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One of the concerns of underground CO2 onshore storage is the triggering of induced seismicity and fault reactivation by the pore pressure increasing. Hence, a comprehensive analysis of the tectonic parameters involved in the storage rock formation is mandatory for safety management operations. Unquestionably, active faults and seal faults depicting the storage bulk are relevant parameters to be considered. However, there is a lack of analysis of the active tectonic strain field affecting these faults during the CO2 storage monitoring. The advantage of reconstructing the tectonic field is the possibility to determine the strain trajectories and describing the fault patterns affecting the reservoir rock. In this work, we adapt a methodology of systematic geostructural analysis to underground CO2 storage, based on the calculation of the strain field from kinematics indicators on the fault planes (ey and ex for the maximum and minimum horizontal shortening, respectively). This methodology is based on a statistical analysis of individual strain tensor solutions obtained from fresh outcrops from the Triassic to the Miocene. Consequently, we have collected 447 fault data in 32 field stations located within a 20 km radius. The understanding of the fault sets' role for underground fluid circulation can also be established, helping further analysis of CO2 leakage and seepage. We have applied this methodology to Hontomín onshore CO2 storage facilities (central Spain). The geology of the area and the number of high-quality outcrops made this site a good candidate for studying the strain field from kinematics fault analysis. The results indicate a strike-slip tectonic regime with maximum horizontal shortening with a 160 and 50 ∘ E trend for the local regime, which activates NE–SW strike-slip faults. A regional extensional tectonic field was also recognized with a N–S trend, which activates N–S extensional faults, and NNE–SSW and NNW–SSE strike-slip faults, measured in the Cretaceous limestone on top of the Hontomín facilities. Monitoring these faults within the reservoir is suggested in addition to the possibility of obtaining a focal mechanism solutions for micro-earthquakes (M<3). [ABSTRACT FROM AUTHOR]

One of the concerns of underground CO2 onshore storage is the triggering of Induced Seismicity and fault reactivation. Hence, a comprehensive analysis of the tectonic parameters involved in the storage rock formation is mandatory for safety management operations. Unquestionably, active faults and seal faults depicting the storage bulk are relevant parameters to be considered. However, there is a lack of analysis of the active tectonic strain field affecting these faults during the CO2 storage monitoring. The advantage of reconstructing the tectonic field is the possibility to determine the strain trajectories and describing the fault patterns affecting the reservoir rock. In this work, we adapt a methodology of systematic geostructural analysis to the underground CO2 storage, based on the calculation of the strain field and defined by the strain field from kinematics indicators on the fault planes (ey and ex for the maximum and minimum horizontal shortening respectively). This methodology is based on statistical analysis of individual strain tensor solutions obtained from fresh outcrops. Consequently, we have collected 447 fault data in 32 field stations located within a 20 km radius. The understanding of the fault sets role for underground fluid circulation can also be established, helping for further analysis about CO2 leakage and seepage. We have applied this methodology to Hontomín onshore CO2 storage facilities (Central Spain). The geology of the area and the number of high-quality outcrops, made this site as a good candidate for studying the strain field from kinematics fault analysis. The results indicate a strike-slip tectonic regime with the maximum horizontal shortening with N160° E and N50° E trend for local regime, which activates NE-SW strike-slip faults and NE-SW compressional faults. A regional tectonic field was also recognized with N-S trend, which activates E-W compressional faults. Monitoring of E-W faults within the reservoir is suggested in addition with the possibility of obtaining focal mechanism solutions for micro earthquakes (M < 3). [ABSTRACT FROM AUTHOR]

