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One main goal of the Continental Deep Drilling Program (KTB) of the Federal Republic of Germany was to establish a continuous stress profile from the surface to the final drilling depth of 9.1 km. To characterize stresses with depth, several independent methods were applied: analyses of borehole failure such as borehole breakouts/drilling-induced tensile fractures; hydraulic fracturing mini-tests at several intervals ≤3 km depth as well as two modified hydraulic tests at 6 and 9 km depth; and core disking and strain retardation of core samples. Focal mechanisms of induced seismic events from fluid injection experiments were inverted for stress estimates at different depths. Since then, the KTB is known as a world-class site with regard to crustal stress data. In particular, stress magnitude estimates are still among the deepest and fewest high-quality estimates derived at crustal depth. The GEOREAL fluid injection experiment aims to characterize the geothermal potential at the KTB site at 4 km depth and to refine the adaptive reservoir stimulation concept employing near-real-time microseismic monitoring with direct feedback on hydraulic parameters. Additionally, a goal of GEOREAL is to investigate spatial and temporal stress variations at this depth. We noticed new borehole breakouts in the open hole section of the pilot well KTB-VB, likely due to the massive fluid production and injection experiments between 2002 and 2005. Together with new logging and seismic data from GEOREAL, these stress estimates will be used to further characterize the stress field from the borehole to the reservoir scale. The GEOREAL hydraulic stimulation will include a series of hydraulic tests at ≥3.9 km to investigate the effect of pressure build-up and release, the role of continuous and periodically varying flow rates, the effect of relaxation phases and maximum injection pressure on the spatio-temporal propagation of induced seismicity. Induced events will be monitored with high precision using a 12-level geophone chain in the KTB main hole at only ~300 m distance to the stimulation interval. This will be used to determine stress estimates from focal mechanism inversion of induced events on a 100-m source scale.To better understand the role of the local stress field we use a 3-D geomechanical-numerical model (10 x 10 x 10 km3) of the KTB. This offers a unique opportunity to utilize the detailed knowledge of the subsurface at the KTB site, in particular due to the existing 3-D structural model, high-quality rock property estimates from laboratory work, high-quality stress magnitude data, and new information from GEOREAL. The model provides a continuous description of the 3-D stress field including its changes due to the variability of rock properties to assess the in-situ stability of the intact rock mass and faults. This allows for further detailed studies that require the undisturbed in-situ stress state as one key observable and input parameter to characterize deep geothermal reservoirs and associated processes such as induced seismicity.

The stress field of Earth's upper crust is crucial for geodynamic processes and of key importance in planning and managing the utilization of the subsurface, such as geothermal energy extraction, stimulation of enhanced geothermal systems, or safety assessment of deep geological repositories. The contemporary 3-D stress state also provides the basis to assess the impact of induced stress changes which can lead to the reactivation of pre-existing faults, the generation of new fractures, or subsidence due to long-term depletion.However, information on the stress state of Earth's crust is sparse and often not available for the areas of interest. So far, the stress tensor orientations and stress regimes have been systematically compiled and provided by the World Stress Map (WSM) project in a public-domain database. Yet, the acquisition of stress tensor orientations is not necessarily accompanied by the determination of the stress magnitudes, which, however, are required when investigating questions related to stability and hazard mitigation strategies of georeservoirs. To estimate the 3-D stress state, geomechanical-numerical modelling is applied. For the calibration of such models, stress magnitude data are essential. A major challenge is to bridge the scale gap between the widely scattered data that is required for model calibration and the high-resolution small-scale geological model in the target area. Ziegler et al. (2016) presented a multistage approach to resolve this challenge. For this, two successively calibrated models are created – one large-scale model with coarse resolution but available stress magnitude data for calibration, and one local model with fine resolution, e.g., based on a 3-D seismic survey of the target area, but without any stress data. Synthetic data obtained through the large-scale model is used to calibrate the small-scale local model.First, we validated the multistage approach by means of generic models to rigorously quantify the associated introduced uncertainties. For this purpose, we implemented a highly simplified model lithology with only vertical stratification and no lateral changes. In a second step, we applied the multistage approach in a real-world setting and demonstrated the applicability on a local model of Unterhaching, south of Munich/Germany, where a geothermal district heating plant is located. Here, a local high-resolution model based on a 3-D seismic survey (Budach et al., 2018) has been successfully calibrated on a regional-scale stress model of the Bavarian Molasse Basin. The results of the local-scale model agree with the large-scale model. At the same time, stress change due to rock property variability, only resolved in the local-scale model, is shown.

Aftershocks are a time-dependent (exponential decay) phenomenon in the aftermath of large earthquakes. In subduction zones, those occurring in the upper plate are of special concern given their potential seismic hazard, as they may produce substantial surface shaking close to highly populated cities. Therefore, the understanding of the mechanisms that drive upper-plate aftershocks is of utmost importance to improving seismic hazard assessment. Transfer of static coseismic stresses has been commonly proposed to explain this; however, they fail to explain their exponential decay over time. This time-dependency is observed in postseismic geodetic measurements, suggesting that the processes that control the postseismic surface deformation also govern or at least are involved in the generation of upper-plate aftershocks. Here, the postseismic surface deformation is dominated by aseismic slip along the fault interface (afterslip), non-linear viscoelastic relaxation in the lower crust and upper mantle, and pore-pressure diffusion in the crust. Despite great research efforts, however, the key driver remains elusive.In this study, we investigate which postseismic mechanism mainly controls the occurrence of aftershocks in the upper plate in subduction zones using the 2014 Mw=8.1 Iquique earthquake, northern Chile, as a study case. We employ a 4D numerical forward model to simulate the transient poroelastic and non-linear viscoelastic relaxation, whose contributions are subtracted from the cumulative Global Navigation Satellite System (GNSS) measurements to then invert for afterslip. Using realistic rock material properties, we first show that this approach explains the surface displacements during the first nine months of postseismic deformation recorded by continuous GNSS. For the same period, we then compute the spatiotemporal Coulomb Failure Stress changes (ΔCFS) that result from individual postseismic processes and compare them with the upper-plate aftershocks using a high-resolution seismicity catalog and focal mechanisms. We show for the first time that the ΔCFS produced by pore-pressure diffusion induced by the mainshock are unambiguously better correlated in space and time with the increase in upper-plate aftershocks than those from afterslip or non-linear viscous relaxation. In addition, pore-pressure diffusion lowers the effective normal stress of the stress tensor more effectively, while its resulting ΔCFS are relatively independent of the fault orientation. The latter would also explain the diversity of faulting styles in the upper plate exhibited by focal mechanisms following the 2014 Iquique earthquake and other subduction zone earthquakes. Our findings provide new insights into the link between pore-pressure diffusion and upper-plate deformation in subduction zones with implications for time-dependent seismic hazard.

This paper presents the first comprehensive analysis of the present- day stress from boreholes near an active continental rifting zone in the Moatize Basin, Mozambique. The state of present-day stress in this area that is located ca. 100 km away from the Eastern African Rift System (EARS) is poorly understood, and most of our knowl- edge is from earthquake focal mechanisms that provide stress information from the deeper part of the lithosphere, and to a lesser extent from surface geological fea- tures in the Malawi region. Considering the limited reliability of earthquake-derived stress orientations near plate boundaries, poor coverage of low to moderate magni- tude earthquakes in eastern Africa, and ambiguity about the latest activity of geo- logical structure; other well- established methods are required to shed light on the active tectonics of EARS. In this study, we analyse stress orientation using log data from 95 vertical boreholes in a mine site to investigate the neotectonic stress pattern of the region. Analysis of 17.9 km of televiewer logs resulted in interpretation of 1188 stress-related borehole failures. The results indicate a mean regional trend of 045° ± 31° for the maximum horizontal stress (SHmax). Our investigation reveals that the regional state of stress in the study area is controlled by superposition of stress sources that act at very different spatial scales. The consistency between our results and predictions by plate-scale geodynamic models of stress orientations confirm that the regional pattern of stress in this area is mainly controlled by first (>500 km) and second (distances between 500 and 100 km) order stress sources (i.e., large tec- tonic forces and lateral density variations). However, high- resolution data used in this study reveal that third (between 100 and 1 km) and fourth (

The distance to failure of the upper crustal rock in the prevalent stress field is of importance to better understand fault reactivation by natural and induced processes as well as to plan and manage georeservoirs. In particular, the contemporary stress state is one of the key ingredients for this assessment. To provide a continuous description of the 3D absolute stress state geomechanical‐numerical models are used. However, stress magnitude data for model calibration are sparse and incomplete and thus, the resulting model uncertainties are large. In order to reduce the uncertainties, we incorporate additional constraints on stress magnitudes to check the plausibility of different data‐based stress states. We use formation integrity tests, borehole breakouts, drilling induced fractures, and observations of seismicity and distinct seismological quiescence. This information is weighted according to its confidence and the agreement with the different modeled stress states is assessed. The information is introduced to a Bayesian approach to estimate weights of the modeled stress states and thereby identify their plausibility. A case study in southern Germany shows the ability of the approach to identify from a wide range of stress states a small number of plausible ones and reject implausible stress states. This significantly reduces the number of stress states and thus lowers the model uncertainties., Plain Language Summary The upper crust of the Earth (upper few kilometers) is subject to a kind of pressure, referred to as stress. When the stress becomes larger than the strength of the rock, the rock breaks. Sometimes this can be measured or even felt as a seismic event. It happens naturally but may also happen due to human activity. To prevent such induced seismic event, it is important to know the stress state. But there is only few information on the magnitude of the stress so we need computer models to predict the stress state. These models are often not very precise since there is only few information on the stress magnitudes and in addition they are often contradicting. We use all stress magnitude information individually to model various stress states. Then we look at other information that is related to the stress state but does not provide stress magnitude information on its own. We compare this information with the modeled stress states to find out whether a stress state agrees with the additional information or not. This allows us to identify a few realistic stress state models out of a wide range of possible ones. This reduces the uncertainties of the stress predictions.

