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This research targets disentangling shallow causes of anthropogenically-induced subsidence in a reclaimed and urbanized coastal plain. The study area is around the city of Almere, in the South Flevoland polder, the Netherlands, which is among the countries’ fastest subsiding areas. The procedure consists of integrating synthetic Interferometric Synthetic Aperture Radar (InSAR) data with high-resolution phreatic groundwater and lithoclass models, and a database containing construction details. The two main parts of the workflow are isolation of the InSAR points of structures without a pile foundation and a data assimilation procedure by Ensemble Smoothing with Multiple Data Assimilation. The shrinkage of surficial clay beds by phreatic groundwater level lowering is identified to be the main cause of shallow subsidence in the area, with an average contribution of 6 mm/year. The history-matched physics-based model predicts that one meter drop in phreatic groundwater level now translates into 10 millimeter of subsidence in the next five years. Also, this study showed that a groundwater deficiency due to severe dry periods should be considered as an accelerator of subsidence in both the short- and long-term planning. To ensure a robust network to estimate future subsidence, we advise on a consistent monitoring strategy of the phreatic groundwater level.

Shallow and deep human subsurface activities contribute to the total subsidence

Reclaimed coastal plains often experience significant subsidence as a result of phreatic water level lowering, which induces oxidation of organic material, shrinkage of clay, and sediment compaction. A primary example in the center of the Netherlands, is the ‘South Flevopolder’ which was reclaimed in 1968 and transformed into an area for residential, industrial and agricultural use. The area subsided over 1.5 m since its reclamation and its surface is still lowering.The city of Almere, with roughly 200.000 inhabitants and a surface area of c. 250 km2, is situated in the South Flevopolder. Most buildings in the city are founded in deeper Pleistocene sand, whilst objects such as parking lots, sport fields and playgrounds are often unfounded and are directly situated on the younger Holocene coastal deposits. Currently, the unfounded objects show subsidence rates as high as 5 mm per year for which the different subsidence rates may be related to subsurface heterogeneities. The upper layers in the area are dominated by clay and sand, up to a few meters in thickness, which overly peat and highly organic layers. The lowering of the phreatic surface results in an erratic pattern of subsidence over the area.We present a workflow to disentangle and parameterize the different contributions of shallow subsidence from Interferometric synthetic-aperture radar (InSAR) measurements. InSAR measurements from founded and unfounded scatterers are separated with a dimensionality reduction technique, t-Distributed Stochastic Neighbor Embedding (t-SNE), followed by an automatic detection of clusters with Hierarchical Density-Based Spatial Clustering of Applications with Noise (HDBSCAN). We have limited ourselves to structures with a construction date of >10 years with respect to the first InSAR measurement date to reduce the effect of construction-related consolidation and isolate the effect of shallow subsidence related to reclamation and phreatic surface level changes. The filtered dataset represents the surface response of unfounded structures in the urbanized reclaimed coastal area.The subsidence processes are disentangled and parameterized with an Ensemble Smoother with Multiple Data Assimilation (ES-MDA). Additional input data for this method is provided by a phreatic groundwater level model and a voxelized lithological model from the surface towards the top of the Pleistocene sand layers.We show that the automated data selection method prevents bias by selecting unfounded objects and the proposed workflow can be of aid when studying shallow subsidence in urbanized areas, where most objects are founded below the level at which shallow subsidence takes place. The results of this study quantify the rate of the different subsidence processes on a spatiotemporal scale and thus provide insights for tailored decision making to mitigate subsidence.

Hydrocarbon reservoirs can be situated below low-lying coastal plains. Extraction from these reservoirs are known to cause substantial amounts of subsidence. Yet, the relative contribution of hydrocarbon extraction to total subsidence is often ignored in many coastal areas around the world. The primary reason for such negligence is because hydrocarbon extraction data are often confidential and therefore difficult to access for scientific research purposes. Incorporating the effects of hydrocarbon extraction in coastal subsidence research is however critical, as reservoirs can be depleted for decades in a row, causing decimeters of subsidence. Furthermore, gas is recently labeled by the European Union as a ‘green energy,’ motivating countries to increase production from low-lying coastal areas. Therefore, taking coastal subsidence by hydrocarbon extraction into account with datasets that are commonly private is essential for understanding regional subsidence processes, and eventually to design mitigation or adaptation strategies. In this study, we present the outline of a workflow being developed to deploy hydrocarbon extraction data for subsidence modelling while acknowledging data privacy constraints. The targeted area is the urbanized coastal plain of Friesland (The Netherlands), which is subsiding by compaction of ca. 2-3 km deep gas reservoirs, as well as by surficial processes such as peat oxidation and clay shrinkage. The core of the method is a Federated Learning framework for Neural Networks on vertically partitioned data including cryptographic techniques. Federated Learning implies that a central model can be trained on data which is only stored locally. Therefore, the data does not leave the premises of the data-owner (in this case the hydrocarbon operator), to protect confidential information. Such a model trains at each dataset and only model-updates are sent back and aggregated to the central server. The trained model and its output are shared between the parties involved. Our workflow comprises a secure learning set-up for gas reservoir pressure depletion. The workflow uses the library FATE (FAir, Transparent and Explainable decision making), which combines secure inner sect (a Multi-Party Computation) techniques with a bottom and top split Neural Network, combining the outputs of the bottom models with an interactive layer. The technique of Neural Network was selected for flexibility in algorithms used, such as future intertwining of the workflow with physical models (e.g., transfer learning and physics informed neural networks). Current work focuses on extracting relevant information on feature importance causing subsidence from the Federate Learning framework without compromising confidentiality. Preliminary results show that a Federated trained model does not significantly increase the prediction error compared to a centrally trained model, suggesting that the developed approach can be a critical step forward in convincing hydrocarbon operators to provide their data in a confidential way. In this way, subsidence by hydrocarbon extraction can be integrated into overall coastal subsidence studies.

