Crustal Stress Research Articles

Glacial Hydro-Isostatic Adjustment at Mid-Ocean Ridges

Summary Recent studies have suggested a link between ice age sea level fluctuations and variations in magma production and crustal faulting along mid-ocean ridges based on the detection of Milankovitch cycle frequencies in topography off several ridges. These fluctuations have also been connected to variability in hydrothermal metal fluxes near ridges. Ice age sea level calculations have shown that the sea level change across glacial cycles will be characterized by significant geographic variability, that is, departures from eustasy, due to the gravitational, deformation and rotational effects of the glacial isostatic adjustment (GIA) process. Using a state-of-the-art GIA simulation that incorporates 3-D variations in Earth viscoelastic structure, including plate boundaries, and updated constraints on the magnitude and geometry of ice mass fluctuations, we predict global sea level changes from Last Glacial Maximum (LGM, 26 ka) to present and from the Penultimate Glacial Maximum (PGM, 143 ka) to the Last Interglacial (LIG, 128 ka). We focus on the results along three ridges: the Mid-Atlantic Ridge, Juan de Fuca Ridge and East Pacific Rise, which are examples of slow, intermediate and fast spreading ridges, respectively. Sea level change across the Mid-Atlantic Ridge shows the greatest variability, ranging from a sea level fall greater than 200 m in Iceland to a maximum rise of ∼150 m in the South Atlantic, with significant non-monotonicity north of the Equator as the ridge weaves across the field of sea level changes. We also calculate changes in crustal normal stress from LGM to present-day across the Mid-Atlantic and Juan de Fuca Ridges and the East Pacific Rise. These results indicate that the contribution from ice mass changes to the crustal stress field can be significant well away from the location of ancient ice complexes. We conclude that any exploration of the hypothesized links to magma production and crustal faulting must consider both ocean and ice loading effects and, more generally, the profound geographic variability of the GIA process.

Experimental and Numerical Investigations of Low-Permeability Sandstone Under Water Injection–Induced Dilation in West Oilfield, South China Sea

With the development of offshore oil fields, the reservoirs of X oilfield in the west of the South China Sea are poor in physical property, serious in pollution, and increasingly prominent in interlayer contradictions. Water injection dilation technology has strongly affected the development of loose sandstone reservoirs. To explore whether this technology applies to the low-permeability sandstone of X oilfield in the west of the South China Sea and the dilation effect and radius of water injection dilation technology on the target reservoir, low confining pressure rock mechanics experiments and numerical simulation of water injection in this reservoir section are carried out. The triaxial shear experiment of low confining pressure shows that the target reservoir sandstone with low-permeability can have a shear strength of 45 MPa when the effective confining pressure is 0.5 MPa, and the target reservoir core can have dilatancy. When the axial strain is 2.5%, the core dilatancy is 1%, and the permeability changes by 1.17 times. It was found that the core volume dilation was obviously under low effective confining pressure, and the permeability is 2 orders higher than in the initial condition. The numerical simulation of the target reservoir shows that the bottom-hole pressure reaches 47.12 MPa at the end of water injection in typical wells. The reservoir was deformed to different degrees around the well, and the top layer was raised by 5.58 mm. This paper characterizes the rock expansion potential and expansion flow capacity of low-permeability sandstone reservoirs from multiple perspectives and establishes a three-dimensional, full-size wellbore formation crustal stress strict matching geological model for offshore expansion wells. We have provided theoretical guidance for on-site construction.

Crustal stress near the Yakutat microplate collision from probabilistic earthquake focal mechanisms