The undisturbed stress state of the subsurface is a key parameter for the assessment of geological barrier integrity in deep geological repositories. A robust knowledge of the stress state is required for forward simulations that predict the evolution of the deep geological repository during building, canister emplacement, and post-closure phase. However, stress magnitude data will always be sparse and point-wise and thus not sufficient to allow a continuous characterization of the stress field. 3D geomechanical-numerical models are incorporated to mitigate this shortcoming. The significance of such a model is tied to the stress magnitude data records whose availability is generally low and subject to measurement errors. In turn, the uncertainties of the modelled 3D stress state are large which lowers the predictive values of the ensuing forward models. Here, we present a novel approach to reduce the uncertainties of a modelled stress state (Ziegler and Heidbach, 2023).In a first step we estimate all possible and realistic stress states with respect to the available stress magnitude data and their uncertainties. This provides an overall quantification of the range of stress states that are supported by data. In a second step we limit this usually very large range of stress states using additional indirect stress information to determine upper and lower limits. Formation integrity tests (FITs) are used as a lower limit for the stress state. Observed borehole wall failures (drilling induced tensile fractures and borehole breakouts) provide an estimate of the quality of a modelled stress state in relation with estimations of the rock strength. Observed seismicity and observed seismological quiescence in connection with a failure criterion is used as an upper constraint of the stress state. These additional data allow to identify the probability of a modelled stress state based on its agreement with observations. A weight is estimated for each modelled stress state in a formalized Bayesian approach. This allows an improvement in the significant interpretation of the initial stress state in terms of its robustness and transparency.Results from an explanatory model and a case study in the Bavarian Molasse Basin show an improved significance in terms of clearly reduced model uncertainties. The amount of uncertainty reduction, however, depends significantly on the quality, suitability and assessment of the additional information. Reference:Ziegler, M. O., & Heidbach, O. (2023). Bayesian quantification and reduction of uncertainties in 3D geomechanical-numerical models. Journal of Geophysical Research: Solid Earth, 128, e2022JB024855. https://doi.org/10.1029/2022JB024855

In 2018, after more than 700 years, the last black coal mine ceased operation in the greater Ruhr region in western Germany. In addition to the repurposing of the unused subsurface infrastructure for heat storage projects, the utilization of deep geothermal resources, located below mining levels in the Devonian Massenkalk formations, is the most promising way of facilitating the green energy transition in this highly populated region. The operation of fluid injection or withdrawal during geothermal production alters subsurface stress and poses questions about the possible reactivation of faults in the greater Ruhr region, with strong implications for seismic hazard potential. Deep geothermal resources depend on permeable pathways (i.e., fault zones and fracture networks) in the subsurface, which are known to have certain intrinsic pre-operational seismic hazard. In this study, we evaluate the probability of reactivation (using the so-called slip tendency and fracture susceptibility indicators) and dilation tendency of major faults accounting for uncertainties of in situ stress conditions, fault geometries, and frictional properties. Furthermore, we investigate the spatio-temporal evolution of the slip tendency of a major fault in the region during geothermal operations utilizing coupled thermo-hydro-mechanical numerical models. This study benefits from a recently published in situ stress database of the greater Ruhr region derived from 429 hydrofracturing tests performed across several coal mines. The assessment of fault reactivation utilizing probabilistic and coupled numerical modelling approaches, as presented in this study, aims at de-risking the exploration of deep geothermal systems in the greater Ruhr region and can hopefully serve as an example for quantification of pre-operational seismic hazards in other regions. Based on preliminary results, we find that the NW-SE striking faults are much more prone to reactivation, in comparison to the NE-SW ones, which are oriented unfavorably under prevailing stress conditions. We also find that thermal stress contributes significantly to fault stability, especially during long-term geothermal production.

For the safe usage of the subsurface the stress state is of great importance, e.g., for borehole stability, mitigation of induced seismicity or the search and long-term safety of a high-level nuclear waste deposit. However, the state of knowledge concerning the stress state in Germany is limited as only unevenly distributed stress measurements are available which frequently provide only one component of the stress tensor. The SpannEnD (Spannungsmodell Endlagerung Deutschland) project aims to improve this knowledge with the help of a 3D geomechanical-numerical model. The model is calibrated on available stress magnitudes and enables a continuum-mechanics based prediction of the stress state and its local variability for Germany.The 3D geomechanical-numerical model comprises the upper lithosphere and contains 22 lithological units parametrized with individual mechanical properties (Young’s modulus and Poisson’s ratio) and densities. Linear elasticity is assumed and the finite element method (FEM) is used to solve the equilibrium of forces. Overall, the model contains about 11 million hexahedral elements resulting in a lateral resolution of 2.5 x 2.5 km2 and a vertical resolution of up to 250 m. The model is calibrated by adaptation of displacement boundary conditions with magnitudes of the minimum (Shmin) and maximum horizontal stresses (SHmax). The model results show an overall good fit with these stress magnitudes used for calibration indicated by a mean of the absolute stress differences of 4.6 MPa for Shmin and 6.4 MPa for SHmax. Furthermore, the results agree well with additional data sets excluded from calibration but used for validation, e.g., with a mean of the absolute stress differences of 1.1 MPa for vertical stress magnitudes and an absolute mean deviation of the orientation of SHmax with regard to World Stress Map data of 11.9°.

For many underground operations such as geothermal energy exploitation, mining, oil and gas production or the storage of high-level radioactive waste, active tectonic or induced seismicity is of concern. Seismicity usually occurs on pre-existing faults that are reactivated under adequate stress conditions. Thus, an assessment of the reactivation potential of faults can aid in the identification of areas particularly prone to the occurrence of seismic events or such areas where adequate geotechnical measures have to be taken to avoid anthropogenic fault reactivation. A tool for the assessment of the fault reactivation potential is the so called slip tendency, which is the ratio between the maximum resolved shear stress on the fault plane and the normal stress. Such an analysis requires information about the stress field acting on the fault plane and information about the fault geometry, fault orientation and frictional properties. Information about these parameters can be very limited, since 3D fault geometries are often only extrapolated from geological surface data. Furthermore, stress data is usually sparse, only available pointwise and unevenly spatially distributed. Geomechanical-numerical modelling can be used to derive a spatially comprehensive description of all six independent components of the stress tensor from the available stress data. For Germany, an estimate of the stress tensor is provided by the geomechanical-numerical model by Ahlers et al. (2022). Furthermore, fault geometries as part of geological models of the German federal states are available for large parts of Germany. We use both the stress data derived from the geomechanical-numerical model and the fault geometry data from the federal state models to calculate slip tendencies for more than 10.000 faults and fault segments. The resulting slip tendency is generally the highest in the northern Upper Rhine Graben area where it routinely reaches values of 0.7 and more. In the Alpine and Alpine Foreland region the slip tendency is generally the lowest with values only very rarely exceeding 0.3. In North Germany slip tendency values range mainly between 0.3 and 0.6 but with both higher and lower values being fairly common. In general, faults striking in NNE-SSW direction and NW-SE direction display the overall highest slip tendencies whereas faults striking in ENE-WSW direction show very low slip tendencies. With increasing depth slip tendencies generally decrease strongly. However, there are still major areas in Germany where either no fault geometries or only insufficient fault geometries are available. Furthermore, pore pressure has a major influence on the slip tendency. For our calculations, we assume hydrostatic pore pressure. While overpressured pore fluid is documented for example for the Molasse Basin in South Germany, no spatially comprehensive pore pressure data set is currently available for the whole of Germany.

