Regional Stress Research Articles

Significant spatial variation of upper crustal anisotropy in Southern Sichuan Basin, China: constraints from local shear wave splitting analysis

Summary The recent upsurge in seismic activity within the southern Sichuan Basin has garnered considerable public attention and simultaneously offers a valuable opportunity for investigating upper crustal anisotropy. Such investigations can provide critical insights into the stress field and crustal deformation in the region. We obtained a total of 1 845 high-quality local shear wave splitting measurements at 15 stations and 2 027 null measurements at 19 stations. These results indicate the presence of a single layer of anisotropy with a horizontal axis of symmetry at a depth of 3 to 7 km. The fast polarization directions display discernible spatial variations that are primarily influenced by the geographical location of earthquakes rather than changes over time. In the vicinity of the Baimazhen Syncline, the fast polarization directions coincide with the strike of the stratum, forming a circular pattern around the core of the syncline, suggesting that the observed anisotropy is structure-controlled. In contrast, stations situated in the southern Weiyuan Anticline and western Baimazhen Syncline display fast directions trending N171.7° E and N45.9° E, respectively. These directions are consistent with the P-axes of the focal mechanism of earthquakes, signifying that the anisotropy in these areas is governed by the regional stress field. The findings of this study not only deepen our understanding of the intricate geological structures in the southern Sichuan Basin but also indicate the need for greater caution when interpreting potential temporal changes in anisotropy in future research.

The temporal and spatial evolution characteristics of induced seismicity in the Changning shale gas field based on dense array

In this paper, we performed microseismicity detection and location using the deep learning method and obtained a high-precision earthquake catalog in the Changning gas field, which is one of the largest shale gas production demonstration areas in China. We found that the spatial and temporal characteristics of seismicity in the region are indicative of its correlation with industrial operations. The distribution of earthquakes at depth reflected variations in reservoir depth and provided valuable constraints on it. The horizontal layered distribution of earthquakes at depth is due to the formation of fracture surfaces from interconnected fractures near the reservoir during hydraulic fracturing (HF) operations, which clearly demonstrates how HF operations impact seismicity. We suggested that the horizontally layered distribution was driven by two fundamental mechanisms: reactivation of pre-existing faults during HF and injection of high-pressure fluids into the reservoir, leading to fracture creation. Several ML ≥4 earthquakes, which did not occur on well-defined seismogenic faults, may have been triggered by pore elastic coupling resulting from regional stress accumulation and fluid injection. Significantly, the ML 4.9 seismic sequence occurring at the basement indicates that fracking has reactivated and promoted pre-existing faults, highlighting the need for further investigation into potential seismic hazards in the region.

Tectonic Activity Analysis of the Laji-Jishi Shan Fault Zone: Insights from Geomorphic Indices and Crustal Deformation Data

Fault segmentation plays a critical role in assessing seismic hazards, particularly in tectonically complex regions. The Laji-Jishi Shan Fault Zone (LJSFZ), located on the northeastern margin of the Tibetan Plateau, is a key structure that accommodates regional tectonic stress. This study integrates geomorphic indices, cross-fault deformation rate profiles, and 3D crustal electrical structure data to analyze the varying levels of tectonic activity across different segments of the LJSFZ. We extracted 160 drainage basins along the strike of the LJSFZ from a 30 m resolution digital elevation model and calculated geomorphic indices, including the hypsometric integral (HI), stream length-gradient index (SL), and channel steepness index (ksn), to assess the variations in tectonic activity intensity along the strike of the LJSFZ. The basins were categorized based on river flow directions to capture potential differences across the fault zone. Our results show that the eastern basins of the LJSFZ exhibit the strongest tectonic activity, demonstrated by significantly higher SL and ksn values compared to other regions. A detailed segmentation analysis along the northern Laji Shan Fault and eastern Jishi Shan Fault identified distinct fault segments characterized by variations in SL and ksn indices. Segments with high SL values (>500) correspond to higher crustal uplift rates (~3 mm/year), while segments with lower SL values exhibit lower uplift rates (~2 mm/year), as confirmed by cross-fault deformation profiles derived from GNSS and InSAR data. This correlation demonstrates that geomorphic indices effectively reflect fault activity intensity. Additionally, 3D crustal electrical structure data further indicate that highly conductive mid- to lower-crustal materials originating from the interior of the Tibetan Plateau are obstructed at segment L3 of the LJSFZ. This obstruction leads to localized intense uplift and enhanced fault activity. These findings suggest that while the regional stress–strain pattern of the northeastern Tibetan Plateau is the primary driver of the segmented activity along the Laji-Jishi Shan belt, the direction of localized crustal flow is a critical factor influencing fault activity segmentation.