Earthquake source characteristics provide a valuable constraint on crustal stress and regional plate tectonics. Earthquake source studies in Yukon and northern British Columbia have been limited by sparse seismic network coverage. In this work, we leverage recent seismic network improvements to estimate focal mechanisms for small and moderate-magnitude (M>2.0) earthquakes from P-wave first-motion polarity data. We invert these data within a probabilistic framework to rigorously quantify mechanism uncertainties. Subsequently, probabilistic earthquake focal mechanisms are used as input for inversions for the orientation of principal stress axes, and stress shape ratio, for spatial windows throughout the region. We implement a novel, data-driven approach to propagate focal mechanism uncertainty in stress inversion. Our results improve the spatial coverage of existing earthquake focal mechanisms and enable an orogenic-scale study of crustal stress near the Yakutat-North America collision. Overall, the region is characterized by a transpressive stress regime. Maximum horizontal compressive stress orientations exhibit a pattern orthogonal to the Yakutat collision syntaxis. Stress inversion results also reveal an abrupt change in orientation east-to-west, which we interpret as a change in stress regime across the Fairweather-Connector-Totschunda fault system that is likely related to coupling between the subducting Yakutat slab and North America. This work improves our understanding of potential earthquake hazards in southwestern Yukon and the surrounding region.

Spatially varying stress regime in the southern Junggar Basin, NW China

The southern Junggar Basin in NW China is an important tectonic unit in the region of the Tibetan Plateau and has been the focus of considerable research into its tectonic processes and geodynamic setting. However, the relationship between deep structural deformation and stress in this region remains unclear. This study investigates the Gaoquan and Hutubi anticlines in the southern Junggar Basin using three-dimensional geophysical data and a finite-element numerical simulation to examine the crustal stress distribution and stress regime at depths of up to 7 km. Numerical simulation results indicate that the stress regime in the southern Junggar Basin changes from west to east. In the western part of the region, including the Gaoquan anticline at depths of 4900–6100 m, the maximum horizontal principal stress shows a peak of 140–200 MPa, the minimum horizontal principal stress is 110–170 MPa, and the vertical principal stress is 115–175 MPa, indicating a mixed stress regime incorporating both compression and strike-slip components. In the eastern part of the region, including the Hutubi anticline at depths of 5400–7800 m, the maximum horizontal principal stress shows a peak of 160–280 MPa, the minimum horizontal principal stress is 155–250 MPa, and the vertical principal stress is 125–215 MPa, indicating a compressive stress regime. The stress magnitude and orientation are affected by the presence of faults and depth in the crust. Combining these results with the regional tectonic setting, it is considered that the geometrical relationship between pre-existing faults and the current stress field is the main control on the west–east differentiation in the stress regime, with spatial variations in the mechanical parameters of the crust and the pressure coefficient being secondary factors. These results provide insights into the relationship between stress and deformation, and support the updated version of the World Stress Map database.

Stress heterogeneity in the eastern Tibetan Plateau and implications for the present-day plateau expansion

The eastward expansion of the Tibetan Plateau has resulted in different earthquake types in the eastern Tibetan Plateau, but the mechanism remains unclear. Here, we construct a three-dimensional visco-elastoplastic finite element model considering the topography to investigate the influence of fault geometry and rheological heterogeneity on stress fields. In our best-fitting model, the minimum principal stress is nearly vertical around the southern Huya fault zone, which is adjacent to the Longmen Shan fault zone, due to the significant mid-lower crust lateral rheological heterogeneity, and the thrust stress regime accounts for the reverse fault and thrust-dominated earthquakes. In this scenario, the eastward horizontal motion of the mid-lower crust is obstructed and facilitates thrust faulting, suggesting the limited eastward expansion of the Tibetan Plateau. In contrast, the northern Huya fault zone, one of the terminal branches of the East Kunlun fault, accommodates the continuous eastward extrusion of the East Kunlun fault, where the stress regime under a more homogenized crust favors the strike-slip faulting process, along with the dominant strike-slip earthquakes. Moreover, the best-fitting of stress regime explains the thrust-dominated 2008 Ms. 8.0 Wenchuan and 2013 Ms. 7.0 Lushan earthquakes on the Longmen Shan fault zone. Combining geophysical and geodetic observations and model analyses, we propose that the hybrid deformation mode in the eastern Tibetan Plateau is accommodated by upper crustal shear and thrusting deformation and mid-lower crustal thickening driven by the gravitational potential energy gradient. Our results elucidate the mechanism for differences in strong historical earthquakes and, more importantly, isolate the effect of fault geometry from those of heterogeneous viscosity on crustal deformation and stress heterogeneity in the eastern Tibetan Plateau.

