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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 ML > 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. Key Points: We statistically analyze temporal properties of seismicity utilizing enhanced seismicity catalogs framing 20 ML > 2.5 earthquakes at The GeysersTriggered aftershocks are mostly restricted to the northwestern part of the geothermal fieldLarger events may be preceded by isolated smaller earthquakes at approximately the same location [ABSTRACT FROM AUTHOR]

The article talks about near-real-time seismic monitoring of fluid injection for control of induced earthquakes. Topics include evolution of seismic and hydraulic energy, seismicity in geothermal projects, and fluid pressure dissipation.

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 ∗ 10−4 s−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 ∗ 10−4 s−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 tip is up to 30 times higher than the strain in the adjacent host rock, rapidly dropping to 5 times higher at larger distance from the inclusion. At both bulk strains, the evolution of microstructural and textural parameters is independent of loading conditions. Our results suggest that loading conditions do not significantly affect material heterogeneity-induced strain localization during its nucleation and transient stages. Highlights • Torsion experiments performed on Carrara marble with single Solnhofen limestone inclusion. • Simulation of high temperature heterogeneity-induced shear zone nucleation. • High temperature creep of marble is the dominant deformation mechanism, but brittle deformation is also observed. • Different loading conditions applied lead to no significant variation in amount and efficiency of localization [ABSTRACT FROM AUTHOR]

Abstract: This paper reviews major findings of the Multidisciplinary Experimental and Modeling Impact Crater Research Network (MEMIN). MEMIN is a consortium, funded from 2009 till 2017 by the German Research Foundation, and is aimed at investigating impact cratering processes by experimental and modeling approaches. The vision of this network has been to comprehensively quantify impact processes by conducting a strictly controlled experimental campaign at the laboratory scale, together with a multidisciplinary analytical approach. Central to MEMIN has been the use of powerful two‐stage light‐gas accelerators capable of producing impact craters in the decimeter size range in solid rocks that allowed detailed spatial analyses of petrophysical, structural, and geochemical changes in target rocks and ejecta. In addition, explosive setups, membrane‐driven diamond anvil cells, as well as laser irradiation and split Hopkinson pressure bar technologies have been used to study the response of minerals and rocks to shock and dynamic loading as well as high‐temperature conditions. We used Seeberger sandstone, Taunus quartzite, Carrara marble, and Weibern tuff as major target rock types. In concert with the experiments we conducted mesoscale numerical simulations of shock wave propagation in heterogeneous rocks resolving the complex response of grains and pores to compressive, shear, and tensile loading and macroscale modeling of crater formation and fracturing. Major results comprise (1) projectile–target interaction, (2) various aspects of shock metamorphism with special focus on low shock pressures and effects of target porosity and water saturation, (3) crater morphologies and cratering efficiencies in various nonporous and porous lithologies, (4) in situ target damage, (5) ejecta dynamics, and (6) geophysical survey of experimental craters. [ABSTRACT FROM AUTHOR]

• St1 deep heat project, Finland, the world's deepest EGS pilot reservoir. • Hydraulic conductivity of crystalline rocks was measured at 5 – 6 km depth. • Natural level of conductivity before stimulation 5 0·10−10 – 5 0·10−9 m/s. • Hydraulic stimulation improved conductivity temporarily by two orders of magnitude. • Stimulation generated >80,000 micro-earthquakes in five clusters. We analyze hydraulic stimulation data from the St1 Deep Heat Project, which comprised drilling wells to >6 km depth in crystalline rock in southern Finland with an aim to study the conditions for an Enhanced Geothermal System. Combining the stimulation and borehole logging data with induced seismicity results allows for a comprehensive analysis of hydraulic conductivity at rarely tested depths of 5 – 6 km. Hydraulic conductivity was observed to be pressure-dependent, heterogeneously distributed and affected by pre-existing fractures and lithological variation. The pre-stimulation formation fracture frequency is in the range of 0 – 8 m −1 with an average of 1.9 m −1. Only a small fraction of the fractures were open and conductive implying a hydraulically heterogeneous medium. The average natural hydraulic conductivity derived from leak-off tests and well tests before stimulation and from cross-hole pressure data is of the order of 5 0·10−10 - 5 0·10−9 m/s (permeability 1· 10−17 – 1·10−16 m2), which is in agreement with permeability models for the brittle crystalline crust at this depth. Hydraulic stimulation increased conductivity to 10−8 – 10−7 m/s, but it gradually decreased back to the natural level after pressure release. Stimulation generated five micro-earthquake clusters at 4.8 – 6.3 km TVD depth. Hydraulic connections between clusters were apparently not attained. The project generated extensive experience and data sets regarding deep drilling, hydrogeological properties, and seismic response to stimulation of crystalline rock in the upper continental crust. Hydraulic conductivity turned out to be the most challenging issue for the St1 EGS development and the EGS project is not continued at the moment. [ABSTRACT FROM AUTHOR]

