@article{ZieglerReiterHeidbachetal.2015, author = {Ziegler, Moritz O. and Reiter, Karsten and Heidbach, Oliver and Zang, Arno and Kwiatek, Grzegorz and Stromeyer, Dietrich and Dahm, Torsten and Dresen, Georg and Hofmann, Gerhard}, title = {Mining-Induced Stress Transfer and Its Relation to a 1.9 Seismic Event in an Ultra-deep South African Gold Mine}, series = {Pure and applied geophysics}, volume = {172}, journal = {Pure and applied geophysics}, number = {10}, publisher = {Springer}, address = {Basel}, issn = {0033-4553}, doi = {10.1007/s00024-015-1033-x}, pages = {2557 -- 2570}, year = {2015}, abstract = {On 27 December 2007, a 1.9 seismic event occurred within a dyke in the deep-level Mponeng Gold Mine, South Africa. From the seismological network of the mine and the one from the Japanese-German Underground Acoustic Emission Research in South Africa (JAGUARS) group, the hypocentral depth (3,509 m), focal mechanism and aftershock location were estimated. Since no mining activity took place in the days before the event, dynamic triggering due to blasting can be ruled out as the cause. To investigate the hypothesis that stress transfer, due to excavation of the gold reef, induced the event, we set up a small-scale high-resolution three-dimensional (3D) geomechanical numerical model. The model consisted of the four different rock units present in the mine: quartzite (footwall), hard lava (hanging wall), conglomerate (gold reef) and diorite (dykes). The numerical solution was computed using a finite-element method with a discretised mesh of approximately elements. The initial stress state of the model is in agreement with in situ data from a neighbouring mine, and the step-wise excavation was simulated by mass removal from the gold reef. The resulting 3D stress tensor and its changes due to mining were analysed based on the Coulomb failure stress changes on the fault plane of the event. The results show that the seismic event was induced regardless of how the Coulomb failure stress changes were calculated and of the uncertainties in the fault plane solution. We also used the model to assess the seismic hazard due to the excavation towards the dyke. The resulting curve of stress changes shows a significant increase in the last in front of the dyke, indicating that small changes in the mining progress towards the dyke have a substantial impact on the stress transfer.}, language = {en} } @article{ZangStephanssonStenbergetal.2017, author = {Zang, Arno and Stephansson, Ove and Stenberg, Leif and Plenkers, Katrin and von Specht, Sebastian and Milkereit, Claus and Schill, Eva and Kwiatek, Grzegorz and Dresen, Georg and Zimmermann, G{\"u}nter and Dahm, Torsten and Weber, Michael}, title = {Hydraulic fracture monitoring in hard rock at 410 m depth with an advanced fluid-injection protocol and extensive sensor array}, series = {Geophysical journal international}, volume = {208}, journal = {Geophysical journal international}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0956-540X}, pages = {790 -- 813}, year = {2017}, abstract = {In this paper, an underground experiment at the Aspo Hard Rock Laboratory (HRL) is described. Main goal is optimizing geothermal heat exchange in crystalline rock mass at depth by multistage hydraulic fracturing with minimal impact on the environment, that is, seismic events. For this, three arrays with acoustic emission, microseismicity and electromagnetic sensors are installed mapping hydraulic fracture initiation and growth. Fractures are driven by three different water injection schemes (continuous, progressive and pulse pressurization). After a brief review of hydraulic fracture operations in crystalline rock mass at mine scale, the site geology and the stress conditions at Aspo HRL are described. Then, the continuous, single-flow rate and alternative, multiple-flow rate fracture breakdown tests in a horizontal borehole at depth level 410 m are described together with the monitoring networks and sensitivity. Monitoring results include the primary catalogue of acoustic emission hypocentres obtained from four hydraulic fractures with the in situ trigger and localizing network. The continuous versus alternative water injection schemes are discussed in terms of the fracture breakdown pressure, the fracture pattern from impression packer result and the monitoring at the arrays. An example of multistage hydraulic fracturing with several phases of opening and closing of fracture walls is evaluated using data from acoustic emissions, seismic broad-band recordings and electromagnetic signal response. Based on our limited amount of in situ tests (six) and evaluation of three tests in Avro granodiorite, in the multiple-flow rate test with progressively increasing target pressure, the acoustic emission activity starts at a later stage in the fracturing process compared to the conventional fracturing case with continuous water injection. In tendency, also the total number and magnitude of acoustic events are found to be smaller in the progressive treatment with frequent phases of depressurization.