Search

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.

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.

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.

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.

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 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.

Knowledge of the present-day crustal in-situ stress field is a key for the understanding of geodynamic processes such as global plate tectonics and earthquakes. It is also essential for the management of geo-reservoirs and underground storage sites for energy and waste. Since 1986, the World Stress Map (WSM) project has systematically compiled the orientation of maximum horizontal stress (S-Hmax). For the 30th anniversary of the project, the WSM database has been updated significantly with 42,870 data records which is double the amount of data in comparison to the database release in 2008. The update focuses on areas with previously sparse data coverage to resolve the stress pattern on different spatial scales. In this paper, we present details of the new WSM database release 2016 and an analysis of global and regional stress pattern. With the higher data density, we can now resolve stress pattern heterogeneities from plate-wide to local scales. In particular, we show two examples of 40 degrees-60 degrees S-Hmax rotations within 70 km. These rotations can be used as proxies to better understand the relative importance of plate boundary forces that control the long wave-length pattern in comparison to regional and local controls of the crustal stress state. In the new WSM project phase IV that started in 2017, we will continue to further refine the information on the S-Hmax orientation and the stress regime. However, we will also focus on the compilation of stress magnitude data as this information is essential for the calibration of geomechanical-numerical models. This enables us to derive a 3-D continuous description of the stress tensor from point-wise and incomplete stress tensor information provided with the WSM database. Such forward models are required for safety aspects of anthropogenic activities in the underground and for a better understanding of tectonic processes such as the earthquake cycle.

The selection of earthquake focal mechanisms (FMs) for stress tensor inversion (STI) is commonly done on a spatial basis, that is, hypocentres. However, this selection approach may include data that are undesired, for example, by mixing events that are caused by different stress tensors when for the STI a single stress tensor is assumed. Due to the significant increase of FM data in the past decades, objective data-driven data selection is feasible, allowing more refined FM catalogues that avoid these issues and provide data weights for the STI routines. We present the application of angular classification with expectation-maximization (ACE) as a tool for data selection. ACE identifies clusters of FM without a priori information. The identified clusters can be used for the classification of the style-of-faulting and as weights of the FM data. We demonstrate that ACE effectively selects data that can be associated with a single stress tensor. Two application examples are given for weighted STI from South America. We use the resulting clusters and weights as a priori information for an STI for these regions and show that uncertainties of the stress tensor estimates are reduced significantly.

The majority of published in-situ stress information in the Australian continent is confined to petroleum provinces where industry technologies facilitate the capture of contemporary crustal stress information. In contrast, the stress pattern of non-petroleum regions such as the Flinders and Mount Lofty Ranges in South Australia, where intraplate deformation is localised, has not been investigated comprehensively so far. The ongoing activities of the mining industry in South Australia has enabled us to access recently drilled boreholes for image logging techniques that have typically been under-utilised by the mineral sector. Herein, we conduct stress analysis by analysing borehole image logs from 16 boreholes in the basement rocks of South Australia as well as two geothermal wells and one coal seam gas well in South Australia's northern Flinders Ranges, the Gawler Craton and the Eyre Peninsula. The resulting dataset of stress orientations is further accomplished by including recent seismological observations, which provide crustal stress information derived from focal mechanism solutions. The results of this study suggest a regional E–W orientation of the maximum horizontal compressive stress that is consistent with numerous observed neotectonic structures in this region. The focal mechanism solutions in this study suggest that the majority of events occur in a thrust faulting stress regime, which is consistent with the observed Quaternary fault scarps. However, our data compilation also indicates the presence of strike-slip and normal faulting stress regimes in the region, which has not been suggested extensively before this study.

Mapping the contemporary stress field of the Mediterranean provides fundamental insights on the complexity of plate tectonic forces throughout the region and at different depths. Despite increased data availability and methodological improvements, most recent comprehensive stress field characterization across the entire Mediterranean dates back to 1995. To extend the regional stress information, we use all earthquake focal mechanisms compiled in the World Stress Map database release 2016 for a formal stress inversion. Our main goals are to (1) improve the resolution of the stress field orientation, (2) evaluate the performance of the recently refined stress inversion methodology in a tectonically complex region, (3) test the hypothesis of a depth-dependent stress orientation heterogeneity, and (4) compare different types of stress and strain observations from surface using the GPS-derived information on the strain rate tensor down to the seismogenic crust using summation of normalized seismic potencies. The obtained stress orientations generally capture the main seismotectonic features, including tectonically complex settings such as the Alpine Orogeny or the Ionian Sea. The orientation of the maximum horizontal stress SHmax tends to be uniform with depth within uncertainties while larger stress heterogeneity (quantified by means of the focal mechanism diversity and misfit angles) is found between 5 and 14 km. Both, the orientation of the largest horizontal shortening axis of the strain field from potency tensors, and horizontal strain rate tensor from GPS data are generally sub-parallel to SHmax orientation, indicating a linear stress/strain relationship and that the orientations of the co-seismic release and interseismic strain accumulation are generally consistent.

