Velocity Weakening Research Articles

Slip‐behavior transitions of a heterogeneous linear fault

AbstractGeological faults have frictional heterogeneity. Here we investigate the slip behavior of such heterogeneous faults in the simplest situation, simulating antiplane shear slip along an infinite linear fault in a 2‐D elastic model space, subjected to constant loading by external stresses characterized by stiffness and plate‐motion velocity. The fault is discretized into many subfaults, each of which has either a velocity‐weakening (VW) or velocity‐strengthening (VS) friction law. We vary the proportion of the fault that is VW, revealing several different regimes of slip behavior. The first regime boundary, where stick‐slip behavior is initiated, is located at the proportion of VW zones such that each VW zone reaches its nucleation size. The other boundary is located where the spatially averaged a − b value of the rate and state friction law is close to zero. Below this density, the VW zone slips seismically, whereas the VS zone shows afterslip. In contrast, above this density, the entire fault slips simultaneously at the seismic slip velocity. We observe transitional behavior at the second boundary, where relatively slower deformation dominates. We also explore the slip behavior of faults using Cantor‐set distributions of frictional parameters. We propose that frictional heterogeneity on the fault may explain not only the diversity of seismic phenomena but also the observed scaling of frictional parameters, such as slip‐weakening distance or the fracture energy of ordinary earthquakes.

Cohesion versus friction in controlling the long‐term strength of a self‐healing experimental fault

AbstractThe Coulomb's failure criterion, which postulates that failure occurs along a fault plane when the applied shear stress overcomes a resistance made of two parts of different nature, cohesion, and friction, remains the standard conceptual framework of faulting mechanics. More recently, rate‐and‐state friction laws became the main modeling tool of the seismic cycle. These laws implicitly assume that only frictional resistance sets the fault strength and its evolution. We therefore raise the question of the role of cohesion and related healing/sealing mechanisms on fault mechanics. We designed an original laboratory experiment based on a model material, an ice thin layer sitting on top of a water tank and mechanically deformed at various rates with a circular Couette‐like geometry. This allowed sliding along the fault plane over arbitrarily large slip distances, and analyzing the competition between faulting and cohesion‐healing rates, whereas frictional resistance, resulting from the fault roughness, is limited in magnitude. We show that cohesion‐healing plays an essential role in the time evolution of the interface strength under constant loading rate. In particular, the magnitudes of shear stress fluctuations are observed to be temperature and velocity weakening. The present experiment shares several properties similar to that of active faults during seismic cycles, suggesting that cohesive healing could control some of these features: scale invariance properties, Gutenberg‐Richter law of rupture event size distribution, Omori's law of energy dissipation, at least after major ruptures, time asymmetry in energy dissipation, and periods of creep under high shear stress alternating with major earthquake‐like ruptures.

Unified rheology of vibro-fluidized dry granular media: From slow dense flows to fast gas-like regimes.

Granular media take on great importance in industry and geophysics, posing a severe challenge to materials science. Their response properties elude known soft rheological models, even when the yield-stress discontinuity is blurred by vibro-fluidization. Here we propose a broad rheological scenario where average stress sums up a frictional contribution, generalizing conventional μ(I)-rheology, and a kinetic collisional term dominating at fast fluidization. Our conjecture fairly describes a wide series of experiments in a vibrofluidized vane setup, whose phenomenology includes velocity weakening, shear thinning, a discontinuous thinning transition, and gaseous shear thickening. The employed setup gives access to dynamic fluctuations, which exhibit a broad range of timescales. In the slow dense regime the frequency of cage-opening increases with stress and enhances, with respect to μ(I)-rheology, the decrease of viscosity. Diffusivity is exponential in the shear stress in both thinning and thickening regimes, with a huge growth near the transition.