}, language = {en} } @article{WangRybackiBonnelyeetal.2021, author = {Wang, Lei and Rybacki, Erik and Bonnelye, Audrey and Bohnhoff, Marco and Dresen, Georg}, title = {Experimental investigation on static and dynamic bulk moduli of dry and fluid-saturated porous sandstones}, series = {Rock mechanics and rock engineering}, volume = {54}, journal = {Rock mechanics and rock engineering}, number = {1}, publisher = {Springer}, address = {Wien}, issn = {0723-2632}, doi = {10.1007/s00603-020-02248-3}, pages = {129 -- 148}, year = {2021}, abstract = {Knowledge of pressure-dependent static and dynamic moduli of porous reservoir rocks is of key importance for evaluating geological setting of a reservoir in geo-energy applications. We examined experimentally the evolution of static and dynamic bulk moduli for porous Bentheim sandstone with increasing confining pressure up to about 190 MPa under dry and water-saturated conditions. The static bulk moduli (K-s) were estimated from stress-volumetric strain curves while dynamic bulk moduli (K-d) were derived from the changes in ultrasonic P- and S- wave velocities (similar to 1 MHz) along different traces, which were monitored simultaneously during the entire deformation. In conjunction with published data of other porous sandstones (Berea, Navajo and Weber sandstones), our results reveal that the ratio between dynamic and static bulk moduli (K-d/K-s) reduces rapidly from about 1.5 - 2.0 at ambient pressure to about 1.1 at high pressure under dry conditions and from about 2.0 - 4.0 to about 1.5 under water-saturated conditions, respectively. We interpret such a pressure-dependent reduction by closure of narrow (compliant) cracks, highlighting thatK(d)/K(s)is positively correlated with the amount of narrow cracks. Above the crack closure pressure, where equant (stiff) pores dominate the void space,K-d/K(s)is almost constant. The enhanced difference between dynamic and static bulk moduli under water saturation compared to dry conditions is possibly caused by high pore pressure that is locally maintained if measured using high-frequency ultrasonic wave velocities. In our experiments, the pressure dependence of dynamic bulk modulus of water-saturated Bentheim sandstone at effective pressures above 5 MPa can be roughly predicted by both the effective medium theory (Mori-Tanaka scheme) and the squirt-flow model. Static bulk moduli are found to be more sensitive to narrow cracks than dynamic bulk moduli for porous sandstones under dry and water-saturated conditions.}, language = {en} } @article{WangKwiatekRybackietal.2020, author = {Wang, Lei and Kwiatek, Grzegorz and Rybacki, Erik and Bonnelye, Audrey and Bohnhoff, Marco and Dresen, Georg}, title = {Laboratory study on fluid-induced fault slip behavior: the role of fluid pressurization rate}, series = {Geophysical research letters : GRL}, volume = {47}, journal = {Geophysical research letters : GRL}, number = {6}, publisher = {Wiley}, address = {Hoboken, NJ}, issn = {0094-8276}, doi = {10.1029/2019GL086627}, pages = {12}, year = {2020}, abstract = {Understanding the physical mechanisms governing fluid-induced fault slip is important for improved mitigation of seismic risks associated with large-scale fluid injection. We conducted fluid-induced fault slip experiments in the laboratory on critically stressed saw-cut sandstone samples with high permeability using different fluid pressurization rates. Our experimental results demonstrate that fault slip behavior is governed by fluid pressurization rate rather than injection pressure. Slow stick-slip episodes (peak slip velocity < 4 mu m/s) are induced by fast fluid injection rate, whereas fault creep with slip velocity < 0.4 mu m/s mainly occurs in response to slow fluid injection rate. Fluid-induced fault slip may remain mechanically stable for loading stiffness larger than fault stiffness. Independent of fault slip mode, we observed dynamic frictional weakening of the artificial fault at elevated pore pressure. Our observations highlight that varying fluid injection rates may assist in reducing potential seismic hazards of field-scale fluid injection projects.
Plain Language Summary Human-induced earthquakes from field-scale fluid injection projects including enhanced geothermal system and deep wastewater injection have been documented worldwide. Although it is clear that fluid pressure plays a crucial role in triggering fault slip, the physical mechanism behind induced seismicity still remains poorly understood. We performed laboratory tests, and here we present two fluid-induced slip experiments conducted on permeable Bentheim sandstone samples crosscut by a fault that is critically stressed. Fault slip is then triggered by pumping the water from the bottom end of the sample at different fluid injection rates. Our results show that fault slip is controlled by fluid pressure increase rate rather than by the absolute magnitude of fluid pressure. In contrast to episodes of relatively rapid but stable sliding events caused by a fast fluid injection rate, fault creep is observed during slow fluid injection. Strong weakening of the dynamic friction coefficient of the experimental fault is observed at elevated pore pressure, independent of fault slip mode. These results may provide a better understanding of the complex behavior of fluid-induced fault slip on the field scale.