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Article ; Online: Deformation Behavior and Seismic Characteristics of Sandy Facies Opalinus Clay During Triaxial Deformation Under Dry and Wet Conditions

Schuster, V. / Rybacki, E. / Bonnelye, A. / Dresen, G.

Rock Mechanics and Rock Engineering

2024

Abstract: Unconsolidated, undrained triaxial deformation tests were performed on sandy facies Opalinus Clay at 50 MPa confining pressure to characterize the effect of water and microfabric orientation on the deformation behavior, mechanical properties, and P-wave ... ...

Abstract	Unconsolidated, undrained triaxial deformation tests were performed on sandy facies Opalinus Clay at 50 MPa confining pressure to characterize the effect of water and microfabric orientation on the deformation behavior, mechanical properties, and P-wave velocity evolution. Dry and wet (≈ 8 and > 95% initial water saturation, respectively) samples with 12.6 ± 0.4 vol% porosity were deformed parallel and perpendicular to the bedding direction at a constant strain rate of 5 × 10–6 s−1. Dry samples revealed semi-brittle behavior and exhibited strain localization at failure, while deformation was more ductile at saturated conditions, promoting stable, slow faulting. Peak strength, Young’s modulus, and number of cumulative acoustic emissions decreased significantly for wet samples compared to dry samples; the opposite was observed for Poisson’s ratio. P-wave velocity anisotropy was significantly altered by differential stress, primarily due to the interplay between pore and fracture closure and stress-induced microcrack formation. For samples that were deformed perpendicular to bedding, we observed a reduction and reversal of P-wave velocity anisotropy with increasing differential stress, whereas anisotropy of parallel samples increased. The results suggest that water saturation reduces the pressure at the brittle-ductile transition and that the elastic properties and anisotropy of sandy facies Opalinus Clay can be significantly altered in an anisotropic stress field, e.g., adjacent to fault zones or tunnel excavations. Changes in elastic anisotropy are primarily controlled by the orientation between the pre-existing microfabric and the maximum principal stress direction, stress magnitude, and the degree of water saturation.
Subject code	669
Publishing country	de
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: Injection-induced fault slip and associated seismicity in the lab

Wang, L. / Kwiatek, G. / Bohnhoff, M. / Rybacki, E. / Dresen, G.

Earth and Planetary Science Letters

Insights from source mechanisms, local stress states and fault geometry

2024

Abstract: Probing source mechanisms of natural and induced earthquakes is a powerful tool to unveil associated rupture kinematics. The source processes of failure and slip instability driven by stress loading are affected by fault geometry, but the source ruptures ...

Abstract	Probing source mechanisms of natural and induced earthquakes is a powerful tool to unveil associated rupture kinematics. The source processes of failure and slip instability driven by stress loading are affected by fault geometry, but the source ruptures of injection-induced seismicity in relation to fault structures and local stress states remain poorly understood. We have conducted a series of fault reactivation and slip experiments on sandstone samples containing faults with different surface roughness (smooth saw-cut fault and fractured rough fault). We impose progressive fluid injection to induce fault slip, and simultaneously monitor the associated acoustic emission (AE) activity. Using high-resolution AE recordings, we perform full moment tensor inversion of all located AE sources, and investigate the changes of AE source characteristics associated with induced fault slip and their relation to fault roughness. For the complex and rough fault, we observe significant non-double-couple components of AE sources and a high degree of focal mechanism heterogeneity. The temporal changes of AE mechanisms associated with injection-induced fault slip on the smooth fault reveal increasing proportions of double-couple components and decreasing variability of AE focal mechanisms when approaching the onset of slip events. The observed inconsistency between the nodal planes of AE sources and the macroscopic fault plane orientation is attributed to the development of secondary fracture networks surrounding the principal slip surface. We analyze changes in the magnitude-frequency characteristics and source mechanisms of AEs with fault-normal distance, showing that for the smooth (mature) fault, Gutenberg–Richter b-value of on-fault seismicity is lower and focal mechanisms are less heterogeneous, compared to off-fault seismicity. Our results emphasize the important role of roughness-related changes in local fault geometry and associated stress heterogeneity for source mechanisms and rupture kinematics of injection-induced ...
Subject code	550
Publishing country	de
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: Transient Creep of Quartz and Granulite at High Temperature Under Wet Conditions

Masuti, S. / Muto, J. / Rybacki, E.

