coated patches are interpreted to record hybrid extension-shear failure, whereas slickensided and slickolite patches record shear and compactional shear failure, respectively. Substantial variability in both Ts and Td across the exposed fault surfaces reflects complex fault morphology that is not easily measured using traditional field techniques but is captured by our photogrammetry data. We document complex fault geometries, with kinematic (displacement) compatibility indicating the various failure modes were active coevally during fault slip. This finding is in direct contrast with the often-assumed concept of faults forming by shear failure on surfaces oriented 30° to σ1. Distinct failure behaviors are consistent with patchworks of volume neutral, volume gain, and volume loss zones along the fault surfaces, indicating that the characterized faults likely represent dual conduit-seal systems for fluid flow.

Influence of lateral heterogeneities on strike-slip fault behaviour: insights from analogue models

Abstract. This study investigates how lithological changes can affect the strike-slip fault propagation patterns using analogue models. Strike-slip fault zones are long structures that may cut across pre-existing tectonic or lithological steep boundaries. How strike-slip faulting is affected by a laterally heterogeneous upper crust is crucial for understanding the evolution of regional and local fault patterns, stress reorientations, and seismic hazard. Our models undergo sinistral distributed strike-slip shear (simple shear) and have been analysed by particle image velocimetry (PIV). We use quartz sand and microbeads as brittle analogue materials over a viscous mixture to distribute the deformation through the model. The first models investigate strike-slip faulting in a homogeneous upper crust using quartz sand or microbeads only. Three further models examine how the presence of a central section which laterally differs in its properties influences strike-slip faulting. The main observations are the following: The homogeneous upper crust shows typical Mohr–Coulomb strike-slip faults, with synthetic fault strikes related to the angle of internal friction of the material used. The heterogeneous upper crust has a profound effect on synthetic fault propagation, interaction, and linkage, as well as the kinematic evolution of antithetic faults that rotate around a vertical axis. The orientation of the central section determines whether antithetic fault activity concentrates along the entire length of the central contact or not. In the first case, fault activity is segmented or the number of different faults formed is increased in distinct domains. In the second case, the properties of the central material determine fault propagation, interaction, and/or linkage across the central domain. These findings have potential implications for nature that have been seen in the NW Iberian Peninsula. In this area, the change in direction of the sinistral faults and the position of the antithetic faults can be explained due to lithological change.

Effects of Flow and Geomechanics Coupling in Faulted Reservoirs for CO2 Storage

Summary Accurately reproducing the coupling of fluid flow in porous media and rock mechanics is crucial for the modeling of CO2 geological storage to properly evaluate and prevent the risk of inducing fault instability during injection operations. As an alternative to using monolithic flow/mechanical suites, the process can be modeled by linking individual codes, that is i, a reservoir simulator for flow and a geomechanical package to account for fluid-induced stress change, which tackle the two problems sequentially. We developed a flexible numerical framework from which different coupling logics can be selected (i.e., one-way coupling, two-way iterative coupling, and two-way explicit coupling), which are characterized by different levels of accuracy and computational costs. A multirate, two-way coupling algorithm, which allows the flow and mechanical simulators to exchange information periodically rather than at every timestep, is also available to reduce the computational cost of two-way coupled simulations. In this work, we use this coupling infrastructure to perform numerical experiments aimed at defining whether sequential iterative coupling is strictly needed or not, and which less expensive logic can be used to attain a similar solution accuracy. First, a synthetic test case is used to illustrate the onset of fault instability during CO2 injection operations for different sets of coupling parameters (type and frequency), rock properties, and fault permeability. It is thus possible to evaluate, for a reasonable range of coupling strength, which depends on fluids and rock properties, the optimal level of coupling. Results are strongly influenced by the coupling strength, and two-way iterative coupling should be selected for tightly coupled systems to accurately reproduce the fault behavior. For a loosely coupled system, instead, the one-way approach should be the preferred choice due to its lower computational cost. Later, we consider CO2 injection into a realistic formation, and we analyze the impact of the coupling frequency on the computational performance. We show that, for complex cases, there is no one-to-one correspondence between the reduction in the number of coupling iterations and the reduction in computational time for increasing coupling period.