The Canadian database on contemporary crustal stress has not been revised systematically in the past two decades. Here we present the results of our new compilation that contains 514 new data records for the orientation data of maximum compressive horizontal stress and 188 data records that were re-assessed. In total the Canadian stress database has now 1667 data records, which is an increase of about 45%. From these data, a new Canadian Stress map as well as one for the Province of Alberta is presented. To analyse the stress pattern, we use the quasi median on the circle as a smoothing algorithm that generates a smoothed stress map of the maximum compressive horizontal stress orientation on a regular grid. The newly introduced quasi interquartile range on the circle estimates the spreading of the data and is used as a measure for the wave-length of the stress pattern. The result of the hybrid wavelength analysis confirms that long spatial wavelength stress patterns (>= 1000 km) exist in large areas in Canada. The observed stress pattern is transmitted through the intra-plate regions. The results reveal that shorter spatial wave length variation of the maximum compressive horizontal stress orientation of less than 200 km, prevails particularly in south-eastern and western Canada. Regional stress sources such as density contrasts, active fault systems, crustal structures, etc. might have a significant impact in these regions. In contrast to these variations, the observed stress pattern in the Alberta Basin is very homogeneous and mainly controlled by plate boundary forces and body forces. The influence of curvature of the Rocky Mountains salient in southern Alberta is minimal. The present-day horizontal stress orientations determined herein have important implications for the production of hydrocarbons and geothermal energy in the Alberta Basin. (C) 2014 Elsevier B.V. All rights reserved.

In the context of examining the potential usage of safe and sustainable geothermal energy in the Alberta Basin, whether in deep sediments or crystalline rock, the understanding of the in situ stress state is crucial. It is a key challenge to estimate the 3-D stress state at an arbitrarily chosen point in the crust, based on sparsely distributed in situ stress data. To address this challenge, we present a large-scale 3-D geomechanical–numerical model (700 km × 1200 km × 80 km) from a large portion of the Alberta Basin, to provide a 3-D continuous quantification of the contemporary stress orientations and stress magnitudes. To calibrate the model, we use a large database of in situ stress orientation (321 SHmax) as well as stress magnitude data (981 SV, 1720 Shmin and 2 (+11) SHmax) from the Alberta Basin. To find the best-fit model, we vary the material properties and primarily the displacement boundary conditions of the model. This study focusses in detail on the statistical calibration procedure, because of the large amount of available data, the diversity of data types, and the importance of the order of data tests. The best-fit model provides the total 3-D stress tensor for nearly the whole Alberta Basin, and allows estimation of stress orientation and stress magnitudes in advance of any well. First-order implications for the well design and configuration of enhanced geothermal systems are revealed. Systematic deviations of the modelled stress from the in situ data are found for stress orientations in the Peace River and the Bow Island Arch as well as for leak-off test magnitudes.

SUMMARY We present the contemporary 3-D background stress field of the Marmara Sea region derived from a geomechanical model. The background stress field (i.e. the component of the absolute stress state that is time-independent over the seismic cycle) primarily depends on the distribution of density and elastic parameters, on the acting far-field stresses from plate boundary forces and on fault geometries. We take these into account to predict the 3-D background stress field including its spatial variations. Technically, our model concept involves the definition of an appropriate initial stress state accounting for the gravitational reference stress state of the crust, which is then changed by plate tectonics until the 3-D background stress field has evolved. The modelled stress field agrees well with observations from earthquake focal mechanism solutions and their formal stress inversion, with orientation of maximum horizontal stress and with the distribution of seismicity in the Marmara Sea. In particular in the vicinity of fault bends the stress field deviates considerably from the regional NW–SE oriented maximum horizontal stress and exhibits variability of the stress regime. Our model results are consistent not only with dynamic observations but also with kinematic ones. Various kinematic observations are understandable from the stress field. We show that a stress regime that indicates normal faulting in the basins is nonetheless reconcilable with almost pure strike-slip motion on the Main Marmara Fault. The distribution of seismicity in the Marmara Sea can be explained in first order by the distribution of critical differential stress, which is closely related to local fault geometries. We refer the wide absence of seismicity between the bend of the Main Marmara Fault near Istanbul and the Central basin to the relatively plane fault geometry of that segment. Normal stress on the Main Marmara Fault is highly variable along strike, which makes segment-wise rupture more likely than a rupture of the seismic gap at once. Comparably low normal stress on the Prince's Islands Segment indicates a shorter interevent time for this fault segment than for the central segment of the Main Marmara Fault.