This paper describes and deploys a workflow to assess the evolution of seismicity associated to injection of cold fluids close to a fault. We employ a coupled numerical thermo-hydro-mechanical simulator to simulate the evolution of pressures, temperatures and stress on the fault. Adopting rate-and-state seismicity theory we assess induced seismicity rates from stressing rates at the fault. Seismicity rates are then used to derive the time-dependent frequency-magnitude distribution of seismic events. We model the seismic response of a fault in a highly fractured and a sparsely fractured carbonate reservoir. Injection of fluids into the reservoir causes cooling of the reservoir, thermal compaction and thermal stresses. The evolution of seismicity during injection is non-stationary: we observe an ongoing increase of the fault area that is critically stressed as the cooling front propagates from the injection well into the reservoir. During later stages, models show the development of an aseismic area surrounded by an expanding ring of high seismicity rates at the edge of the cooling zone. This ring can be related to the “passage” of the cooling front. We show the seismic response of the fault, in terms of the timing of elevated seismicity and seismic moment release, depends on the fracture density, as it affects the temperature decrease in the rock volume and thermo-elastic stress change on the fault. The dense fracture network results in a steeper thermal front which promotes stress arching, and leads to locally and temporarily high Coulomb stressing and seismicity rates. We derive frequency-magnitude distributions and seismic moment release for a low-stress subsurface and a tectonically active area with initially critically stressed faults. The evolution of seismicity in the low-stress environment depends on the dimensions of the fault area that is perturbed by the stress changes. The probability of larger earthquakes and the associated seismic risk are thus reduced in low-stress environments. For both stress environments, the total seismic moment release is largest for the densely spaced fracture network. Also, it occurs at an earlier stage of the injection period: the release is more gradually spread in time and space for the widely spaced fracture network.

The coastal plains of the Netherlands are subject to anthropogenic and natural subsidence with rates which are an order of magnitude higher than sea-level rise. Because one-third of the Netherlands lies below mean sea level, subsidence may threaten the country’s subsistence with major socio-economic consequences. Subsidence is a normal natural process but is overprinted and accelerated by anthropogenic activities which drives deep or shallow processes. Deep processes are caused by the extraction of hydrocarbons and salt, whereas shallow processes are primarily caused by lowering of phreatic groundwater levels. At present, the relative contribution of each process to total subsidence (i.e. natural plus anthropogenic) is unclear. Such information is important for stakeholders to support decision making on subsidence mitigation and it should be substantiated with independent scientific studies.We present the outline of a hybrid Artificial Intelligence (AI) big data and model workflow to disentangle different subsidence forcing and we report on preliminary results for an area covering a gas field in the peat-rich Friesland coastal plain. The proposed workflow is a hybrid approach between a knowledge-based physical model and machine-learning techniques. The big input data comprises a suite of static (structural model) and time-dependent subsurface data (phreatic groundwater level, reservoir pressure), and geodetic measurements. Geomechanical models provide the connection between the drivers (groundwater levels and reservoir pressures) and the surface movement.

The Netherlands is subject to anthropogenically induced deep-source and shallow subsidence. Deep sources are related to the extraction of hydrocarbons or salt mining activities, whereas shallow subsidence comprise compaction, shrinkage and oxidation of clay and peat under progressive lowering groundwater levels. At TNO – Geological Survey of the Netherlands, deep-source and shallow subsidence are presently investigated separately. Forward and inverse modelling techniques are generally deployed to forecast subsidence caused by deep sources, whereas shallow subsidence is predicted using the high-resolution geological 3-D subsurface model GeoTOP. A new approach is proposed which encompasses forward and inverse modelling techniques and GeoTOP. Such combination will yield a powerful shallow subsidence forecasting model, which would be a critical step forward in analyzing shallow subsidence in the Netherlands on a regional scale. In the present contribution, we sketch the setup of this new approach that combines subsidence measurements, GeoTOP subsurface data, and data assimilation of subsidence with the help of state-of-the-art forward and inverse modelling techniques. The setup uses ensemble technology to catch uncertainties of parameters, different model choices, and implicit correlations. With such a setup, forecasts can be faithfully accompanied with a quality measure that enables to judge its relevance and confidence range.