Journal of Geophysical Research: Solid Earth

2023

Abstract: Transient creep of crustal rocks is important to explain time-dependent geological processes such as postseismic deformation following a large continental earthquake. While the steady-state creep flow law parameters of quartz and feldspar, major minerals ...

Abstract	Transient creep of crustal rocks is important to explain time-dependent geological processes such as postseismic deformation following a large continental earthquake. While the steady-state creep flow law parameters of quartz and feldspar, major minerals in the upper and lower crust, are well known, the physical mechanism behind transient creep and the corresponding flow law parameters are poorly understood. We quantify the flow law parameters for both quartz and granulite (mixture of plagioclase and pyroxene) under wet conditions with a nonlinear Burgers model using a Markov chain Monte Carlo (MCMC) method. Modeling results yield an activation energy of 70 ± 20 kJ/mol and a stress exponent of 2.0 ± 0.1 for transient creep of quartz aggregates. For granulite/feldspar, we find activation energies of 280 ± 30 and 220 ± 20 kJ/mol and stress exponents of 1.0 ± 0.2 and 0.9 ± 0.1 under mid (1050–1100°C) and high (1125–1150°C), temperature conditions, respectively. The stress exponents and activation energies of transient creep are consistently smaller than those of steady-state creep for both quartz and granulite/feldspar. Combined with results for transient creep of olivine that were previously obtained (Masuti & Barbot, 2021, https://doi.org/10.1186/s40623-021-01543-9), we suggest that the activation energies and stress exponents of transient creep are smaller than those of steady-state creep for volumetrically important silicate minerals of the crust and upper mantle. Extrapolation of the estimated flow law parameters of granulite/feldspar to natural conditions suggests that transient creep may dominate during the postseismic period and lasts longer than previously thought.
Subject code	621
Publishing country	de
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article: Semi‐Brittle Deformation of Carrara Marble: Hardening and Twinning Induced Plasticity

Rybacki, E. / Niu, L. / Evans, B.

Journal of geophysical research. 2021 Dec., v. 126, no. 12

2021

Abstract: Semi‐brittle flow occurs when crystal plasticity and cataclastic mechanisms operate concurrently and may be common at mid‐levels of the Earth's crust. Using a Paterson gas‐deformation apparatus, we performed 67 conventional triaxial deformation ... ...

Abstract	Semi‐brittle flow occurs when crystal plasticity and cataclastic mechanisms operate concurrently and may be common at mid‐levels of the Earth's crust. Using a Paterson gas‐deformation apparatus, we performed 67 conventional triaxial deformation experiments on dry samples of Carrara marble in the temperature range of T = 20–800°C; confining pressures (PC) were 30, 50,100, 200, and 300 MPa, and strain rates (ε˙ $\dot{\varepsilon }$) were 10⁻³, 10⁻⁴, 10⁻⁵, and 10⁻⁶ s⁻¹. Axial strains (ε) were ≲0.12. The measured (differential) stress, Δσ=σ1−PC ${\Delta}\sigma ={\sigma }_{1}-{P}_{\mathrm{C}}$, changes with strain at most applied conditions. At ε = 0.05, both stress and the hardening coefficient (h), that is, the rate of increase of stress with strain, increase as T decreases and PC increases. At T ≲ 400°C, h is quite large and the sensitivity of Δσ ${\Delta}\sigma $ on ε˙ $\dot{\varepsilon }$ is low, while both are sensitive to increasing pressure. In this temperature range, the mechanical behavior of the marble is very similar to that exhibited by high‐strength, high‐ductility, hexagonal metals that deform by processes called twinning induced plasticity (TWIP). Twinning and dislocation motion are abundant in the samples, as are inter‐ and intracrystalline microfractures. The concurrent activation of these deformation mechanisms leads to complex relationships of Δσ ${\Delta}\sigma $ and h with the applied T ‐ PC ‐ ε˙ $\dot{\varepsilon }$ conditions. This behavior suggests that peak stresses for calcite rocks deforming by semi‐brittle processes will occur at PC ‐ T conditions of the middle crust, and that they might be more strongly influenced by total strain rather than by strain rate.
Keywords	calcite ; deformation ; geophysics ; mechanical properties ; plasticity ; research ; temperature
Language	English
Dates of publication	2021-12
Publishing place	John Wiley & Sons, Ltd
Document type	Article
Note	JOURNAL ARTICLE
ISSN	2169-9313
DOI	10.1029/2021JB022573
Database	NAL-Catalogue (AGRICOLA)

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Article ; Online: High-temperature creep of magnetite and ilmenite single crystals

Till, J. / Rybacki, E.

