@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{KwiatekMartinezGarzonDresenetal.2015, author = {Kwiatek, Grzegorz and Martinez-Garzon, Patricia and Dresen, Georg and Bohnhoff, Marco and Sone, Hiroki and Hartline, Craig}, title = {Effects of long-term fluid injection on induced seismicity parameters and maximum magnitude in northwestern part of The Geysers geothermal field}, series = {Journal of geophysical research : Solid earth}, volume = {120}, journal = {Journal of geophysical research : Solid earth}, number = {10}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9313}, doi = {10.1002/2015JB012362}, pages = {7085 -- 7101}, year = {2015}, abstract = {The long-term temporal and spatial changes in statistical, source, and stress characteristics of one cluster of induced seismicity recorded at The Geysers geothermal field (U.S.) are analyzed in relation to the field operations, fluid migration, and constraints on the maximum likely magnitude. Two injection wells, Prati-9 and Prati-29, located in the northwestern part of the field and their associated seismicity composed of 1776 events recorded throughout a 7year period were analyzed. The seismicity catalog was relocated, and the source characteristics including focal mechanisms and static source parameters were refined using first-motion polarity, spectral fitting, and mesh spectral ratio analysis techniques. The source characteristics together with statistical parameters (b value) and cluster dynamics were used to investigate and understand the details of fluid migration scheme in the vicinity of injection wells. The observed temporal, spatial, and source characteristics were clearly attributed to fluid injection and fluid migration toward greater depths, involving increasing pore pressure in the reservoir. The seasonal changes of injection rates were found to directly impact the shape and spatial extent of the seismic cloud. A tendency of larger seismic events to occur closer to injection wells and a correlation between the spatial extent of the seismic cloud and source sizes of the largest events was observed suggesting geometrical constraints on the maximum likely magnitude and its correlation to the average injection rate and volume of fluids present in the reservoir.}, 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{HofmannZimmermannFarkasetal.2019, author = {Hofmann, Hannes and Zimmermann, G{\"u}nter and Farkas, M{\´a}rton P{\´a}l and Huenges, Ernst and Zang, Arno and Leonhardt, Maria and Kwiatek, Grzegorz and Martinez-Garzon, Patricia and Bohnhoff, Marco and Min, Ki-Bok and Fokker, Peter and Westaway, Rob and Bethmann, Falko and Meier, Peter and Yoon, Kern Shin and Choi, JaiWon and Lee, Tae Jong and Kim, Kwang Yeom}, title = {First field application of cyclic soft stimulation at the Pohang Enhanced Geothermal System site in Korea}, series = {Geophysical journal international}, volume = {217}, journal = {Geophysical journal international}, number = {2}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0956-540X}, doi = {10.1093/gji/ggz058}, pages = {926 -- 949}, year = {2019}, abstract = {Large-magnitude fluid-injection induced seismic events are a potential risk for geothermal energy developments worldwide. One potential risk mitigation measure is the application of cyclic injection schemes. After validation at small (laboratory) and meso (mine) scale, the concept has now been applied for the first time at field scale at the Pohang Enhanced Geothermal System (EGS) site in Korea. From 7 August until 14 August 2017 a total of 1756 m(3) of surface water was injected into Pohang well PX-1 at flow rates between 1 and 10 l s(-1), with a maximum wellhead pressure (WHP) of 22.8 MPa, according to a site-specific cyclic soft stimulation schedule and traffic light system. A total of 52 induced microearthquakes were detected in real-time during and shortly after the injection, the largest of M-w 1.9. After that event a total of 1771 m(3) of water was produced back from the well over roughly 1 month, during which time no larger-magnitude seismic event was observed. The hydraulic data set exhibits pressure-dependent injectivity increase with fracture opening between 15 and 17 MPa WHP, but no significant permanent transmissivity increase was observed. The maximum magnitude of the induced seismicity during the stimulation period was below the target threshold of M-w 2.0 and additional knowledge about the stimulated reservoir was gained. Additionally, the technical feasibility of cyclic injection at field scale was evaluated. The major factors that limited the maximum earthquake magnitude are believed to be: limiting the injected net fluid volume, flowback after the occurrence of the largest induced seismic event, using a cyclic injection scheme, the application of a traffic light system, and including a priori information from previous investigations and operations in the treatment design.