Fault Strength Research Articles

Simple topographic parameter reveals the along-trench distribution of frictional properties on shallow plate boundary fault

The critical taper model best describes the first-order mechanics of subduction zone wedges. The wedge geometry, which is conventionally defined by two parameters, slope angle and basal dip angle, accounts for the strength of megathrust. By applying this theoretical model, fault frictional properties and earthquake occurrences can be compared among subduction zones, and within a single subduction zone, and the spatial distribution or temporal change of fault strength can be investigated. Slope angle can be accurately estimated from bathymetry data, but basal dip angle must be inferred from subsurface structure, which requires highly accurate depth-migrated seismic reflection profiles. Thus, application of the critical taper model is often limited by an insufficient number of highly accurate profiles, and the spatial distribution of frictional coefficients must be inferred from relatively few data. To improve this situation, we revisited the theoretical formula of the critical taper model. We found that the effect of basal dip angle on the critical taper model is small, and slope angle can be a proxy for the effective friction when the pore fluid pressure ratio is high, internal friction is small, or both. These conditions are met in many subduction zones. The validity of the approximation can be checked with a parameter newly introduced in this study. Therefore, this finding allows use of variations in slope angle, which could be obtained accurately from only the bathymetry as an approximation for relative variations in the effective coefficient of basal friction, if the targeted subduction meets the validity. We applied this approximation to the Japan Trench and estimated the variations in the friction coefficient distribution on the shallow plate boundary fault from 71 data points. We found that the area where the friction coefficient was smaller than the mean corresponded to a segment, where a large coseismic shallow rupture occurred during the 2011 Tohoku-Oki earthquake (Mw 9.0). Thus, by approximating tapered wedge geometry with a simple topographic parameter that can be obtained from existing global bathymetry, we can quickly estimate the distribution of frictional properties on a plate boundary fault along a trench and related seismic activity.Graphical

Three-dimensional finite element modelling on strain localization around the Mabian earthquake swarm, Sichuan Province

SUMMARYThe Mabian fault zone, distanced ∼200 km to the east of the Xianshuihe–Xiaojiang fault system, is located in the western vicinity of the relatively stable South China Block. Since 1917, about 54 M > 4.7 earthquakes, including the 1974 Ms = 7.1 Mabian event have occurred around this fault zone, suggesting that significant strain is localized within the Mabian fault zone. Here, we built a 3-D finite element model to investigate the main parameters that possibly control strain localization around the Mabian fault zone averaged over the active deformation timescale. In the model, the Xianshuihe–Xiaojiang fault system is specified as a discontinuous contact interface for its motion governed by a Coulomb-friction law, and the crustal rheology is simplified as a frictional upper crust underlain by a viscoelastic lower crust. In addition, global positioning system (GPS) data are used to mimic the horizontal tectonic loading, and the model base is supported by a hydrostatic pressure. Numerical results show that with the weak fault strength and the low viscosity contrast between the Tibetan plateau and the South China Block, strain rates from motion of the southeastern Tibetan plateau could be propagated across the Xianshuihe–Xiaojiang fault system more widely within the Mabian fault zone. Constrained by the estimates on slip rates of the faults and on rheological structures of the crust, our optimal model predicts the effective friction coefficient of the Xianshuihe–Xiaojiang fault of 0.05–0.1. Under this condition, relative motion across the Xianshuihe–Xiaojiang fault system is largely partitioned by the geometric bend near the central portion of the fault system, resulting in a relatively high strain rate of 2.1–3×10–8 yr–1 accumulating around the Mabian fault zone. Keeping the weak strength of the fault, numerical results also show that if the central portion of the Xianshuihe–Xiaojiang fault system follows the Daliangshan fault, strain accumulation around the Mabian fault zone could be significantly reduced. It thus can be concluded that the strain partitioning from the weak strength and the special geometry of the Xianshuihe–Xiaojiang fault system must play a crucial role in active deformation around the Mabian area out of the Tibetan plateau deformation domain. This in turn means that in the Xianshuihe–Xiaojiang fault system, the Anninghe–Zemuhe fault is still the main boundary between the southeastern Tibetan plateau and the South China Block.