The upper Earth crust is increasingly used by mankind, to extract, transport, store or dispose materials, energy, or waste. Regardless of the objective, long term safety and stability is essential and thus, the contemporary stress state of the upper crust is one of the key variables. To estimate a continuous description of the 3‑D stress tensor, geomechanical numerical models are used. The most important parameters to set up such models are the knowledge of the underground structures, the distribution of rock properties as well as the stress data, on which the models are calibrated. In the model, the vertical stress results from the gravitational volume forces due to the density distribution and the horizontal stresses from the Poisson effect as well as appropriate lateral displacement boundary conditions. The latter are determined by finding a best-fit with respect to given stress magnitude data of the maximum and minimum horizontal stress SHmax and Shmin, respectively.A unique dataset of stress magnitude data has been recently acquired within the exploration phase for deep geological repository of radioactive waste in Switzerland. Nine cored boreholes in three potential siting areas have been drilled and besides a wide range of logging runs, and laboratory tests of rock properties, more than 120 Mini-Hydraulic Fracturing (MHF) and Sleeve Re-Opening (SR) tests were successfully performed in different stratigraphic units to estimate the magnitudes of Shmin and SHmaxHere, we present a 3‑D geomechanical-numerical model that shows both, the best-fit to the measured stress magnitudes as well as the range of stress magnitude variability in the volume of the different stratigraphic units. This variability results from MHF/SR measurements uncertainties and from the variation of rock properties within the lithologies. Furthermore, one has to assess how representative each MHF/SR measurement is for a larger rock volume. To represent the stress variability within the lithologies, many model simulations that cover the distribution of possible rock parameters were performed. The distribution is given by the cumulative density function (CDF) for the Youngs modulus and the Poisson number for each stratigraphic unit. Based on the range of model simulations we visualize the variation of the stress components along virtual well paths in analogy to the statistical variation. Such plots allow to quantify and visualize the potential variation of the present-day stress state within the stratigraphic column because of the petro-physical variability within the stratigraphic units. Furthermore, using the CDF, we can assign to each model simulation a probability that allows us also to estimate a probability distribution of the stress variability in the different units.

We analyze the spatiotemporal evolution of seismicity during a sequence of moderate (an Mw 4.7 foreshock and Mw 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.

Large earthquakes impose differential stresses in the crust and upper mantle that are transiently relaxed during the postseismic phase mostly due to afterslip on the fault interface, viscoelastic relaxation in the lower crust and upper mantle, and poroelastic rebound in the upper crust. During the last years, the wealth of geophysical and geodetic observations, as well as great effort in forward and inverse modelling have allowed a better comprehension of the role of these mechanisms during the postseismic period. However, it is still an open question to what extent postseismic processes contribute to the surface deformation signal, especially during the early postseismic period. In this study, we use GNSS and InSAR observations collected in the first 48 days following the 2010 Maule earthquake in Chile along with a model approach that integrates afterslip, poroelasticity, and temperature-controlled power-law (non-linear viscosity) rheology. The afterslip distribution is obtained from a geodetic data inversion after removing the poro-viscoelastic component by forward modelling to the geodetic data. We find that our model approach explains the geodetic cumulative signal 14% better than a pure elastic model inverting for afterslip. This improvement is mainly produced by the better fit to the geodetic signal at the volcanic and back-arc regions due to the inclusion of non-linear viscoelastic processes, which can explain > 60% of the observed surface displacements in these regions. We also show that poroelastic processes play a significant role locally, specifically near the region where the coseismic slip was largest. Here, poroelastic processes explain most of the cumulative observed GNSS uplift signal and produce surface landward patterns that affect the horizontal GNSS component by up to 15% in the opposite direction. If poroelastic processes are ignored, our results reveal that the resulting afterslip amplitude is both amplified and suppressed by up to 40% in regions of ~50 x 50 km2. Our findings have implications for the calculation of the postseismic slip budget, and therefore the seismic hazard assessment of future earthquakes.

Seismic hazard during subsurface operations is often related to the reactivation of pre-existing tectonic faults. The analysis of the slip tendency, i.e., the ratio of shear to normal stress acting on the fault plane, allows an assessment of the reactivation potential of faults. We use the total stresses that result from a large-scale 3D geomechanical–numerical model of Germany and adjacent areas to calculate the slip tendency for three 3D fault geometry sets with increasing complexity. This allows us to draw general conclusions about the influence of the fault geometry on the reactivation potential. In general, the fault reactivation potential is higher in Germany for faults that strike NW–SE and NNE–SSW. Due to the prevailing normal stress regime in the geomechanical–numerical model results, faults dipping at an angle of about 60∘ generally show higher slip tendencies in comparison to steeper or shallower dipping faults. Faults implemented with a straight geometry show higher slip tendencies than those represented with a more complex, uneven geometry. Pore pressure has been assumed to be hydrostatic and has been shown to have a major influence on the calculated slip tendencies. Compared to slip tendency values calculated without pore pressure, the consideration of pore pressure leads to an increase in slip tendency of up to 50 %. The qualitative comparison of the slip tendency with the occurrence of seismic events with moment magnitudes Mw>3.5 shows areas with an overall good spatial correlation between elevated slip tendencies and seismic activity but also highlights areas where more detailed and diverse fault sets would be beneficial.

The Main Marmara Fault (MMF) in NW Turkey south of Istanbul is a segment of the North Anatolian Fault Zone (NAFZ) that constitutes a right-lateral continental transform fault. Several well-documented strong (M7+) earthquakes indicate that the MMF poses a great risk to the Istanbul metropolitan region. A 150 km long stretch of the MMF has not ruptured since 1766 and the recurrence time of 250 yrs for M7+ events derived from historical records indicate that the fault is overdue. We introduce a new project addressing how the rheological configuration of the lithosphere in concert with active fluid dynamics within the crust and mantle influence the present-day deformation along the MMF in the Marmara Sea region. We test the following hypotheses: (1) the seismic gap is related to the mechanical segmentation along the MMF which originates from the rheological configuration of the crust and lithosphere; (2) variations in deformation mechanisms with depth in response to variations in temperature and (fluid) pressure exert a first-order control on the mode of seismic activity along the MMF, and, (3) stress and strain concentrations due to strength and structural variability along the MMF can be used as an indicator for potential nucleation areas of expected earthquakes. To assess what mechanisms control the deformation along the MMF, we use data from the ICDP GONAF observatory (International Continental Drilling Programme – Geophysical Observatory at the North Anatolian Fault) and a combined work flow of data integration and process modelling to derive a quantitative description of the physical state of the MMF and its surrounding crust and upper mantle. Seismic and strain observations from the ICDP-GONAF site are integrated with regional observations on active seismicity, on the present-day deformation field at the surface, on the deep structure (crust and upper mantle) and on the present-day stress and thermal fields. This will be complemented by numerical forward simulations of coupled thermo-hydraulic-mechanical processes based on the observation-derived 3D models to evaluate the key controlling factors for the present-day mechanical configuration of the MMF and to contribute to a physics-based seismic hazard assessment.

Between 1986 and 1995, 429 hydrofracturing tests have been carried out in six now-abandoned coal mines and two coal bed methane boreholes at depths between 600 and 1950 m within the greater Ruhr region in western Germany. From these tests, stress magnitudes and orientations of the stress tensor are derived. The majority of hydrofracturing tests were carried out from mine galleries away from mine workings in a relatively undisturbed rock mass. These data along with detailed information have been disclosed recently. In combination with already published material, we provide the first comprehensive stress database of the greater Ruhr region. Our study summarises the results of the extensive in situ stress test campaign and assigns quality to each data record using the established quality ranking schemes of the World Stress Map project. The stress magnitudes suggest predominantly strike-slip stress regime, where the magnitude of the minimum horizontal stress, Shmin, is half of the magnitude of the maximum horizontal stress, SHmax, implying that the horizontal differential stress is high. We observe no particular change in the stress gradient at depth throughout the Carboniferous layers and no significant difference between tests carried out in coal mines and deep boreholes. The mean SHmax orientation varies between 133 ± 13∘ in the westernmost located Friedrich Heinrich coal mine and 168 ± 23∘ in the easternmost located Westphalia coal mine. The mean SHmax orientation, based on 87 data records from this and already published studies, of 161 ± 43∘ is in good agreement with the regional stress orientation observed in northwestern Europe. The presented public database provides in situ stress magnitude and stress orientation data records that are essential for the calibration of geomechanical numerical models on regional and/or reservoir scales for, among others, assessing stability issues of borehole trajectories, caverns, and georeservoirs in general. For an application example of this database, we estimate slip and dilation tendencies of major geological discontinuities, discovered during the 700-year-long coal mining activities in the region. The result, although burdened by high uncertainties, shows that the discontinuities striking in the N–S and NW–SE directions have a higher slip tendency compared to the ones striking ENE–WSW and NNW–SSE, whereas a high dilation tendency is observed for discontinuities striking NNW–SSE and a low dilation tendency for the ones striking ENE–WSW. The stress orientation database is available under https://doi.org/10.24406/fordatis/200 (Kruszewski et al., 2022a), the stress magnitude database is available under https://doi.org/10.24406/fordatis/201 (Kruszewski et al., 2022b), whereas the hydrofracturing test reports are available under https://doi.org/10.24406/fordatis/222 (Kruszewski et al., 2022c).