}, language = {en} } @article{BentzMartinezGarzonKwiateketal.2019, author = {Bentz, Stephan and Martinez-Garzon, Patricia and Kwiatek, Grzegorz and Dresen, Georg and Bohnhoff, Marco}, title = {Analysis of Microseismicity Framing M-L > 2.5 Earthquakes at The Geysers Geothermal Field, California}, series = {Journal of geophysical research : Solid earth}, volume = {124}, journal = {Journal of geophysical research : Solid earth}, number = {8}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9313}, doi = {10.1029/2019JB017716}, pages = {8823 -- 8843}, year = {2019}, abstract = {Preparatory mechanisms accompanying or leading to nucleation of larger earthquakes have been observed at both laboratory and field scales, but conditions favoring the occurrence of observable preparatory processes are still largely unknown. In particular, it remains a matter of debate why some earthquakes occur spontaneously without noticeable precursors as opposed to events that are preceded by an extended failure process. In this study, we have generated new high-resolution seismicity catalogs framing the occurrence of 20 M-L > 2.5 earthquakes at The Geysers geothermal field in California. To this end, a seismicity catalog of the 11 days framing each large event was created. We selected 20 sequences sampling different hypocentral depths and hydraulic conditions within the field. Seismic activity and magnitude frequency distributions displayed by the different earthquake sequences are correlated with their location within the reservoir. Sequences located in the northwestern part of the reservoir show overall increased seismic activity and low b values, while the southeastern part is dominated by decreased seismic activity and higher b values. Periods of high injection coincide with high b values and vice versa. These observations potentially reflect varying differential and mean stresses and damage of the reservoir rocks across the field. About 50\% of analyzed sequences exhibit no change in seismicity rate in response to the large main event. However, we find complex waveforms at the onset of the main earthquake, suggesting that small ruptures spontaneously grow into or trigger larger events.}, 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{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{ZieglerHeidbachZangetal.2017, author = {Ziegler, Moritz O. and Heidbach, Oliver and Zang, Arno and Martinez-Garzon, Patricia and Bohnhoff, Marco}, title = {Estimation of the differential stress from the stress rotation angle in low permeable rock}, series = {Geophysical research letters}, volume = {44}, journal = {Geophysical research letters}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0094-8276}, doi = {10.1002/2017GL073598}, pages = {6761 -- 6770}, year = {2017}, abstract = {Rotations of the principal stress axes are observed as a result of fluid injection into reservoirs. We use a generic, fully coupled 3-D thermo-hydro-mechanical model to investigate systematically the dependence of this stress rotation on different reservoir properties and injection scenarios. We find that permeability, injection rate, and initial differential stress are the key factors, while other reservoir properties only play a negligible role. In particular, we find that thermal effects do not significantly contribute to stress rotations. For reservoir types with usual differential stress and reservoir treatment the occurrence of significant stress rotations is limited to reservoirs with a permeability of less than approximately 10(-12)m(2). Higher permeability effectively prevents stress rotations to occur. Thus, according to these general findings, the observed principal stress axes rotation can be used as a proxy of the initial differential stress provided that rock permeability and fluid injection rate are known a priori.}, 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{BentzKwiatekMartinezGarzonetal.2020, author = {Bentz, Stephan and Kwiatek, Grzegorz and Martinez-Garzon, Patricia and Bohnhoff, Marco and Dresen, Georg}, title = {Seismic moment evolution during hydraulic stimulations}, series = {Geophysical research letters}, volume = {47}, journal = {Geophysical research letters}, number = {5}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0094-8276}, doi = {10.1029/2019GL086185}, pages = {9}, year = {2020}, abstract = {Analysis of past and present stimulation projects reveals that the temporal evolution and growth of maximum observed moment magnitudes may be linked directly to the injected fluid volume and hydraulic energy. Overall evolution of seismic moment seems independent of the tectonic stress regime and is most likely governed by reservoir specific parameters, such as the preexisting structural inventory. Data suggest that magnitudes can grow either in a stable way, indicating the constant propagation of self-arrested ruptures, or unbound, for which the maximum magnitude is only limited by the size of tectonic faults and fault connectivity. Transition between the two states may occur at any time during injection or not at all. Monitoring and traffic light systems used during stimulations need to account for the possibility of unstable rupture propagation from the very beginning of injection by observing the entire seismicity evolution in near-real time and at high resolution for an immediate reaction in injection strategy. Plain Language Summary Predicting and controlling the size of earthquakes caused by fluid injection is currently the major concern of many projects associated with geothermal energy production. Here, we analyze the magnitude and seismic moment evolution with injection parameters for prominent geothermal and scientific projects to date. Evolution of seismicity seems to be largely independent of the tectonic stress background and seemingly depends on reservoir specific characteristics. We find that the maximum observed magnitudes relate linearly to the injected volume or hydraulic energy. A linear relation suggests stable growth of induced ruptures, as predicted by current models, or rupture growth may no longer depend on the stimulated volume but on tectonics. A system may change between the two states during the course of fluid injection. Close-by and high-resolution monitoring of seismic and hydraulic parameters in near-real time may help identify these fundamental changes in ample time to change injection strategy and manage maximum magnitudes.