We investigate mixed mode fracture initiation and propagation in experimentally deformed granite and sandstone. We performed a series of asymmetric loading tests to induce fractures in cylindrical specimens at confining pressures up to 20 MPa. Loading was controlled using acoustic emission (AE) feedback control, which allows studying quasi-static fracture propagation for several hours. Location of acoustic emissions reveals distinct differences in spatial-temporal fracture evolution between granite and sandstone samples. Before reaching peak stress in experiments performed on granite, axial fractures initiate first at the edge of the indenter and then propagate through the entire sample. Secondary inclined fractures develop during softening of the sample. In sandstone, inclined shear fractures nucleate at peak stress and propagate through the specimen. AE source type analysis shows complex fracturing in both materials with pore collapse contributing significantly to fracture growth in sandstone samples. We compare the experimental results with numerical models to analyze stress distribution and energy release rate per unit crack surface area in the samples at different stages during fracture growth. We thereby show that for both rock types the energy release rate increases approximately linearly during fracture propagation. The study illuminates how different material properties modify fracture initiation direction under similar loading conditions. [ABSTRACT FROM AUTHOR]

Estimating the expected size of the largest earthquake on a given fault is complicated by dynamic rupture interactions in addition to geometric and stress heterogeneity. However, a statistical assessment of the potential of seismic events to grow to larger sizes may be possible based on variations in magnitude distributions. Such variations can be described by the b-value, which quantifies the proportion of small- to large-magnitude events. The values of b vary significantly if stress changes are large, but additional factors such as geometric heterogeneity may affect the growth potential of seismic ruptures. Here, we examine the influence of fault roughness on b-values, focal mechanisms, and spatial localization of laboratory acoustic emission (AE) events during stick-slip experiments. We create three types of roughness on Westerly granite surfaces and study AE event statistics during triaxial loading of the lab faults. Because both roughness and stress variations are expected to influence b, we isolate roughness contributions by analyzing AEs at elevated stresses close to stickslip failure. Our results suggest three characteristics of seismicity on increasingly rough faults: (1) seismicity becomes spatially more distributed, (2) b-values increase, and (3) focal mechanisms become more heterogeneous, likely caused by underlying stress field heterogeneity within the fault zones. Localized deformation on smooth faults, on the other hand, promotes larger rupture sizes within the associated homogeneous stress field, which is aligned with the macroscopic stress orientation. The statistics of earthquake magnitude distributions may help quantify these fault states and expected rupture sizes in nature. [ABSTRACT FROM AUTHOR]

We studied spatiotemporal b -value variations along the North Anatolian Fault Zone (NAFZ) in northwestern Turkey with a focus on the combined 1999 Izmit and Düzce rupture and the eastern Sea of Marmara. We used a local seismicity catalog of the Izmit-Düzce region covering a time span from 2.5 years prior to the Izmit until 14 months after the Düzce mainshock and a four-year hypocenter catalog in the eastern Sea of Marmara. We consistently calculated moment magnitudes to ensure a homogeneous dataset and applied strict quality criteria. This allows studying variations of b -values throughout the region and at different stages of the seismic cycle. With a standard gridding technique b -value maps, depth sections and time series were calculated which reveal a very heterogeneous b -value distribution in the study area. The variety of b -value observations cannot be interpreted unambiguously, given that the b -value most likely depends on a combination of fault-zone characteristics like local stress conditions, heterogeneity of the crust and damage distribution. By presenting a comprehensive set of possible interpretations we point out that a biased discussion of the results towards stress or another individual parameter may lead to erroneous conclusions. Furthermore, the applied data discretization scheme influences the appearance of the final b -value distribution leading to potential misinterpretations. [ABSTRACT FROM AUTHOR]