SUMMARY We investigate by means of a 3-D geomechanical model the relationship between structural elements and contemporary kinematics in the Marmara Sea region, northwest Turkey. The recently imaged fault system beneath the Marmara Sea is incorporated into the model as frictional surfaces with varying strike and dip. The Main Marmara Fault is implemented as through-going and is accompanied by mostly non-vertical second-order faults. Topography, basement-topography and the Moho become mechanically effective through changes in density and elastic parameters across these horizons. The model is subjected to gravity and kinematic boundary conditions. The ultimate goal of this study is to set up a 3-D model that is consistent with both, kinematic observations and stress data. The stress results are presented in a complementary paper. In this paper we present the modelled long-term 3-D kinematics in terms of fault slip rates, rotations, vertical motion and sense of fault slip. The model results agree with Global Positioning System velocities, geological fault slip rates, palaeomagnetic measurements and with the observed pattern of subsidence and uplift. Furthermore, our tectonically driven vertical velocities can be linked to landscape and basin evolution and to features of sedimentation. Our results indicate that the Main Marmara Fault can be interpreted as a through-going fault that slips almost purely in a strike-slip sense. Nevertheless, and not contradictory to the previous statement, there is significant dip-slip motion at some sections of the Main Marmara Fault. The agreement of the modelled 3-D kinematics with model-independent observations supports that the main structural details of the fault system are accounted for. Sensitivity analysis of model parameters reveals that changes in rock properties and the initial stress state have minor influence on the 3-D kinematics. We conclude that the 3-D structure of the fault system is the key control of the kinematics. The slip rate of the Main Marmara Fault from our model is lower than previous estimates and shows high variability along strike (12.8–17.8 mm a–1). The latter indicates that stress accumulation is non-uniform along strike.

The crustal stress pattern of Romania provides key insights into whether the Vrancea slab with its seismogenic volume between 70 and 175 km depth is still coupled to the crust and thus acts as a stress guide, or whether it is already in a state of detachment from the crust. Knowledge of the state of the slab under Vrancea is particularly critical because the slab attached to the crust can result in future strong earthquake occurrence in the crust and even in the currently aseismic zone between 40 km and 70 km depth, potentially causing severe damage. Our analysis of the contemporary tectonic stress observations in the context of potential stress sources and the comparison with numerical modelling shows that the crustal stress pattern in Romania is heterogeneous and does not contain a long wave-length stress pattern that would be expected if there is a strong present-day coupling between the subducted slab and the upper plate, or if lateral plate boundary forces would control the regional stress pattern. Therefore, we conclude that the crustal stress pattern of Romania is characterised by small differential horizontal stresses where local stress sources (third-order effects) are responsible for the observed heterogeneity of stress orientations and that the subducted slab under Vrancea is only weakly coupled to the crust.

The World Stress Map (WSM) project is a global compilation of information on the contemporary crustal stress field from a wide range of stress indicators. The WSM database release 2008 contains 21,750 stress data records that are quality-ranked using an updated and refined quality-ranking scheme. Almost 17,000 of these data records have A–C quality and are considered to record the orientation of maximum horizontal compressional stress SH to within ±25°. As this is almost a triplication of data records compared with the first WSM database release in 1992, we reinvestigate the spatial wave-length of the stress patterns with a statistical analysis on a global 0.5° grid. The resulting smoothed global stress map displays both; the mean SH orientation that follows from the maximum smoothing radius for which the standard deviation is 2000 km) exist for example in North America and NE Asia. These have been used in earlier analyses to conclude that the global stress pattern is primarily controlled by plate boundary forces that are transmitted into the intraplate region. However, our analysis reveals that rather short wave-length of the stress pattern