Stress modeling for the upper and lower crust along the Anninghe, Xianshuihe, and Longmenshan Faults in southeastern Tibetan plateau

Earthquake occurrence depth in the crust is related to stress, temperature, and brittle–ductile transition, which is also near the transition depth of the upper to lower crust. The composition variation between the upper and lower crust causes remarkable changes of rheological properties and variation in stress distribution. Clarifying the detailed stress distribution in the upper and lower crust is crucial for understanding the brittle–ductile transition and the stress environment of the seismogenic zone. The Southeastern Tibetan Plateau (SETP), with wide spread of active strike−slip faults and clustered earthquakes, provides a natural field for investigating the relationships between crustal stresses, deformation behaviors, and earthquake mechanics. By considering the rheological properties of granite and anorthite, this paper established stress models with different boundary depths (15, 20 and 25 km) between the upper and lower crust along the Anninghe, Xianshuihe, and Longmenshan Faults in the SETP with a horizontal strain of 6 × 10−4 extracted from in situ stress data. The stress model with different geothermal gradients and a boundary depth of 20 km between the upper and lower crust suggests two distinct types of the brittle–ductile transition below these three faults. Simultaneously, the stress model can account for the continuity of earthquake depth distribution below the Longmenshan Fault and the seismic gap below the Anninghe and Xianshuihe Faults. The continuity of earthquake depth distribution or seismic gap below these three faults can be explained by their different geothermal gradients. These findings provide new insights for understanding the stress environment of the seismogenic zone in the SETP. Our model reveals the relationships between differential stress, seismicity, brittle–ductile transition, and boundary depth of the upper and lower crust in the continental crust, and connects the multiple observations from geophysics and geology. Furthermore, our model provides insights for studying multiple processes in the continental crust, such as crustal deformation, fault slip, and earthquake occurring.

Present-day stress field, strain rate field and seismicity of the Pannonian region: overview and integrated analysis

We present an overview of recent findings on the seismicity, stress field and strain rate field of the Pannonian region. We furthermore show new inferences for deformation mechanisms and lithospheric rheology via an integrated analysis of the datasets. The N-NW motion of the Dinarides and the opposite, SW motion of the E-and S-Carpathians induce shortening in the western and central Pannonian Basin while leading to regional shearing around the basin's SE boundary. This induces moderate seismic activity related to strike-slip and reverse faulting. Maximum horizontal stress and geodetic shortening directions generally agree, implying that upper crustal stresses and surface deformation correspond to the same forces. 2D rheological models confirm that the shallow upper crust is the only brittle layer in the Pannonian lithosphere, while the brittle-ductile transition zone could be as shallow as 6-9.5 km in the Danube Basin. Comparison of seismic moment rates and moment rate predictions from geodetic strain rates show that deep Pannonian sub-basins accumulate major seismic deficits. This could be explained by dominantly aseismic deformation of the weak upper crust, as supported by a comparison with case studies from different geodynamic environments. Stronger mountainous regions in the study area show no to moderate seismic deficits.

Empirical evidence for multi-decadal transients affecting geodetic velocity fields and derived seismicity forecasts in Italy

This study critically examines the use of geodetic strain rates for forecasting long-term earthquake rates in a slow-deforming region such as Italy, challenging the prevailing assumption of their temporal stationarity in interseismic stages for seismic hazard analyses. Typically, earthquake-rate models derived from geodesy assume stationary interseismic loading rates, with stress rates in the upper crust proportional to geodetic strain rates, leading to earthquake rates directly proportional to these strain rate tensors. However, our analysis unveils a pronounced correlation between the epicenters of earthquakes that occurred in the past 60–120 years and areas forecasted for higher future earthquake rates based on geodetic strain rates. This correlation appears weak and scattered in analyses of even older earthquakes. To corroborate our findings, we select the 2009 L’Aquila earthquake (mw = 6.3) to prove that its apparently marginal viscoelastic relaxation significantly alters the time series of adjacent benchmarks for the following ~ 30–60 years, explaining the high correlation between recent earthquakes and strain rate peaks. Our findings require a methodological shift in interpreting geodetic data for earthquake forecasting, emphasizing the two-component (plate-tectonics-driven stationary long-term deformation, and decadal transient viscoelastic relaxation after an earthquake) nature of crustal stress accumulation recorded in geodetic data. We underscore the potential of geodesy-derived forecasts to provide deeper insights into seismic hazards, stressing the importance of acknowledging the long-term temporal variability inherent in geodetic measurements.