Homoclinic bifurcation of a rate-weakening patch in a viscoelastic medium and effect of strength contrast

The time-dependent viscoelastic deformation of host rocks is important when considering the dynamics of fault behavior, specifically in brittle–ductile transitional regions or shallow subduction zones, because it relaxes stress heterogeneity and affects loading to the fault. For a rate-and-state fault embedded in a Maxwell viscoelastic medium, a previous study discovered a transition from repeated earthquakes to the permanent stuck of a rate-weakening patch (EQ–ST transition) with decreased viscoelastic relaxation time tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${t}_{\ ext{c}}$$\\end{document}. This transition differs from the well-known seismic–aseismic transition explained by the Hopf bifurcation at the critical stiffness of an elastic system. To better understand the EQ–ST transition, quantifying the effect of heterogeneous frictional strength is important, because this effect is characteristic to the viscoelastic medium and is absent in the elastic limit. Previous experimental studies suggest a potential contrast in frictional strength Δf∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {f}_{*}$$\\end{document} in such a way that a rate-weakening patch is stronger than a rate-strengthening region containing clay minerals. Here, we conducted two-dimensional, fully dynamic earthquake sequence simulations for a fault in a Maxwell viscoelastic medium; we investigated the EQ–ST transition in the two-dimensional parameter space of tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${t}_{\ ext{c}}$$\\end{document} and Δf∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {f}_{*}$$\\end{document}. With Δf∗=0.3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {f}_{*}=0.3$$\\end{document}, the EQ–ST transition occurred at about 1 order of magnitude larger tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${t}_{\ ext{c}}$$\\end{document} than in the case with Δf∗=0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {f}_{*}=0$$\\end{document}. We constructed a coarse-grained model with only two degrees of freedom based on the spatial average. Consequently, the coarse-grained model behaves remarkably similar to the continuum model, and the EQ–ST transition is associated with a homoclinic bifurcation. The EQ–ST boundary in the parameter space can be quantitatively explained by considering elastic loading due to creep in the rate-strengthening region and unloading by viscoelastic relaxation of the stress heterogeneity comparable to Δf∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {f}_{*}$$\\end{document}. A larger rate-weakening patch is anticipated to become aseismic earlier as tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${t}_{\ ext{c}}$$\\end{document} decreases, because the elastic loading rate is inversely correlated with the patch size. This may be qualitatively consistent with the change in the size distribution of events around the brittle–ductile transition in observations and laboratory experiments; however, further investigations on, for example, the interactions of events and changes in frictional parameters with depth are required for quantitative discussion.Graphical abstract

Analysis of stress changes on the Poso fault caused by large inland earthquakes: Case study of the 2017 Mw 6.6 Poso and the 2018 Mw 7.5 Palu-Donggala earthquakes

The 2017 Mw 6.6 Poso and the 2018 Mw 7.5 Palu-Donggala represent significant seismic events in central Sulawesi Island. These events transferred the stress to the surrounding faults, which can promote or inhibit the critical condition in a fault, thus evaluating the stress transfer from these earthquakes is crucial for a comprehensive understanding of fault behaviour. In this study, we investigate the effect of these large events by resolving the accumulated Coulomb stress change onto a multi-strike segment of the Poso fault. Additionally, we also used GPS measurement (Global Strain Rate Model) to estimate the increase in tectonic stress rate due to coupling of triple-junction plate convergence. The 2017 Poso earthquake induces positive stress changes (7 kPa to 27 kPa) in the central segment, negative changes (–4 kPa to –10 kPa) in the northern segment, and less significant effects in the southern segment. The 2018 Palu-Donggala earthquake imparts a notably high negative Coulomb stress (–16 kPa to –27 kPa) across the entire northern segment. Cumulatively, both earthquakes reduce Coulomb stress predominantly in the northern part of the Poso fault, suggesting a delayed critical point. Furthermore, the analysis of the Coulomb stress changes time function indicates that stress accumulation in the northern Poso fault is more modulated by large earthquakes, while stress accumulation in the southern segments may be more controlled by longterm tectonic stress loading. The stress accumulation model developed in this study can provide a guidance in better understanding further earthquake risk mitigation in central Sulawesi.