Although it is well accepted that mass redistribution on Earth's surface due to erosion and sedimentation affects the rate of crustal deformation, the influence of surface processes on the slip behaviour of individual faults remains poorly understood. Here we use the new tool CASQUS, which combines landscape evolution modelling with three-dimensional tectonic models, to investigate how erosion and sediment deposition affect fault behaviour in extensional tectonic regimes. A systematic set of experiments with models consisting of an isostatically responding elastic upper crust containing a single normal fault shows that the slip accumulation on the fault is accelerated by erosion and sedimentation. Depending on fault geometry, extension rate and parameters controlling erosion and sediment deposition, the slip rate of the normal fault is up to ~ 15% higher when surface processes are taken into account. The slip acceleration can be explained by an increase in the differential stress caused by erosion of the fault footwall and sedimentation in the hanging wall, which alter the faulting-induced flexure of the crust. We observe a similar slip rate increase in a more complex model of a growing horst bounded by three faults of different length. Our results show that a positive feedback exists between surface processes and tectonics and imply that geologically derived fault slip rates that integrate over several earthquake cycles may contain a contribution from surface processes.

Knowledge of the full present-day stress tensor and pore pressure has significant applications in the exploration and production of conventional and unconventional hydrocarbon reservoirs. The Darling Basin of New South Wales, Australia, is an old sedimentary basin (Late Cambrian/Silurian to Early Carboniferous) in which there was limited information about the present-day stress field prior to this study. In this paper we evaluate the contemporary stress field of the Darling Basin using a dataset from recent exploration wells and perform a geomechanical risk assessment with respect to borehole stability, fracture/fault generation and reactivation. Our interpretations of borehole failures in borehole image logs reveal a prevailing east-west orientation of the maximum horizontal stress throughout the Darling Basin. The estimates of the magnitudes for the vertical, minimum and maximum horizontal stress in the studied wells indicate a transition between thrust and strike-slip faulting stress regime at 600-700 m depths, where the magnitude of vertical stress and minimum horizontal stress are close to each other. However, the presence of borehole breakouts and drilling-induced tensile fractures, that we observe in the image logs at greater depths (900-2100 m) indicate a transition into a strike-slip tectonic stress regime below a depth range of approximately 700-900 m. These findings are in agreement with overcoring stress measurements east and west of the investigated wells. Furthermore, there are several Neogene-to-Recent geological structures in the study area that indicate thrust faulting with an east-west oriented maximum horizontal stress orientation around this old sedimentary basin. The consistency between the orientation of maximum horizontal stress determined from wellbore data and neotectonic structures is significant, and implies that horizontal stress orientations derived from very recent geological features may be valuable inputs to geomechanical models in the absence of wellbore or other data. However, the recent surface geological structures suggest a thrust faulting stress regime that is in slight contrast to the transition between thrust and strike-slip stress regime (SH > Sh similar to Sv) indicated by petroleum data, and highlights a potential pitfall of using neotectonic structures in geomechanical models. In particular, careful attention and verification should be made when using neotectonic structures for input, calibration or confirmation of geomechanical models, especially in intraplate tectonic settings such as Australia. (C) 2016 Elsevier Ltd. All rights reserved.

The stress field at depth is a relevant parameter for the design of subsurface constructions and reservoir management. Yet the distortion of the regional stress field due to local-scale features such as sedimentary and tectonic structures or topography is often poorly constrained. We conduct a stress sensitivity analysis using 3-D numerical geomechanical modelling with an elasto-plastic material law to explore the impact of such site-specific features on the stress field in a sedimentary sequence of the Swiss Alpine foreland. The model's dimensions are 14 x 14 x 3 km(3) and it contains 10 units with different mechanical properties, intersected by two regional fault zones. An initial stress state is established involving a semi-empirical relationship between the ratio of horizontal to vertical stress and the overconsolidation ratio of argillaceous sediments. The model results indicate that local topography can affect the stress field significantly to depths greater than the relief contrasts at the surface, especially in conjunction with horizontal tectonic loading. The complexity and frictional properties of faults are also relevant. The greatest variability of the stress field arises across the different sedimentary units. Stress magnitudes and stress anisotropy are much larger in stiffer formations such as massive limestones than in softer argillaceous formations. The stiffer formations essentially carry the load of the far-field forces and are therefore more sensitive to changes of the boundary conditions. This general characteristic of stress distribution in the stiff and soft formations is broadly maintained also with progressive loading towards the plastic limit. The stress field in argillaceous sediments within a stack of formations with strongly contrasting mechanical properties like in the Alpine foreland appears to be relatively insensitive to changes in the tectonic boundary conditions and is largely controlled by the maximum stiffness contrast with respect to the load-bearing formations., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe, 522