Naturally and experimentally deformed samples of granite show that the deformation mechanisms of plagioclase and K-feldspar are different. To investigate these mechanisms and the brittle–plastic transition that takes place in granitic rocks composed of quartz, plagioclase, and K-feldspar, five leucosome granite samples were deformed with a constant strain rate of 10 −5 s −1 at different temperatures from 850 °C to 1050 °C and confining pressure (CP) of 300 MPa using a Paterson-type gas deformation apparatus. To consider pressure effects, two more samples were deformed at 950 °C but with different CPs, one with CP = 100 kPa and the other with CP = 100 MPa. In addition, an eighth sample was deformed under torsion shear at 950 °C with CP = 400 MPa. Microstructures of an undeformed sample and experimentally deformed samples were analyzed using an optical microscope and a scanning electron microscope. The granite is composed of about 36 volume percent quartz, ∼26% plagioclase, ∼34% microcline, ∼3% muscovite, and ∼1% chlorite. The stress–strain curves for all but two samples display weakening. The two exceptions are the sample that deformed with steady-state creep under a CP of 100 MPa and the sample that displayed brittle fracture under a CP of 100 kPa. For the other samples, peak strengths decreased with increasing temperature or lower CP. Microstructures show that samples underwent a brittle–plastic transition with increasing temperature. Samples fractured by cataclastic flow at 850 °C with CP = 300 MPa and at 950 °C with CP = 100 kPa. Microcline deformed by cataclastic flow at 900–1050 °C with CP = 100–400 MPa accompanied by dislocation glide at temperatures of 1000 °C and 1050 °C. At 900–1050 °C with CP = 100–400 MPa, plagioclase displayed bulging recrystallization and grain boundary migration recrystallization and quartz deformed by subgrain rotation recrystallization. Diffusion rims were observed between quartz, plagioclase, and microcline grain boundaries at 900–1050 °C with CP = 100–400 MPa. Partial melting appeared around plagioclase, microcline, and quartz grains at 1000 °C and 1050 °C. Exsolution of albite from microcline took place in a sample deformed at 950 °C and CP = 100 MPa. Electron backscatter diffraction analysis showed that quartz has a lattice preferred orientation in both undeformed and experimentally deformed samples. The basal 〈a〉 slip of the quartz c-axis was found in undeformed samples. The c-axis of quartz closes to the rhomb 〈a〉 slip with a secondary in the basal 〈a〉 slip in samples deformed at 900 °C and 950 °C with CP’s of 300 MPa and 100 MPa respectively. The c-axis of quartz changes to prism 〈a〉 slip in samples deformed at 950–1050 °C with CP = 300 MPa and does the same for a sample deformed with torsion shear at 950 °C with CP = 400 MPa. Microstructures show that plagioclase and microcline deform differently. Microcline is a strong phase compared with quartz and plagioclase. Therefore, microcline is important for the brittle–plastic transition in granitic rocks and the depth of that transition in the middle crust will be greater where the granitic rocks in the crust contain large amounts of microcline. [ABSTRACT FROM AUTHOR]

Solid-solid mineral reaction rates are influenced by the microfabrics of reactant phases and concurrent deformation. To investigate this interplay in carbonate systems, we performed annealing and deformation experiments on polycry stalline and single-crystal calcite and magnesite, forming dolomite (Dol) and magnesio-calcite (Mg-Cal). At a fixed temperature of T = 750 °C and confining pressure of P = 400 MPa, samples were either annealed for 29 h, or deformed in triaxial compression or torsion for 18 h using a Paterson-type gas deformation apparatus. At the contact interface of the starting reactants, Dol reaction rims and poly cry stalline Mg-Cal layers were formed. The widths of the layers were in the ranges 4-117 and 30-147 μm, respectively, depending on the microstructure of starting materials and experimental conditions. Annealing experiments with polycrystalline reactants in contact with each other resulted in a ~22-fold increase in Dol rim thickness compared to a contact between two single crystals and a larger Mg-Cal layer width by a factor of 5 (cf. ). This suggests that the micro structure of magnesite controls migration of the reaction front. For polycrystalline starting materials, axial stress accelerated Mg-Cal growth rates but not Dol growth rates. Highly strained torsion samples showed Dol formation along grain boundaries in Mg-Cal as well as in the polycrystalline calcite reactant. A reduction of Dol rim thickness between polycrystalline reactants deformed in torsion is possibly caused by concurrent grain coarsening of polycrystalline magnesite. Dol and Mg-Cal growth kinetics between single crystals were unaffected by the addition of ~0.3 wt% water. The experiments demonstrate that Dol reaction kinetics strongly correlate with magnesite reactant grain sizes, while Mg-Cal growth depends on the calcite reactant grain sizes. The dolomite-forming mineral reaction kinetics are not significantly affected by concurrent deformation. In contrast, deformation enhances Mg-Cal formation, especially at small calcite grain sizes that promote efficient grain boundary diffusion. Therefore, the fastest reactions forming Dol and Mg-Cal in nature are expected to occur in very fine-grained reactants. Concurrent deformation may drastically enhance reaction kinetics if grain size reduction of the reactants occurs by, for example, cataclasis or dynamic recrystallization. [ABSTRACT FROM AUTHOR]