Geophysical downhole logging analysis within the shallow-depth ICDP STAR drilling project (central Italy)

Abstract. The International Continental Scientific Drilling Program (ICDP) STAR (A Strainmeter Array Along the Alto Tiberina Fault System) drilling project aims to study the seismic and aseismic fault slip behavior of the active low-angle Alto Tiberina normal fault (ATF) in the northern Apennines, central Italy, by drilling and instrumenting six shallow boreholes (maximum depth 160 m) with seismometers and strainmeters. During the STAR fieldwork, a geophysical downhole logging campaign was carried out to define the optimal target depth for instrument deployment and formation rock characterization. In particular, the main objectives of this study were to define in situ physical properties of the rocks and the tectonic discontinuity geometry along the boreholes. The downhole logging data provide new findings and knowledge, especially with regards to physical properties such as resistivity and gamma-ray and wave velocity. The collected parameters were compared to the results of literature data collected in similar lithologies, as well as with the results of logging performed in deeper wells drilled for commercial purposes. The physical properties of the Mesozoic–early Tertiary calcareous formations show low gamma-ray values and high compressional (Vp) and shear wave (Vs) velocities (up to 5.3 and 2.9 km s−1, respectively), whereas the overlying clay-rich Late Tertiary formations exhibit high gamma-ray and low resistivity values as well as relatively low Vp and Vs values (up to 3.5 and 2.0 km s−1, respectively). The results obtained from the analysis of the orientations of the tectonic structures, measured along the six boreholes, show good agreement with the orientations of the present-day extensional stress field, which is NE–SW-oriented. Our study allowed us to bridge the gap between the physical properties obtained from literature data and those obtained from the deep well measurements, representing a possible case history for future projects. These new outcomes represent an almost unexplored window of data and will contribute to the advancement of knowledge of the physical properties of the rocks at shallow depths, which are typically overlooked.

Slip characteristics of planar and rough granite fractures under unloading normal force

Unloading processes are common in natural systems. Intense unloading activities can alter the frictional equilibrium of faults and induce their instabilities. Understanding the slip behavior of faults under stress unloading conditions is helpful in guiding engineering practices. We conducted a series of direct shear experiments under linear-unloading normal force conditions considering the influences of initial normal forces, initial shear forces, and normal unloading rates on planar and rough granite fractures. The experimental results showed that planar fracture exhibits sudden slip events during normal unloading, while rough fracture mostly displays stable sliding behavior. The planar fracture demonstrates an exponential increase in sliding distance and velocity at the end of each slip event. The rough fracture usually exhibits a quasi-static stage before rapid slip events. In addition, the accumulative sliding distance at the slip activation moment (at the first moment when sliding velocity is greater than 0.05 mm/s) for the planar fracture decreases with lower normal unloading rate, larger shear force and larger normal force, while its variation trend for rough fracture is opposite. These findings provide valuable insights into fault slip behavior under stress unloading, aiding in mitigating associated risks in engineering applications.

Analysis of deformation evolution characteristics of the northern Lajishan fault before the Jishishan MS6.2 earthquake based on GNSS observations

A MS6.2 earthquake struck Jishishan Country in Gansu Province on 18 December 2023. The earthquake occurred on the eastern branch of the Northern Lajishan fault (NLJSF) which has never experienced such a violent seismic event in recorded history. In this study, multiple periods of GNSS campaign observation data and continuous observation data are collected to reveal the crustal deformation evolution characteristics of the eastern branch of the NLJSF before the earthquake, focusing on the dynamic behavior of the causative fault, regional strain parameter features, and the variation of GNSS baselines across the NLJSF. Velocity field data show that the differential movement on both sides of the fault zone before the earthquake accumulated energy for the occurrence of the earthquake. Cross-fault profile analyses indicate a noticeable weakening of horizontal shortening on both fault flanks, suggesting a nearing fault lockup. The baseline time series and strain parameter time series results both show that the study area is mainly NEE-trending compression deformation. In addition, before the earthquake, multiple baseline results and strain parameter time series deviated from the linear trend and gradually flattened, indicating a strain accumulation in the study area. The overall crustal deformation evolution shows a relatively high earthquake risk before the Lajishan earthquake.