}, language = {en} } @article{WangKwiatekRybackietal.2020, author = {Wang, Lei and Kwiatek, Grzegorz and Rybacki, Erik and Bohnhoff, Marco and Dresen, Georg}, title = {Injection-induced seismic moment release and laboratory fault slip}, series = {Geophysical research letters}, volume = {47}, journal = {Geophysical research letters}, number = {22}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0094-8276}, doi = {10.1029/2020GL089576}, pages = {11}, year = {2020}, abstract = {Understanding the relation between injection-induced seismic moment release and operational parameters is crucial for early identification of possible seismic hazards associated with fluid-injection projects. We conducted laboratory fluid-injection experiments on permeable sandstone samples containing a critically stressed fault at different fluid pressurization rates. The observed fluid-induced fault deformation is dominantly aseismic. Fluid-induced stick-slip and fault creep reveal that total seismic moment release of acoustic emission (AE) events is related to total injected volume, independent of respective fault slip behavior. Seismic moment release rate of AE scales with measured fault slip velocity. For injection-induced fault slip in a homogeneous pressurized region, released moment shows a linear scaling with injected volume for stable slip (steady slip and fault creep), while we find a cubic relation for dynamic slip. Our results highlight that monitoring evolution of seismic moment release with injected volume in some cases may assist in discriminating between stable slip and unstable runaway ruptures.}, language = {en} } @article{WangDresenRybackietal.2020, author = {Wang, Lei and Dresen, Georg and Rybacki, Erik and Bonnelye, Audrey and Bohnhoff, Marco}, title = {Pressure-dependent bulk compressibility of a porous granular material modeled by improved contact mechanics and micromechanical approaches}, series = {Acta materialia}, volume = {188}, journal = {Acta materialia}, publisher = {Elsevier}, address = {Amsterdam}, issn = {1359-6454}, doi = {10.1016/j.actamat.2020.01.063}, pages = {259 -- 272}, year = {2020}, abstract = {The change of the mechanical properties of granular materials with pressure is an important topic associated with many industrial applications. In this paper we investigate the influence of hydrostatic pressure (P-e) on the effective bulk compressibility (C-eff) of a granular material by applying two modified theoretical approaches that are based on contact mechanics and micromechanics, respectively. For a granular material composed of rough grains, an extended contact model is developed to elucidate the effect of roughness of grain surfaces on bulk compressibility. At relatively low pressures, the model predicts that the decrease of bulk compressibility with pressure may be described by a power law with an exponent of -1/2 (i.e., C-eff proportional to P-e(1/2) ), but deviates at intermediate pressures. At elevated pressures beyond full contact, bulk compressibility remains almost unchanged, which may be roughly evaluated by continuum contact mechanics. As an alternative explanation of pressure-dependent bulk compressibility, we suggest a micromechanical model that accounts for effects of different types of pore space present in granular materials. Narrow and compliant inter-granular cracks are approximated by three-dimensional oblate spheroidal cracks with rough surfaces, whereas the equant and stiff pores surrounded by three and four neighboring grains are modeled as tubular pores with cross sections of three and four cusp-like corners, respectively. In this model, bulk compressibility is strongly reduced with increasing pressure by progressive closure of rough-walled cracks. At pressures exceeding crack closure pressure, deformation of the remaining equant pores is largely insensitive to pressure, with almost no further change in bulk compressibility. To validate these models, we performed hydrostatic compression tests on Bentheim sandstone (a granular rock consisting of quartz with high porosity) under a wide range of pressure. The relation between observed microstructures and measured pressure-dependent bulk compressibility is well explained by both suggested models.