}, 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{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{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{DurandBentzKwiateketal.2020, author = {Durand, Virginie and Bentz, Stephan and Kwiatek, Grzegorz and Dresen, Georg and Wollin, Christopher and Heidbach, Oliver and Martinez-Garzon, Patricia and Cotton, Fabrice Pierre and Nurlu, Murat and Bohnhoff, Marco}, title = {A two-scale preparation phase preceded an M-w 5.8 earthquake in the sea of marmara offshore Istanbul, Turkey}, series = {Seismological research letters}, volume = {91}, journal = {Seismological research letters}, number = {6}, address = {Boulder}, issn = {0895-0695}, doi = {10.1785/0220200110}, pages = {3139 -- 3147}, year = {2020}, abstract = {We analyze the spatiotemporal evolution of seismicity during a sequence of moderate (an M-w 4.7 foreshock and M-w 5.8 mainshock) earthquakes occurring in September 2019 at the transition between a creeping and a locked segment of the North Anatolian fault in the central Sea of Marmara, northwest Turkey. To investigate in detail the seismicity evolution, we apply a matched-filter technique to continuous waveforms, thus reducing the magnitude threshold for detection. Sequences of foreshocks preceding the two largest events are clearly seen, exhibiting two different behaviors: a long-term activation of the seismicity along the entire fault segment and a short-term concentration around the epicenters of the large events. We suggest a two-scale preparation phase, with aseismic slip preparing the mainshock final rupture a few days before, and a cascade mechanism leading to the nucleation of the mainshock. Thus, our study shows a combination of seismic and aseismic slip during the foreshock sequence changing the strength of the fault, bringing it closer to failure.}, language = {en} } @article{WeberAbuAyyashAbueladasetal.2009, author = {Weber, Michael H. and Abu-Ayyash, Khalil and Abueladas, Abdel-Rahman and Agnon, Amotz and Alasonati-Taš{\´a}rov{\´a}, Zuzana and Al-Zubi, Hashim and Babeyko, Andrey and Bartov, Yuval and Bauer, Klaus and Becken, Michael and Bedrosian, Paul A. and Ben-Avraham, Zvi and Bock, G{\"u}nter and Bohnhoff, Marco and Bribach, Jens and Dulski, Peter and Ebbing, Joerg and El-Kelani, Radwan J. and Foerster, Andrea and F{\"o}rster, Hans-J{\"u}rgen and Frieslander, Uri and Garfunkel, Zvi and G{\"o}tze, Hans-J{\"u}rgen and Haak, Volker and Haberland, Christian and Hassouneh, Mohammed and Helwig, Stefan L. and Hofstetter, Alfons and Hoffmann-Rothe, Arne and Jaeckel, Karl-Heinz and Janssen, Christoph and Jaser, Darweesh and Kesten, Dagmar and Khatib, Mohammed Ghiath and Kind, Rainer and Koch, Olaf and Koulakov, Ivan and Laske, Maria Gabi and Maercklin, Nils}, title = {Anatomy of the Dead Sea transform from lithospheric to microscopic scale}, issn = {8755-1209}, doi = {10.1029/2008rg000264}, year = {2009}, abstract = {Fault zones are the locations where motion of tectonic plates, often associated with earthquakes, is accommodated. Despite a rapid increase in the understanding of faults in the last decades, our knowledge of their geometry, petrophysical properties, and controlling processes remains incomplete. The central questions addressed here in our study of the Dead Sea Transform (DST) in the Middle East are as follows: (1) What are the structure and kinematics of a large fault zone? (2) What controls its structure and kinematics? (3) How does the DST compare to other plate boundary fault zones? The DST has accommodated a total of 105 km of left-lateral transform motion between the African and Arabian plates since early Miocene (similar to 20 Ma). The DST segment between the Dead Sea and the Red Sea, called the Arava/Araba Fault (AF), is studied here using a multidisciplinary and multiscale approach from the mu m to the plate tectonic scale. We observe that under the DST a narrow, subvertical zone cuts through crust and lithosphere. First, from west to east the crustal thickness increases smoothly from 26 to 39 km, and a subhorizontal lower crustal reflector is detected east of the AF. Second, several faults exist in the upper crust in a 40 km wide zone centered on the AF, but none have kilometer-size zones of decreased seismic velocities or zones of high electrical conductivities in the upper crust expected for large damage zones. Third, the AF is the main branch of the DST system, even though it has accommodated only a part (up to 60 km) of the overall 105 km of sinistral plate motion. Fourth, the AF acts as a barrier to fluids to a depth of 4 km, and the lithology changes abruptly across it. Fifth, in the top few hundred meters of the AF a locally transpressional regime is observed in a 100-300 m wide zone of deformed and displaced material, bordered by subparallel faults forming a positive flower structure. Other segments of the AF have a transtensional character with small pull-aparts along them. The damage zones of the individual faults are only 5-20 m wide at this depth range. Sixth, two areas on the AF show mesoscale to microscale faulting and veining in limestone sequences with faulting depths between 2 and 5 km. Seventh, fluids in the AF are carried downward into the fault zone. Only a minor fraction of fluids is derived from ascending hydrothermal fluids. However, we found that on the kilometer scale the AF does not act as an important fluid conduit. Most of these findings are corroborated using thermomechanical modeling where shear deformation in the upper crust is localized in one or two major faults; at larger depth, shear deformation occurs in a 20-40 km wide zone with a mechanically weak decoupling zone extending subvertically through the entire lithosphere.}, 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} }