Dynamic simulation of CO2-injection-induced fault rupture with slip-rate dependent friction coefficient

Poro-elastic stress and effective stress reduction associated with deep underground fluid injection can potentially trigger shear rupture along pre-existing faults. We modeled an idealized CO2 injection scenario, to assess the effects on faults in the first phase of a generic CO2 aquifer storage operation. We used coupled multiphase fluid flow and geomechanical numerical modeling to evaluate the stress and pressure perturbations induced by fluid injection and the response of a nearby normal fault. Slip-rate dependent friction and inertial effects have been taken into account during rupture. Contact elements have been used to take into account the frictional behavior of the rupture plane. We investigated different scenarios of injection rate to induce rupture on the fault, employing various fault rheologies. Published laboratory data on CO2-saturated intact and crushed rock samples, representative of a potential target aquifer, sealing formation and fault gouge, have been used to define a scenario where different fault rheologies apply at different depths. Nucleation of fault rupture takes place at the bottom of the reservoir, in agreement with analytical poro-elastic stress calculations, depending on injection-induced reservoir inflation and the tectonic stress scenario. For the stress state considered here, the first triggered rupture always produces the largest rupture length and slip magnitude, both of which correlate with the fault rheology. Velocity weakening produces larger ruptures and generates larger magnitude seismic events. Heterogeneous faults have been considered including velocity-weakening or velocity strengthening sections inside and below the aquifer, with the upper sections being velocity-neutral. Nucleation of rupture in a velocity-strengthening section results in a limited rupture extension, both in terms of maximum slip and rupture length. For a heterogeneous fault with nucleation in a velocity-weakening section, the rupture may propagate into the overlying velocity-neutral section, if the extent of velocity-weakening and associated friction drop are large enough.

An empirically based steady state friction law and implications for fault stability

Empirically based rate‐and‐state friction laws (RSFLs) have been proposed to model the dependence of friction forces with slip and time. The relevance of the RSFL for earthquake mechanics is that few constitutive parameters define critical conditions for fault stability (i.e., critical stiffness and frictional fault behavior). However, the RSFLs were determined from experiments conducted at subseismic slip rates (V < 1 cm/s), and their extrapolation to earthquake deformation conditions (V > 0.1 m/s) remains questionable on the basis of the experimental evidence of (1) large dynamic weakening and (2) activation of particular fault lubrication processes at seismic slip rates. Here we propose a modified RSFL (MFL) based on the review of a large published and unpublished data set of rock friction experiments performed with different testing machines. The MFL, valid at steady state conditions from subseismic to seismic slip rates (0.1 µm/s < V < 3 m/s), describes the initiation of a substantial velocity weakening in the 1–20 cm/s range resulting in a critical stiffness increase that creates a peak of potential instability in that velocity regime. The MFL leads to a new definition of fault frictional stability with implications for slip event styles and relevance for models of seismic rupture nucleation, propagation, and arrest.

Frictional properties of phyllosilicate‐rich mylonite and conditions for the brittle‐ductile transition

AbstractTo understand frictional properties of natural phyllosilicate‐rich mylonite along fault depth, friction experiments on simulated mylonite gouge were conducted at temperatures of 100–600°C, effective normal stresses of 100–300 MPa, and loading rates of 0.04–1.0 µm/s. Experimental results show that at temperatures above 200°C, frictional strength of the mylonite gouge exhibits systematic increase with temperature, in the range of 0.52–0.73. Under 200–300 MPa normal stress conditions, velocity dependence of mylonite gouge shows a transition from initial velocity‐strengthening (Regime 1) to velocity‐weakening behavior (Regime 2) at a temperature between 200 and 300°C and transitions back to velocity‐strengthening behavior (Regime 3) as temperature increases. The latter transition is also found to be promoted by slower loading rates. Friction stability of the mylonite gouge also exhibits a strong pressure sensitivity. While velocity‐strengthening behavior in Regime 3 is absent under 100 MPa normal stress condition, stable frictional behavior is significantly enhanced at higher effective normal stresses, with the transition temperature to velocity strengthening decreasing with effective normal stress. Microstructural analysis shows that the transition of velocity dependence from velocity weakening to velocity strengthening corresponds to transition from cataclastic flow to semibrittle process featured by mylonitic structures (S‐C fabrics). The formation of mylonitic fabrics is due to plastic deformation of phyllosilicates combined with thermally activated grain size reduction of hard clasts (quartz and plagioclase). Our results may help constrain depth range of seismogenesis within phyllosilicate‐rich fault zone and imply that variation of effective normal stress may affect faulting behavior.

Tribological origin of squeal noise in lubricated elastomer–glass contact

An experiment of squeal noise on a lubricated elastomer/glass contact is presented. The experimental device, stiff enough to be non-intrusive on the system response up to several kHz, provides the contact force, vibration velocity and contact image with a high sampling rate. It is shown that the self-excited oscillation responsible of squeal involves a single mode and that its apparition is induced by the velocity weakening of friction. Direct observation of the contact by microscopy highlights that the contact is highly heterogeneous and the number of contact spots decreases with the sliding velocity during squeal.