The study of crustal stress examines the causes and consequences of in-situ stress in the Earth’s crust. Stress at any given point has several geological sources, including ‘short-term and local-scale’ and ‘long-term, ongoing and wide-scale’ source. In order to better characterise the crustal stress state, the analyses of both local- and wide-scale sources, and the consequences of their superposition are required. The global compilation of stress data in the World Stress Map database has increased significantly since its first release in 1992 and its analysis revealed large rotations of the stress tensor in several intraplate settings.Large-scale stress analysis, so called first-order, (> 500 km) provides information on the key drivers of the stress state that result from large density contrasts and plate boundary forces. The analyses of stress at smaller-scales (< 500 km) have numerous applications in reservoir geomechanics, geo-storage sites, civil engineering and mining industry. To date, numerous studies have investigated the stress analysis from different perspectives. However, the stress, in geosciences, is still enigmatic because it is a scale-dependant parameter. It means, stress variations can be studied at both the ‘spatial-scale’ and ‘temporal-scale’. This paper aims to investigate the crustal stress pattern with a particular emphasis on the orientation of maximum horizontal stresses at various spatial-scales, ranging from continental scales down to basin, field and wellbore scales, to better evaluate the role of various stress sources and their applications in the Earth’s crust. The stress analyses conducted in this work shows that stress pattern at large-scales do not necessarily represent the in-situ stress pattern at smaller-scales. Similarly, analysis of just a couple of borehole measurements in one area might not yield a good representation of the regional stress pattern.

We present initial results from a physically self-consistent 3D geomechanical model for the Fitzroy Trough in the Canning Basin, Western Australia. Our research aims to improve the understanding of in-situ stresses in the trough, in particular the overall stress regime and spatial changes of the stress magnitudes due to basin geometry. We investigate the in-situ stresses first by utilising the classic “one-dimensional” approach to geomechanics and second by building a 3D geomechanical-numerical model. The boundary conditions of the 3D model are calibrated against stress data from wells within the trough. We show that the resulting overall stress regime is most likely strike-slip, although stress data from a small subset of wells would suggest a reverse faulting regime when considered in isolation. Further, basement depth and distance to basin boundary influence the stress magnitudes and lead to significant lateral variability of the stress magnitudes.

Natural seismicity and tectonic activity are important processes for the site-selection and for the long-term safety assessment of a nuclear waste repository, as they can influence the integrity of underground structures significantly. Therefore, it is crucial to gain insight into the reactivation potential of faults. The two key factors that control the reactivation potential are (a) the geometry and properties of the fault such as strike direction and friction angle, and (b) the orientations and magnitudes of the recent stress field and future changes to it due to exogenous processes such as glacial loading as well as anthropogenic activities in the subsurface. One measure of the reactivation potential of faults is the ratio of resolved shear stress to normal stresses at the fault surface, which is called slip tendency. However, the available information on fault properties and the stress field in Germany is sparse. Geomechanical numerical modelling can provide a prediction of the required 3D stress tensor in places without stress data. Here, we present slip tendency calculations on major faults based on a 3D geomechanical numerical model of Germany and adjacent regions of the SpannEnD project (Ahlers et al., 2021). Criteria for the selection of faults relevant to the scope of the SpannEnD project were identified and 55 faults within the model area were selected. For the selected faults, simplified geometries were created. For a subset of the selected faults, vertical profiles and seismic sections could be used to generate semi-realistic 3D fault geometries. Slip tendency calculations using the stress tensor from the SpannEnD model were performed for both 3D fault sets. The slip tendencies were calculated without factoring in pore pressure and cohesion, and were normalized to a coefficient of friction of 0.6. The resulting values range mainly between 0 and 1, with 6 % of values larger than 0.4. In general, the observed slip tendency is slightly higher for faults striking in the NW and NNE directions than for faults of other strikes. Normal faults show higher slip tendencies than reverse and strike slip faults for the majority of faults. Seismic events are generally in good agreement with the regions of elevated slip tendencies; however, not all seismicity can be explained through the slip tendency analysis.

Faults are crucial structures in the subsurface with respect to seismic hazards or the exploitation of the subsurface. However, even though it is clear that the released elastic energy changes the stress field, it is not well known at what distance these change leave a significant imprint on the stress tensor components. In particular, it is assumed that stress tensor rotations are a measure of these changes. Furthermore, from a technical point of view, the implementation of faults in geomechanical models is a challenging task. There are several implementation concepts are to mimic faults in geomechanical models. The two main classes are the continuous approach (soft of low plastic elements) and the discontinuous approach (contact surfaces). However, only partial aspects of the complex behaviour of faults or fault zones are represented by these techniques.Knowing this limitation, we investigate the influence of the implementation concepts, fault properties and numerical resolution on the resulting stress field in the vicinity of a fault. The main focus of the generic models is to investigate, up to which distance from a fault, significant stress changes of the stress tensor components can be observed. In doing so, the respective models undergo a deformation that produces a similar stress state. The resulting stress magnitudes are investigated along a horizontal line at a depth of 660m, parallel to the shortening direction.The result indicates, that stress magnitude pattern varies significantly close to the modelled fault, depending on the used implementation concept. However, beyond 500 m distance from the fault, the changes in stresses are The results indicate, that faults alone have limited effect on the far-field stress pattern. On the other hand, data of stress magnitudes or the stress tensor orientation close to a fault (

A decisive criterion for the selection and the long-term safety of a deep geological repository for high radioactive waste is the crustal stress state and its future changes. The basis of any prognosis is the recent crustal stress state, but the state of knowledge in Germany is quite low in this respect. There are stress orientation data provided by the World Stress Map (WSM, Heidbach et al., 2018) and stress magnitude data from a database (Morawietz et al., 2020) for Germany, both providing selective information on the recent stress field. However, these data are often incomplete, of low quality and spatially unevenly distributed. Therefore, a 3D continuous description is not possible with these data so far, at most for the orientation of the maximum horizontal stress (SHmax), but not for the most important magnitudes of the minimum (Shmin) and SHmax. In the course of the SpannEnD project, a geomechanical–numerical 3D model of Germany is created, with which a continuous description of the complete tensor of the recent stress field in Germany is possible. The model covers an area of 1250×1000 km2 from Poland in the east, to France in the west, from Italy in the south to Scandinavia in the north. The depth extent is 100 km. Even though the focus is primarily on Germany, the model area was chosen to be so wide to minimize boundary effects and for a simplified definition of the displacement boundary conditions, which are ideally oriented perpendicular or parallel to the orientation of SHmax. The model contains a total of 21 units: The upper part of the lithospheric mantle, the lower crust, four laterally overlapping units of the upper crust, and 14 stratigraphic units of the sedimentary cover. The stratigraphic subdivision of the sedimentary cover is only done in the core area of the model; because this area is the focus of our study, our calibration data are mainly from this region and well-resolved geometry data are available. Outside of the core area, the sediments are grouped into an undifferentiated unit. The units are parameterized with density and elastic material parameters (Poisson's ratio and Young's modulus). The model has a lateral resolution of 2.5×2.5 km2 and a vertical resolution of a maximum of 240 m; in total it includes 11.1 million hexahedral elements. The equilibrium of forces between body and surface forces is solved by finite element method. The model is calibrated with Shmin and SHmax magnitudes from the WSM and data from the stress magnitude database. First, an initial stress state is generated and in a second step displacement boundary conditions are defined at the model edges, which are adjusted until a best-fit to the calibration data is found. The results show good agreement with both the SHmax orientation data from the WSM and the magnitudes of the two principal horizontal stresses (Shmin and SHmax) from the magnitude database.