Numerical modeling of the long-term poromechanical performance of a deep enhanced geothermal system in northern Québec

This study numerically investigates the thermo-poromechanical effects in a Canadian geothermal reservoir caused by long-term fluid production and injection. Using finite element modeling, it explores pore pressure diffusion and thermal dynamics, incorporating both the geological structure of the rock mass and faults. The simulations utilize the IAPWS (International Association for the Properties of Water and Steam) equations to model fluid density and viscosity, ensuring realistic representations of heterogeneous pressure fields. The system replicates a doublet configuration within a faulted zone, featuring two hydraulically stimulated fractures. The primary aim is to assess the likelihood of fault reactivation under varying in-situ stress conditions over a 100-year geothermal operation. Results show that stress distribution is largely influenced by thermal stresses along the fluid circulation pathway, with fluid velocity and temperature gradients affecting reservoir stability. Minimal pore pressure changes highlight the dominant role of thermal stresses in controlling fault behavior. The analysis indicates no potential for fault reactivation, as slip tendency values remain below the critical threshold, even when accounting for reduced mechanical properties using the Hoek-Brown criterion. Thermal effects continue to influence the surrounding rock throughout the operational period, suggesting that the reservoir maintains mechanical stability conducive to sustained geothermal production and injection. These findings provide valuable insights into the long-term safety and behavior of geothermal reservoirs, offering important implications for future geothermal energy development and management strategies.

The Palu-Koro fault behaviour monitoring associated with the 2018 Palu earthquake based on the multi-temporal planetscope and Landsat 8 satellite images

The Palu-Koro Fault on Sulawesi Island possesses an extensive record of earthquake-related activity, notably the Palu earthquake on September 28, 2018, which was particularly destructive. This study investigates the evolution of this fault by using high-resolution PlanetScope and Landsat 8 Operational Land Imager/Therma Infrared Sensor (OLI/TIRS) images. By investigating the interseismic, coseismic, and postseismic stages of the earthquake's habits, this paper aims to obtain an in-depth understanding of its behavior. The coseismic displacement analysis, which was carried out alongside the optical image correlation technique, indicated major displacements throughout the Palu-Koro Fault, with the largest displacement of roughly 7 m. To ensure the accuracy of the results, internal verification standards, such as a reliability criterion of >30% and a mean structural similarity index (MSSIM) of 1, were used. Landsat 8 imagery was processed using the land surface temperature method to enhance the understanding of the earthquake phases. Prior to the earthquake, the results suggested a rise in temperature, which peaked during the coseismic phase and decreased progressively during the postseismic phase. Intriguingly, the temperature behavior revealed the possibility of using information from remote sensing as an alternative approach to identify the fault distribution in Palu City. Overall, this study demonstrates the utility of remote sensing data for analyzing the dynamics of the Palu-Koro Fault and understanding each stage of the 2018 Palu earthquake. By integrating high-resolution satellite imagery with sophisticated image processing techniques, this paper provides crucial insights into earthquake activity and its impact in this area.

Evaluating interseismic deformation patterns in the North Tabriz Fault (Iran) using enhanced fitting of velocity field and analysis of surface deformation

Understanding the mechanisms and locations of interseismic strain accumulation along faults is essential for assessing earthquake hazards. However, the mechanical response during the transition from deep fault locking to creep behavior remains uncertain. Estimating the slip deficit within these transition zones is challenging. This study challenges the assumption of a constant depth distribution of interseismic slip rate along the fault over time and proposes variable locking depths as an alternative model. By rejecting the constant locking depth assumption, singularity issues during stress theorem resolution are resolved. To address this, we employ a methodology considering creep propagation within a fully elastic medium. This approach incorporates long-term deformation resulting from viscoelastic flow in the upper mantle and lower crust. Including viscoelastic effects improves the fit to interseismic deformation rates, yielding lower locking depths compared to fully elastic models. To conduct the investigation, the GPS velocity field is recovered using the forward problem and the boundary element method. Subsequently, a physics-based inversion approach, deep interseismic creep, is employed to determine interseismic deformation patterns on a strike-slip fault. Furthermore, this study examined the correlation between the dislocation parameters and their relationship, as well as established the probability distributions associated with each faulting parameter. This research highlights the importance of considering variable locking depths in understanding interseismic strain accumulation and the transition to creep behavior along faults. The findings contribute to improved earthquake hazard assessment and mitigation strategies by providing valuable insights into fault behavior mechanics along the North Tabriz Fault.