Image correlation techniques have provided new ways to analyse the distribution of deformation in analogue models of tectonics in space and time. Here, we demonstrate, using a new version of our software package (TecPIV), how the correlation of successive time-lapse images of a deforming model allows not only to evaluate the components of the strain-rate tensor at any time in the model but also to calculate the finite displacements and finite strain tensor. We illustrate with synthetic images how the algorithm produces maps of the velocity gradients, small-strain tensor components, incremental or instantaneous principal strains and maximum shear. The incremental displacements can then be summed up with Eulerian or Lagrangian summation, and the components of the 2-D finite strain tensor can be calculated together with the finite principal strain and maximum finite shear. We benchmark the measures of finite displacements using specific synthetic tests for each summation mode. The deformation gradient tensor is calculated from the deformed state and decomposed into the finite rigid-body rotation and left or right finite-stretch tensors, allowing the deformation ellipsoids to be drawn. The finite strain has long been the only quantified measure of strain in analogue models. The presented software package allows producing these finite strain measures while also accessing incremental measures of strain. The more complete characterisation of the deformation of tectonic analogue models will facilitate the comparison with numerical simulations and geological data and help produce conceptual mechanical models. [ABSTRACT FROM AUTHOR]

The Western Alps show persistent seismicity and deformation despite the end of the orogenesis. The first stations of the RENAG permanent GNSS network were thus installed in the French Alps since the end of the 1990s. Moreover, three temporary surveys took place in 1996, 2006 and 2011 in the Briançon area, ~50x50 km² wide, in the South-Western French Alps, where a steady moderate seismic activity along the Briançon seismic arc is observed. The horizontal velocity field computed from these first three surveys showed an east-west extension in the network, compatible in amplitude and in orientation with the seismic deformation of the area. A fourth campaign was led in 2016, yielding 20 years of observation span. The measures may now reach a sufficient accuracy to assess if the extension found within the Briançon network is localized on any particular tectonic feature. Several faults in this area indeed showed recent activity in normal faulting. Assessing the localization of the deformation may lead to better understand the origin and processes of the present-day deformation of the Alpine belt. To address this issue, the present study aimed at increasing the resolution of both the velocity and the strain field over the network, by exploiting the long measurement span and the exceptional redundancy of our campaign measurements : 1) A robust velocity field was computed from the combination of the different campaign and permanent data. A single strain tensor was then derived, revealing 20±2 nanostrain/yr, which is consistent with previous studies, and also more accurate. 2) Two inversion methods were then compared to assess the distribution of the deformation over the area. Despite their coherence on the orientation of the deformation, the size of the network compared to the small amplitude of the signal did not enable us to derive significant values of strain over the width of the grid cells. 3) We then tried to localize the deformation by fitting a model to a GPS velocity profile perpendicular to the faults. Despite the high uncertainties on the GPS velocities, a two fault model seems to be the best fit to the data. The localization of the potential faults being the best constrained parameter in this forward model, we used it as apriori information in a block modeling in order to compute fault slip rates as the velocities at the interface of the blocks. The velocities on the interfaces are found to be strongly dependent on the model parameters but remain below 1 mm/yr. Thanks to the unusual density of the network and long time span of its instrumentation, we were able to increase the accuracy of the deformation pattern observed through the Briançon area. Our results suggest that the extension is localized on two nearby tectonic features, even if the fault parameters retrieved from our models differ slightly from geological and seismotectonics clues. These results encourage us to believe that the next survey will add sufficient signal-to-noise ratio to achieve the data convergence towards the regional tectonic signal. [ABSTRACT FROM AUTHOR]

Mapping the contemporary stress field orientation in the Mediterranean region can provide fundamental insights on the complexity of plate tectonic forces in this region at different depths. Despite increased data availability and methodological improvements, most recent stress field characterization across the entire Mediterranean dates back to 1995. We consistently map stress field orientations and related parameters for the Mediterranean utilizing all focal mechanisms from the World Stress Map database release 2016. Our main goals are (1) to resolve the stress field orientation for this region at unprecedented finer scale, (2) to evaluate the performance of our stress inversion methodology in one of the most tectonically complex regions on Earth, (3) to compare different types of stress and strain observations covering the entire depth range from surface (e.g. GPS data) to mantle (e.g. shear wave splitting in the mantle). The obtained stress field orientations generally capture correctly the main seismotectonic features of each area, including tectonically complex settings such as the Alpine Orogeny or the Ionian Sea. SHMax orientations and stress regimes tend to be uniform with depth within uncertainties, but larger stress heterogeneity is resolved for the upper 1-10 km. Both coseismic elastic strain field from potency tensors and strain field from GPS data are highly consistent with the stress field orientation, indicating that stress and strain maintain a linear relationship in this area and that inelastic strain release is consistent with the elastic stress or strain accumulation. The Italian Peninsula displays the largest discrepancies between these parameters, potentially indicating stress/strain changes with depth, a prominent role of aseismic deformation, and/or a non-linear relation between stress and strain. Increasing discrepancy between stress field orientation and fast shear wave propagation is found from eastern (sub-parallel) to western (sub-perpendicular) Mediterranean, eventually indicating different causes of anisotropy. Implications of these results with respect to seismic hazard will be discussed. [ABSTRACT FROM AUTHOR]