The stress state is a key component for the safety and stability of deep geological repositories for the storage of nuclear waste. For the stability assessment and prediction over the repository lifetime, the stress state is put in relation to the rock strength. This assessment requires knowledge of both the future stress changes and the current in situ stress state. Due to the limited number of in situ stress data records, 3D geomechanical models are used to obtain continuous stress field prediction. However, meaningful interpretation of the stress state model requires quantification of the associated uncertainties that result from the geological, stress and rock-property data. This would require thousands of simulations which in a high-resolution model is called an exhaustive approach. Here we present a feasible approach to reduce computation time significantly. The exhaustive approach quantifies uncertainties that are due to variabilities in stress data records. Therefore, all available data records within a model volume are used individually in separate simulations. Due to the inherent variability in the available data, each simulation represents one of many possible stress states supported by data. A combination of these simulations allows estimation of an individual probability density function for each component of the stress tensor represented by an average value and a standard deviation. If weighting of the data records can be performed, the standard deviation can usually be reduced and the significance of the model result is improved. Alternatively, a range of different stress states supported by the data can be provided with the benefit that no outliers are disregarded, but this comes at the cost of a loss in precision. Both approaches are only feasible since the number of stress data records is limited. However, it is indicated that large uncertainties are also introduced by variabilities in rock properties due to natural intra-lithological lateral variations that are not represented in the geomechanical model or due to measurement errors. Quantification of these uncertainties would result in an exhaustive approach with a high number of simulations, and we use an alternative, feasible approach. We use a generic model to quantify the stress state uncertainties from the model due to rock property variabilities. The main contributor is the Young's module, followed by the density and the Poisson ratio. They affect primarily the σxx and σyy components of the stress tensor, except for the density, which mainly affects the σzz component. Furthermore, a relative influence of the stress magnitudes, the tectonic stress regime and the absolute magnitude of rock properties is observed. We propose to use this information in a post-computation assignment of uncertainties to the individual components of the stress tensor. A range of lookup tables need to be generated that compile information on the effect of different variabilities in the rock properties on the components of the stress tensor in different tectonic settings. This allows feasible quantification of uncertainties in a geomechanical model and increases the significance of the model results significantly. Der Spannungszustand ist eine Schlüsselkomponente für die Sicherheit und Stabilität einer geologischen Tiefenlagerung von radioaktivem Abfall. Um die Stabilität zu beurteilen und diese auch für die erwartete Dauer der Lagerung vorherzusagen, wird der Spannungszustand in Beziehung zur Gesteinsfestigkeit gesetzt. Für diese Beurteilung müssen sowohl der in Zukunft erwartete als auch der derzeit bestehende in-situ Spannungszustand bekannt sein. Da in Bezug auf den in-situ Spannungszustand nur wenige Daten vorliegen, kommen 3D-geomechanische Modelle zum Einsatz, um die zukünftigen Spannungsänderungen kontinuierlich vorhersagen zu können. Eine aussagekräftige Interpretation des Modells des Spannungszustands erfordert allerdings eine Quantifizierung der damit einhergehenden Ungewissheiten, die sich als Folge der geologischen Gegebenheiten, der Spannungsdatenpunkte und Gesteinseigenschaften ergeben. Eine umfassende Quantifizierung dieser Ungewissheiten erfordert Tausende Rechnungen, welches in einem hochauflösenden Modell nicht praktikabel ist. Hier stellen wir einen machbaren Ansatz vor, mit dem sicher der Zeitaufwand für die Modellierung auf einen realisierbaren Rahmen beschränken lässt. Mit dem umfassenden Ansatz lassen sich Ungewissheiten quantifizieren, die sich aus den Variabilitäten in den aufgezeichneten Spannungsdaten ergeben. Daher werden alle verfügbaren Datensätze aus einem Volumenmodell individuell in separaten Szenarien genutzt. Wegen der natürlichen Variabilität der verfügbaren Daten repräsentiert jede Simulation einen von vielen möglichen Spannungszuständen, der sich aus den Daten ergibt. Eine Kombination dieser Simulationen ermöglicht es, eine individuelle Wahrscheinlichkeitsdichtefunktion für jede Komponente des Spannungstensors zu modellieren, und zwar jeweils mit einem Durchschnittswert und einer Standardabweichung. Wenn sich die Eingangsdaten gewichten lassen, lässt sich die Standardabweichung meist reduzieren und die Signifikanz des Modellergebnisses verbessern. Alternativ lässt sich ein Bereich an Spannungszuständen angeben, welche auf den Daten basieren. Das ermöglicht es, keine Ausreißer zu übersehen, geht aber mit einer geringeren Genauigkeit einher. Beide Ansätze sind nur praktikabel, weil es nur begrenzt viele Spannungsmagnitudendaten gibt. Hinzu kommt jedoch eine große Quelle für Ungewissheiten durch unterschiedliche Gesteinseigenschaften, die sich aus den natürlichen lateralen Variationen im Gestein ergeben oder ein Resultat von Messfehlern sind. Um diese Ungewissheiten zu quantifizieren, wäre ein sehr umfangreicher Ansatz mit zahlreichen Simulationen erforderlich; wir hingegen möchten einen alternativen, praktikablen Ansatz nutzen. Wir verwenden ein generisches Modell, um die Ungewissheiten des Spannungszustands zu quantifizieren, die sich aus den unterschiedlichen Gesteinseigenschaften ergeben. Am wichtigsten ist hierbei das Elastizitätsmodul, an zweiter Stelle die Dichte und die Poisson-Zahl. Diese beeinflussen vor allem die σxx und σyy Komponenten des Spannungstensors, mit Ausnahme der Dichte, die vor allem die σzz-Komponente beeinflusst. Außerdem lässt sich ein relativer Einfluss der Größenordnungen der Spannungen, des tektonischen Spannungsregimes und der absoluten Magnitude der Gesteinseigenschaften beobachten. Wir schlagen vor, anhand dieser Informationen im Anschluss an die Modellierung eine Zuordnung der Ungewissheiten zu den individuellen Komponenten des Spannungstensors vorzunehmen. Dafür werden zahlreiche Tabellen erstellt, welche die Effekte von verschiedenen Variabilitäten der Gesteinseigenschaften auf die Komponenten des Spannungstensors bei verschiedenen tektonischen Gegebenheiten umfassend darstellen. Das ermöglicht es, Ungewissheiten in einem geomechanischen Modell mit einem praktikablen Verfahren zu quantifizieren und die Aussagekraft der Ergebnisse des Modells deutlich zu verbessern.

The contemporary stress state in the upper crust is of great interest for geotechnical applications and basic research likewise. However, our knowledge of the crustal stress field from the data perspective is limited. For Western Central Europe basically two datasets are available: Orientations of the maximum horizontal stress (SHmax) and the stress regime as part of the World Stress Map (WSM) database (Heidbach et al., 2018) as well as a complementary compilation of stress magnitude data of Germany and adjacent regions (Morawietz et al., 2020). However, these datasets only provide pointwise, incomplete and heterogeneous information of the 3D stress tensor. Here, we present a geomechanical-numerical model that provides a continuous description of the contemporary 3D crustal stress state on a regional scale for Western Central Europe. The model covers an area of about 1000 × 1250 km2 and extends to a depth of 100 km containing seven lithostratigraphic units, with specific material properties (density and elastic rock properties) and laterally varying thicknesses: A sedimentary unit, four different units of the upper crust, the lower crust and the lithospheric mantle. The model is calibrated by the two datasets to achieve a best-fit regarding the SHmax orientations and the minimum horizontal stress magnitudes (Shmin). The modelled orientations of SHmax are almost entirely within the uncertainties of the WSM data used and the Shmin magnitudes fit to various datasets well. Only the SHmax magnitudes show locally significant deviations, primarily indicating too low values in the lower part of the model. The model is open for further refinements regarding model geometry, e.g., additional layers with laterally varying material properties, and incorporation of future stress measurements. In addition, it can provide the initial stress state for local geomechanical models with a higher resolution.

In September 2019 a sequence of two moderate earthquakes (Mw4.7 and Mw5.8) occurred in the central Sea of Marmara (Turkey), SW of Istanbul. These events took place ate the transition between a creeping and a locked segment of the North Anatolian Fault. To investigate in detail the spatiotemporal evolution of the seismicity, we apply a matched-filter technique to continuous waveforms, thus reducing the magnitude threshold for detection. Sequences of foreshocks preceding the two mainshocks are clearly seen, exhibiting different behaviors: a migration of the seismicity along the entire fault segment on the long-term (several days before the mainshocks) and a concentration around the epicenters of the large events on the short-term (during the few hours preceding the mainshocks). We infer that both seismic and aseismic slip during the foreshock sequences change the stress state on the fault, bringing it closer to failure. Our observations also suggest that the Mw 4.7 event contributed to weaken the fault as part of the preparation process of the Mw 5.8 earthquake.

