TY - JOUR A1 - Davidsen, Joern A1 - Kwiatek, Grzegorz A1 - Charalampidou, Elli-Maria A1 - Goebel, Thomas A1 - Stanchits, Sergei A1 - Rueck, Marc A1 - Dresen, Georg T1 - Triggering Processes in Rock Fracture JF - Physical review letters N2 - We study triggering processes in triaxial compression experiments under a constant displacement rate on sandstone and granite samples using spatially located acoustic emission events and their focal mechanisms. We present strong evidence that event-event triggering plays an important role in the presence of large-scale or macrocopic imperfections, while such triggering is basically absent if no significant imperfections are present. In the former case, we recover all established empirical relations of aftershock seismicity including the Gutenberg-Richter relation, a modified version of the Omori-Utsu relation and the productivity relation-despite the fact that the activity is dominated by compaction-type events and triggering cascades have a swarmlike topology. For the Gutenberg-Richter relations, we find that the b value is smaller for triggered events compared to background events. Moreover, we show that triggered acoustic emission events have a focal mechanism much more similar to their associated trigger than expected by chance. Y1 - 2017 U6 - https://doi.org/10.1103/PhysRevLett.119.068501 SN - 0031-9007 SN - 1079-7114 VL - 119 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Dresen, Georg A1 - Kwiatek, Grzegorz A1 - Goebel, Thomas A1 - Ben-Zion, Yehuda T1 - Seismic and aseismic preparatory processes before large stick-slip failure JF - Pure and applied geophysics N2 - Natural earthquakes often have very few observable foreshocks which significantly complicates tracking potential preparatory processes. To better characterize expected preparatory processes before failures, we study stick-slip events in a series of triaxial compression tests on faulted Westerly granite samples. We focus on the influence of fault roughness on the duration and magnitude of recordable precursors before large stick-slip failure. Rupture preparation in the experiments is detectable over long time scales and involves acoustic emission (AE) and aseismic deformation events. Preparatory fault slip is found to be accelerating during the entire pre-failure loading period, and is accompanied by increasing AE rates punctuated by distinct activity spikes associated with large slip events. Damage evolution across the fault zones and surrounding wall rocks is manifested by precursory decrease of seismic b-values and spatial correlation dimensions. Peaks in spatial event correlation suggest that large slip initiation occurs by failure of multiple asperities. Shear strain estimated from AE data represents only a small fraction (< 1%) of total shear strain accumulated during the preparation phase, implying that most precursory deformation is aseismic. The relative contribution of aseismic deformation is amplified by larger fault roughness. Similarly, seismic coupling is larger for smooth saw-cut faults compared to rough faults. The laboratory observations point towards a long-lasting and continuous preparation process leading to failure and large seismic events. The strain partitioning between aseismic and observable seismic signatures depends on fault structure and instrument resolution. KW - Earthquakes KW - rupture KW - stick–slip tests KW - seismic KW - aseismic Y1 - 2020 U6 - https://doi.org/10.1007/s00024-020-02605-x SN - 0033-4553 SN - 1420-9136 VL - 177 IS - 12 SP - 5741 EP - 5760 PB - Springer CY - Basel ER - TY - JOUR A1 - Martinez-Garzon, Patricia A1 - Kwiatek, Grzegorz A1 - Sone, Hiroki A1 - Bohnhoff, Marco A1 - Dresen, Georg A1 - Hartline, Craig T1 - Spatiotemporal changes, faulting regimes, and source parameters of induced seismicity: A case study from the Geysers geothermal field JF - Journal of geophysical research : Solid earth N2 - 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. KW - thermal effect KW - focal mechanisms KW - geothermal KW - pore pressure KW - fluid-induced seismicity KW - reservoir characterization Y1 - 2014 U6 - https://doi.org/10.1002/2014JB011385 SN - 2169-9313 SN - 2169-9356 VL - 119 IS - 11 SP - 8378 EP - 8396 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Kwiatek, Grzegorz A1 - Martinez-Garzon, Patricia A1 - Dresen, Georg A1 - Bohnhoff, Marco A1 - Sone, Hiroki A1 - Hartline, Craig T1 - Effects of long-term fluid injection on induced seismicity parameters and maximum magnitude in northwestern part of The Geysers geothermal field JF - Journal of geophysical research : Solid earth N2 - 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. KW - fluid-induced seismicity KW - maximum magnitude KW - reservoir characterization KW - source parameters KW - passive seismic monitoring Y1 - 2015 U6 - https://doi.org/10.1002/2015JB012362 SN - 2169-9313 SN - 2169-9356 VL - 120 IS - 10 SP - 7085 EP - 7101 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Martinez-Garzon, Patricia A1 - Kwiatek, Grzegorz A1 - Bohnhoff, Marco A1 - Dresen, Georg T1 - Impact of fluid injection on fracture reactivation at The Geysers geothermal field JF - Journal of geophysical research : Solid earth N2 - 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. Y1 - 2016 U6 - https://doi.org/10.1002/2016JB013137 SN - 2169-9313 SN - 2169-9356 VL - 121 SP - 7432 EP - 7449 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Bentz, Stephan A1 - Martinez-Garzon, Patricia A1 - Kwiatek, Grzegorz A1 - Dresen, Georg A1 - Bohnhoff, Marco T1 - Analysis of Microseismicity Framing M-L > 2.5 Earthquakes at The Geysers Geothermal Field, California JF - Journal of geophysical research : Solid earth N2 - 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. KW - induced seismicity KW - earthquake nucleation KW - The Geysers KW - earthquake sequences Y1 - 2019 U6 - https://doi.org/10.1029/2019JB017716 SN - 2169-9313 SN - 2169-9356 VL - 124 IS - 8 SP - 8823 EP - 8843 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Kwiatek, Grzegorz A1 - Saarno, Tero A1 - Ader, Thomas A1 - Blümle, Felix A1 - Bohnhoff, Marco A1 - Chendorain, Michael A1 - Dresen, Georg A1 - Heikkinen, Pekka A1 - Kukkonen, Ilmo A1 - Leary, Peter A1 - Leonhardt, Maria A1 - Malin, Peter A1 - Martinez-Garzon, Patricia A1 - Passmore, Kevin A1 - Passmore, Paul A1 - Valenzuela, Sergio A1 - Wollin, Christopher T1 - Controlling fluid-induced seismicity during a 6.1-km-deep geothermal stimulation in Finland JF - Science Advances N2 - 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. Y1 - 2019 U6 - https://doi.org/10.1126/sciadv.aav7224 SN - 2375-2548 VL - 5 IS - 5 PB - American Association for the Advancement of Science CY - Washington ER - TY - JOUR A1 - Martinez-Garvon, Patricia A1 - Bohnhoff, Marco A1 - Mencin, David A1 - Kwiatek, Grzegorz A1 - Dresen, Georg A1 - Hodgkinson, Kathleen A1 - Nurlu, Murat A1 - Kadirioglu, Filiz Tuba A1 - Kartal, Recai Feyiz T1 - Slow strain release along the eastern Marmara region offshore Istanbul in conjunction with enhanced local seismic moment release JF - Earth & planetary science letters N2 - 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. KW - strain transient KW - slow slip KW - transform faults KW - seismic hazard KW - strainmeter data KW - Sea of Marmara Y1 - 2019 U6 - https://doi.org/10.1016/j.epsl.2019.01.001 SN - 0012-821X SN - 1385-013X VL - 510 SP - 209 EP - 218 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Martínez-Garzón, Patricia A1 - Kwiatek, Grzegorz A1 - Bohnhoff, Marco A1 - Dresen, Georg T1 - Volumetric components in the earthquake source related to fluid injection and stress state JF - Geophysical research letters N2 - 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. KW - non-double-couple components KW - induced seismicity KW - geothermal KW - stress state KW - tensile opening KW - pore pressure Y1 - 2017 U6 - https://doi.org/10.1002/2016GL071963 SN - 0094-8276 SN - 1944-8007 VL - 44 IS - 2 SP - 800 EP - 809 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Bentz, Stephan A1 - Kwiatek, Grzegorz A1 - Martinez-Garzon, Patricia A1 - Bohnhoff, Marco A1 - Dresen, Georg T1 - Seismic moment evolution during hydraulic stimulations JF - Geophysical research letters N2 - 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. Y1 - 2020 U6 - https://doi.org/10.1029/2019GL086185 SN - 0094-8276 SN - 1944-8007 VL - 47 IS - 5 PB - American Geophysical Union CY - Washington ER -