Drought-induced seismicity modulation in the New Madrid Seismic Zone, central United States

The New Madrid Seismic Zone (NMSZ) in the central United States is a seismically active intraplate region composed of two fault segments: the Reelfoot and Cottonwood Grove fault segments. It has witnessed several major earthquakes, including a devastating 1811–1812 earthquake sequence of three ~M7 events. Nearly 200 years later, earthquakes still continue today in this complex seismic zone. This seismic zone is located in a domain with higher hydrological load than surrounding regions, which may play a crucial role in seismicity modulation. However, the hydrological unloading or loading-induced seismicity modulation and the underlying earthquake dynamics on this stable plate interior remain equivocal. Our study demonstrates that increased climate variability and drought-induced hydrological unloading can be potential drivers for the crustal stress change in the upper Mississippi embayment and seismicity modulation in the NMSZ. The seismicity rate associated with the Reelfoot fault segment shows ~60% increase during drought-induced prolonged drier periods, linked to La Niña cycles, than during the relatively wetter periods. However, such a feature is lacking for the seismicity associated with the Cottonwood Grove fault segment. We argue that the near-lithostatic pore pressure condition explicitly on the Reelfoot fault segment leads to an increase in the amplitude of the velocity perturbation, making this fault segment more susceptible to seismicity modulation on a multi-annual or annual time scale by the resonance destabilization process.

Crustal stress buildup/relaxation and pore pressure in preparation and sequential brittle-shear fault rupture — Implications for a general theory of earthquake nucleation

Compelling geoscience evidence has heightened the appreciation of abnormal (suprahydrostatic, sub/quasi/supralithostatic) pore pressure and multimode (brittle-shear) fault rupture leading to seismicity. However, preparation and rupture nucleation are yet to be adequately constrained and quantitatively modeled. This challenges crucial schemes, notably physics-based earthquake forecasting/prediction. In this multidisciplinary study, the transitions and associated critical points linking the preexisting fluid overpressure in the fault patches, temporal crustal stress, and sequential brittle-shear fault failure were considered. In preparation, tectonic loading was accompanied by processes such as off-fault yielding, permeability enhancement, and foreshocks that facilitate local/regional stress relaxation. Subsequent equalization of stress and the preexisting local overpressure triggers hydraulic fracturing that destabilizes major asperities. Almost instant shear failure follows with the spatially varying rupture velocity/intensity of the frictional slip because of localized asperity stress and syn-slip fluid-/melt-driven fracturing/dilation and lubrication. Based on the aforementioned critical points, quantitative modeling associated stress drop with the evolution of the stress and pore pressure. Equations for the time to onset of stress relaxation and time to rupture were derived using fluid flow and viscoelastic models. The seismic moment was estimated with classical seismological relations after modifications accounting for the smaller surface area during frictional slip. For retrospective testing, two cases of induced seismicity (2016 Fairview, Oklahoma, USA, and 2017 Pohang, South Korea) and multiple cases of natural seismicity (including the 2024 Noto Peninsula earthquake, Japan) were considered. The replication of triggering mechanisms, source properties, and time to rupture suggested that stress temporal relaxation and triggered anomalies (STRATA) encompass fundamental hydromechanical processes in seismogenesis. The setting and scale invariance of STRATA suggest it might be a general theory of earthquake nucleation. Based on the identified preparatory processes/retrospective validations, a physics-based earthquake forecasting/prediction scheme was developed. The nurseries/hypocenters of impending earthquakes are identified through the simultaneous consideration of locally preexisting fluid overpressure and spatiotemporal analysis of stress relaxation. Event size and rupture timing are estimated with the derived relations herein.