A coupled THMC (thermal-hydrological-mechanical-chemical) model is developed and applied to explore the potential feasibility of using scCO 2 (supercritical carbon dioxide) as a working fluid in geothermal reservoirs . This is achieved by examining the evolution of the kinetics of mineral precipitation-dissolution and its associated impact on the evolution of the rock permeability and porosity. The pH of the reservoir rapidly reduces from 7 to ∼4.5–5 due to the fast dissolution of calcite . Chemical reactions and mineral dissolution and precipitation near the injector are suppressed by the plug-flow penetration of anhydrous scCO 2 displacing the original pore fluid . A conceptual three-zone model is proposed to illustrate the kinetic process of feldspar dissolution and precipitation depending on timing. The initial high concentration of K + prompts feldspar to precipitate in the first stage by consuming K+ until 1y , Feldspar were dissolved into precipitations of illite , smectite , and siderite at 1-6 y , with albite , muscovite and kaolinite mostly precipitated in the last stage 6–10 y. The precipitations of secondary clay minerals and quartz serve to maintain the integrity of caprock sealing. Continuous scCO2 injection under fully coupled THMC model shows a 1.4-times enhancement of fracture permeability and 1.2-times enhancement of matrix permeability dominated by chemical dissolution and thermal unloading process. The pronounced thermal drawdown is the principal factor in enhancing permeability and porosity near injection well. Furthermore, the expansive capability of CO 2 provides extra benefits in enhancing formation pressure to ensure consistent high flow rates, while achieving a higher thermal energy extraction efficiency and preventing scaling issues in wellbore. The mass concentration of scCO2 in the production well increased to 0.82 after 1.2 × 10 8s also leads to the enhancement of fluid enthalpy up to 6.5 × 10 5 J/kg, due to the high heat capacity of scCO2. The injected CO2 are sequestered at ∼2 × 107 kg at t = 2 × 108s (6.34y) as the solubility trapping mechanism.

Enhanced Geothermal Systems (EGS) involve hydraulic stimulation of the permeability of deep low-permeable rock formations. This causes the reactivation and opening of pre-existing natural fracture networks and the formation of new fractures. During hydraulic stimulation, injection pressures at the bottom of the injection well can reach overpressures of up to several tens of MPa. The associated rise in reservoir pressures may trigger felt induced seismicity, as large-scale critically stressed fault structures can be reactivated. We here employ a 3D hydro-mechanical model coupling the software codes of TOUGHREACT and FLAC3D and combine it with Dieterich’s formulation for the rate of earthquake nucleation, to create a conceptual model to simulate the effect of stimulation activities on fault Coulomb stressing and associated induced seismicity rates. We discuss the effect of the hydromechanical properties such as fault and damage zone transmissivity and elastic properties on the relative contribution of pore pressure diffusion versus poroelasticity to fault loading. Our modelling approach shows that poroelastic effects can significantly contribute to fault loading, specifically in cases of low fault transmissivity. In this context, we discuss the potential contribution of poroelasticity to the occurrence of seismicity on a previously unmapped sealing fault associated to hydraulic stimulation at the Pohang EGS site in the Southeast of Korea. Our study demonstrates that a quantitative understanding of the stress response and induced seismicity upon injection operations such as the hydraulic stimulation at Pohang requires the incorporation of both pore pressure diffusion and poroelastic effects.

Acoustoelasticity measurements in a sample of room dry Berea sandstone are conducted at various loading frequencies to explore the transition between the quasi‐static ( ) and dynamic (few kilohertz) nonlinear elastic response. We carry out these measurements at multiple confining pressures and perform a multivariate regression analysis to quantify the dependence of the harmonic content on strain amplitude, frequency, and pressure. The modulus softening (equivalent to the harmonic at 0f) increases by a factor 2–3 over 3 orders of magnitude increase in frequency. Harmonics at 2f, 4f, and 6f exhibit similar behaviors. In contrast, the harmonic at 1f appears frequency independent. This result corroborates previous studies showing that the nonlinear elasticity of rocks can be described with a minimum of two physical mechanisms. This study provides quantitative data that describes the rate dependency of nonlinear elasticity. These findings can be used to improve theories relating the macroscopic elastic response to microstructural features.