Physics and Chemistry of Minerals

2020

Abstract: We performed deformation experiments on dry natural single crystals of magnetite and ilmenite to determine the rheological behavior of these oxide minerals as a function of temperature, orientation, and oxygen fugacity. Samples were deformed at ... ...

Abstract	We performed deformation experiments on dry natural single crystals of magnetite and ilmenite to determine the rheological behavior of these oxide minerals as a function of temperature, orientation, and oxygen fugacity. Samples were deformed at temperatures of 825–1150 ∘C to axial strains of up to 15–24% under approximately constant stress conditions up to 120 MPa in a dead-load-type creep rig at ambient pressure in a controlled gas atmosphere. Oxygen fugacity ranged from 10−9.4 to 10−4 atm. Ilmenite creep was insensitive to oxygen fugacity, while magnetite displayed a strong, non-monotonic oxygen fugacity dependence, with creep rates varying as f−0.7O2 and f0.4O2 at more reducing and more oxidizing conditions, respectively. Dislocation creep rates of magnetite single crystals were weakly dependent on crystallographic orientation with stress exponents that varied between 2.8 and 4.3 (mean 3.5 ± 0.4). Magnetite compressed parallel to <100>, <110>, and <111> axes exhibited apparent activation energies of 315±5, 345±30, and 290±5 kJ/mol, respectively. We estimated fO2-independent magnetite activation energies of 715 ± 150, 725 ± 145, and 690 ± 150 kJ/mol for <100>, <110>, and <111> orientations, respectively, in the region of negative fO2-dependence. Ilmenite single crystals were compressed parallel, normal, and inclined to the c-axis. Stress exponents of 3.4, 4.3, and 3.9 indicate dislocation creep with activation energies of 420 ± 35, 345 ± 30, and 360 ± 40 kJ/mol, respectively, for these orientations. Mechanical anisotropy in ilmenite is notably higher than in magnetite, as expected from its lower crystal symmetry. Constitutive equations were formulated for ilmenite and magnetite creep.
Subject code	669
Language	English
Publishing country	de
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS)

Herrmann, J. / Schuster, V. / Cheng, C. / Milsch, H. / Rybacki, E.

Geosciences

2022

Abstract: We experimentally determined the hydraulic properties of fractures within various rock types, focusing on a variety of Variscan rocks. Flow-through experiments were performed on slate, graywacke, quartzite, granite, natural fault gouge, and claystone ... ...

Abstract	We experimentally determined the hydraulic properties of fractures within various rock types, focusing on a variety of Variscan rocks. Flow-through experiments were performed on slate, graywacke, quartzite, granite, natural fault gouge, and claystone samples containing an artificial fracture with a given roughness. For slate samples, the hydraulic transmissivity of the fractures was measured at confining pressures, pc, at up to 50 MPa, temperatures, T, between 25 and 100 °C, and differential stress, σ, acting perpendicular to the fracture surface of up to 45 MPa. Fracture transmissivity decreases non-linearly and irreversibly by about an order of magnitude with increasing confining pressure and differential stress, with a slightly stronger influence of pc than of σ. Increasing temperature reduces fracture transmissivity only at high confining pressures when the fracture aperture is already low. An increase in the fracture surface roughness by about three times yields an initial fracture transmissivity of almost one order of magnitude higher. Fractures with similar surface roughness display the highest initial transmissivity within slate, graywacke, quartzite and granite samples, whereas the transmissivity in claystone and granitic gouge material is up to several orders of magnitude lower. The reduction in transmissivity with increasing stress at room temperature varies with composition and uniaxial strength, where the deduction is lowest for rocks with a high fraction of strong minerals and associated high brittleness and strength. Microstructural investigations suggest that the reduction is induced by the compaction of the matrix and crushing of strong asperities. Our results suggest that for a given surface roughness, the fracture transmissivity of slate as an example of a target reservoir for unconventional EGS, is comparable to that of other hard rocks, e.g., granite, whereas highly altered and/or clay-bearing rocks display poor potential for extracting geothermal energy from discrete fractures.
Subject code	550
Language	English
Publishing country	de
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: Semi‐brittle deformation of Carrara marble

Rybacki, E. / Niu, L. / Evan, B.