}, language = {en} } @article{SchusterRybackiBonnelyeetal.2021, author = {Schuster, Valerian and Rybacki, Erik and Bonnelye, Audrey and Herrmann, Johannes and Schleicher, Anja Maria and Dresen, Georg}, title = {Experimental deformation of opalinus clay at elevated temperature and pressure conditions}, series = {Rock mechanics and rock engineering}, volume = {54}, journal = {Rock mechanics and rock engineering}, publisher = {Springer}, address = {Wien}, issn = {0723-2632}, doi = {10.1007/s00603-021-02474-3}, pages = {4009 -- 4039}, year = {2021}, abstract = {The mechanical behavior of the sandy facies of Opalinus Clay (OPA) was investigated in 42 triaxial tests performed on dry samples at unconsolidated, undrained conditions at confining pressures (p(c)) of 50-100 MPa, temperatures (T) between 25 and 200 degrees C and strain rates (epsilon) (over dot ) of 1 x-10(-3)-5 x-10(-6) -s(-1). Using a Paterson-type deformation apparatus, samples oriented at 0 degrees, 45 degrees and 90 degrees to bedding were deformed up to about 15\% axial strain. Additionally, the influence of water content, drainage condition and pre-consolidation was investigated at fixed p(c)-T conditions, using dry and re-saturated samples. Deformed samples display brittle to semi-brittle deformation behavior, characterized by cataclastic flow in quartz-rich sandy layers and granular flow in phyllosilicate-rich layers. Samples loaded parallel to bedding are less compliant compared to the other loading directions. With the exception of samples deformed 45 degrees and 90 degrees to bedding at p(c) = 100 MPa, strain is localized in discrete shear zones. Compressive strength (sigma(max)) increases with increasing pc, resulting in an internal friction coefficient of approximate to 0.31 for samples deformed at 45 degrees and 90 degrees to bedding, and approximate to 0.44 for samples deformed parallel to bedding. In contrast, pre-consolidation, drainage condition, T and epsilon(over dot )do not significantly affect deformation behavior of dried samples. However, sigma(max) and Young's modulus (E) decrease substantially with increasing water saturation. Compared to the clay-rich shaly facies of OPA, sandy facies specimens display higher strength sigma(max) and Young's modulus E at similar deformation conditions. Strength and Young's modulus of samples deformed 90 degrees and 45 degrees to bedding are close to the iso-stress Reuss bound, suggesting a strong influence of weak clay-rich layers on the deformation behavior.}, language = {en} } @article{SchusterRybackiBonnelyeetal.2021, author = {Schuster, Valerian and Rybacki, Erik and Bonnelye, Audrey and Dresen, Georg}, title = {Authors' Reply to the Discussion by Crisci et al. (2021): Experimental deformation of Opalinus Clay at elevated temperature and pressure conditions}, series = {Rock mechanics and rock engineering}, volume = {55}, journal = {Rock mechanics and rock engineering}, publisher = {Springer}, address = {Wien}, issn = {0723-2632}, doi = {10.1007/s00603-021-02675-w}, pages = {467 -- 469}, year = {2021}, language = {en} } @misc{SchuckSchleicherJanssenetal.2020, author = {Schuck, Bernhard and Schleicher, Anja Maria and Janssen, Christoph and Toy, Virginia G. and Dresen, Georg}, title = {Fault zone architecture of a large plate-bounding strike-slip fault}, series = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {1}, issn = {1866-8372}, doi = {10.25932/publishup-51244}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-512441}, pages = {32}, year = {2020}, abstract = {New Zealand's Alpine Fault is a large, platebounding strike-slip fault, which ruptures in large (M-w > 8) earthquakes. We conducted field and laboratory analyses of fault rocks to assess its fault zone architecture. Results reveal that the Alpine Fault Zone has a complex geometry, comprising an anastomosing network of multiple slip planes that have accommodated different amounts of displacement. This contrasts with the previous perception of the Alpine Fault Zone, which assumes a single principal slip zone accommodated all displacement. This interpretation is supported by results of drilling projects and geophysical investigations. Furthermore, observations presented here show that the young, largely unconsolidated sediments that constitute the footwall at shallow depths have a significant influence on fault gouge rheological properties and structure.}, language = {en} } @article{SchuckSchleicherJanssenetal.2020, author = {Schuck, Bernhard and Schleicher, Anja Maria and Janssen, Christoph and Toy, Virginia G. and Dresen, Georg}, title = {Fault zone architecture of a large plate-bounding strike-slip fault}, series = {Solid Earth}, volume = {11}, journal = {Solid Earth}, number = {1}, publisher = {Copernicus Publications}, address = {G{\"o}ttingen}, issn = {1869-9529}, doi = {10.5194/se-11-95-2020}, pages = {95 -- 124}, year = {2020}, abstract = {New Zealand's Alpine Fault is a large, platebounding strike-slip fault, which ruptures in large (M-w > 8) earthquakes. We conducted field and laboratory analyses of fault rocks to assess its fault zone architecture. Results reveal that the Alpine Fault Zone has a complex geometry, comprising an anastomosing network of multiple slip planes that have accommodated different amounts of displacement. This contrasts with the previous perception of the Alpine Fault Zone, which assumes a single principal slip zone accommodated all displacement. This interpretation is supported by results of drilling projects and geophysical investigations. Furthermore, observations presented here show that the young, largely unconsolidated sediments that constitute the footwall at shallow depths have a significant influence on fault gouge rheological properties and structure.