On 27 December 2007, a $$\text{M}_w$$ 1.9 seismic event occurred within a dyke in the deep-level Mponeng Gold Mine, South Africa. From the seismological network of the mine and the one from the Japanese–German Underground Acoustic Emission Research in South Africa (JAGUARS) group, the hypocentral depth (3,509 m), focal mechanism and aftershock location were estimated. Since no mining activity took place in the days before the event, dynamic triggering due to blasting can be ruled out as the cause. To investigate the hypothesis that stress transfer, due to excavation of the gold reef, induced the event, we set up a small-scale $$(450\times 300\times 310\;\text{m}^3)$$ high-resolution three-dimensional (3D) geomechanical numerical model. The model consisted of the four different rock units present in the mine: quartzite (footwall), hard lava (hanging wall), conglomerate (gold reef) and diorite (dykes). The numerical solution was computed using a finite-element method with a discretised mesh of approximately $$10^6$$ elements. The initial stress state of the model is in agreement with in situ data from a neighbouring mine, and the step-wise excavation was simulated by mass removal from the gold reef. The resulting 3D stress tensor and its changes due to mining were analysed based on the Coulomb failure stress changes on the fault plane of the event. The results show that the seismic event was induced regardless of how the Coulomb failure stress changes were calculated and of the uncertainties in the fault plane solution. We also used the model to assess the seismic hazard due to the excavation towards the dyke. The resulting curve of stress changes shows a significant increase in the last $${\sim}50\,\text{m}$$ in front of the dyke, indicating that small changes in the mining progress towards the dyke have a substantial impact on the stress transfer.

Iceland is located on the Mid-Atlantic Ridge which is the plate boundary between the Eurasian and the North American plates. It is one of the few places on earth where an active spreading centre is located onshore but the stress pattern has not been extensively investigated so far. In this paper we present a comprehensive compilation of the orientation of maximum horizontal stress (S-Hmax). In particular we interpret borehole breakouts and drilling induced fractures from borehole image logs in 57 geothermal wells onshore Iceland. The borehole results are combined with other stress indicators including earthquake focal mechanism solutions, geological information and overcoring measurements resulting in a dataset with 495 data records for the S-Hmax orientation. The reliability of each indicator is assessed according to the quality criteria of the World Stress Map project The majority of S-Hmax orientation data records in Iceland is derived from earthquake focal mechanism solutions (35%) and geological fault slip inversions (26%). 20% of the data are borehole related stress indicators. In addition minor shares of S-Hmax orientations are compiled, amongst others, from focal mechanism inversions and the alignment of fissure eruptions. The results show that the S-Hmax orientations derived from different depths and stress indicators are consistent with each other. The resulting pattern of the present-day stress in Iceland has four distinct subsets of S-Hmax orientations. The S-Hmax orientation is parallel to the rift axes in the vicinity of the active spreading regions. It changes from NE-SW in the South to approximately N-S in central Iceland and NNW-SSE in the North. In the Westfjords which is located far away from the ridge the regional S-Hmax rotates and is parallel to the plate motion. (C) 2016 Elsevier B.V. All rights reserved.

We have studied the stress pattern of Italy using a dataset with 590 data records from the World Stress Map database release 2008 and 106 new data records to test the hypothesis the mean orientation of maximum horizontal stress SH is different at different depth sections. For this, we split the dataset into a shallow (0–6 km) and a deep (6–40 km) depth section. For the data analysis we used a new statistical tool that calculates the mean SH orientation on a 0.2° grid. The tool takes into account the distance to each grid point, number and quality of the data records within the search radius, and the radial distribution. The result is a smoothed Italian stress map that displays both; the mean SH orientation and the wave-length of the stress pattern. The stress pattern does not vary in depth except for two areas (Sardinia and southern Apulia). Therefore stress data from different depths can be used to estimate the mean SH orientation and the wave-length of the stress pattern in Italy. Furthermore, the smoothed Italian stress map reveals that most of Italy has short wave-length stress patterns (