One important criterion for the characterization of a potential nuclear waste repository is the crustal stress field. However, stress data are sparse and usually incomplete regarding the six independent components of the stress tensor. The World Stress Map (WSM) is a valuable compilation of stress data, but it does not include information about stress magnitudes as only the orientation of the maximum horizontal stress (SHmax) is provided. To receive a comprehensive and continuous 3D description of the stress field in a particular area, geomechanical-numerical modelling is required. Key objectives of the SpannEnD project (Spannungsmodell Endlagerung Deutschland) is to provide such a model for Germany and to develop methods for robust stress predictions at the local scale.The SpannEnD model is based on finite element techniques and comprises a 3D lithosphere-scale structural model of Germany. The lateral extent of the model covers a pentagon-shaped area of Central Europe with dimensions of 1000 x 1250 km². The model has been chosen significantly larger than Germany to reduce boundary effects in the study area. Furthermore, on the base of the observed stress orientation pattern, the boundaries have been defined parallel or perpendicular to the known orientation of SHmax to simplify the definition of the boundary conditions. The vertical extent of the model is from the surface to a depth of 100 km, incorporating several sedimentary layers, several basement units and the Mohorovičić discontinuity. The mesh is laterally homogenous with a resolution of about 4 km and vertically inhomogeneous with a decreasing resolution with increasing depth, to provide the finest mesh in the layers of the greatest interest, near the surface. These units also provide the most stress data measurements to calibrate the model. Furthermore, a selected number of important faults is implemented in the model. This structural model is discretized into about 4 million elements. For the calibration of the model we use a new compilation of stress magnitude data. We present the workflow, the model geometry, and some first results.

After large earthquakes at subduction zones, the plate interface continues moving due to mostly frictional afterslip processes. Below depths of 60 km, little frictional afterslip is to be expected on the plate interface due to low shear strength, lack of apparent geodetic interseismic locking, and low seismic moment release from aftershocks. However, inversion models that consider an elastic crust above a mantle with viscoelastic rheology result in a significant portion of afterslip at depths > 60 km. In this study, we present a forward 3D geomechanical-numerical model with power-law rheology that simulates dislocation creep processes for the crust and upper mantle in combination with an afterslip inversion. The linear rheology case is also considered for comparison. We estimate the cumulative viscoelastic relaxation and the afterslip distribution for the first six years following the 2010 Mw 8.8 Maule earthquake in Chile. The cumulative afterslip distribution is obtained from the inversion of the residual surface displacements between continuous GPS (cGPS) observations and predicted displacements from viscoelastic forward modelling. We investigate three simulations: two with the same dislocation creep parameters in the slab and upper mantle but different ones in the continental crust, and another with elastic properties in the crust and slab and a linear viscoelastic upper mantle. Our preferred simulation is the one with power-law rheology in the crust and upper mantle with a weak continental crust since the corresponding afterslip distribution shows the best overall fit to the cGPS displacements (cumulative and time series) as well as having a good correlation with aftershock activity. In this simulation, most of the viscoelastic relaxation occurs in the continental lower crust beneath the volcanic arc due to dislocation creep processes. The resulting afterslip pattern from the inversion is reduced at depths > 60 km, which correlates well with the spatial distribution of cumulative seismic moment release from aftershocks. We conclude that by allowing for non-linear stress relaxation in the continental lower crust due to dislocation creep processes, the resulting afterslip distribution is in better agreement with the physical constraints from the shear strength of the plate interface at depth, the predicted locking degree, and the aftershock activity.

The undisturbed stress state of a potential site for nuclear waste disposal is of key importance for the assessment of long-term stability of the geotechnical installations and for seismic hazard assessment. In particular, the stability of pre-existing faults within and near a repository can only be evaluated with the knowledge of the initial stress state. Information on stress magnitudes is rare and unevenly distributed. Thus, 3D geomechanical-numerical modelling is used to estimate the stress state in an area of interest. However, due to the limitation of available data, the modelled stress state has a large uncertainty which has not been rigorously quantified yet. We present an approach to quantify the uncertainties in a 3D geomechanical-numerical modelled stress field. We combine the available SHmax and Shmin data records to pairs. For each pair we compute an individual model scenario. At each location in the model each scenario contains the full stress tensor. Then, from all model scenarios we compute an average value and a standard deviation for each component of the full stress tensor at each location within the model. This provides a comprehensive assessment of the stress state and its uncertainties.Furthermore, we present an approach to reduce the previously quantified uncertainties in the model results: We use additional borehole observables (Formation Integrity Tests) and observed seismicity and - if available - its focal mechanisms. These observables cannot provide any data records on the stress state. Yet, the information that can be extracted is valuable as it contains upper boundaries for the magnitudes of the minimum principal stress (Formation Integrity Tests) and the maximum principal stress/differential stress (seismicity), respectively. These boundaries are compared to the stress states in the individual model scenarios. Then, each scenario is assigned a weight based on its agreement with the additional data. This allows computing a weighted average and a standard deviation. The resulting standard deviation is clearly smaller compared to the unweighted approach and small changes in the average stress state are observed. Thus, even with only limited data record availability, a quantification and even a significant reduction of uncertainties in the modelling results is possible which increases the significance and value of the model.

The Sea of Marmara is a tectonically active basin that straddles the North Anatolian Fault Zone (NAFZ), a major strike-slip fault that separates the Eurasian and Anatolian tectonic plates. The Main Marmara Fault (MMF), which is part of the NAFZ, contains an approximately 150 km long seismotectonic segment that has not ruptured since 1766. A key question for seismic hazard and risk assessment is whether or not the next rupture along this segment is likely to produce one major earthquake or a series of smaller earthquakes. Geomechanical characteristics such as along-strike variations in rock strength may provide an important control on seismotectonic segmentation. We find that variations in lithospheric strength throughout the Marmara region control the mechanical segmentation of the MMF and help explain its long-term seismotectonic segmentation. In particular, a strong crust that is mechanically coupled to the upper mantle spatially correlates with aseismic patches, where the MMF bends and changes its strike in response to the presence of high-density lower crustal bodies. Between the bends, mechanically weaker crustal domains that are decoupled from the mantle indicate a predominance of creeping. These results are highly relevant for the ongoing debate regarding the characteristics of the Marmara seismic gap, especially in view of the seismic hazard (Mw > 7) in the densely populated Marmara region.

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 m2. 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.

The unusual present-day crustal stress pattern of the Australian continent has been the subject of scientific debate for over 25 years. The orientation of maximum horizontal present-day stress (S Hmax ) in continental Australia is unlike all other major tectonic plates in that the stress pattern shows regional variability and it is not oriented sub-parallel to the direction of absolute plate motion. Previous studies on the stress pattern of Australia revealed that the complex stress pattern of the continent is controlled, at a first-order, by the superposition of plate tectonic forces exerted at the plate boundaries. However, prior analysis of the contemporary Australian crustal stress pattern have been unable to model or explain the stress pattern observed in most of eastern Australia, and has not extensively addressed the numerous smaller scale variations in stress orientation. The recent development of unconventional reservoirs in Australia has resulted in a greatly increased amount of new data for stress analysis in previously unstudied or poorly-constrained areas in eastern Australia. In addition, stress analysis in conventional hydrocarbon, mineral and geothermal exploration in all other parts of the continent provides the opportunity to review and update the Australian Stress Map (ASM). This study presents the new release of the ASM, with a total of 2150 stress data records in Australia (increased from 594 data records in 2003). The 2016 ASM contains 1359 data records determined from the interpretation of drilling-related stress indicators, 650 from earthquake focal mechanism solutions, 139 from shallow engineering measurements and two from geological indicators. The results reveal four distinct regional trends for the S Hmax orientation in Australia including a NNE-SSW S Hmax trend in northern and northwestern Australia, which rotates to a prevailing E-W orientation in most Western and South Australia. The orientation of S Hmax in eastern Australia is primarily ENE-WSW and swings to NW-SE in southeastern Australia. A comparison between the new ASM database and neotectonic features further confirms the role of contemporary stress in recent deformation of the Australian crust. The 2016 ASM reveals significant discrepancies between newly observed S Hmax orientations and predictions by published geomechanical-numerical models for the Australian continent. Forces generated at the boundary of Indo-Australian Plate remain the major control on the regional crustal stress pattern in continental Australia, however, the increased data density, particularly in eastern Australia, reveals numerous local perturbations of the stress field that were not previously clearly captured. Hence, the key findings of the new release are that local (intra-plate) stress sources are more significant than previously recognized, particularly in eastern Australian basins, and cause substantial local scale rotations of the present-day crustal stress pattern that have not been factored into existing Australian stress models.

The World Stress Map (WSM) compiles orientations of the maximum horizontal stress SHmax and provides the only public global database of this kind. To make the SHmax orientation data from a wide range of stress indicators comparable, a quality ranking scheme has been developed. However, for the assessment of subsurface stability, not only the orientations but also data of the principal stress magnitudes are essential to calibrate 3D geomechanical-numerical models that deliver a continuous description of the complete 3D stress tensor. Thus, a comprehensive extension of the WSM database with quality-ranked stress magnitude data is needed. In a pilot study, we compiled an open-access stress magnitude database for Germany and adjacent regions, consisting of 568 data records. Indicators of stress magnitudes are diverse and include e.g. hydraulic fracturing and overcoring. To make data from different sources comparable, we developed a quality ranking scheme for stress magnitude data for the first time. In contrast to the established WSM quality ranking for SHmax orientation data records, estimates of stress magnitudes cannot be averaged over large rock volumes or depth ranges. Instead, each point-wise information has to be considered separately. Thus, we developed a new approach for the quality ranking scheme of Shmin magnitude data records which considers both the type of stress magnitude indicator and the degree of information availability. We present the results of our work including the data quality ranking scheme, which will serve as a template for a global stress compilation within the framework of the WSM project. The next countries and regions that we will explore are Australia, Scandinavia and India. We invite you to contribute to this project in your area or country of interest and to join the WSM team as an official collaborator.