Mantle exhumation at magma-poor rifted margins controlled by frictional shear zones

The transition zone from continental crust to the mature mid-ocean ridge spreading center of the Iberia-Newfoundland magma-poor rifted margins is mostly composed of exhumed mantle characterized by highs and domes with varying elevation, spacing and shape. The mechanism controlling strain localization and fault migration explaining the geometry of these peridotite ridges is poorly understood. Here we show using forward geodynamic models that multiple out-of-sequence detachments with recurring dip reversal form during magma-poor rifting and mantle exhumation as a consequence of the strength competition between weak frictional-plastic shear zones and the thermally weakened necking domain beneath the exhuming footwall explaining geometry of these peridotite ridges. Model behaviour also shows that fault types and detachment styles vary with spreading rate and fault strength and confirm that these results can be compared to other magma poor passive margins such as along Antarctica-Australia and to ultra-slow mid-ocean spreading systems as the South-West Indian Ridge.

The crustal and uppermost mantle dynamics of the Tengchong–Baoshan region revealed by P-wave velocity and azimuthal anisotropic tomography

SUMMARY The Tengchong–Baoshan region (TBR) is located at the margin of the southeastern Tibetan Plateau (SETP) and is subject to the complex interaction among multiple tectonic blocks. Therefore, the TBR is approximate for investigating the mechanism responsible for the interaction among the SETP and its surrounding blocks. Based on eikonal equation-based seismic tomography, we obtain high-resolution P-wave velocity and azimuthal anisotropy at depths of 0–40 km beneath the TBR. We discover a large magma reservoir (with horizontal scale of ∼50 km and vertical scale of ∼15 km) in the middle-lower crust beneath the Tengchong volcanic zone, which is directly fed by hot materials from the uppermost mantle. We also detect a shallow magma chamber in the uppermost crust above the large reservoir. These magma reservoirs at different depths constitute a multiscale magma system. Furthermore, we observe a low-velocity belt beneath the Weixi–Qiaohou–Weishan and Nantinghe faults and postulate that it corresponds to lower crustal flow and hot mantle upwelling. Additionally, we infer that two M > 7.0 earthquakes that have occurred in Baoshan block nucleated in the transition zone between low- and high-velocity anomalies. Based on the azimuthal anisotropy tomography results, we consider that incoherent deformation in the crust causes the uneven accumulation of stress, which facilitated the occurrence of these two events. The 2021 Ms 6.4 Yangbi earthquake also occurred in the transition zone between low- and high-velocity anomalies. Based on the P-wave velocity tomography results, we suggest that hot material decreases the fault strength and leads to the instability of the fault.

Dynamic Rupture Simulation of the 1833 Songming, Yunnan, China,M8.0 Earthquake: Effects From Stepover Location and Overlap Distance

AbstractStepover structures are widely distributed in natural fault systems. The jump distance is typically easier to identify than both the stepover location and the overlap distance because of the presence of lakes and sedimentary basins around the stepover. Using the curved grid finite‐difference method, we first conducted a 3D spontaneous dynamic rupture simulation of the 1833 Songming earthquake on a continuous model of the western Xiaojiang fault to estimate the regional stress azimuth. Then, we performed rupture process simulations with different stepover structure models on the western Xiaojiang fault to investigate the effects of the stepover location and overlap distance on the dynamic rupture process. In addition, we analyzed the fault strength ratioSand rupture velocity of the preferred fault models and discussed the occurrence of supershear rupture during the Songming event. Finally, we simulated the strong ground motion with the preferred fault models for the 1833 Songming earthquake. Our simulation results suggest that the optimal regional maximum principal stress azimuth during the Songming event was N20°W. Our simulations further reveal that the Qingshuihai stepover is located in the middle of Qingshuihai Lake, and the overlap distance is 6 km; in contrast, the location of the Yangzonghai stepover is located in the southern part of Yangzonghai Lake, and the overlap distance is not constrained. Furthermore, our numerical simulation indicates that the Songming earthquake exhibited supershear rupture; however, this supershear rupture is not sustained and is easily affected by the friction parameters and nucleation location.