Sublithostatic pore fluid pressure in the brittle–ductile transition zone of Mesozoic Yingxiu-Beichuan fault and its implication for the 2008 Mw 7.9 Wenchuan earthquake

In order to understand the mechanism for occurrence of large earthquakes in the Longmen Shan region, we indirectly estimated the flow stress and pore fluid pressure in the brittle–ductile transition zone by studying exhumed granitic rocks which experienced Mesozoic ductile deformation, and constructed rheological profiles for the brittle regime and transition zone. The samples were collected from a small pluton that includes granites and deformed granitic rocks overthrust at an outcrop along the southern segment of the Yingxiu-Beichuan fault. The outcrop profile consists of granitic gneiss, protomylonite and mylonite. We observed that heterogeneous ductile deformation occurred in the brittle–ductile transition zone, and that the flow stress ranges from 15 to 80MPa. Moreover, the fault in that zone experienced a temporary brittle deformation, which might indicate a high strain rate during the co-seismic process and early post-seismic creep. Secondary fluid inclusions were found and measured to define the possible range of the capture temperature and fluid pressure. Sublithostatic pore fluid pressure was determined at the capture temperatures of 330–350°C during the process of filling and/or healing of microcracks. According to constructed rheological profiles and related mechanisms, high, sublithostatic pore fluid pressure is likely to significantly weaken the fault and to be related to inception of a brittle fault slip above the brittle–ductile transition zone. A high strain rate driven by the coseismic slip in the brittle regime may lead to a brittle fault slip in the brittle–ductile transition zone, and then plastic deformation in the transition zone may resume gradually during post-seismic creep. The focal depth of the 2008 Mw 7.9 Wenchuan earthquake may be controlled by a velocity weakening to velocity strengthening transition in frictional slip (brittle regime) of granite around a temperature of 350°C.

Velocity‐ and slip‐dependent weakening in simulated fault gouge: Implications for multimode fault slip

AbstractIn addition to the velocity dependence of friction, slip dependence may play a major role before and during earthquake slip in fault zones. We performed laboratory friction experiments on simulated fault gouges, measuring both the velocity and slip dependence of friction in velocity step tests. The pure velocity‐dependent component of friction measured over short displacements shows both velocity strengthening and velocity weakening friction, depending on the amount of slip considered. However, we observe that increases in sliding velocity can induce slip weakening behavior which overwhelms the velocity dependence resulting in large overall weakening, especially at rates &gt; 1 µm/s. On natural tectonic faults, this suggests that a velocity perturbation, such as coseismic rupture propagating onto a fault patch, could induce instability via large slip weakening. Therefore, a fault which is experiencing a transient slip or slow earthquakes may be more easily induced to slip coseismically if a dynamic rupture from large earthquake propagates onto the fault.

Lithification facilitates frictional instability in argillaceous subduction zone sediments

Previous work suggests that in subduction zones, the onset of large earthquake nucleation at depths>~5–10km is likely driven by a combination of factors associated with the process of lithification. At these depths, lithification processes affect the entire fault system by modifying the mechanical properties of both the plate boundary fault zone and the wall-rock. To test the hypothesis that lithification of subduction zone sediments produces rocks capable of earthquake nucleation via diagenesis and low-grade metamorphism, we conducted friction experiments on fossil subduction zone sediments recovered from exposures in the Shimanto Belt in SW Japan. These meta-sediments represent accreted and subducted material which has experienced maximum temperatures of 125 to 225°C, which are representative of seismogenic depths along the active Nankai subduction megathrust in the foreland of the Shimanto Belt. We find that intact Shimanto rock samples, which preserve the influence of diagenetic and metamorphic processes, exhibit the potential for unstable slip under in-situ pressure conditions. Powdered versions of the same samples tested under the same conditions exhibit only velocity-strengthening friction, thus demonstrating that destroying the lithification state also removes the potential for unstable slip. Using advanced porosity loss to quantify the lithification process, we demonstrate that increased velocity weakening correlates with increasingly advanced lithification. In combination with documented frictionally stable behavior of subduction zone sediments from shallower depths, our results provide evidence that the sediment lithification hypothesis can explain the depth-dependent onset of large earthquake nucleation along subduction zone megathrusts.