When cracks propagate in nature it is rarely in perfectly isotropic material, and in some cases, anisotropy can make a strong difference in terms of wave propagation or strength. For that purpose, it is crucial to study fracture propagation in anisotropic material and to address questions such as : what will be the influence of anisotropy on deformation mechanisms? On the pattern of deformation? Which parameter could be used to quantify crack propagation?In order to try to answer these questions, we performed a set of experiments, at the microscale, aimed at deforming uniaxially samples of anisotropic shales. A hole (0.9mm) was drilled on cylindrical samples (Φ=8mm, l=16mm) with a flattened surface in order to create artificial stress concentration. Three samples were prepared with different bedding orientations with respect to the loading axis (0°, 45°, 90°). Digital images are then taken around the hole during the experiments using an optical set up with a pixel resolution of 0.55µm and a magnification factor of 10. The full strain tensor is then extracted with Digital Image Correlation (DIC) method. These experiments allowed us to see fracture initiation and propagation with different patterns depending on the bedding orientation. From our experiments we then extracted mechanical parameters such as anisotropic fracture toughness.We also performed some calculations within the elastic domain in order to understand strain patterns by solving the problem of infinite plane under uniaxial loading with different bedding orientations assuming a transversely isotropic material.Our study allows to understand how some complex structures that are encountered both in the lab (discontinuous fractures often seen in shales) and in natural faults (such as anastomosed faults) develop. Our understanding of the mechanisms involved are also supported by other experiments performed under SEM observations. [ABSTRACT FROM AUTHOR]

It is of utmost importance to determine the smaller-scale deformation caused by themovement on a large fault, especially if the fault in question is only visible in, for instance,seismic, and no other method is available to determine the former. Sub-seismic deformationimproves or decreases the permeability of a fault, and it is therefore invaluable to geothermalprospecting, safety issues of radioactive waste depositories and CO2 sequestration, to name afew applications. This is where 3-D kinematic restoration plays a key role, because restoringa structure to its original pre-fault geometry in three dimensions (3-D retro-deformation), canalso allow the illumination of the sub-seismic deformation in the surrounding volume.Nevertheless, care must applied to choose the right algorithm for retro-deformation, use thecorrect kinematic parameters for the modelling and understand the meaning of theresults.We show an example of determining the areal distribution and magnitude of strain during theretro-deformation of faults within 3-D seismic volumes from the Otway passive margin inAustralia and the Alpine Molasse basin in Germany. In both cases, we use the inclined-shearalgorithm —widely used in research and industry, but poorly investigated— to restore theseismic-scale faults. By kinematically restoring the tetrahedral volumes between the seismichorizons, we are able to determine orientation and magnitude of the strain tensor at everynode point defined by tetrahedral volume. The magnitudes of strain during forward andretro-deformation are identical, and therefore directly related to the density of sub-seismicstructures such as fractures. We show graphically the effect of the incline shear angle, as themain parameter, on the strain magnitude and discuss its relevance for such modelling. [ABSTRACT FROM AUTHOR]