Physics informed neural network can retrieve rate and state friction parameters from acoustic monitoring of laboratory stick-slip experiments

Various machine learning (ML) and deep learning (DL) techniques have been recently applied to the forecasting of laboratory earthquakes from friction experiments. The magnitude and timing of shear failures in stick-slip cycles are predicted using features extracted from the recorded ultrasonic or acoustic emission (AE) signals. In addition, the Rate and State Friction (RSF) constitutive laws are extensively used to model the frictional behavior of faults. In this work, we use data from shear experiments coupled with passive acoustic (variance, kurtosis, and AE rate) interleaved with active source ultrasonic monitoring (transmitted wave amplitude) to develop physics-informed neural network (PINN) models incorporating the RSF law and AE rate generation equation with wave amplitude serving as a proxy for friction state variable. This PINN framework allows learning RSF parameters from stick-slip experiments rather than measuring them through a series of velocity step experiments. We observe that when the stick-slip cycles are irregular, the PINN models outperform the data-driven DL models. Transfer learning (TL) PINN models are also developed by pre-training on data collected at one normal stress level followed by forecasting shear failures and retrieving RSF parameters at other stress levels (i.e., with different recurrence intervals) after retraining on a limited amount of new data. Our findings suggest that TL models perform better compared to standalone models. Both standalone and TL PINN-estimated RSF parameters and their ground truth values show excellent agreements thus demonstrating that RSF parameters can be retrieved from laboratory stick-slip experiments using the corresponding acoustic data and that the transmitted wave amplitude provides a good representation of the evolving frictional state during stick-slips.

Heterogeneous mineralogical composition and fault behaviour: A systematic study in ternary fault rock compositions

The heterogeneous mineralogical compositions of fault gouges, formed during fault evolution, influence frictional properties and slip behaviour. While the influence of individual mineral phases on friction has been extensively studied, the impact of varying systematically mineral phases in gouge mixtures on macroscopic frictional behaviour remains unclear. Thus, we performed 34 frictional experiments on fault gouges composed of three representative mineral phases: muscovite (platy phyllosilicate), quartz (granular silicate) and calcite (granular carbonate), known for their markedly distinct frictional properties. Using a biaxial rock deformation apparatus (BRAVA), we performed tests on fault gouges with grain sizes <125 μm under normal stresses of 50–100 MPa, at room temperature and water-saturated conditions. Our data indicate that the mineralogical composition of fault gouges significantly affects frictional strength, healing, and stability with a non-trivial pattern. Increasing the muscovite content leads to a decrease in frictional strength, from 0.62 for pure calcite and 0.56 for pure quartz to 0.33 of pure muscovite, along with reduced frictional healing and a velocity-strengthening behaviour. This weakening is promoted by a transition from localized deformation along discrete shear planes in granular-rich fault gouges to distributed deformation within the entire gouge layer with increasing muscovite content. At fixed muscovite content, frictional properties depend on the dominant granular phase. Calcite-dominated mixtures exhibit more marked frictional weakening rather than quartz-dominated ones, suggesting a non-linear mixing law between friction coefficient and muscovite content. This different trend is likely due to favourable conditions for fluid-assisted pressure-solution of calcite and foliation development, unlike quartz. When only the granular phases are mixed, we observe complex behaviour with the indentation of quartz into calcite, resulting in higher values of healing rates than those of pure end-member mixtures.Our findings provide robust insights into microphysical processes strongly dependent on the complex mineralogical compositions usually observed along natural faults.

Deep Tectonic Environment Analysis of the Lingshan Conjugate Earthquake within the Qinzhou Fold Belt, South China: Insights Derived from 3D Resistivity Structure Model