Studies have documented several rapid changes along the Pacific–North American, Pacific–Antarctic and Pacific–Australian plate boundaries in latest Miocene to earliest Pliocene time consistent with a sudden clockwise rotation of Pacific plate velocity relative to hotspots during this period. We test the hypothesis that this change in plate motion was initiated by cessation of subduction along the northern Melanesian arc due to the collision between the arc and the Ontong Java plateau. This hypothesis has long been formulated but never tested quantitatively. We use a geomechanical model of the lithosphere to determine the changes in plate boundary forces that are necessary to obtain the observed change in the Pacific plate motion. Our model results show that the change in motion can be explained by a clockwise rotation of the slab-related (basal-strength) component of plate driving force. The change of slab-related force from a post-6 Ma to a pre-6 Ma setting is perpendicular to the arc and points towards the Australian plate. The force per unit length is in the range of currently accepted values for subduction zones. Since there have been no other relevant changes at subduction zones along the Pacific plate boundary during the latest Miocene, we relate this change in slab-related force to the former southward-dipping Pacific plate slab along the northern Melanesian arc system which is now detached. Our model results suggest that rapid changes in plate motion can be triggered by slab detachment, with consequences for plate boundary processes even at great distances from the event.

SUMMARY We model microseismicity triggered by fluid injection on the basis of the theory of poroelasticity accounting for the external stress field. Consideration of the fully coupled poroelastic field equations enables us to apply a Coulomb failure criterion using pore fluid pressure and stress tensor as well as the coefficient of friction. The poroelastic fields are calculated with the finite-element method simulating fluid injection with constant injection rate into a 2-D domain. The influence of diffusivity, injection rate and stress field on the occurrence of microseismicity is analysed and compared to simulations based on pore fluid pressure diffusion only. We show that an anisotropic initial stress field causes elongated microseismic clouds. These clouds are indistinguishable from those generated in poroelastic solids under isotropic stress but exhibiting anisotropic hydraulic diffusivity. This similarity shows that microseismicity distributions dependent on both, the hydraulic properties and the coupling of pore fluid pressure to the stress field. In particular, neglecting the influence of the external stress field may lead to overestimation of the anisotropy of diffusivity tensor components. Furthermore, the results of our numerical simulations are strongly sensitive to changes of fluid injection rate.

We apply here a forecasting model to the Italian region for the spatio-temporal distribution of seismicity based on a smoothing Kernel function, Coulomb stress variations, and a rate-and-state friction law. We tested the feasibility of this approach, and analyzed the importance of introducing time-dependency in forecasting future events. The change in seismicity rate as a function of time was estimated by calculating the Coulomb stress change imparted by large earthquakes. We applied our approach to the region of Italy, and used all of the cataloged earthquakes that occurred up to 2006 to generate the reference seismicity rate. For calculation of the time-dependent seismicity rate changes, we estimated the rate-and-state stress transfer imparted by all of the ML≥4.0 earthquakes that occurred during 2007 and 2008. To validate the results, we first compared the reference seismicity rate with the distribution of ML≥1.8 earthquakes since 2007, using both a non-declustered and a declustered catalog. A positive correlation was found, and all of the forecast earthquakes had locations within 82% and 87% of the study area with the highest seismicity rate, respectively. Furthermore, 95% of the forecast earthquakes had locations within 27% and 47% of the study area with the highest seismicity rate, respectively. For the time-dependent seismicity rate changes, the number of events with locations in the regions with a seismicity rate increase was 11% more than in the regions with a seismicity rate decrease.

The present-day state of stress in Western Europe is considered to be controlled by forces acting at the plate boundaries. It is assumed that the Alpine orogen only influence the regional pattern of present-day stress in Western Europe within the Alps themselves. We examine the present-day maximum horizontal stress orientation in the Molasse Basin in the Alpine foreland in order to investigate the possible influence of the Alps on the far-field stress pattern of Western Europe. Four-arm caliper and image logs were analysed in 137 wells, in which a total of 1348 borehole breakouts and 59 drilling-induced fractures were observed in 98 wells in the German Molasse Basin. The borehole breakouts and drilling-induced fractures reveal that stress orientations are highly consistent within the Molasse Basin and that the present-day maximum horizontal stress orientation rotates from N–S in southeast Germany (002°N ± 19°) to approximately NNW–SSE in southwest Germany and the Swiss Molasse Basin (150°N ± 24°). The present-day maximum horizontal stress orientation in the Molasse Basin is broadly perpendicular to the strike of the Alpine front, indicating that the stress pattern is probably controlled by gravitational potential energy of Alpine topography rather than by plate boundary forces. The present-day maximum horizontal stress orientations determined herein have important implications for the production of hydrocarbons and geothermal energy in the German Molasse Basin, in particular that hydraulically-induced fractures are likely to propagate N–S and that wells deviated to the north or south may have reduced wellbore instability problems.