The Bavarian Molasse Basin is one of the largest European areas for low-enthalpy hydrothermal applications. In the last decades, more than 20 geothermal applications for district heating and even power generation have been established, and more are planned or under construction. At about one third of the projects, seismicity of up to Ml 2.4 has been observed after the onset of production and reinjection of fluids while no seismicity has been observed at the other project sites. In order to assess the potential for induced seismicity at a specific site, three parts of information are required: (1) The initial stress state, (2) the changes in the stress state due to production and reinjection, (3) a fault geometry (strike, dip), and (4) a fault properties in terms of a failure criterion. While a modelling of the production and/or injection induced stress changes is performed commonly and basic information on the failure behaviour of rocks is available, information on stress magnitudes is rare and unevenly distributed. Thus, 3D geomechanical-numerical modelling is used to estimate the stress state in a target area based on the few data records available.We present a 3D geomechanical-numerical model of the initial stress state in the Bavarian Molasse Basin in order to assess the individual potential for induced seismicity at different geothermal sites. Our model area contains several lithological units with different rock properties. Additionally, in our approach, we quantify the uncertainties introduced by the variability of the 13 stress magnitude data records used for the calibration of the model. We further reduce the large uncertainties by introduction of additional observables that limit the range of acceptable stress states. From the model results, we extract the stress state and its uncertainties at the two geothermal sites Aschheim/Feldkirchen/Kirchheim and Poing that are in a distance of 4 km. While seismicity of up to Ml 2.1 has been observed in Poing, the other site remained seismically quiet. Our modelled stress state at the two sites in combination with according failure criteria on optimally oriented faults is in agreement with the seismological observations. Even considering uncertainties of 2σ, the modelled stress state in Aschheim/Feldkirchen/Kirchheim is stable, while in Poing even the average stress state is already critical. These results indicate the relevance of a geomechanical assessment of sites of subsurface applications in order to minimize the potential for induced seismicity.

A long standing debate in seismology revolves around the nonexistent heat flow anomaly across the San Andreas fault. Given the fault’s average slip rate and age, a strong San Andreas fault, i.e. characterized by a relatively high static friction coefficient of µ>=0.6, should produce a significant local heat flow anomaly across the fault [1]. Since the work of Lachenbruch and Sass [1], this anomaly has not been observed and although many possible causes for the lack of a heat flow anomaly have been explored, the static or dynamic weakness of the San Andreas fault remains a favorable explanation [2,3].Recently, we have introduced the ENergy COnserving Seismicity (ENCOS) framework that relates elastic deformation energy loading rates to the long-term average energy release of the seismic process. Within the presented implementation of ENCOS for Southern California with an elastic loading rate between 300 MW and 1.9 GW, the two most significant parameters are the static friction coefficient and the average efficiency. In particular, they are the most significant sources of uncertainty in harnessing the GPS-derived strain rates and the stress data within the ENCOS framework.Here, we show how ENCOS can be leveraged in combination with the constraints from heat flow measurements and observed seismicity to restrict the parameter space of the average efficiency and the static friction coefficient. This can help to reduce the uncertainty of the ENCOS model parameters, such as the elastic deformation energy loading rate, and opens a new viewpoint on the heat flow paradox.[1] Lachenbruch, A. H., and Sass, J. H. (1980), Heat flow and energetics of the San Andreas Fault Zone, J. Geophys. Res., 85(B11), 6185–6222.[2] Scholz, C. H. (2013). The Strength of the San Andreas Fault: A Critical Analysis. In Earthquakes: Radiated Energy and the Physics of Faulting (eds R. Abercrombie, A. McGarr, G. Di Toro and H. Kanamori).[3] E. E. Brodsky et al. (2020), The State of Stress on the Fault Before, During, and After a Major Earthquake, Annu. Rev. Earth Planet. Sci. 48:2.1–2.26.

Estimating earthquake occurrence rates from the accumulation rate of seismic moment is an established tool of seismic hazard analysis. We propose an alternative, fault-agnostic approach based on the conservation of energy: the Energy-Conserving Seismicity Framework (ENCOS). Working in energy space has the advantage that the radiated energy is a better predictor of the damage potential of earthquake waves than the seismic moment release. In a region, ENCOS balances the stationary power available to cause earthquakes with the long-term seismic energy release represented by the energy-frequency distribution's first moment. Accumulation and release are connected through the average seismic efficiency, by which we mean the fraction of released energy that is converted into seismic waves. Besides measuring earthquakes in energy, ENCOS differs from moment balance essentially in that the energy accumulation rate depends on the total stress in addition to the strain rate tensor. To validate ENCOS, we exemplarily model the energy-frequency distribution around Southern California. We estimate the energy accumulation rate due to tectonic loading assuming poroelasticity and hydrostasis. Using data from the World Stress Map and assuming the frictional limit to estimate the stress tensor, we obtain a power of 0.8 GW. The uncertainty range, 0.3–2.0 GW, originates mainly from the thickness of the seismogenic crust, the friction coefficient on preexisting faults, and models of Global Positioning System (GPS) derived strain rates. Based on a Gutenberg-Richter magnitude-frequency distribution, this power can be distributed over a range of energies consistent with historical earthquake rates and reasonable bounds on the seismic efficiency.

The North Anatolian Fault (NAF) below the Sea of Marmara, also known as the Main Marmara Fault (MMF), has repeatedly produced major (M>7) earthquakes in the past. Currently, the MMF corresponds to a seismic gap between the locus of the most recent M>7 ruptures of the 1912 Ganos (M 7.3) and 1999 Izmit (M 7.4) earthquakes. This seismic gap has a recurrence time of approximately 250 years and has not ruptured since 1766. Consequently, it poses a major seismic hazard to the Marmara region, including the megacity Istanbul. The Marmara seismic gap is considered to be locked in the eastern and central segments of the MMF, while the western segment is partly creeping. In the context of seismic hazard and risk assessment, one of the main questions is, if either the Marmara seismic gap will rupture in a single large earthquake or in several ones due to segmentation along the MMF. In part this depends on the physical properties of the lithosphere below the Sea of Marmara as they are a key control of the contemporary stress state. To contribute to this discussion, we present 3‑D lithospheric-scale thermal and rheological models of the Sea of Marmara. These models are based on published 3‑D density models that indicate lateral and vertical crustal heterogeneities below the Sea of Marmara (Gholamrezaie et al., 2019). The density models consist of two layers of sediments, upper and lower crystalline crustal layers, and two crustal dome-shaped, high-density bodies that spatially correlate with major bends along the MMF. We show that these crustal heterogeneities may cause the lithospheric strength to vary significantly along the MMF, supporting the hypothesis that the fault is mechanically segmented. In addition, our results indicate a spatial correlation between observed aseismic fault patches (Wollin et al., 2018) and the location of the high-density bodies. These bodies are colder and stronger than the surrounding crystalline crust, and may thus represent the lateral bounds of the locked MMF segment.

In this study we utilize regional and teleseismic earthquake moment tensor solutions in order to infer the contemporary crustal stress in the Greek region. We focus on crustal earthquakes and select only solutions with good waveform fits and well-resolved nodal planes. A data set of 1614 focal mechanisms is used as input to a regional-scale damped stress inversion algorithm over a grid whose node spacing is 0.35°. Several resolution and sensitivity tests are performed in order to ascertain the robustness of our results. Our findings show that for most of the Greek region the largest principal stress σ1 is vertically oriented and that the minimum principal stress axis σ3 are subhorizontal with a predominant N-S orientation. In the SW Peloponnese the orientation of σ3 axes rotates clockwise and in SE Aegean counterclockwise. These results are in agreement with the generally accepted model that slab rollback combined with gravitational spreading of the Aegean lithosphere are the main causes of the extension. Transitions between different faulting types in NW Greece or in the Aegean occur within narrow zones in the order of tens of kilometers. A visual comparison of the principal horizontal stress axes and the principal strain axes derived from GPS observations shows good agreement, suggesting that the crust in the Greek region behaves largely in an elastic manner.