The Qinzhou fold belt, situated at the contact zone between the Yangtze and Cathaysia blocks in South China, was affected by the 1936 Lingshan M6¾ earthquake and the 1958 Lingshan M5¾ earthquake, both of which occurred within the conjugate structure. Understanding the deep seismogenic setting and causal mechanism of the Lingshan conjugate earthquake is of great significance for assessing the seismic disaster risk in the region. In this study, we utilized 237 magnetotelluric datasets and employed three-dimensional electromagnetic inversion to characterize the deep-seated three-dimensional resistivity structure of the Qinzhou fold belt and the Lingshan seismic zone. The results reveal that: (1) The NE-trending faults within the Qinzhou fold belt and adjacent areas are classified as trans-crustal faults. The faults exhibit crust-mantle ductile shear zones in their deeper sections, which are essential in governing regional tectonic deformation and seismic activity; (2) The electrical structure of the Qinzhou fold belt is in line with the tectonic characteristics of a composite orogenic belt, having experienced several phases of tectonic modification. The southeastern region is being influenced by mantle-derived magmatic activities originating from the Leiqiong area over a significant distance; (3) In the Lingshan seismic zone, the NE-trending Fangcheng-Lingshan fault is a trans-crustal fault and the NW-trending Zhaixu fault is an intra-crustal fault. The electrical structure pattern “two low, one high” in the zone has a significant impact on the deep tectonic framework of the area and influences the deformation behavior of shallow faults; and (4) The seismogenic structure of the 1936 Lingshan M6¾ earthquake was the Fangcheng-Lingshan fault. The earthquake’s genesis was influenced by the coupling effect of tectonic stress and deep thermal dynamics. The seismogenic structure of the 1958 Lingshan M5¾ earthquake was the Zhaixu fault. The earthquake’s genesis was influenced by tectonic stress and static stress triggering from the 1936 Lingshan M6¾ earthquake. The conjugate rupture mode in the Lingshan seismic zone is influenced by various factors, including differences in physical properties, rheology of deep materials, and the scale and depth of fault development.

Quantifying Creep on the Laohushan Fault Using Dense Continuous GNSS

The interseismic behavior of faults (whether they are locked or creeping) and their quantitative kinematic constraints are critical for assessing the seismic hazards of faults and their surrounding areas. Currently, the creep of the eastern segment of the Laohushan Fault in the Haiyuan Fault Zone at the northeastern margin of the Tibetan Plateau, as revealed by InSAR observations, lacks confirmation from other observational methods, particularly high-precision GNSS studies. In this study, we utilized nearly seven years of observation data from a dense GNSS continuous monitoring profile (with a minimum station spacing of 2 km) that crosses the eastern segment of the Laohushan Fault. This dataset was integrated with GNSS data from regional continuous stations, such as those from the Crustal Movement Observation Network of China, and multiple campaign measurements to calculate GNSS baseline change time series across the Laohushan Fault and to obtain a high spatial resolution horizontal crustal velocity field for the region. A comprehensive analysis of this primary dataset indicates that the Laohushan Fault is currently experiencing left-lateral creep, characterized by a partially locked shallow segment and a deeper locked segment. The fault creep is predominantly concentrated in the shallow crustal region, within a depth range of 0–5.7 ± 3.4 km, exhibiting a creep rate of 1.5 ± 0.7 mm/yr. Conversely, at depths of 5.7 ± 3.4 km to 16.8 ± 4.2 km, the fault remains locked, with a loading rate of 3.9 ± 1.1 mm/yr. The shallow creep is primarily confined within 3 km on either side of the fault. Over the nearly seven-year observation period, the creep movement within approximately 5 km of the fault’s near field has shown no significant time-dependent variation, instead demonstrating a steady-state behavior. This steady-state creep appears unaffected by postseismic effects from historical large earthquakes in the adjacent region, although the deeper (far-field) tectonic deformation of the Laohushan Fault may have been influenced by the postseismic effects of the 1920 Haiyuan M8.5 earthquake.

Band Relevance Factor (BRF): A novel automatic frequency band selection method based on vibration analysis for rotating machinery

Monitoring rotating machinery has become a fundamental activity in the industry given the high criticality in production processes. Extracting useful information from signals is a key factor for effective monitoring. Several studies in the areas of Informative Frequency Selection bands (IFB) and Feature Extraction/Selection have demonstrated the need to identify the bands of interest in vibration signals. However, typical methods in these areas focus on identifying bands where impulsive excitations are present or analyzing the relevance of features after signal extraction. Therefore, they do not focus on other regions that may be related to changes in the dynamic behavior of machines or faults. Furthermore, the methods generally require parameter adjustments and are not automatic. To overcome these problems, the present study proposes a new approach called Band Relevance Factor (BRF). BRF aims to perform an automatic selection of all relevant frequency bands for a vibration analysis of a rotating machine based on spectral entropy. In other words, automatically identify all frequency bands that are related to changes in the behavior of the machines or faults. The results are presented through a relevance ranking and can be visually analyzed through a heatmap. The effectiveness of the approach is validated on a synthetically dataset and on two real datasets, showing that BRF is capable of automatically identifying bands that present relevant information for the analysis of rotating machinery.