The strain rate tensor across the Indian Plate boundary in Himalaya and part of north Burmese arc is calculated from the horizontal velocities of 1252 GPS stations and represented as continuous strain rate map. From the interpolated scalar strain field data, the areas displaying greater than 104 nstrain/yr are marked as 10 seismic source zones. The low b-values (0.53 to 0.73) and related seismo-tectonics indicate compressive nature of these zones. The high strain zones marked in the foothill Himalaya are areas of fault interactions between Himalayan Thrusts of Tertiary age with transverse older Proterozoic reactivated faults (an integral part of basement ridges on under-thrusted rigid Indian plate). Fault interaction is also prominent in EHS area and in Sagaing Fault, where faults from two different tectonic domains are interacting and getting activated by overall clockwise rotational mass movement around plate interface. For calculating seismic vulnerability, we have computed the geodetic moment rates from the measured geodetic strain rates by empirical formula within 15±2 Km seismic volume in these zones, and compared geodetic with seismic moment rate from earthquake catalogue of 100 years. From the moment deficit, the estimated magnitude of earthquake in 10 seismic source zones are calculated which ranges from 6.5 to 7.4. As per our analysis, the Arunachal Himalaya, Sagaing fault, EHS and Himachal Himalaya are posing high seismic threat with occurrence of shallow focus 7.1 Mw earthquake at any point of time [ABSTRACT FROM AUTHOR]

Hydration reactions influence rock density and rheology and have important consequences for geodynamic (serpentinization, eclogitization) and geoengineering processes (expansive cement used to seal boreholes). Volume increases produced in hydration reactions may generate sufficient tensile and shear stress to fracture both the rock undergoing the reaction and the surrounding host rock. We performed in situ dynamic X-ray synchrotron microtomography experiments to investigate reaction-induced fracturing. We used a novel deformation apparatus, the Hades rig, which allows imaging rocks samples at in situ conditions during fluid-rock interactions. We performed three experiments on hydration of periclase at 180 or 190 degrees C, under a confinement of 10 or 80 MPa, a pore fluid pressure of 5 MPa or 75 MPa, and with or without differential stress, respectively. The sample assembly consists of a periclase cylinder inserted into a central hole within a serpentinite cylinder. The reaction from periclase to brucite results in a large volume increase (110%), pushing the periclase/brucite against the serpentinite and ultimately breaking it. Using time-resolved three-dimensional imaging, we quantify the spatial and temporal distribution of the reaction-induced fractures. We perform digital volume correlation (DVC) analysis to obtain the incremental strain tensors throughout the hydration and fracturing process. We assess the distribution of normal tangential stress within the serpentinite using the boundary element method code Fric2D. The numerical model shows that failure initiates on the inside wall for a hydration pressure of 30 MPa, as in the experiments. The DVC results show mixed-mode strain localization. The von Mises strain, indicative of shear, increases by a larger percentage than the contractive or dilative strain components as the reaction-induced fractures grow. The magnitude distribution of von Mises strain follows a power law relationship, indicative of stress interactions during fracturing. This experimental finding sheds insights on the mechanisms of microseismicity measured in areas undergoing active serpentinization and the mechanisms of borehole sealing when using expansive cement. [ABSTRACT FROM AUTHOR]

The moment tensor is a useful representation of seismic sources. It can be used for efficient solution of forward and inverse problems of seismic wave excitation and propagation. The full moment tensor solutions based on seismic waveform inversion e.g. induced by hydraulic fracturing and borehole breakout processes can be used to understand the volumetric failure mechanisms associated with microseismic events. The characteristics of the moment tensor can be linked to the stress state in the reservoir rocks. Yet, the physical interpretation of the seismic moment tensor remains somewhat ambiguous. The moment tensor as a seismic source has been interpreted by introducing a stress or strain "glut" of some inelastic nature which cannot be described by the Hooke's law. Here, we suggest an alternative interpretation of this inelastic strain in the framework of time-independent non-associated plasticity theory. Thus, the previously introduced an inelastic strain "glut" acquires a meaning of plastic strain tensor. We show that this interpretation is practical since: i) it is consistent with continuum mechanics approach, such as applied in numerical geomechanical modeling of fault and fracture growth; 2) captures results of laboratory deformation and acoustic emission experiments; 3) explains the observed non-double-couple component of moment tensor solutions (dilatant plastic failure); 4) gives correct first-order estimates of scalar moment of moment tensor. As a numerical setup, we consider a classical problem of wellbore failure when stress is applied externally. Our analytical model provides first-order insights into the magnitude of scalar moment and scaling relations. Numerical finite element model in 2D plane strain combines both the yield function and flow potential describing the shear and tensile failure regimes. We observe and classify several modes of deformation, obtained in numerical experiments such as localized (shear bands) and distributed failure around wellbore. We systematically explore the effects of boundary conditions, heterogeneity of material parameters and elastic anisotropy. The source type and distribution of seismic events, obtained in our numerical model, agrees with observations of borehole breakouts, tunnel collapse and early growth of fluid-filled cracks such as hydraulic fractures and magmatic dikes. [ABSTRACT FROM AUTHOR]