Frictional dynamics of viscoelastic solids driven on a rough surface.

We study the effect of viscoelastic dynamics on the frictional properties of a (mean-field) spring-block system pulled on a rough surface by an external drive. When the drive moves at constant velocity V, two dynamical regimes are observed: at fast driving, above a critical threshold V(c), the system slides at the drive velocity and displays a friction force with velocity weakening. Below V(c) the steady sliding becomes unstable and a stick-slip regime sets in. In the slide-hold-slide driving protocol, a peak of the friction force appears after the hold time and its amplitude increases with the hold duration. These observations are consistent with the frictional force encoded phenomenologically in the rate-and-state equations. Our model gives a microscopical basis for such macroscopic description.

Dynamic weakening of smectite‐bearing faults at intermediate velocities: Implications for subduction zone earthquakes

AbstractThe hydrous clay mineral smectite, which is pervasive in sediments on subducting oceanic plates, is thought to weaken and stabilize subduction thrust faults. However, these frictional properties of smectite alone cannot explain the large coseismic slip in the vicinity of a trench. Here we performed friction experiments to demonstrate the rate dependence of friction at slip rates from 30 µm/s to 1.3 m/s for water‐saturated smectite‐quartz mixtures with various smectite contents, so as to shed light on the frictional response of smectite‐bearing faults at intermediate to high slip rates. At slip rates of 30 to 150 µm/s, the friction coefficients decreased gradually from 0.5–0.6 to 0.1 with an increase in smectite content from 20 to 50 wt %. In contrast, at slip rates higher than 1.3 mm/s, friction exhibited marked slip weakening, resulting in low friction coefficients of 0.1–0.05, even for low smectite contents (roughly &lt;30 wt %). Drastic slip weakening occurred at smectite contents of 10–30 wt % at slip rates of ~10 mm/s, which is 1 to 2 orders of magnitude lower than the slip rate at which slip weakening was observed in previous experiments on various rock types. The intermediate velocity weakening could be attributed to a rise in pore pressure caused by both shear‐enhanced compaction and microscopic thermal pressurization of pore fluids. This process could weaken the fault even below seismic slip rates, leading to an acceleration of fault motion and potentially facilitating large coseismic slip and a stress drop in the vicinity of a trench.

Shear localization, velocity weakening behavior, and development of cataclastic foliation in experimental granite gouge

Microstructural aspects of room-temperature deformation in experimental Westerly granite gouge were studied by a set of velocity stepping rotary-shear experiments at 25 MPa normal stress. The experiments were terminated at: (a) 44 mm, (b) 79 mm, and (c) 387 mm of sliding, all involving variable-amplitude fluctuations in friction. Microstructural attributes of the gouge were studied using scanning (SEM) and scanning transmission electron microscopy (STEM), image processing, and energy dispersive X-ray (EDX) analyses. The gouge was velocity weakening at sliding distances >10 mm as a core of cataclasites along a through-going shear zone developed within a mantle of less deformed gouge in all experiments. Unlike in experiment (a), the cataclasites in experiments (b) and (c) progressively developed a foliation defined by stacks of shear bands. The individual bands showed an asymmetric particle-size grading normal to shearing direction. These microstructures were subsequently disrupted and reworked by high-angle Riedel shears. While the microstructural evolution affected the effective thickness and frictional strength of the gouge, it did not affect its overall velocity dependence behavior. We suggest that the foliation resulted from competing shear localization and frictional slip hardening and that the velocity dependence of natural fault gouge depends upon compositional as well as microstructural evolution of the gouge.

Energy consumption of the brushing process for PCB manufacturing based on a friction model

Industries are now required to use ‘green’ manufacturing due to emerging environmental concerns. In this study, the de-burring process for printed circuit board manufacturing was analyzed, and its energy consumption was examined based on a friction model. For de-burring of drilled micro-vias, a mechanical brushing process is typically used due to its performance and cost-effectiveness. However, it is very complex to analyze the energy consumption of a brushing process because a brush simply sweeps the surface with rotating metal wires, unlike conventional cutting processes. To construct an energy consumption model of a brushing process, its energy consumption was examined empirically considering a friction model. The brush was assumed to be an elastic body, and velocity strengthening and weakening of the friction coefficients were predicted. Experiments were performed to match the data, and the energy consumption model was validated. The power consumption of the brushing process showed a concave shape with respect to the rotational speed, and the overall power could be reduced by up to 40% by controlling the process parameters. The process parameters for the minimum energy consumption could be derived using the model considering local constraints such as burr height.