Strength of Dry and Wet Quartz in the Low-Temperature Plasticity Regime: Insights From Nanoindentation.

At low‐temperature and high‐stress conditions, quartz deformation is controlled by the kinetics of dislocation glide, that is, low‐temperature plasticity (LTP). To investigate the relationship between intracrystalline H2O content and the yield strength of quartz LTP, we have integrated spherical and Berkovich nanoindentation tests at room temperature on natural quartz with electron backscatter diffraction and secondary‐ion mass spectrometry measurements of intracrystalline H2O content. Dry (<20 wt ppm H2O) and wet (20–100 wt ppm H2O) crystals exhibit comparable indentation hardness. Quartz yield strength, which is proportional to indentation hardness, seems to be unaffected by the intracrystalline H2O content when deformed under room temperature, high‐stress conditions. Pre‐indentation intracrystalline microstructure may have provided a high density of dislocation sources, influencing the first increments of low‐temperature plastic strains. Our results have implications for fault strength at the frictional‐viscous transition and during transient deformation by LTP, such as seismogenic loading and post‐seismic creep.

Evolution of Fault-Zone Hydromechanical Properties in Response to Different Cementation Processes

Abstract Progressive cementation and sealing of fault-localized fractures impact crustal mass transport and the recovery of fault strength following slip events. We use discrete fracture network (DFN) models to examine how fracture sealing during end-member cementation mechanisms (i.e., reaction- versus transported-limited cementation) influences the partitioning of fluid flow through time. DfnWorks was used to generate randomized fracture networks parameterized with fracture orientation data compiled from field studies. Single-phase flow simulations were performed for each network over a series of timesteps, and network parameters were modified to reflect progressive fracture sealing consistent with either reaction- or transport-limited crystal growth. Results show that when fracture cementation is reaction-limited, fluid flow becomes progressively channelized into a smaller number of fractures with larger apertures. When fracture cementation is transport-limited, fluid flow experiences progressive dechannelization, becoming more homogeneously distributed throughout the fracture network. These behaviors are observed regardless of the DFN parameterization, suggesting that the effect is an intrinsic component of all fracture networks subjected to the end-member cementation mechanisms. These results have first-order implications for the spatial distribution of fluid flow in fractured rocks and recovery of permeability and strength during fault/fracture healing in the immediate aftermath of fault slip.

Fault rock heterogeneity can produce fault weakness and reduce fault stability

Geological heterogeneity is abundant in crustal fault zones; however, its role in controlling the mechanical behaviour of faults is poorly constrained. Here, we present laboratory friction experiments on laterally heterogeneous faults, with patches of strong, rate-weakening quartz gouge and weak, rate-strengthening clay gouge. The experiments show that the heterogeneity leads to a significant reduction in strength and frictional stability in comparison to compositionally identical faults with homogeneously mixed gouges. We identify a combination of weakening effects, including smearing of the weak clay; differential compaction of the two gouges redistributing normal stress; and shear localization producing stress concentrations in the strong quartz patches. The results demonstrate that geological heterogeneity and its evolution can have pronounced effects on fault strength and stability and, by extension, on the occurrence of slow-slip transients versus earthquake ruptures and the characteristics of the resulting events, and should be further studied in lab experiments and earthquake source modelling.