[1] A 3-dimensional Finite-Element-Model of Merapi Volcano was developed to calculate the surface deformation field in response to internal pressure variations. The model geometry is based on a new Digital Elevation Model with a grid size of 15 m and an accuracy of 7 m. This allows an investigation of local effects due to radially striking features like deeply eroded canyons, separated by narrow ridges. Close to the ridge margins, topographic disturbances are invoked with amplitudes comparable to the expected signal on a smooth surface. Comparison of the model results with observed tilt records at four locations reveals that topographically induced tilts exhibit a certain pattern that differs systematically from the spatial characteristics of tilt disturbances induced by meteorology. We conclude that a monitoring of the difference of two instruments located at opposite ridge margins provides a simple test to discriminate externally from internally driven tilt anomalies.

[1] Recent numerical models that couple global mantel circulation with lithosphere dynamics show that growth of the central Andes controls the 30% reduction of convergence velocity between the Nazca and South America plates observed over the past 10 Ma. The increase of gravitational potential energy due to topographic growth is also a major control on the stress pattern. Here we use numerical models which reproduce the Nazca/South America convergence history to predict the change of stress pattern in the central Andes for the past 10 Ma. Comparison of the modeled stress orientations at present-day with the observed ones results in ±23.9° mean deviation. Based on this good agreement we attempt to predict paleostress orientations 10 Ma ago. Interestingly, the modeled stress orientations 3.2 Ma ago are very similar to the present-day orientations. From this result we infer that stress rotations occurred between 10 and 3.2 Ma ago, when topography was considerably lower.

We calculated the stress evolution for the central part of the Dead Sea Fault System (30.5°N–34.5°N) from 551 to 2005 through modelling the static stress changes due to 14 historical earthquakes with MS ≥ 6.0 superimposed by tectonic loading from relative plate motion. From the results of our numerical models, we identified two segments with high positive changes in Coulomb failure stress (ΔCFS) exceeding 4 MPa, a ∼ 90-km-long segment of the Jordan Fault south of the Sea of Galilee and a ∼ 30-km-long segment east of the Dead Sea. These segments could result in MS = 7.4 and MS = 6.8 earthquakes, respectively. In contrast to similar stress evolution models of other continental transform faults, our results support only partly the hypothesis that coseismically induced static stress changes control the spatial succession of earthquakes. We calculated on each rupture plane the mean of ΔCFS and the maximum ΔCFS due to the stress changes of all preceding earthquakes. The results of the calculated mean ΔCFS values reveal that only 6 out of 13 earthquakes could have been triggered the subsequent earthquake. From the analysis of the maximum ΔCFS values, we find that 8 out of 13 earthquakes could have triggered the subsequent earthquake. Since our model results are sensitive to location and magnitude of historical earthquakes and the tectonic loading rates of the fault segments, we emphasize the need for further paleoseismological studies and current slip rate estimation from continuously observing GPS arrays and geological investigations. Taking into account that six large cities (Beirut, Damascus, Haifa, Tel Aviv, Amman, and Jerusalem) are in close proximity with distances between 30 and 150 km to the Jordan segment, the seismic risk is probably higher than accounted before.

[1] The 2001 Mw = 8.4 southern Peru subduction earthquake marked the beginning of a transient postseismic surface motion in direction of the coseismic displacement at the permanent GPS station Arequipa. In general this motion is assigned to afterslip. Our hypothesis is that the observed transient signal can be explained by stress relaxation processes in the overriding plate. We use a 2D finite element model incorporating non-linear viscoelastic Maxwell rheology. Our model results indicate that coseismically induced stresses are relieved by viscoelastic stress relaxation in the lower crust. The trenchward directed creep motion is transferred to the upper crust due to elastic coupling leading there to an instantaneous relief of elastic stresses. In contrast to the existing conceptual models for stress relaxation, which incorporate shear stresses, we conclude that tensional elastic stresses throughout the crust and upper mantle are the main driving forces for the transient GPS signal.