Mitigation of induced seismicity associated with large-scale energy production from subsurface resources requires models beyond the current state of the art. In countries with long histories of conventional gas production, such as the Netherlands, issues are mainly associated with depletion-induced seismicity caused by fault reactivation due to differential compaction of reservoir compartments that are juxtaposed at faults. Geomechanical modeling approaches can be extended and integrated to better assess the occurrence of depletion-induced seismicity and provide a basis to modify operations with the aim of mitigating seismicity. Static models of fault stability and reactivation can be used to analyze the location and likelihood of fault reactivation during injection or depletion operations. Dynamic models of fault rupture and seismic wave propagation can be used to analyze the characteristics of induced seismicity from seismic source to expressions (motions) at surface or potential sensor locations. Ensemble-based optimization workflows can be used to maximize production under constraints imposed by thresholds for induced seismicity. The combination of these modeling approaches can be used to determine fault sections that are most prone to induce seismicity, analyze the fault rupture process and associated characteristics of induced seismicity, and optimize well production strategies for minimizing induced seismicity. Integrated geomechanical modeling of depletion-induced seismicity and production optimization workflows can lead to gas production strategies that maximize total gas production while minimizing seismic risk. © 2018 by The Society of Exploration Geophysicists.

Principal slip surfaces in faults have measurable roughness generated during slip. The roughness both records previous events and poses the boundary conditions for future rupture. Digital, high-precision roughness data are now available at the field scale (tens of centimeters to tens of meters) for at least 22 faults, and at the laboratory scale (millimeters to tens of centimeters) for a subset of these. We quantify the slip surface roughness by measuring the aspect ratio, which is the average asperity height divided by the profile length. Higher aspect ratios indicate rougher surfaces. From the field studies, two major trends have emerged: (1) fault surface roughness lies in a restricted range with aspect ratios in the slip-parallel direction of 0.07%–0.5% for profiles of 1 m length, and (2) fault surfaces are rougher at small scales than large ones. These features can both be interpreted as fingerprints of scale-dependent strength, which sets a limit to the aspect ratio of the surface. The measurements imply that shear strength scales with the observation scale, L , as L –0.4 . The new understanding of the physical controls on roughness allows generalization of the extant measurements of a wide array of faults.

Strong feedbacks link temperature (T), hydrologic flow (H), mechanical deformation (M), and chemical alteration (C) in fractured rock. These processes are interconnected as one process affects the initiation and progress of another. Dissolution and precipitation of minerals are affected by temperature and stress, and can result in significant changes in permeability and solute transport characteristics. Understanding these couplings is important for oil, gas, and geothermal reservoir engineering, for CO2 sequestration, and for waste disposal in underground repositories and reservoirs. To experimentally investigate the interactions between THMC processes in a naturally stressed fracture, we report on heated (25°C up to 150°C) flow-through experiments on fractured core samples of Westerly granite. These experiments examine the influence of thermally and mechanically activated dissolution of minerals on the mechanical (stress/strain) and transport (permeability) responses of fractures. The evolutions of the permeability and relative hydraulic aperture of the fracture are recorded as thermal and stress conditions’ change during the experiments. Furthermore, the efflux of dissolved mineral mass is measured periodically and provides a record of the net mass removal, which is correlated with observed changes in relative hydraulic fracture aperture. During the experiments, a significant variation of the effluent fluid chemistry is observed and the fracture shows large changes in permeability to the changing conditions both in stress and in temperature. We argue that at low temperature and high stresses, mechanical crushing of the asperities and the production of gouge explain the permeability decrease although most of the permeability is recoverable as the stress is released. While at high temperature, the permeability changes are governed by mechanical deformation as well as chemical processes, in particular, we infer dissolution of minerals adjacent to the fracture and precipitation of kaolinite.

Flow rate dictates permeability enhancement during fluid pressure oscillations in laboratory experiments Thibault Candela 1,2 , Emily E. Brodsky 2 , Chris Marone 1 , Derek Elsworth 1 1.Dept Geosciences, Penn State Univ, University Park, PA, United States. 2.Dept Earth Sciences, UC Santa Cruz, Santa Cruz, CA, United States.

Fault slip surfaces and earthquake surface ruptures are not flat but show corrugations at all scales, also called roughness. These corrugations control the distribution of stress and strength on the fault plane and around. Here, we review the scaling properties of faults and earthquake slip roughness. Interestingly, both fault and earthquake datasets show power‐law geometrical properties with similar scaling exponents, which indicate the existence of long‐range spatial correlations. Several mechanisms are identified that control fault roughness, such as wear during slip and growth of a fault, by linking shear fracture segments. Scaling exponents are also found in numerical models where a rupture propagates into an elastic medium that contains mechanical heterogeneities: the roughness scaling emerges from the long‐range elastic interactions and the pinning of the slip front by heterogeneities. Finally, several questions remain open on the effect of roughness on fault slip and could be the topic of future studies: (i) Is aseismic slip controlled by roughness? (ii) Is small scale‐roughness (below millimeter scale) different from larger scale roughness and how does it record processes occurring during dynamic rupture and/or during fault healing? (iii) How do plastic and elastic processes interact to modify roughness and faulting?