Occurrences of low-stress drop earthquakes in the eastern Ladakh-Karakoram zone of the Trans Himalaya and their tectonic implications

The Ladakh-Karakoram zone is a unique testing ground for understanding the geodynamic evolution of the Himalaya-Karakoram orogeny. Despite the accumulation of a large amount of strain energy that originated due to the India-Asia collision along the Karakoram fault, earthquakes of M ≥ 7.0 are considerably less in the Karakoram Fault Zone (LKZ) compared to the Himalayan seismic belt in the northwest Himalaya. The spectral analysis of microearthquakes data (1.9 < ML < 4.3) recorded during 2009–2012 by 10 seismological stations of the LKZ reveals that the region produces low-stress drop earthquakes. Analysis of selected 51 earthquakes shows the seismic moment (Mo) within the range of 1.2 × 1012–4.3 × 1015 Nm with stress drop values varying from ∼ 0.06 to 64.36 bar. The low stress-drop earthquakes have significant implications in understanding seismogenesis. The earthquake cluster near the Tso-Morari Crystallines is associated with the brittle shear failure on the faults (e.g. Zildat, and Karzok Faults), and the low-stress drop of the earthquakes are explained by the partial stress drop model. The low effective fault strength due to fault weakening mechanisms in the studied segment of the Karakoram Fault zone promotes aseismic creeping patches producing low-stress drop earthquakes at the expense of generating large earthquakes. This study provides the scaling relations between important source parameters for the first time in the region which can serve as useful inputs for the assessment of earthquake hazards.

Fault roughness controls sliding instability

The frictional strength of faults controls the stability and the dynamics of slip in diverse natural phenomena including earthquakes, induced seismicity, and landslides. It has been shown that geological faults and rock discontinuities in general are rough in a wide range of scales but the dependency of slip dynamics on surface roughness for a large spectrum of heights has never been measured. Here we show specifically how slip dynamics dramatically varies as a function of surface roughness using direct shear experiments performed on laboratory-generated faults in Diabase. We demonstrate experimentally that under relatively low normal stress of 5 MPa, stick-slip oscillations, commonly referred to as laboratory earthquakes, only occur in a very limited range of roughness values within which a specific level, defined here as the critical roughness, triggers the highest amplitude of oscillations. Sliding across roughness higher or lower than critical is typically stable. Using monitored vertical motions through slip (dilation) coupled with numerical modelling we show that sliding on extremely “smooth” surfaces is typically stable because the very small height of asperities does not allow for the nucleation and motion arrest required for ‘stick’ phases to ensue, whereas sliding on extremely rough “fractured” surfaces is typically characterized by shearing through the tall asperities. Sliding across “sawcut” surfaces, however, is found to be particularly susceptible to stick slip deformation, and the shear motion is shown to be purely dilatant where the dilation and compression of the sliding interface are in phase with the stick slip oscillations. We conclude therefore that dilatant shear across moderately rough interfaces is a prerequisite for stick slip oscillations and consequently, for sliding instability.

Low-effective fault strength of a blind detachment beneath the Indo-Burmese Arc (NE-India) induced by frictional–viscous flow

Low-effective fault strength of a blind detachment beneath the Indo-Burmese Arc (NE-India) induced by frictional–viscous flow

Regional Heat Flow Analysis Reveals Frictionally Weak Dead Sea Fault

AbstractThe strength of faults was vigorously debated for years, but lately a growing number of studies suggest that faults are weaker than originally suggested. Nonetheless, only a handful of natural faults have been studied in detail, and only one, the San Andreas, is a strike‐slip fault. Here, we reanalyze 268 surface heat flow measurements taken in the proximity of the southern Dead Sea Transform fault to evaluate its friction. To account for large terrain relief, and the presence of salt diapirs, we apply 3‐D terrain and salt diapir corrections. Based on these corrected heat flow values we estimate that the long‐term frictional resistance of the Dead Sea fault is 0.28 ± 0.17. This low value is similar to friction estimates from the San Andreas fault and several subduction zones.