Regional stress field in the SE margin of the Tibetan Plateau revealed by the focal mechanisms of small and moderate earthquakes

In this study, we investigate the stress field in the southeastern margin of the Tibetan Plateau. We first determine the focal mechanism solutions of 1537 small and moderate (3.2 ≤ MW ≤ 6.7) regional earthquakes from January 2009 to June 2021, and then use the focal mechanisms to invert for the spatial variation of crustal stress field by a damped linear inversion method. Our result suggests that in the southeastern margin of the Tibetan Plateau the seismogenic zone is in the upper crust above 15-km depth, and the stress field is predominantly strike-slip in the Sichuan-Yunnan Rhombic Block (SYRB). The maximum compressional stress axis is oriented in a fan-shaped pattern, rotating clockwise from nearly east-west in the Songpan-Ganzi Terrain in the north to northwest-southeast in the SYRB to nearly north-south across the Red River Fault in the Indo-China Block (ICB), consistent with the GPS-derived surface strain rate. The stress field around the border of the Tibetan Plateau with high elevation relief appears to be largely caused by gravitational effect with the maximum extensional axis perpendicular to the topography gradient. The stress field in the vicinity of the Longmenshan Fault Zone and in the Yangtze Craton is mainly thrust as a result of the eastward expansion of the Tibetan Plateau and the resistance of the Sichuan Basin. Near the epicenter of the 2008 Wenchuan earthquake and in the northeastern end of the Longmenshan Fault Zone, the thrust stress field shows spatial variations as a result of the perturbation by complex geometry and the post-seismic healing process. Our result provides multi-resolution images of the stress field for better understanding about the mechanisms of seismic activity and crustal deformation in the southeastern margin of Tibetan Plateau.

Quantitative analysis of crustal deformation, seismic strain, and stress estimation in Iran via earthquake mechanisms

Quantitative analysis of crustal deformation, seismic strain, and stress estimation in Iran via earthquake mechanisms

Damage characteristics of gas extraction boreholes under different crustal stresses and their effects on gas extraction

AbstractTo investigate the effects of different crustal stresses on the stability of gas extraction boreholes, several equations were deduced. Additionally, the coupled flow‐solid model of gas extraction was constructed, given the elasticity−plasticity deformation of the coal body. Using the numerical simulation and analysis software of Comsol, a study was conducted on the deformation and damage of gas extraction boreholes under different crustal stresses. The results are as follows: with vertical stress increasing, the concentration area of shear stress, deformation amount, and the area of the plastic zone around the hole increase. Varied lateral pressure coefficients led to differing distribution positions of shear stress concentration areas and plastic zones around the hole. An increase in lateral pressure coefficient shifted the distribution positions of shear stress maximum. In addition, plastic zone, and permeability ratio maximum change from the left and right sides of the borehole to the upper and lower sides, and the larger the lateral pressure coefficient is, the worse the stability of the upper and lower sides of the borehole is. Higher lateral pressure coefficients inhibited extraction, with the effect becoming more pronounced over time. Elevated vertical stress and lateral pressure coefficients hindered negative pressure propagation in the borehole.

Drill tools sticking prediction based on adaptive long short-term memory

As one of the most severe disasters in deep coal mining, rockburst can be prevented through drill-hole pressure relief. However, the coal mine is characterized by high crustal stress and changeable mechanical properties of surrounding rock, which will cause drill rod deflection phenomenon, then lead to rod-deflection sticking accidents. This paper proposes a prediction method based on adaptive long short-term memory (ALSTM) for rod-deflection sticking accidents to improve drilling efficiency and reduce sticking accidents. Firstly, the sticking data is collected through the intelligent drilling condition simulation experimental platform, and then the sticking features are extracted based on the sticking data. Secondly, the sticking factor is constructed, and the sticking critical line is set. Thirdly, the good-point set and the proposed random perturbation algorithm are employed to improve the spotted hyena optimizer (SHO) to obtain the improved SHO (ISHO). Finally, we use the ISHO to optimize the hyperparameters of the long short-term memory and then establish the sticking prediction model based on ALSTM. The experimental results show that the proposed prediction model meets the demands for sticking prediction very well.