One way to improve our understanding of the reservoir behavior at depth is to analyze the ground surface response during reservoir exploitation. During reservoir exploitation, pressure changes can lead to compaction/dilation at depth resulting in ground surface displacements. Disentangling the information encrypted in the surface displacement data should therefore provide information about the reservoir behavior at depth. In the last decade we developed a workflow imbricating fast geomechanical forward models and a surface displacement data assimilation scheme to improve our (i) understanding of the subsurface processes and (ii) improve predictions of reservoir behavior in terms of reservoir management. Here we present two case studies to illustrate the versatility and robustness of the workflow. The first case study identified undepleted gas compartments in a strongly faulted and compartmentalized reservoir in a Dutch gas field through inversion of combined surface levelling and InSAR data. The second case study constrained spatial variation in aquifer activity around another Dutch gas field by directly employing ascending and descending PS-InSAR line-of-sight data. We also show the benefit of using multiple data assimilation for constraining parameters in forward models that contain non-linearity. Additionally, we present a novel approach for an inversion workflow that includes uncertainties in every step of the inversion process in a single integrated inversion procedure.

[1] Many faults and fractures in various natural and man-made materials share a remarkable common fractal property in their morphology. We report on the roughness of faults in rocks by analyzing the out-of-plane fluctuations of slip surfaces. They display a statistical power-law relationship with a nearly constant fractal exponent from millimeter scale micro-fractures in fault zones to coastlines measuring thousands of kilometers that have recorded continental breakup. A possible origin of this striking fractal relationship over 11 orders of magnitude of length scales is that all faulting processes in rocks share common characteristics that play a crucial role in the shaping of fault surfaces, such as the effects of elastic long-range stress interactions and stress screening by mechanical heterogeneities during quasi-static fracture growth.

The dynamics and geometry of slip along a fault govern the distribution of heterogeneities and, inturn, these heterogeneities organize slip. Some of these heterogeneities are morphological and are fossilized in the topography of the fault plane, i.e., its roughness. In the present study, our goal is to gain a better understanding of the processes involved during the creation and destruction of fault roughness. Our analysis is focused on the Dixie Valley (Nevada) normal fault, which has a particular outcrop that offers the opportunity to characterize the relationship between fault surface geometry and fault zone internal architecture. The fault morphology was measured in the field using a Light Detection And Ranging (LiDAR) apparatus. The data indicate that the fault surface topography is self-affine and characterized by two power law exponents, one parallel ( H / / = 0.6 ) and one perpendicular ( H ⊥ = 0.8 ) to the slip direction. Accordingly, self-affinity implies that the fault surface appears smoother as the spatial scale increases. More precisely, this self-affine property indicates that the standard deviation of the roughness amplitude scales as L 0.6 where L is the distance along the fault in the slip direction. We propose that fault roughness generation is dominated by damage processes that leave an imprint on fault geometry in the form of elongated lenses. Indeed, the fault zone displays a network of anastomosed discrete slip surfaces that bound bumpy lenses of damaged rock elongated in the direction of slip and that give the fault a rough topography. Such lenses are also observed in many other faults worldwide. Symmetry axis measurements of the lenses found in the Dixie Valley fault reveal that their geometry is scale-dependent. The maximum thickness T of the lenses scales with their length l, measured in the slip direction, as T ∝ l 0.6 . Based on previous experimental studies on the fault growth process, we suggest that the multi-scale aggregation of lenses explains why the standard deviation of the roughness amplitude of the fault surface scales as L 0.6 . We propose that elastic interactions related to the linkage of many discrete slip surfaces, controlling the generation of multi-scale bumpy lenses, are one of the physical processes at the origin of fault roughness.

Assessing the healing rate of a fault is relevant to the knowledge of the seismic machinery. However, measuring fault healing at the depths where it occurs still remains inaccessible. We have designed an analog laboratory experiment of a simulated rough fault that undergoes healing and investigate the relative roles of interface chemical reactivity and sliding velocity on the healing rate. Slide-hold-slide experiments are conducted on a bare interface with various materials in contact (glass/glass, salt/glass, and salt/salt) with or without the presence of a reactive fluid and the slider-surface pull-off force is measured. Our results show that the interface strengthens with hold time, whatever the conditions of the experiments. In addition, we quantify the effect of chemical reactivity on the healing rate. Considering the glass/glass case as a reference, we show that the healing rate is increased by a factor of 2 for the salt/glass case; by a factor of 3 for the salt/salt case; and by about a factor of 20 when saturated brine is added on a salt/salt interface. We also measure that the sliding velocity affects the healing rate for salt/salt interfaces at room humidity. A careful optical monitoring of the interface allows a direct observation of the contact growth characteristics associated to each type of materials. Finally, the large differences of healing rate are interpreted through a mechanistic approach, where the various experimental conditions allow separating different healing mechanisms: increase of adhesion of the contacts by welding, contact growth due to creep or due to neck growth driven by surface tension.