THMD analysis of fluid injection-induced fault reactivation and slip in EGS

A pseudo-3D coupled model with fault using coupled thermo-hydro-mechanical-damage (THMD) modeling processes is established by the framework of TOUGHREACT-FLAC3D. Four scenarios are proposed to evaluate the effect of deviatoric stress, injection temperature, and fracture spacing on conducting damage, inducing the fault slip, and changing the permeability evolution. Results indicates the larger initial deviatoric stress, the larger the area of ​​reservoir damage, the larger the area of ​​fault damage, the earlier the occurrence of fault damage, and the faster the damage passes through the fault and connects with the withdrawal well. It is found that the normal stress release and strength weakening are the dominated reasons of destabilizing the fault and demonstrated the ultimate slip resistance strength of fault is close to 23 MPa. Research on injection temperature has found that the main reasons for the damage evolution of reservoirs are the varying sensitivity of fluid viscosity to temperature and the variation of deviatoric stress. Both damage evolution and deviatoric stress influence the variation of fault slip distance. Similarly, we also investigate the effect of fracture spacing on damage evolution, which shows that the decreasing fracture spacing induces the increase of reservoir permeability which obviously decreases the barrier effect of the fault and widens the damage front surface after passing through the fault.

Seismogenic crustal structure affected by the Hainan mantle plume

Mantle plumes are one of the most influential first-order tectonic systems on Earth, playing a prominent role in controlling local magmatic activity and seismicity. Recent studies have shown that the Hainan mantle plume originates from the lower mantle and its volcanic activity is still intense during the Holocene. However, the effect of the Hainan plume on local seismicity is still unclear. In this study, we conducted the first wide-angle seismic survey in the source area of the 1969 earthquake doublet (M 5.1 and 5.2) offshore Hainan Island. A high-resolution 3-D P-wave velocity model of the crust is obtained using a large amount of Pg and PmP arrival times recorded by a portable array consisting of 17 ocean bottom seismographs (OBSs). Our results show that the 1969 earthquake doublet occurred in high-to-low velocity transitional zones which correspond to active faults. Prominent low-velocity anomalies are revealed in the crust of the study area, which may reflect magmatic intrusion channels. On the basis of multiple geophysical evidence including multi-channel seismic reflection profiles, geothermal data, and seismic tomography results, we infer that the low-velocity anomalies in the source area may reflect magmas and fluids from the Hainan plume, and high-angle normal faults in the crust serve as conduits for ascending of the magmas and fluids. The 1969 earthquake doublet offshore Hainan Island can be attributed to the magma-hydrothermal alteration processes in the fault zones, which led to the significant reduction of the fault strength and friction coefficient, facilitating the shear failure propagation. Furthermore, the seismicity in and around Hainan Island exhibits distinct swarm-like behaviors, which is also evidence for the Hainan plume affecting the crustal seismicity.

An interdisciplinary view of the long-term evolution of repository systems across scales: the iCROSS project

Abstract. The interdisciplinary project “Integrity of nuclear waste repository systems – Cross-scale system understanding and analysis (iCROSS)” combines research competencies of Helmholtz scientists related to the topics of nuclear, geosciences, biosciences and environmental simulations in collaborations overarching the research fields energy and earth and environment. The focus is to understand and analyze close-to-real long-term evolutionary pathways of radioactive waste repositories across nanoscales to repository scales. The project is subdivided into work packages dealing with laboratory studies, field experiments in underground research laboratories (URLs), advanced modelling studies and the integration and alignment of data and information using virtual reality methods. In this sense, the project structure aims at a holistic view on relevant processes across scales in order to comprehensively simulate potential repository evolutions. Within the multi-barrier system of a repository for heat-generating radioactive waste, a number of complex reactions proceed, including dissolution, redox processes, biochemical reactions, gas evolution and solid/liquid interface and (co)precipitation reactions. At the same time, thermal and external mechanical stress has an impact on the conditions in a deep geological repository. All those processes are highly coupled, with multiple interdependencies on various scales and have a strong impact on radionuclide mobility and retention. In recent years, substantial progress was achieved in describing coupled thermal-hydro-mechanical-chemical-biological (THM/CB) processes in numerical simulations. A realistic and concise description of these coupled processes on different time and spatial scales is, at present, a largely unresolved scientific and computational challenge. The close interaction of experimental and simulation teams aims at a more accurate quantification and assessment of processes and thus, the reduction of uncertainties and of conservative assumptions and eventually to a close-to-real perception of the repository evolution. One focus of iCROSS is directed to relevant processes in a clay rock repository. In this context, the iCROSS team became a full member of the international Mont Terri consortium and worked in close collaboration with international and German institutions in URL projects. Respective experiments specifically deal with coupled processes at the reactive interfaces in a repository near field (e.g. the steel/bentonite and bentonite/concrete interfaces). Within iCROSS, the impact of secondary phase formation on radionuclide transport is investigated. At Mont Terri, experiments are in preparation to study radionuclide transport phenomena in clay rock formations within temperature gradients and in facies exhibiting significant heterogeneities on different scales (nm to cm). Beside those studies, high resolution exploration methods for rock characterization are developed and tested and the effect of temperature and other boundary conditions on the strength, creep properties and healing of faults within Opalinus clay are quantified. Multiphysics models coupled to reactive transport simulation have been further developed and applied to laboratory and field experiments. Results are digitally analyzed and illustrated in a visualization center, in order to enhance the comprehension of coupled processes in repository systems across scales. The present contribution provides an overview on the project and reports selected results. The impact of considering complex coupled processes in repository subsystems for the assessment of the integrity of a given (generic) repository arrangement is discussed.