A friction to flow constitutive law and its application to a 2‐D modeling of earthquakes

AbstractEstablishment of a constitutive law from friction to high‐temperature plastic flow has long been a challenging task for solving problems such as modeling earthquakes and plate interactions. Here we propose an empirical constitutive law that describes this transitional behavior using only friction and flow parameters, with good agreements with experimental data on halite shear zones. The law predicts steady state and transient behaviors, including the dependence of the shear resistance of fault on slip rate, effective normal stress, and temperature. It also predicts a change in velocity weakening to velocity strengthening with increasing temperature, similar to the changes recognized for quartz and granite gouge under hydrothermal conditions. A slight deviation from the steady state friction law due to the involvement of plastic deformation can cause a large change in the velocity dependence. We solved seismic cycles of a fault across the lithosphere with the law using a 2‐D spectral boundary integral equation method, revealing dynamic rupture extending into the aseismic zone and rich evolution of interseismic creep including slow slip prior to earthquakes. Seismic slip followed by creep is consistent with natural pseudotachylytes overprinted with mylonitic deformation. Overall fault behaviors during earthquake cycles are insensitive to transient flow parameters. The friction‐to‐flow law merges “Christmas tree” strength profiles of the lithosphere and rate dependency fault models used for earthquake modeling on a unified basis. Strength profiles were drawn assuming a strain rate for the flow regime, but we emphasize that stress distribution evolves reflecting the fault behavior. A fault zone model was updated based on the earthquake modeling.

Frictional strength, rate-dependence, and healing in DFDP-1 borehole samples from the Alpine Fault, New Zealand

The Alpine Fault in southern New Zealand is a major plate-boundary fault zone that, according to various lines of evidence, may be nearing the end of its seismic cycle and approaching earthquake failure. In order to better characterize this fault and obtain a better understanding of its seismic potential, two pilot boreholes have been completed as part of a larger drilling project. Samples representative of the major lithologic subdivisions of the fault zone at shallow (~100m) depths were recovered and investigated in laboratory friction experiments. We show here that materials from within and very near the principal slip zone (PSZ) tend to exhibit velocity strengthening frictional behavior, and restrengthen (heal) rapidly compared to samples of the wall rock recovered near the PSZ. Fluid saturation causes the PSZ to be noticeably weaker than the surrounding cataclasites (μ=0.45), and eliminates the velocity-weakening behavior in these cataclasites that is observed in dry tests. Our results indicate that the PSZ has the ability to regain its strength in a manner required for repeated rupture, but its ability to nucleate a seismic event is limited by its velocity-strengthening nature. We suggest that seismogenic behavior at depth would require alteration of the PSZ material to become velocity weakening, or that earthquake nucleation occurs either within the wall rock or at the interface between the wall rock and the PSZ. Coseismic ruptures which propagate to the surface have been inferred for previous earthquakes on the Alpine Fault, and may be facilitated by the slightly weaker PSZ material and/or high rates of healing in the PSZ which support fault locking and eventual stress drops.

Thermodynamic aspects of rock friction

Rate- and state-dependent friction law for velocity-step tests is analyzed from a thermodynamic point of view. A simple macroscopic non-equilibrium thermodynamic model with a single internal variable reproduces instantaneous jump and relaxation. Velocity weakening appears as a consequence of a plasticity related nonlinear coefficient. Permanent part of displacement corresponds to plastic strain, and relaxation effects are analogous to creep in thermodynamic rheology.