Highlights: [•] We analyzed the particle size distribution (PSD) ejected material from dry and wet hypervelocity impact experiments. [•] Power law fits are well suited to quantify the obtained PSDs. [•] Impact velocity and impact energy seem to have no effects on the fragmentation. [•] Water saturation has a positive effect on fragmentation. [Copyright &y& Elsevier]

Abstract: Particle size distribution (PSD) is an often used parameter to describe and quantify fragmentation of deformed rock. Our analyses of shock deformed sandstone show that dynamic fragmentation influences the PSD, expressed as fractal dimension (D-value). Image analysis was used to derive fractal dimensions from a hypervelocity impact cratering experiment (2.5 mm steel sphere, 4.8 km/s) and a planar shock recovery experiment (2.5 GPa). The D-values in the cratering experiment decrease from 1.74 at the crater floor to 0.84 at a distance of 7.2 mm to the crater floor. The D-values found in this experiment are closely related to the microstructural features found at distinct distances from the crater floor. The obtained values are in good agreement with the D-values reported for fault zones, impact sites and deformation experiments. The D-value measured in the shock recovery experiment is 2.42. Such high D-values were usually attributed to abrasive processes related to high strain. Since the strain in our experiment is only ∼23% we suggest that at highly dynamic deformation very high d -values can be reached at small strain. To quantify this, numerical impact modelling has been used to estimate strain rates for the impact experiment. This is related to the activation of more inherent flaws and fracture bifurcation at very high strain rates ∼>102 s−1. [Copyright &y& Elsevier]

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. • Ductile strain localization is inhibited in compressional step-over regions between heterogeneities. • Stress concentration at heterogeneities' tips can lead to high temperature microcracking. • Brittle precursors facilitate ductile localization if heterogeneously distributed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Dynamic failure in the laboratory is commonly preceded by many foreshocks which accompany premonitory aseismic slip. Aseismic slip is also thought to govern earthquake nucleation in nature, yet, foreshocks are rare. Here, we examine how heterogeneity due to different roughness, damage and pore pressures affects premonitory slip and acoustic emission characteristics. High fluid pressures increase stiffness and reduce heterogeneity which promotes more rapid slip acceleration and shorter precursory periods, similar to the effect of low geometric heterogeneity on smooth faults. The associated acoustic emission activity in low-heterogeneity samples becomes increasingly dominated by earthquake-like double-couple focal mechanisms. The similarity of fluid pressure increase and roughness reduction suggests that increased stress and geometric homogeneity may substantially shorten the duration of foreshock activity. Gradual fault activation and extended foreshock activity is more likely observable on immature faults at shallow depth., (© 2024. The Author(s).)

Earthquakes are rupture-like processes that propagate along tectonic faults and cause seismic waves. The propagation speed and final area of the rupture, which determine an earthquake's potential impact, are directly related to the nature and quantity of the energy dissipation involved in the rupture process. Here, we present the challenges associated with defining and measuring the energy dissipation in laboratory and natural earthquakes across many scales. We discuss the importance and implications of distinguishing between energy dissipation that occurs close to and far behind the rupture tip, and we identify open scientific questions related to a consistent modeling framework for earthquake physics that extends beyond classical Linear Elastic Fracture Mechanics., (© 2024. The Author(s).)

Surface roughness ubiquitously prevails in natural faults across various length scales. Despite extensive studies highlighting the important role of fault geometry in the dynamics of tectonic earthquakes, whether and how fault roughness affects fluid-induced seismicity remains elusive. Here, we investigate the effects of fault geometry and stress heterogeneity on fluid-induced fault slip and associated seismicity characteristics using laboratory experiments and numerical modeling. We perform fluid injection experiments on quartz-rich sandstone samples containing either a smooth or a rough fault. We find that geometrical roughness slows down injection-induced fault slip and reduces macroscopic slip velocities and fault slip-weakening rates. Stress heterogeneity and roughness control hypocenter distribution, frequency-magnitude characteristics, and source mechanisms of injection-induced acoustic emissions (AEs) (analogous to natural seismicity). In contrast to smooth faults where injection-induced AEs are uniformly distributed, slip on rough faults produces spatially localized AEs with pronounced non-double-couple source mechanisms. We demonstrate that these clustered AEs occur around highly stressed asperities where induced local slip rates are higher, accompanied by lower Gutenberg-Richter b -values. Our findings suggest that real-time monitoring of induced microseismicity during fluid injection may allow identifying progressive localization of seismic activity and improve forecasting of runaway events., Competing Interests: Competing interests statement:The authors declare no competing interest.

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.