Relative Structural Complexity of Plate‐Boundary Fault Systems Controls Incremental Slip‐Rate Behavior of Major Strike‐Slip Faults

AbstractA comparison of published incremental fault slip rates from four major strike‐slip faults with the proximity and number of other active faults in the surrounding plate boundary systems shows that the behavior of the primary fault is correlated with the structural complexity of its tectonic setting. To do this, we characterize the relative structural complexity of the fault network surrounding a fault location of interest by defining the coefficient of complexity, which quantifies the density and displacement rates of the faults in the plate‐boundary network at specified radii around the site of interest. We show that the relative constancy of incremental slip rates measured along primary faults of the Alpine, San Andreas, North Anatolian, and Dead Sea fault systems reflects the proximity, number, and activity of their close neighbors. Specifically, faults that extend through more structurally complex plate boundary fault systems exhibit more irregular slip behavior than faults that pass through simpler settings. We suggest that these behaviors are likely a response to more complex stress interactions within more structurally complicated regional fault systems, as well as possible temporal changes in fault strength and/or kinematic interactions amongst mechanically complementary faults within a system that collectively accommodates overall relative plate motion. Our results provide a potential means for better evaluating the future behavior of large plate‐boundary faults in the absence of well‐documented incremental slip‐rate behavior, and may help improve the use of geological slip‐rate data in probabilistic seismic hazard assessments.

Spatio‐Temporal Evolution of Earthquake Static Stress Drop Values in the 2016–2017 Central Italy Seismic Sequence

AbstractThe static stress drop of an earthquake is an indicator of the stress state of a specific fault before rupture initiation. The stress state is primarily controlled by the ambient stress field, fault strength, fault complexity, and the presence of fluids. This study aims to investigate the spatio‐temporal distribution of static stress drop values of the 2016–2017 multi‐fault rupture seismic sequence in central Italy, which includes three earthquakes with Mw ≥ 5.9 (Amatrice, Visso, and Norcia earthquakes), and over 95,000 aftershocks (M 0.5–6.5). We estimate stress drop values using a circular crack model with corner frequency and seismic moment estimates from single‐spectra fitting, a cluster‐event method, and spectral‐ratio fitting. The temporal distribution of stress drop values shows an apparent increase of stress drop following a large earthquake (Mw ≥ 5.9). The spatial distribution shows comparably high stress drop values for early aftershocks surrounding the mainshock rupture area. High stress drop events correlate with fault complexity, such as fault intersections at depth and reactivated thrust fronts. We observe a constant stress drop for Mw ≥ ∼3, in contrast to previous studies. Instrument response and signal‐to‐noise bandwidth limitations likely govern the observed decrease in stress drop with decreasing magnitude for events with Mw ≤ 3. The spatio‐temporal distribution of stress drop values in a complex seismic sequence could support a more complete understanding of the earthquake rupture process and the evolution of seismic sequences. It could also highlight areas where stress loading is focused, which would have implications for short and intermediate term seismic hazard estimates.