}, language = {en} } @article{SchuckJanssenSchleicheretal.2018, author = {Schuck, Bernhard and Janssen, C. and Schleicher, Anja Maria and Toy, Virginia G. and Dresen, Georg}, title = {Microstructures imply cataclasis and authigenic mineral formation}, series = {Journal of structural geology}, volume = {110}, journal = {Journal of structural geology}, publisher = {Elsevier}, address = {Oxford}, issn = {0191-8141}, doi = {10.1016/j.jsg.2018.03.001}, pages = {172 -- 186}, year = {2018}, abstract = {The Alpine Fault is capable of generating large (MW > 8) earthquakes and is the main geohazard on South Island, NZ, and late in its 250-291-year seismic cycle. To minimize its hazard potential, it is indispensable to identify and understand the processes influencing the geomechanical behavior and strength-evolution of the fault. High-resolution microstructural, mineralogical and geochemical analyses of the Alpine Fault's core demonstrate wall rock fragmentation, assisted by mineral dissolution, and cementation resulting in the formation of a fine-grained principal slip zone (PSZ). A complex network of anastomosing and mutually cross-cutting calcite veins implies that faulting occurred during episodes of dilation, slip and sealing. Fluid-assisted dilatancy leads to a significant volume increase accommodated by vein formation in the fault core. Undeformed euhedral chlorite crystals and calcite veins that have cut footwall gravels demonstrate that these processes occurred very close to the Earth's surface. Microstructural evidence indicates that cataclastic processes dominate the deformation and we suggest that powder lubrication and grain rolling, particularly influenced by abundant nanoparticles, play a key role in the fault core's velocity-weakening behavior rather than frictional sliding. This is further supported by the absence of smectite, which is reasonable given recently measured geothermal gradients of more than 120 °C km-1 and the impermeable nature of the PSZ, which both limit the growth of this phase and restrict its stability to shallow depths. Our observations demonstrate that high-temperature fluids can influence authigenic mineral formation and thus control the fault's geomechanical behavior and the cyclic evolution of its strength.}, language = {en} } @misc{ReigberWassermannScherbaumetal.2002, author = {Reigber, Christoph and Wassermann, Joachim and Scherbaum, Frank and Dresen, Georg and Zang, Arno and Altenberger, Uwe and Rahmstorf, Stefan and Klein, Armin and Oberh{\"a}nsli, Roland and Herzig, Reinhard}, title = {Portal = Vulkane, Erdbeben, Klima: Der Planet Erde birgt viele Geheimnisse}, number = {01-02/2002}, organization = {Universit{\"a}t Potsdam, Referat f{\"u}r Presse- und {\"O}ffentlichkeitsarbeit}, issn = {1618-6893}, doi = {10.25932/publishup-43954}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-439544}, pages = {39}, year = {2002}, abstract = {Aus dem Inhalt: - Vulkane, Erdbeben, Klima: Der Planet Erde birgt viele Geheimnisse - Eine Entdeckungsreise durch die Uni Potsdam auf CD-ROM - Gewitter im Gehirn - Herr der Pflanzen: Wolfgang Pifrement}, language = {de} } @article{NardiniRybackiKrauseetal.2020, author = {Nardini, Livia and Rybacki, Erik and Krause, Michael and Morales, Luiz F.G. and Dresen, Georg}, title = {Control of the geometric arrangement of material heterogeneities on strain localization at the brittle-to-ductile transition in experimentally deformed carbonate rocks}, series = {Journal of Structural Geology}, volume = {135}, journal = {Journal of Structural Geology}, publisher = {Pergamon Press}, address = {Oxford ; Frankfurt, M.}, issn = {0191-8141}, doi = {10.1016/j.jsg.2020.104038}, year = {2020}, abstract = {Triaxial high temperature (900 °C) deformation experiments were conducted at constant strain rate in a Paterson-type deformation apparatus on cylinders of Carrara marble with two right or left stepping, non-overlapping weak inclusions of Solnhofen limestone, oriented at 45° to the cylinders' longitudinal axes. Applying different values of confinement (30, 50, 100 and 300 MPa) we induced various amounts of brittle deformation in the marble matrix and investigated the effect of brittle precursors on the initiation and development of heterogeneity-induced high temperature shear zones. Viscosity contrast between the matrix and the inclusions induces local stress concentration at the tips of these latter. The initial arrangement of the inclusions results in either an overpressured (contractional) or underpressured (extensional) domain in the step-over region of the sample. At low confinement (30 and 50 MPa) abundant brittle deformation is observed, but the spatial distribution of microfractures is dependent on the kinematics of the step-over region: microcracks occur either along the shearing plane between inclusions (in extensional bridge samples), or broadly distributed outside the step-over region (contractional bridge samples). Accordingly, ductile deformation localizes along the inclusions plane in the extensional bridge samples as opposed to distributing over large areas of the matrix in the contractional bridge samples. If microcracking is suppressed (high confinement), strain is accommodated by viscous creep and strain progressively de-localizes in extensional bridge samples. Our experiments demonstrate that brittle precursors enhance the degree of localization in the ductile deformation regime, but only if the interaction of pre-existing heterogeneities induces an extensional mean stress regime in between.