Coupling control technology of anchoring and unloading in deep intense-mining and large-deformation roadway: a case study.

In order to control the deformation of surrounding rock in deep high-stress and intense-mining roadways, taking a deep coal roadway with continuous deformation as an example, the characteristics of crustal stress, coal strength, and mining influence of roadway are obtained by underground tests. The combined failure mechanism of coal roadway surrounding-rock is revealed by differential stress of deep and shallow anchor cables. We propose that the improvement of surrounding rock control for coal roadway is adopting the coupling control technology of anchoring and unloading. The stress distribution and evolution laws of lateral surrounding rock of unloading holes are obtained by numerical simulation and theoretical calculation, and reasonable unloading-hole spacing of 4.0m is comprehensively determined. A mechanical model of roadway roof beam under fixed support at both ends is constructed and the important role of anchor cable beam-truss in controlling the stability of coal roadway is obtained. The rationality of coupling control technology of anchoring and unloading and parameters has been verified by engineering test and mine pressure observation, providing technical references for surrounding rock control in deep intense-mining and large-deformation roadways.

Defining the geomechanical operating limits for subsurface CO2 storage

Carbon capture and storage (CCS) is a critical component of proposed pathways to limit global warming, though considerable upscaling is required to meet emissions reduction targets. Quantifying and managing the risks of fault reactivation is a leading barrier to scaling global CCS projects from current levels of ~40 million tonnes of carbon dioxide(CO2) per year (to target levels of several gigatonnes of CO2 per year), because CO2 injection into reservoirs can result in increased pore-fluid pressure and temperature changes, which can reduce the strength of rocks and faults and induce brittle failure. This can result in induced seismicity, whilst hydraulic fracturing of seals could provide pathways for CO2 leakage. Consequently, identifying favourable geomechanical conditions (typically determined through data on pre-injection rock stress, mechanical and elastic properties, and pore-fluid pressures) to minimise deformation of reservoirs and seals represents a key challenge in the selection of safe and effective sites for CCS projects. Critically, however, such geomechanical data are typically spatially limited (i.e. restricted to wells) and mainly consist of pre-injection crustal stress orientation measurements, rather than a full 3D description of the stress tensor and related geomechanical properties. This paper reviews some key geomechanical issues and knowledge gaps (particularly those associated with data availability and limitations) that need to be understood to enable successful reservoir and seal management for CCS projects. We also highlight recent advances in multi-scale and dimensional geomechanical modelling approaches that can be used to assess sites for the secure storage of CO2 as well as other gases, including hydrogen.

Study on Shear Failure Process and Zonal Disintegration Mechanism of Roadway under High Ground Stress: A Numerical Simulation via a Strain-Softening Plastic Model and the Discrete Element Method

Fracture expansion in rock masses can be observed by monitoring the break of contacts between the bounding particles via the discrete element method. The latter’s realization in this study via the PFC2D program tracked the evolution process of the zonal disintegration in an exemplary roadway-surrounding rock affected by mining. Besides, the damage evolution pattern in a high-stress soft rock roadway was simulated by the FLAC2D program using a strain-softening plastic model, revealing the effects of rock mass strength, stress state, and anchor support on the zonal disintegration of the roadway. Numerical simulation results show that in a roadway with high-level stress, the obvious fractures spread from the roadway surface to the depth of the surrounding rock along a series of geometric planes and cut the surrounding rock into rock mass blocks. Under high crustal stress, conjugate shear fractures occur near the roadway surfaces and form a closed-loop fractured zone after intersecting the conjugate fracture faces. The closed fractured zone becomes a free face, from which conjugate shear fractures develop, forming new closed fractured zones in the deep surrounding rock. By repeatedly generating the closed fracture zones, a fracture network appears in the roadway-surrounding rock. The development of zonal disintegration of roadway-surrounding rock mainly depends on the rock mass strength and its stress state. Zonal disintegration only occurs when the crustal stress of the roadway-surrounding rock exceeds its strength. When the horizontal stress is low and the vertical stress exceeds the rock mass strength, zonal disintegration only occurs on two sides of the roadway. When the vertical stress is low and the horizontal stress exceeds the rock’s mass strength, it only appears on the roof and floor. When the values of cohesion, internal friction angle, and tensile strength are reduced in the same proportion, cohesion has the greatest impact on the expansion of the zonal disintegration zone, followed by the internal friction angle, while the tensile strength effect is the least. In anchor-supported roadways undergoing zonal disintegration processes, the intact zone blocks slide relatively along the fracture surface during the process of loosening and deformation of the surrounding rock, making the anchor rods susceptible to tensile, shear, and bending actions.