Journal of Geophysical Research: Solid Earth

Hardening and twinning induced plasticity

2021

Abstract: Semi-brittle flow occurs when crystal plasticity and cataclastic mechanisms operate concurrently and may be common at mid-levels of the Earth’s crust. Using a Paterson gas-deformation apparatus, we performed 67 conventional triaxial deformation ... ...

Abstract	Semi-brittle flow occurs when crystal plasticity and cataclastic mechanisms operate concurrently and may be common at mid-levels of the Earth’s crust. Using a Paterson gas-deformation apparatus, we performed 67 conventional triaxial deformation experiments on dry samples of Carrara marble in the temperature range of T = 20 - 800°C; confining pressures (PC) were 30, 50 100, 200, and 300 MPa, and strain rates (urn:x-wiley:21699313:media:jgrb55357:jgrb55357-math-0001) were 10-3, 10-4, 10-5, and 10-6 s-1. Axial strains (ε ) were ≲ 0.12. The measured (differential) stress, urn:x-wiley:21699313:media:jgrb55357:jgrb55357-math-0002, changes with strain at most applied conditions. At ε = 0.05, both stress and the hardening coefficient (h), i.e., the rate of increase of stress with strain, increase as T decreases and PC increases. At T ≲ 400°C, h is quite large and the sensitivity of urn:x-wiley:21699313:media:jgrb55357:jgrb55357-math-0003 on urn:x-wiley:21699313:media:jgrb55357:jgrb55357-math-0004 is low, while both are sensitive to increasing pressure. In this temperature range, the mechanical behavior of the marble is very similar to that exhibited by high-strength, high-ductility, hexagonal metals that deform by processes called twinning induced plasticity (TWIP). Twinning and dislocation motion are abundant in the samples, as are inter- and intra-crystalline micro-fractures. The concurrent activation of these deformation mechanisms leads to complex relationships of urn:x-wiley:21699313:media:jgrb55357:jgrb55357-math-0005 and h with the applied T- PC - urn:x-wiley:21699313:media:jgrb55357:jgrb55357-math-0006 conditions. This behavior suggests that peak stresses for calcite rocks deforming by semi-brittle processes will occur at PC - T conditions of the middle crust, and that they might be more strongly influenced by total strain rather than by strain rate.
Subject code	669
Publishing country	de
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: Influence of differential stress on the growth of wet enstatite and enstatite-forsterite reaction rims

Rybacki, E. / Helpa, V.

Mineralogy and Petrology

2019

Publishing country	de
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Book ; Article ; Online: Stress strain data of experimentally deformed Carrara marble in the semi-brittle field

Rybacki, E. / Niu, L. / Evans, B.

2021

Abstract: The data set contains stress-strain data of Carrara marble experimentally deformed in triaxial compression at temperatures of 20 – 800°C, confining pressures of 30 – 300 MPa, and strain rates between 10-3 and 10-6 s-1. This range covers conditions, at ... ...

Abstract	The data set contains stress-strain data of Carrara marble experimentally deformed in triaxial compression at temperatures of 20 – 800°C, confining pressures of 30 – 300 MPa, and strain rates between 10-3 and 10-6 s-1. This range covers conditions, at witch marble deforms in the semi-brittle regime, i.e., strength depends on all parameters, but with different sensitivity. Semi-brittle deformation behavior is expected to be important in the mid continental crust. The experiments were conducted in the Experimental Rock Deformation Laboratory of the GFZ German Research Centre for Geosciences in Potsdam, Germany. The data are separated into 91 individual ASCII files, one for each sample. The corresponding temperature, pressure and strain rate conditions are listed in Tab. 1. of the data description and in the associated work by Rybacki et al. (submitted).
Language	English
Publisher	GFZ Data Services
Publishing country	de
Document type	Book ; Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: Authors’ Reply to the Discussion by Crisci et al. on “Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions Mechanical Properties and the Influence of Rock Fabric”

Schuster, V. / Rybacki, E. / Bonnelye, A. / Dresen, G.

Rock Mechanics and Rock Engineering

2021

Publishing country	de
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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