Friction during earthquakes: 25 years of experimental studies

Dynamic fault strength τ (rock friction in the broad sense) and its evolution with seismic slip and slip rate are among the most relevant parameters in earthquake mechanics. Given the large slip rate (1 m s−1 on average), displacement (up to tens of meters), effective stress (tens of MPa), typical of seismic faulting at depth, thermo-mechanical effects become outstanding: dynamic fault strength is severely affected by fluid and rock phase changes, extreme grain size reduction, and the production of amorphous and unstable materials in the slipping zone. Here, first we will summarize the most relevant findings about dynamic fault strength during seismic slip mainly obtained thanks to the exploitation of dedicated experimental machines (i.e., rotary shear apparatus). However, the interpretation of this experimental dataset remains debated because of technical limitations which impede us to measure fundamental parameters such as temperature, strain rate, pore fluid pressure and grain size in the slipping zone. Without a sound estimate of these physical parameters, any constitutive law proposed to describe the evolution of dynamic fault strength during simulated seismic slip remains speculative. Then, we will discuss the results of some recent experiments which exploit new technical approaches to overcome the main limitations of the previous studies. The experimental approach, together with field studies of the geometry and architecture of exhumed faults and modelling, remains our most powerful tool to investigate seismic-related deformation mechanisms in both natural and human-induced earthquakes.

A Unified Framework for Earthquake Sequences and the Growth of Geological Structure in Fold‐Thrust Belts

AbstractObservations of fold growth in fold‐thrust belt settings show that brittle deformation can be localized or distributed. Localized shear is associated with frictional slip on primary faults, while distributed brittle deformation is recognized in the folding of the bulk medium. The interplay of these processes is clearly seen in fault‐bend folds, which are folds cored by a fault with an abrupt change in dip (e.g., a ramp‐décollement system). While the kinematics of fault‐bend folding were described decades ago, the dynamics of these structures remain poorly understood, especially the evolution of fault slip and off‐fault deformation over different periods of the earthquake cycle. In order to investigate the dynamics of fault‐bend folding, we develop a numerical modeling framework that combines a long‐term elasto‐plastic model of folding in a layered medium with a rate‐state frictional model of fault strength evolution in order to simulate geologically and mechanically consistent earthquake sequences. In our simulations, slip on the ramp‐décollement fault and inelastic fold deformation are mechanically coupled processes that build geologic structure. As a result, we observe that folding of the crust (like fault slip) does not occur steadily in time but is modulated by earthquake cycle stresses. We suggest combining seismological and geodetic observations with geological fault models to uncover how elastic and inelastic crustal deformation generate fault‐bend folds. We find that distinguishing between the elastic and inelastic response of the crust to fault slip is possible only in the postseismic period following large earthquakes, indicating that for most fault systems this information currently remains inaccessible.

Fault Friction During Simulated Seismic Slip Pulses

AbstractTheoretical studies predict that during earthquake rupture faults slide at non‐constant slip velocity, however it is not clear which source time functions are compatible with the high velocity rheology of earthquake faults. Here we present results from high velocity friction experiments with non‐constant velocity history, employing a well‐known seismic source solution compatible with earthquake source kinematics. The evolution of friction in experiments shows a strong dependence on the applied slip history, and parameters relevant to the energetics of faulting scale with the impulsiveness of the applied slip function. When comparing constitutive models of strength against our experimental results we demonstrate that the evolution of fault strength is directly controlled by the temperature evolution on and off the fault. Flash heating predicts weakening behavior at short timescales, but at larger timescales strength is better predicted by a viscous creep rheology. We use a steady‐state slip pulse to test the compatibility of our strength measurements at imposed slip rate history with the stress predicted from elastodynamic equilibrium. Whilst some compatibility is observed, the strength evolution indicates that slip acceleration and deceleration might be more rapid than that imposed in our experiments.