Surface ruptures of the 2022 Guanshan-Chihshang earthquakes in central Longitudinal Valley area, eastern Taiwan

AbstractThe Mw 6.4 and 6.8 Guanshan-Chihshang earthquakes occurred on 17 and 18 September 2022 resulted in prominent surface ruptures within the Longitudinal Valley in eastern Taiwan, particularly along the Yuli fault. Approximately 18 h after the mainshock, we began to document the surface rupture near Yuli Town. Our result suggests the surface rupture formed a confined single left-lateral trace in the town of Yuli, characterized by a series of en échelon right-stepping left-lateral faulting geometry. The rupture of 2022 roughly matches the locations of 1951 surface ruptures inside Yuli Town, with a similar amount of left-lateral cross-fault displacement. North and South of the Yuli residential area, we identified several sections of the surface rupture distributed in the water-saturated paddy fields. The maximum left-lateral displacement recorded across the rupture can reach 1.4 m just south of Yuli, with the fault scarp resembling a high-angle west-dipping fault geometry. In addition to the co-seismic surface ruptures, our repeating cross-fault measurements show significant post-seismic shallow after-slip along the Yuli fault. The amount of post-seismic deformation within 3 months after the mainshock is close to, or even higher than the co-seismic cross-fault displacement, consistent with local witness accounts and post-event field photos which showed continuous damage and displacement of building floors and roads after the earthquake. Such shallow post-seismic slips were also observed along the main fault trace in the 2014 South Napa earthquake, and likely represent the shallow elastoplastic behavior of the sub-vertical fault in the young alluvial sediments.

Maximum fault-free enforcement in Petri nets using supervisory control

Abstract Fault diagnosis and maximum fault-free execution are essential for the development and operation of computer-integrated systems covering aircraft systems, power grid systems, production processes, etc. This paper focuses on the problem of fault diagnosis and maximum fault-free enforcement of systems modeled by labelled Petri nets. Given a system modeled by a labelled Petri net that may enter deadlocks, an extended basis reachability graph that contains sufficient information to characterize deadlocks is used to compress the state space and verify the diagnosability for the considered system. Furthermore, the proposed graph offers sufficient and necessary conditions for fault-free enforcement and deadlock-free enforcement. Finally, a lock-free event set and a supervisor are designed for a system such that any possible fault or dead behaviour is prohibited in the controlled system.

Dependence of Simulated Fault Gouge Frictional Behavior on Mineral Surface Chemistry Quantified by Cation Exchange Capacity

AbstractThe slip behavior of crustal faults is known to be controlled by the composition of the fault gouge, but the exact mechanisms, especially considering the role of water‐rock interactions, are still under investigation. Here, we use a geochemical approach measuring the cation exchange capacity (CEC) of several phyllosilicate minerals and non‐clays, using CEC as a proxy for the ability to bind water to the mineral surfaces and/or develop electrostatic forces between particles. Laboratory shearing experiments show that low CEC materials (&lt;3 mEq/100 g) tend to exhibit high friction, low cohesion, and velocity‐weakening frictional behavior. Phyllosilicate minerals exhibit CEC values up to 78 mEq/100 g and correspondingly lower friction coefficients, higher cohesion, and a tendency for velocity‐strengthening friction. Zeolite behavior is atypical, exhibiting a high CEC value typical of phyllosilicates but the strength and frictional characteristics of a non‐clay with low CEC. This suggests that the structure of the mineral is important for non‐phyllosilicates. For phyllosilicates, our results can be explained by water bound to mineral surfaces, creating bridges of hydrogen or van der Waals bonds between particles. The enhanced particle bonding for high CEC materials is consistent with high cohesion under zero effective stress conditions, and lowered friction by trapping bound water between the mineral surfaces under normal load. Bound water may explain the tendency for velocity‐strengthening friction in high CEC materials by hindering a Dieterich‐type time‐dependent frictional strengthening mechanism.