}, language = {en} } @article{NardiniRybackiDoehmannetal.2018, author = {Nardini, Livia and Rybacki, Erik and D{\"o}hmann, Maximilian J.E.A. and Morales, Luiz F.G. and Brune, Sascha and Dresen, Georg}, title = {High-temperature shear zone formation in Carrara marble}, series = {Tectonophysics}, volume = {749}, journal = {Tectonophysics}, publisher = {Elsevier}, address = {Amsterdam [u.a.]}, issn = {0040-1951}, doi = {10.1016/j.tecto.2018.10.022}, pages = {120 -- 139}, year = {2018}, abstract = {Rock deformation at depths in the Earth's crust is often localized in high temperature shear zones occurring at different scales in a variety of lithologies. The presence of material heterogeneities is known to trigger shear zone development, but the mechanisms controlling initiation and evolution of localization are not fully understood. To investigate the effect of loading conditions on shear zone nucleation along heterogeneities, we performed torsion experiments under constant twist rate (CTR) and constant torque (CT) conditions in a Paterson-type deformation apparatus. The sample assemblage consisted of cylindrical Carrara marble specimens containing a thin plate of Solnhofen limestone perpendicular to the cylinder's longitudinal axis. Under experimental conditions (900 °C, 400 MPa confining pressure), samples were plastically deformed and limestone is about 9 times weaker than marble, acting as a weak inclusion in a strong matrix. CTR experiments were performed at maximum bulk shear strain rates of ~ 2*10-4s-1, yielding peak shear stresses of ~ 20 MPa. CT tests were conducted at shear stresses of ~ 20 MPa resulting in bulk shear strain rates of 1-4*10-4s-1. Experiments were terminated at maximum bulk shear strains of ~ 0.3 and 1.0.Strain was localized within the Carrara marble in front of the inclusion in an area of strongly deformed grains and intense grain size reduction. Locally, evidences for coexisting brittle deformation are also observed regardless of the imposed loading conditions. The local shear strain at the inclusion tipis up to 30 times higher than the strain in the adjacent host rock, rapidly dropping to 5times higher at larger distance from the inclusion. At both investigated bulk strains, the evolution of microstructural and textural parameters is independent of loading conditions. Ourresults suggest that loading conditions do not significantly affect material heterogeneity-induced strain localization during its nucleation and transient stages.}, language = {en} } @article{MartinezGarzonKwiatekBohnhoffetal.2017, author = {Mart{\´i}nez-Garz{\´o}n, Patricia and Kwiatek, Grzegorz and Bohnhoff, Marco and Dresen, Georg}, title = {Volumetric components in the earthquake source related to fluid injection and stress state}, series = {Geophysical research letters}, volume = {44}, journal = {Geophysical research letters}, number = {2}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0094-8276}, doi = {10.1002/2016GL071963}, pages = {800 -- 809}, year = {2017}, abstract = {We investigate source processes of fluid-induced seismicity from The Geysers geothermal reservoir in California to determine their relation with hydraulic operations and improve the corresponding seismic hazard estimates. Analysis of 869 well-constrained full moment tensors (M-w 0.8-3.5) reveals significant non-double-couple components (>25\%) for about 65\% of the events. Volumetric deformation is governed by cumulative injection rates with larger non-double-couple components observed near the wells and during high injection periods. Source mechanisms are magnitude dependent and vary significantly between faulting regimes. Normal faulting events (M-w<2) reveal substantial volumetric components indicating dilatancy in contrast to strike-slip events that have a dominant double-couple source. Volumetric components indicating closure of cracks in the source region are mostly found for reverse faulting events with M-w>2.5. Our results imply that source processes and magnitudes of fluid-induced seismic events are strongly affected by the hydraulic operations, the reservoir stress state, and the faulting regime.