The postseismic deformation captured with continuous Global Positioning System (cGPS) monitoring following many recent mega-thrust events has been shown to be a signal composed of two dominant processes: afterslip on the plate interface and viscoelastic relaxation of the continental and oceanic mantles in response to the coseismic stress perturbation. Following the south-central Chile 2010 Maule Mw 8.8 earthquake, the time series from the regional cGPS network show a distinct curvature in the pathway of the horizontal motion that is not easily fit by a stationary decaying pattern of afterslip in combination with viscoelastic relaxation. Here we show that with realistic assumptions about the long-term decay of the afterslip signal, the postseismic signal can be decomposed into three first-order contributing processes: plate interface re-locking, plate interface afterslip, and mantle viscoelastic relaxation. From our analyses we conclude that the plate interface recovers its interseismic locking state rapidly (model space ranges between an instant recovery and a period of 1 year); a finding that supports laboratory experimental evidence as well as some recent studies of aftershocks and postseismic surface deformation. Furthermore, re-locking is the main cause of the curvature in the cGPS signal, and this study presents a plausible range of geodetic re-locking rates following a megathrust earthquake.

The present-day stress state is a key parameter in numerous geoscientific research fields including geodynamics, seismic hazard assessment, and geomechanics of georeservoirs. The Taranaki Basin of New Zealand is located on the Australian Plate and forms the western boundary of tectonic deformation due to Pacific Plate subduction along the Hikurangi margin. This paper presents the first comprehensive wellbore-derived basin-scale in situ stress analysis in New Zealand. We analyze borehole image and oriented caliper data from 129 petroleum wells in the Taranaki Basin to interpret the shape of boreholes and determine the orientation of maximum horizontal stress (S-Hmax). We combine these data (151 S-Hmax data records) with 40 stress data records derived from individual earthquake focal mechanism solutions, 6 from stress inversions of focal mechanisms, and 1 data record using the average of several focal mechanism solutions. The resulting data set has 198 data records for the Taranaki Basin and suggests a regional S-Hmax orientation of N068 degrees E (22 degrees), which is in agreement with NW-SE extension suggested by geological data. Furthermore, this ENE-WSW average S-Hmax orientation is subparallel to the subduction trench and strike of the subducting slab (N50 degrees E) beneath the central western North Island. Hence, we suggest that the slab geometry and the associated forces due to slab rollback are the key control of crustal stress in the Taranaki Basin. In addition, we find stress perturbations with depth in the vicinity of faults in some of the studied wells, which highlight the impact of local stress sources on the present-day stress rotation.

After large earthquakes at subduction zones, the plate interface continues moving due to mostly frictional afterslip or simply afterslip processes. Below approximately 60 km depth, the seismic moment release at the plate interface is quite small indicating that the shear strength is low and stable sliding is the prevailing process. This agrees with the lack of significant interseismic locking at deeper segments (>60 km) resulting from the inversion of geodetic data and thus low afterslip can be expected. However, inversion models that employ linear viscoelastic mantle rheology and an elastic crust result in significant afterslip at depths >60 km. In this paper, we present a combination of a 3D forward geomechanical model with power-law rheology that simulates postseismic relaxation with dislocation creep processes in the crust and upper mantle and an afterslip inversion. We estimate the cumulative viscoelastic relaxation and the afterslip distribution for the first six years following the 2010 Mw 8.8 Maule earthquake in Chile. The cumulative afterslip distribution is obtained from the inversion of the residual surface displacements between the observed displacements from the continuous GPS (cGPS) and the ones from the forward modelling. We investigate five simulations, four with different dislocation creep parameters for the crust, slab, and upper mantle and one with elastic properties for the crust and slab, and a linear viscoelastic upper mantle for comparison. Our preferred simulation considers a weak crust since it shows the best fit to the cumulative cGPS postseismic displacements, a good fit to the time-series, and, in particular, a good spatial correlation between afterslip and aftershock activity. In this simulation, most of the viscoelastic relaxation occurs in the continental lower crust beneath the volcanic arc due to dislocation creep processes. The resulting afterslip pattern from the inversion is reduced at depths >60 km, which correlates to the low cumulative seismic moment that is released from aftershocks at these depths. Furthermore, the cumulative afterslip moment release from this simulation corresponds to 10% of the main shock in six years, which is approximately half of the moment release that results from models with an elastic crust and linear viscosity in the upper mantle. We conclude that an integrated analysis by considering power-rheology with dislocation creep processes in the continental crust and upper mantle along with aftershock activity may be used to constrain location and magnitude postseismic relaxation processes better.

The surface deformation associated with the 2010 M-w 8.8 Maule earthquake in Chile was recorded in great detail before, during and after the event. The high data quality of the continuous GPS (cGPS) observations has facilitated a number of studies that model the postseismic deformation signal with a combination of relocking, afterslip and viscoelastic relaxation using linear rheology for the upper mantle. Here, we investigate the impact of using linear Maxwell or power-law rheology with a 2D geomechanical-numerical model to better understand the relative importance of the different processes that control the postseismic deformation signal. Our model results reveal that, in particular, the modeled cumulative vertical postseismic deformation pattern in the near field (< 300 km from the trench) is very sensitive to the location of maximum afterslip and choice of rheology. In the model with power-law rheology, the afterslip maximum is located at 20-35 km rather than > 50 km depth as suggested in previous studies. The explanation for this difference is that in the model with power-law rheology the relaxation of coseismically imposed differential stresses occurs mainly in the lower crust. However, even though the model with power-law rheology probably has more potential to explain the vertical postseismic signal in the near field, the uncertainty of the applied temperature field is substantial, and this needs further investigations and improvements.

The World Stress Map (WSM) is a global compilation of information on the present-day crustal in situ stress within the upper 40 km of the lithosphere. It is a collaborative project between academia and industry that aims to characterize the crustal stress pattern and to understand the causes of these stresses. The present-day stress information in the WSM database is derived from a variety of methods, primarily earthquake focal mechanism, borehole breakouts and drilling-induced tensile fractures (from borehole image or multi-arm caliper log data), in situ stress measurements (overcoring, hydraulic fracturing), and geologic indicators such as fault slip and volcanic vent alignment. The in situ stress information in the WSM database is compiled in a standardized format and quality-ranked for reliability and comparability on a global scale.

The Sea of Marmara, in Northwest Turkey, is a transition zone where the dextral North Anatolian Fault Zone (NAFZ) propagates westward from the Anatolian plate to the Aegean plate. The area is of interest in the context of seismic hazard in the vicinity of Istanbul, a metropolitan area with about 15 million inhabitants. Geophysical observations indicate that the crust is heterogeneous beneath the Marmara Basin, but a detailed characterization of the crustal heterogeneities is still missing. To assess if and how crustal heterogeneities are related to the NAFZ segmentation below the Marmara Sea, we develop a new crustal-scale 3D density model which integrates geological and seismological data and is additionally constrained by 3D gravity modelling. This model indicates that the observed gravitational anomalies originate from significant density heterogeneities within the crust. Two layers of sediments, one syn-kinematic and one pre-kinematic with respect to the Marmara Sea formation are underlain by a heterogeneous crystalline crust. A felsic upper crystalline crust (average density of 2720 kg m−3) and an intermediate to mafic lower crystalline crust (average density of 2890 kg m−3) appear to be crosscut by two large, dome-shaped mafic high-density bodies (average density of 3050 kg m−3) of considerable thickness above a rather uniform lithospheric mantle (3300 kg m−3). The spatial correlation between the bent segments of the fault and the location of the high-density bodies suggests that the distribution of lithological heterogeneities within the crust controls the rheological behaviour along the NAFZ, and consequently, influences fault segmentation and propagation dynamics.

This study aims at evaluating the spatial and temporal distribution of 26 micro-seismic events which were triggered by hydraulic stimulation at the geothermal site of Gros Schonebeck (Germany). For this purpose, the alteration of the in-situ stress state and the related change of slip tendency for existing fault zones due to stimulation treatments and reservoir operations is numerical simulated. Changes in slip tendency can potentially lead to reactivation of fault zones, the related movement can lead to the occurrence of seismic events. In the current numerical study, results obtained based on the thermal–hydraulic–mechanical coupled simulation are compared to field observations. In particular, the study focuses on describing the fault reactivation potential: (1) under in-situ stress conditions; (2) during a waterfrac stimulation treatment; and (3) during a projected 30 years production and injection period at the in-situ geothermal test-site Gros Schonebeck. The in-situ stress state indicates no potential for fault reactivation. During a waterfrac stimulation treatment, micro-seismic events were recorded. Our current evaluation shows an increase of slip tendency during the treatment above the failure level in the direct vicinity of the micro-seismic events. During the projected production and injection period, despite increased thermal stress, the values for slip tendency are below the threshold for fault reactivation. Based on these results, and to prove the applied method to evaluate the observed micro-seismic events, a final discussion is opened. This includes the in-situ stress state, the role of pre-existing fault zones, the adopted criterion for fault reactivation, and a 3D rock failure criterion based on true triaxial measurements.