Flash heating and weakening could operate during earthquake nucleation and propagation. We performed 27 friction experiments in a compression-torsion apparatus on ring-shaped limestone samples at sub-seismic to seismic slip rates ≤340 mm/s, centimetric displacements and normal stresses of ≤8 MPa. Friction decreases dramatically at slip rates of 50–150 mm/s. Flash weakening was contemporaneous with a peripheral temperature rise of ∼90 °C measured with an infrared camera. The peripheral temperature yields a lower limit to the slipping zone temperature. The decrease in friction may result from weakening of the asperity contacts due to decarbonation of calcite induced by (1) flash heating or (2) mechanically-activated reactions. However, X-Ray powder diffraction and Raman Spectroscopy analyses do not reveal the presence of decarbonation products in the slipping zone. Instead, White Light Interferometry and Field Emission Scanning Electron Microscope observations reveal the presence of a smooth sliding surface made of nanometric-particles. The mechanical data can be fit by the rate- and state-dependent friction model or by a quadratic model (the latter proposed for powder lubrication). We conclude that flash heating and weakening and powder lubrication may operate together to decrease dynamic friction in limestone in experiments and, for the conditions investigated here, in nature.

We propose that a controlling parameter of static stress drop during an earthquake is related to the scaling properties of the fault-surface topography. Using high resolution laser distance meters, we have accurately measured the roughness scaling properties of two fault surfaces in different geological settings (the French Alps and Nevada). The data show that fault-surface topography is scale dependent and may be accurately described by a self-affine geometry with a slight anisotropy characterized by two extreme roughness exponents ( H R ), H ||=0.6 in the direction of slip and H ⊥=0.8 perpendicular to slip. Disregarding plastic processes like rock fragmentation and focusing on elastic deformation of the topography, which is the dominant mode at large scales, the stress drop is proportional to the deformation, which is a spatial derivative of the slip. The evolution of stress-drop fluctuations on the fault plane can be derived directly from the self-affine property of the fault surface, with the length scale ( λ ) as std Δσ ( λ )∝ λ H R -1. Assuming no characteristic length scale in fault roughness and a rupture cascade model, we show that as the rupture grows, the average stress drop, and its variability should decrease with increasing source dimension. That is for the average stress drop Δσ ( r )∝ r H R -1, where r is the radius of a circular rupture. This result is a direct consequence of the elastic squeeze of fault asperities that induces the largest spatial fluctuations of the shear strength before and after the earthquake at local (small) scales with peculiar spatial correlations.

SUMMARY We present analysis of the spatial correlations of seismological slip maps and fault topography roughness, illuminating their identical self-affine exponent. Though the complexity of the coseismic spatial slip distribution can be intuitively associated with geometrical or stress heterogeneities along the fault surface, this has never been demonstrated. Based on new measurements of fault surface topography and on statistical analyses of kinematic inversions of slip maps, we propose a model, which quantitatively characterizes the link between slip distribution and fault surface roughness. Our approach can be divided into two complementary steps: (i) Using a numerical computation, we estimate the influence of fault roughness on the frictional strength (pre-stress). We model a fault as a rough interface where elastic asperities are squeezed. The Hurst exponent Hτ, characterizing the self-affinity of the frictional strength field, approaches Hτ = H// −1, where H// is the roughness exponent of the fault surface in the direction of slip. (ii) Using a quasi-static model of fault propagation, which includes the effect of long-range elastic interactions and spatial correlations in the frictional strength, the spatial slip correlation is observed to scale as Hs = Hτ + 1, where Hs represents the Hurst exponent of the slip distribution. Under the assumption that the origin of the spatial fluctuations in frictional strength along faults is the elastic squeeze of fault asperities, we show that self-affine geometrical properties of fault surface roughness control slip correlations and that Hs = H//. Given that H// = 0.6 for a wide range of faults (various accumulated displacement, host rock and slip movement), we predict that Hs = 0.6. Even if our quasi-static fault model is more relevant for creeping faults, the spatial slip correlations observed are consistent with those of seismological slip maps. A consequence is that the self-affinity property of slip roughness may be explained by fault geometry without considering dynamical effects produced during an earthquake.

Recent observations suggesting the influence of creep on earthquakes nucleation and arrest are strong incentives to investigate the physical mechanisms controlling how active faults slip. We focus here on deriving generic characteristics of shallow creep along the Haiyuan fault, a major strike‐slip fault in China, by investigating the relationship between fault slip and geometry. We use optical images and time series of Synthetic Aperture Radar data to map the surface fault trace and the spatiotemporal distribution of surface slip along the creeping section of the Haiyuan fault. The fault trace roughness shows a power‐law behavior similar to that of the aseismic slip distribution, with a 0.8 roughness exponent, typical of a self‐affine regime. One possible interpretation is that fault geometry controls to some extent the distribution of aseismic slip, as it has been shown previously for coseismic slip along active faults. Creep is characterized by local fluctuations in rates that we define as creep bursts. The potency of creep bursts follows a power‐law behavior similar to the Gutenberg–Richter earthquake distribution, whereas the distribution of bursts velocity is non‐Gaussian, suggesting an avalanche‐like behavior of these slip events. Such similarities with earthquakes and lab experiments lead us to interpret the rich dynamics of creep bursts observed along the Haiyuan fault as resulting from long‐range elastic interactions within the heterogeneous Earth’s crust.