}, language = {en} } @article{MartinezGarzonKwiatekSoneetal.2014, author = {Martinez-Garzon, Patricia and Kwiatek, Grzegorz and Sone, Hiroki and Bohnhoff, Marco and Dresen, Georg and Hartline, Craig}, title = {Spatiotemporal changes, faulting regimes, and source parameters of induced seismicity: A case study from the Geysers geothermal field}, series = {Journal of geophysical research : Solid earth}, volume = {119}, journal = {Journal of geophysical research : Solid earth}, number = {11}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9313}, doi = {10.1002/2014JB011385}, pages = {8378 -- 8396}, year = {2014}, abstract = {The spatiotemporal, kinematic, and source characteristics of induced seismicity occurring at different fluid injection rates are investigated to determine the predominant physical mechanisms responsible for induced seismicity at the northwestern part of The Geysers geothermal field, California. We analyze a relocated hypocenter catalog from a seismicity cluster where significant variations of the stress tensor orientation were previously observed to correlate with injection rates. We find that these stress tensor orientation changes may be related to increased pore pressure and the corresponding changes in poroelastic stresses at reservoir depth. Seismic events during peak injections tend to occur at greater distances from the injection well, preferentially trending parallel to the maximum horizontal stress direction. In contrast, at lower injection rates the seismicity tends to align in a different direction which suggests the presence of a local fault. During peak injection intervals, the relative contribution of strike-slip faulting mechanisms increases. Furthermore, increases in fluid injection rates also coincide with a decrease in b values. Our observations suggest that regardless of the injection stage, most of the induced seismicity results from thermal fracturing of the reservoir rock. However, during peak injection intervals, the increase in pore pressure may likewise be responsible for the induced seismicity. By estimating the thermal and hydraulic diffusivities of the reservoir, we confirm that the characteristic diffusion length for pore pressure is much greater than the corresponding length scale for temperature and also more consistent with the spatial extent of seismicity observed during different injection rates.}, language = {en} } @article{MartinezGarzonKwiatekBohnhoffetal.2016, author = {Martinez-Garzon, Patricia and Kwiatek, Grzegorz and Bohnhoff, Marco and Dresen, Georg}, title = {Impact of fluid injection on fracture reactivation at The Geysers geothermal field}, series = {Journal of geophysical research : Solid earth}, volume = {121}, journal = {Journal of geophysical research : Solid earth}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9313}, doi = {10.1002/2016JB013137}, pages = {7432 -- 7449}, year = {2016}, abstract = {We analyze the spatiotemporal distribution of fault geometries from seismicity induced by fluid injection at The Geysers geothermal field. The consistency of these faults with the local stress field is investigated using (1) the fault instability coefficient I comparing the orientation of a fault with the optimal orientation for failure in the assumed stress field and (2) the misfit angle beta between slip vectors observed from focal mechanisms and predicted from stress tensor. A statistical approach is applied to calculate the most likely fault instabilities considering the uncertainties from focal mechanisms and stress inversion. We find that faults activated by fluid injection may display a broad range in orientations. About 72\% of the analyzed seismicity occurs on faults with favorable orientation for failure with respect to the stress field. However, a number of events are observed either to occur on severely misoriented faults or to slip in a different orientation than predicted from stress field. These events mostly occur during periods of high injection rates and are located in proximity to the injection wells. From the stress inversion, the friction coefficient providing the largest overall instability is mu = 0.5. About 91\% of the events are activated with an estimated excess pore pressure <10 MPa, in agreement with previous models considering the combined effect of thermal and poroelastic stress changes from fluid injection. Furthermore, high seismic activity and largest magnitudes occur on favorably oriented faults with large instability coefficients and low slip misfit angles.}, language = {en} } @article{MartinezGarvonBohnhoffMencinetal.2019, author = {Martinez-Garvon, Patricia and Bohnhoff, Marco and Mencin, David and Kwiatek, Grzegorz and Dresen, Georg and Hodgkinson, Kathleen and Nurlu, Murat and Kadirioglu, Filiz Tuba and Kartal, Recai Feyiz}, title = {Slow strain release along the eastern Marmara region offshore Istanbul in conjunction with enhanced local seismic moment release}, series = {Earth \& planetary science letters}, volume = {510}, journal = {Earth \& planetary science letters}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0012-821X}, doi = {10.1016/j.epsl.2019.01.001}, pages = {209 -- 218}, year = {2019}, abstract = {We analyze a large transient strainmeter signal recorded at 62.5 m depth along the southern shore of the eastern Sea of Marmara region in northwestern Turkey. This region represents a passage of stress transfer from the Izmit rupture to the Marmara seismic gap. The strain signal was recorded at the Esenkoy site by one of the ICDP-GONAF (International Continental Drilling Programme - Geophysical Observatory at the North Anatolian Fault) strainmeters on the Armutlu peninsula with a maximum amplitude of 5 microstrain and lasting about 50 days. The onset