The Helvetic nappe system of the Western Swiss Alps formed during the Alpine orogeny by the deformation of half-grabens in the European passive margin. This system exhibits both thin- and thick-skinned tectonics. Several studies on the mechanics of fold and thrust belts agree that the rheology of the basement-cover sequence and the lithospheric layering has a major impact on the deformation style of thin- and thick-skinned systems. Previous results from two-dimensional numerical models have shown that the viscosity ratio between the basement and the sedimentary cover, together with the depth of the brittle-ductile transition zone strongly control the deformation behavior. Furthermore, field observations for the Helvetic nappe system indicate that the bulk deformation of many nappes was dominated by ductile creep. Nevertheless, two-dimensional numerical models and cross-sections are not sufficient to describe and investigate the complete deformational behavior during nappe formation because they ignore the effect of lateral stratigraphic variations in three-dimensional (3D) space. Therefore, we employ 3D thermo-mechanical numerical modeling with a viscoplastic rheology to constrain the impact of (I) lateral variation of half-graben depth and (II) temperature gradient on nappe formation during continental collision. The formation of a 3D accretionary wedge is modeled by the application of velocity boundary conditions with a velocity discontinuity, which provides a bulk deformation similar to typical sandbox models of accretionary and orogenic wedge formation. In order to quantify the deformation we compute the 3D finite strain tensor in every point of the model domain. From the 3D finite strain tensor we calculate the strain magnitude and the principal strain orientations (dip direction and angle) which can be compared to geological field data to gain further insight into the deformational behavior. [ABSTRACT FROM AUTHOR]

Chemical compaction is a relevant factor influencing groundwater resources, geological repositories for waste and CO2 sequestration as well as oil and gas reservoirs. In this context sheet silicates are hypothesized to reinforce chemical compaction by dissolution-precipitation creep (DPC) and thereby influence the hydraulic properties as well as the direction of fluid flow. To test these ideas we designed uniaxial compaction experiments in x-ray transparent oedometers, using samples where a layer of a salt/biotite mixture is sandwiched in between two marginal salt layers.Here we present time-resolved (4D) microtomographic data collected with our in-house microCT scanner over 1600 hours of compaction. These data allow us to analyse slow chemical compaction processes on the grain scale, as well as explore the effects of chemical and mechanical feedbacks on the evolving material. The samples' dynamically evolving transport properties can be characterised via pore segmentation and a percolation analysis. Using advanced quantitative volume correlation techniques we determine a locally resolved displacement field, calculate strain tensors on the grain scale and track separated biotite grains during progressive shortening. Our analyses show that biotite grains influence the dynamic evolution of porosity in the sample, clearly causing a reduction of porosity in their vicinity. However, they do not cause strain localization and axial shortening is homogeneously distributed within our sample. This means that porosity reduction is not achieved by pore collapse but by precipitation of salt that has been transported into the biotite layer.Our observations invite a renewed discussion of of the effect of phyllosilicates on DPC, with a particular emphasis on the length scales of the processes involved. We claim that, in our experiments, the diffusive transport processes invoked in classical theoretical models of DPC are superseded by chemo-mechanical feedbacks that arise on longer length scales. These feedbacks drive salt diffusion from the marginal pure salt layers into the central salt-biotite mixture over distances of several hundred μm and several grain diameters. Such a mechanism was postulated by Merino et al. (1983, Contrib Mineral Petrol 82, 360-370). [ABSTRACT FROM AUTHOR]