Earthquakes occur along faults in response to plate tectonic movements, but paradoxically, are not widely recognized in the geological record, severely limiting our knowledge of earthquake physics and hampering accurate assessments of seismic hazard. Light-reflective (so-called mirror like) fault surfaces are widely observed geological features, especially in carbonate-bearing rocks of the shallow crust. Here we report on the occurrence of mirror-like fault surfaces cutting dolostone gouges in the Italian Alps. Using friction experiments, we demonstrate that the mirror-like surfaces develop only at seismic slip rates (∼1 m/s) and for applied normal stresses and sliding displacements consistent with those estimated on the natural faults. Under these experimental conditions, the frictional power density dissipated in the samples is comparable to that estimated for natural earthquakes (1–10 MW/m 2 ). Our results indicate that mirror-like surfaces in dolostone gouges are a signature of seismic faulting, and can be used to estimate power dissipation during ancient earthquake ruptures.

In comparison with standard nonlinear ultrasonic methods such as frequency mixing or resonance based measurements that allow one to extract average, bulk variations of modulus and attenuation versus strain level, dynamic acousto-elasticity (DAE) allows to obtain the elastic behavior over the entire dynamic cycle, detailing the full nonlinear behavior under tension and compression, including hysteresis and memory effects. To improve our understanding of these phenomena, this work aims at comparing static and dynamic acousto-elasticity to evaluate the influence of strain rate. To this purpose, we perform acousto-elasticity on a sample of Berea sandstone and a glass beads pack, oscillating them from 0.001 to 10 Hz. These results are then compared to DAE measurements made in the kHz range. We observe that the average decrease in modulus increases with frequency, meaning that conditioning effects are higher at high strain rate, when relaxation characteristic time is higher than the oscillation period. This result, together with previous quasi-static measurements (Claytor et al., GRL 2009) showing that the hysteretic behavior disappears when the protocol is performed at a very low strain-rate, confirms that a rate dependent nonlinear elastic model has to considered for a more complete description (Gusev et al., PRB 2004).

[1] We report on the topographic roughness measurements of five exhumed faults and thirteen surface earthquake ruptures over a large range of scales: from 50 μm to 50 km. We used three scanner devices (LiDAR, laser profilometer, white light interferometer), spanning complementary scale ranges from 50 μm to 10 m, to measure the 3-D topography of the same objects, i.e., five exhumed slip surfaces (Vuache-Sillingy, Bolu, Corona Heights, Dixie Valley, Magnola). A consistent geometrical property, i.e., self-affinity, emerges as the morphology of the slip surfaces shows at first order, a linear behavior on a log-log plot where axes are fault roughness and spatial length scale, covering five decades of length-scales. The observed fault roughness is scale dependent, with an anisotropic self-affine behavior described by four parameters: two power law exponents H, constant among all the faults studied but slightly anisotropic (H∥ = 0.58 ± 0.07 in the slip direction and H⊥ = 0.81 ± 0.04 perpendicular to it), and two pre-factors showing variability over the faults studied. For larger scales between 200 m and 50 km, we have analyzed the 2-D roughness of the surface rupture of thirteen major continental earthquakes. These ruptures show geometrical properties consistent with the slip-perpendicular behavior of the smaller-scale measurements. Our analysis suggests that the inherent non-alignment between the exposed traces and the along or normal slip direction results in sampling the slip-perpendicular geometry. Although a data gap exists between the scanned fault scarps and rupture traces, the measurements are consistent within the error bars with a single geometrical description, i.e., consistent dimensionality, over nine decades of length scales.