Geostatistical Analysis of Lineament Domains: The Study Case of the Apennine Seismic Province of Italy

Regional-scale swarms of subparallel linear topographic features, known as lineament domains, are a common feature of planetary surfaces. Lineament domains are superficial manifestations of the crustal stress field trajectory. Notably, one of the effects of active tectonics is seismicity. Italy is one of the most seismically active regions in the Mediterranean, with many destructive earthquakes that have occurred in past centuries. Here, we assess the seismic meaning of the main lineament domain in the tectonically active region of Central Italy. We describe the use of an automated analysis of satellite imagery coupled with spatial grid analysis to identify three lineament domains of the Central Apennines. Spatial and azimuthal comparisons of the main lineament domain (i.e., the Apennine Domain), with the known locations of earthquakes (moment magnitude of Mw > 5.5) that occurred during the past century, revealed the most seismically active tectonic areas and their spatial distributions. Further, we present a conceptual seismo-geodynamic model for the Central Apennines, which is characterized by regional arching and explains the presence of an extensional tectonic regime in the upper crustal layer of the active Apennines fold-and-thrust belt.

Natural fracture density controls productivity in shale reservoirs

Abstract Shale reservoirs host ubiquitous multi-scale natural fractures created by factors such as pore pressure build-up after petroleum generation and palaeo crustal stress changes during tectonic episodes. Natural fractures are among postulated drivers of stimulated shale well productivity and related variability. Quantifying the aforementioned role and optimizing recovery call for litho-structural assessments. In this interdisciplinary study, natural fracture density (fractures per unit length) for North and South American shales (Marcellus, Eagle Ford, Haynesville, Barnett, Fayetteville and Vaca Muerta) was estimated from published observations of outcrops, cores and borehole images. Associated production for the latest horizontal wells, drilled in the most productive locations (sweet spots), was normalized by lateral length and reservoir thickness. It was found to correlate positively with the density of small-scale natural fractures. Durations of transient linear flow, diagnosed from production data, were play-specific, negatively correlated with small-scale natural fracture density and led to realization of picodarcy matrix permeability. Conversely, large-scale (tectonic) fractures limit stimulation efficiency and pose environmental/induced seismicity risks. Therefore, stimulation-driven reactivation of small-scale fractures facilitates drainage and enhances well productivity. Relatedly, reservoir flow regimes and production decline curves are intricately controlled by interplay of natural fracture density and matrix permeability. Variability of these parameters calls for acreage-tailored stimulations.

Recent advances in characterizing the crustal stress field and future applications of stress data: perspectives from North America

Abstract The stress field controls patterns of crustal deformation, including which faults are likeliest to cause earthquakes or transmit fluids. Since the 1950s, maps of maximum horizontal stress ( S Hmax ) orientations have advanced dramatically, and the style of faulting (relative principal stress magnitudes) has recently been mapped in some regions as well. This perspectives paper summarizes developments in characterizing stress orientations and (relative) magnitudes, including new seismic and borehole methods, as well as progress in identifying the causes of stress variations. Despite these advances, adding far more spatiotemporal detail would allow geoscientists to address many of today's key challenges regarding natural hazards, energy development, and geodynamics. In particular, it is critically important to characterize stress heterogeneity at multiple scales while also recognizing the coherent variability of the stress field. The second part of the paper considers how more detailed stress datasets could prove essential to addressing some of the grand questions in geoscience, including deciphering the poorly understood feedbacks between crustal dynamics and surface processes, improving earthquake and eruption forecasts, and determining the origins and shared properties of plate boundaries.