of the strain signal coincided with the origin time of a M-w 4.4 earthquake offshore Yalova, which occurred as part of a seismic sequence including eight M-w >= 3.5 earthquakes. The Mw 4.4 event occurred at a distance of about 30 km from Esenkoy on June 25th 2016 representing the largest earthquake in this region since 2008. Before the event, the maximum horizontal strain was subparallel to the regional maximum horizontal stress derived from stress inversion of local seismicity. During the strain transient, we observe a clockwise rotation in the local horizontal strain field of about 20 degrees. The strain signal does not correlate with known environmental parameters such as annual changes of sea level, rainfall or temperature. The strain signal could indicate local slow slip on the Cinarcik fault and thus a transfer of stress to the eastern Marmara seismic gap.}, language = {en} } @article{KwiatekSaarnoAderetal.2019, author = {Kwiatek, Grzegorz and Saarno, Tero and Ader, Thomas and Bl{\"u}mle, Felix and Bohnhoff, Marco and Chendorain, Michael and Dresen, Georg and Heikkinen, Pekka and Kukkonen, Ilmo and Leary, Peter and Leonhardt, Maria and Malin, Peter and Martinez-Garzon, Patricia and Passmore, Kevin and Passmore, Paul and Valenzuela, Sergio and Wollin, Christopher}, title = {Controlling fluid-induced seismicity during a 6.1-km-deep geothermal stimulation in Finland}, series = {Science Advances}, volume = {5}, journal = {Science Advances}, number = {5}, publisher = {American Association for the Advancement of Science}, address = {Washington}, issn = {2375-2548}, doi = {10.1126/sciadv.aav7224}, pages = {11}, year = {2019}, abstract = {We show that near-real-time seismic monitoring of fluid injection allowed control of induced earthquakes during the stimulation of a 6.1-km-deep geothermal well near Helsinki, Finland. A total of 18,160 m(3) of fresh water was pumped into crystalline rocks over 49 days in June to July 2018. Seismic monitoring was performed with a 24-station borehole seismometer network. Using near-real-time information on induced-earthquake rates, locations, magnitudes, and evolution of seismic and hydraulic energy, pumping was either stopped or varied-in the latter case, between well-head pressures of 60 and 90 MPa and flow rates of 400 and 800 liters/min. This procedure avoided the nucleation of a project-stopping magnitude M-W 2.0 induced earthquake, a limit set by local authorities. Our results suggest a possible physics-based approach to controlling stimulation-induced seismicity in geothermal projects.}, language = {en} } @article{KwiatekMartinezGarzonPlenkersetal.2018, author = {Kwiatek, Grzegorz and Martinez-Garzon, Patricia and Plenkers, K. and Leonhardt, Maria and Zang, Arno and von Specht, Sebastian and Dresen, Georg and Bohnhoff, Marco}, title = {Insights into complex subdecimeter fracturing processes occurring during a water injection experiment at depth in Aspo Hard Rock Laboratory, Sweden}, series = {Journal of geophysical research : Solid earth}, volume = {123}, journal = {Journal of geophysical research : Solid earth}, number = {8}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9313}, doi = {10.1029/2017JB014715}, pages = {6616 -- 6635}, year = {2018}, abstract = {We investigate the source characteristics of picoseismicity (M-w < -2) recorded during a hydraulic fracturing in situ experiment performed in the underground Aspo Hard Rock Laboratory, Sweden. The experiment consisted of six stimulations driven by three different water injection schemes and was performed inside a 28-m-long, horizontal borehole located at 410-m depth. The fracturing processes were monitored with a variety of seismic networks including broadband seismometers, geophones, high-frequency accelerometers, and acoustic emission sensors thereby covering a wide frequency band between 0.01 and 100,000Hz. Here we study the high-frequency signals with dominant frequencies exceeding 1000 Hz. The combined seismic network allowed for detection and detailed analysis of 196 small-scale seismic events with moment magnitudes M-W < -3.5 (source sizes of decimeter scale) that occurred solely during the stimulations and shortly after. The double-difference relocated hypocenter catalog as well as source parameters were used to study the physical characteristics of the induced seismicity and then compared to the stimulation parameters. We observe a spatiotemporal migration of the picoseismic events away and toward the injection intervals in direct correlation with changes in the hydraulic energy (product of fluid injection pressure and injection rate). We find that the total radiated seismic energy is extremely low with respect to the product of injected fluid volume and pressure (hydraulic energy). The radiated seismic energy correlates well with the hydraulic energy rate. The obtained fault plane solutions for particularly well-characterized events signify the reactivation of preexisting rock defects under influence of increased pore fluid pressure on fault plane orientations in good correspondence with the local stress field orientation.}, language = {en} }