International audience; Accurate description of the topography of active fault surfaces represents an important geophysical issue because this topography is strongly related to the stress distribution along fault planes, and therefore to processes implicated in earthquake nucleation, propagation, and arrest. To date, due to technical limitations, studies of natural fault roughness either performed using laboratory or field profilometers, were obtained mainly from 1-D profiles. With the recent development of Light Detection And Ranging (LIDAR) apparatus, it is now possible to measure accurately the 3-D topography of rough surfaces with a comparable resolution in all directions, both at field and laboratory scales. In the present study, we have investigated the scaling properties including possible anisotropy properties of several outcrops of two natural fault surfaces (Vuache strike-slip fault, France, and Magnola normal fault, Italy) in limestones. At the field scale, digital elevation models of the fault roughness were obtained over surfaces of 0.25 m2 to 600 m2 with a height resolution ranging from 0.5 mm to 20 mm. At the laboratory scale, the 3-D geometry was measured on two slip planes, using a laser profilometer with a spatial resolution of 20 lm and a height resolution less than 1 lm. Several signal processing tools exist for analyzing the statistical properties of rough surfaces with self-affine properties. Among them we used six signal processing techniques: (i) the root-mean-squares correlation (RMS), (ii) the maximum-minimum height difference (MM), (iii) the correlation function (COR), (iv) the RMS correlation function (RMS-COR), (v) the Fourier power spectrum (FPS), and (vi) the wavelet power spectrum (WPS). To investigate quantitatively the reliability and accuracy of the different statistical methods, synthetic self-affine surfaces were generated with azimuthal variation of the scaling exponent, similar to that which is observed for natural fault surfaces. The accuracy of the signal processing techniques is assessed in terms of the difference between the ‘‘input'' self-affine exponent used for the synthetic construction and the ‘‘output'' exponent recovered by those different methods. Two kinds of biases have been identified: Artifacts inherent to data acquisition and intrinsic errors of the methods themselves. In the latter case, the statistical results of our parametric study provide a quantitative estimate of the dependence of the accuracy with system size and directional morphological anisotropy. Finally, based on this parametric study, we used the most reliable techniques (RMS-COR, FPS, WPS) to analyze field data. These three methods provide complementary results. The FPS and WPS methods determine a robust characterization of the fault surface roughness in the direction of striations and perpendicular to them. The RMS-COR method allows investigation of the azimuth dependence of the scaling exponent. For both field and laboratory data, the topography perpendicular to the slip direction displays a similar scaling exponent H\ = 0.8. However, our analysis indicates that for the Magnola fault surface the scaling roughness exponent parallel to the mechanical striation is identical at large and small scales H// = 0.6–0.7, whereas for the Vuache fault surface it is characterized by two different self-affine regimes at small and large scales. We interpret this cross-over length scale as a witness of different mechanical processes responsible for the creation of fault topography at different spatial scales.

Article history: It is well-established that seismic waves can increase the permeability in natural systems, yet the mechanism remains poorly understood. We investigate the underlying mechanics by generating well- controlled, repeatable permeability enhancement in laboratory experiments. Pore pressure oscillations, simulating dynamic stresses, were applied to intact and fractured Berea sandstone samples under confining stresses of tens of MPa. Dynamic stressing produces an immediate permeability enhancement ranging from 1 to 60%, which scales with the amplitude of the dynamic strain (7 × 10 − 7 to 7 × 10 − 6 ) followed by a gradual permeability recovery. We investigated the mechanism by: (1) recording deformation of samples both before and after fracturing during the experiment, (2) varying the chemistry of the water and therefore particle mobility, (3) evaluating the dependence of permeability enhancement and recovery on dynamic stress amplitude, and (4) examining micro-scale pore textures of the rock samples before and after experiments. We find that dynamic stressing does not produce permanent deformation in our samples. Water chemistry has a pronounced effect on the sensitivity to dynamic stressing, with the magnitude of permeability enhancement and the rate of permeability recovery varying with ionic strength of the pore fluid. Permeability recovery rates generally correlate with the permeability enhancement sensitivity. Microstructural observations of our samples show clearing of clay particulates from fracture surfaces during the experiment. From these four lines of evidence, we conclude that a flow-dependent mechanism associated with mobilization of fines controls both the magnitude of the permeability enhancement and the recovery rate in our experiments. We also find that permeability sensitivity to dynamic stressing increases after fracturing, which is a process that generates abundant particulate matter in situ. Our results suggest that fluid permeability in many areas of the Earth's crust, particularly where pore fluids favor particle mobilization, should be sensitive to dynamic stressing.

The sustainability of geothermal fields is based on a paradox. On one side, fractures are targeted on heat-flow improvement, and on the other side, the same fractures are avoided because of induced seismicity risk. In this context, we developed analytical approaches for estimating (1) thermo–poro-elastic stresses in a fractured geothermal system, and (2) seismicity rates based on the model of Dieterich (J Geophys Res 99:2601–2618, 1994). We modeled cold water injected at a constant rate into a single fracture surrounded by hot impermeable layers. The rationale for focusing on one single isolated fracture was that flow in the vicinity of injection wells is often concentrated in a couple of fractures instead of being homogeneously distributed. Heat flow appeared to be dominated by advection inside the fracture and conduction outside it. Poro- and thermo-elastic stresses around the single fracture were estimated separately following two independent analytical approaches; and for any potential fault around the single fracture, the induced Coulomb stress rates were resolved. The role of thermal stresses appeared to be the leading one. We show that thermal-stressing rates can induce an increase in the rate of seismicity of more than a 1000-fold at distances up to 200 m from the single fracture. Our fast forward models are suitable for data assimilation and they open the route for heat-flow optimization while keeping seismicity at a relatively low magnitude.

International audience; High resolution topography measurements of the Vuache-Sillingy fault (Alps, France) reveal a characteristic roughness of the fault zone. We investigate the effect of roughness on the rheology of a planar shear configuration by using a model system consisting of a visco-elastic layer embedded into a rigid solid. The model is discussed in the context of several geological cases: a damage fault zone, a fault smeared with a clay layer, and a shear zone with strain weakening. Using both analytical approaches and finite element simulations, we calculate to linear order the relation between wall roughness and the viscous dissipation in the fault zone as well as the average shear rate.