Frictional behavior of simulated biotite fault gouge under hydrothermal conditions

Abstract We investigated frictional properties of biotite under hydrothermal conditions by shearing simulated biotite gouge sandwiched between saw-cut driving blocks, using a testing system with argon as confining medium. Experiments were conducted under an effective normal stress of 200 MPa, with pore pressure of 30 MPa. Temperatures were varied from room temperature to 600 °C, with shear displacement rate stepped at standard velocities (1.22–0.122 μm/s) and slow velocities (0.244–0.0488 μm/s) to acquire rate dependence of friction. Our results show that the friction coefficient of biotite ranges from 0.25 to 0.44, significantly lower than that of muscovite documented in previous studies. The steady-state rate dependence shows velocity strengthening at temperatures of 25 °C to 200 °C, and transitions to velocity weakening at temperatures above 300 °C. The velocity weakening is quite minor with (a − b) ranging from − 4 × 10− 4 to − 9 × 10− 4, and stick–slip motions at temperatures from 400 °C to 600 °C are found to be related to submicron dc values corresponding to critical stiffness higher than the actual stiffness surrounding the fault, thus leading to unstable slips. Microstructures of deformed samples show three types corresponding respectively to three different temperature ranges. While brittle structures are dominantly associated with deformation at 100 °C, significant plastic deformation occurred at higher temperatures. Deformation at temperatures of 200 °C to 400 °C are characterized with shear fracture zones accompanied with plastic deformation mostly associated with basal glide. Intensively deformed zones are the characteristic structures for samples deformed at 500 °C to 600 °C, accompanied with shear fracture zones and moderately deformed zones. It is evident that biotite alone is not weak enough to explain a weak fault like the San Andreas Fault, thus overpressured pore fluid or another weaker mineral is needed to produce a weak fault with apparent friction coefficient of 0.1–0.2. The around-neutral velocity weakening found for biotite may be appropriate in modeling fault creep transients in the mid crust.

Quasi‐dynamic versus fully dynamic simulations of earthquakes and aseismic slip with and without enhanced coseismic weakening

AbstractPhysics‐based numerical simulations of earthquakes and slow slip, coupled with field observations and laboratory experiments, can, in principle, be used to determine fault properties and potential fault behaviors. Because of the computational cost of simulating inertial wave‐mediated effects, their representation is often simplified. The quasi‐dynamic (QD) approach approximately accounts for inertial effects through a radiation damping term. We compare QD and fully dynamic (FD) simulations by exploring the long‐term behavior of rate‐and‐state fault models with and without additional weakening during seismic slip. The models incorporate a velocity‐strengthening (VS) patch in a velocity‐weakening (VW) zone, to consider rupture interaction with a slip‐inhibiting heterogeneity. Without additional weakening, the QD and FD approaches generate qualitatively similar slip patterns with quantitative differences, such as slower slip velocities and rupture speeds during earthquakes and more propensity for rupture arrest at the VS patch in the QD cases. Simulations with additional coseismic weakening produce qualitatively different patterns of earthquakes, with near‐periodic pulse‐like events in the FD simulations and much larger crack‐like events accompanied by smaller events in the QD simulations. This is because the FD simulations with additional weakening allow earthquake rupture to propagate at a much lower level of prestress than the QD simulations. The resulting much larger ruptures in the QD simulations are more likely to propagate through the VS patch, unlike for the cases with no additional weakening. Overall, the QD approach should be used with caution, as the QD simulation results could drastically differ from the true response of the physical model considered.

3D effects of strain vs. velocity weakening on deformation patterns in accretionary wedges

Abstract Shear zones are weaker than surrounding rocks. Whether weakening depends on accumulated strain or strain rate is debated. We used a three-dimensional numerical code with a visco-plastic/brittle rheology to investigate the influence of strain and strain-rate weakening (often referred to as velocity weakening) on the evolution of thin-skinned fold-and-thrust belts. Two model setups are used: (i) a uniform setup to recognize the effects of each weakening mode on the structural evolution and dynamics of fold-and-thrust belts and (ii) a dual setup with two adjacent domains of decollement strength to investigate the structural response of laterally varying systems. Strain weakening and velocity weakening lead to remarkably different structural patterns. In particular, strain weakening favours out-of-sequence shear zones whilst some amount of finite plastic strain has to be achieved before the structural development initiates weakening. In contrast, velocity weakening starts with the onset of high strain-rate bands and does not favour out-of-sequence deformation. We also tested separately the influences of shortening velocity, cover sequence thickness and weakening style and amount on the structural evolution of the dual systems. The shortening velocity has a minor effect in strain-weakened models. Velocity weakening and a thinner cover sequence enhance the occurrence of strike-slip shear zones. We conclude that the weakening mode strongly influences the fault patterns and the dynamics of thrust wedges. Inversely, natural fault patterns are keys to discriminate weakening processes.