Dyke emplacement under mixed loading conditions: Insights from the Dharwar Craton, India

Dykes are intrusive igneous bodies that play crucial role in the supply and ascent of magma to the Earth’s crust. Magma can intrude along pre-existing anisotropies such as fractures or foliations present within the host rock or it may create its own path by fracturing the host rock. In the latter scenario, when fractures are formed by the pressure exerted by the invading magma, a dyke’s outcrop shape and geometry are diagnostic of the conditions under which it evolved. Here, we report mafic dykes emplaced within the younger granites of Dharwar Craton, peninsular India. Outcrop attributes of these dykes are characteristic of emplacement under conditions of mixed mode loading. We discuss different discrete modes of fracture formation and their possible combinations to understand the generation and eventual emplacement of dykes under mixed mode loading. This leads to the development of a comprehensive sequence of progressive dyke evolution under mixed mode I-III loading and thereby distinguishing incremental orders of dyke horn formation. We further apply this knowledge along with collected field evidence on dyke body geometries to propose an evolutionary model of dyke formation and emplacement within the Chitradurga granite under varying regional stress fields of the Chitradurga Schist Belt, Western Dharwar Craton, India. We infer, that the dykes initiated as extensional fractures within an earlier NE-SW directed compressive stress field and were subsequently sheared sinistrally by the effect of the adjacent Chitradurga Shear Zone on account of a later E-W to ESE-WNW directed compression.

Crust and Mantle Flow From Central Tibetan Plateau to the Indo‐Burma Subduction Zone

AbstractThe extremely oblique Indo‐Burma subduction zone exhibits dextral strike‐slip faulting along the Sagaing, Kabaw, and Churachandpur‐Mao Faults as well as east‐west shortening between the Sagaing Fault and Bengal Basin. Through regional stress analysis, considering areas from central Tibet, around the eastern Himalaya Syntaxis, to Burma, it has been determined that the principal compressive stress directions align with the principal strain rates. The northeast‐southwest oriented compressive stress direction from the western Shan Plateau continues into Burma. Notably, P axes align with the topographic gradients, and T axes are sub‐parallel to the topographic contours in the Shan Plateau region south of 27°N. These stress patterns are consistent with a gravitational potential energy induced crustal and mantle flow. The alignment of the fast shear wave with the maximum strain rate and the colinear NW‐SE to E‐W fast direction of the SKS wave and T axis determined from focal mechanisms in the Shan Plateau suggest that the mantle lithosphere deforms in concert with the crust. We suggest crust and mantle flow south of the Red River Fault has resulted in widening of the lithosphere in the Shan Plateau in an east‐west direction. Therefore, the Sagaing Fault has bowed approximately 50–100 km westward if we assume that the Sagaing Fault was originally straight. Our results of regional stress inversion are consistent with late Miocene to present E‐W shortening in the Indo‐Burma subduction zone resulting from the release of gravitational potential energy from the central Tibetan Plateau.

Morphological and Structural Characterization of Shortening Landforms on Mars

AbstractThe lithosphere of Mars accommodates horizontal shortening through folding and faulting, producing landforms described as wrinkle ridges or lobate scarps. Despite this nomenclature, we lack a deep understanding of the drivers of morphological differences observed between landform types. This study aims to develop a quantitative model for shortening landform classification based on surface morphology, subsurface architecture, and strain accommodation, facilitating interpretations of where and how lithospheric stresses are recorded. We developed this model by mapping 100 shortening landforms in a Geographic Information System, recording 12 unique geomorphic parameters such as length and asymmetry, and estimating the strain of each landform. We conducted a Discriminant Function Analysis (DFA) using surface morphometrics. This DFA produced a predictive linear function for categorizing wrinkle ridges and lobate scarps and for quantifying which landforms were exemplars within those categories. The three most influential variables on the surface morphometry DFA were the maximum width, forelimb slope, and back limb length. We then modeled the subsurface structural geology of 50 landforms using MOVE Structural Geology Modeling Software and conducted a second DFA based on subsurface metrics. DFA was most influenced by the dip and depth of the lower ramp base. When both surface morphology and subsurface geometry are input into single DFA, wrinkle ridges and lobate scarps can be distinguished quantitatively 96% of the time. Our results also show that lobate scarps accommodate more strain and imply that studies should consider landform type when interpreting local, regional, and global geological stress histories.

Tectonic evolution and source rocks development of the super oil-rich Bohai Bay Basin, East China

The Bohai Bay Basin, as a super oil-rich basin in the world, is characterized by cyclic evolution and complex regional tectonic stress field, and its lifecycle tectonic evolution controls the formation of regional source rocks. The main pre-Cenozoic stratigraphic system and lithological distribution are determined through geological mapping, and the dynamics of the pre-Cenozoic geotectonic evolution of the Bohai Bay Basin are investigated systematically using the newly acquired high-quality seismic data and the latest exploration results in the study area. The North China Craton where the Bohai Bay Basin is located in rests at the intersection of three tectonic domains: the Paleo-Asian Ocean, the Tethys Ocean, and the Pacific Ocean. It has experienced the alternation and superposition of tectonic cycles of different periods, directions and natures, and experienced five stages of the tectonic evolution and sedimentary building, i.e. Middle–Late Proterozoic continental rift trough, Early Paleozoic marginal-craton depression carbonate building, Late Paleozoic marine–continental transitional intracraton depression, Mesozoic intracontinental strike-slip–extensional tectonics, and Cenozoic intracontinental rifting. The cyclic evolution of the basin, especially the multi-stage compression, strike-slip and extensional tectonics processes in the Hercynian, Indosinian, Yanshan and Himalayan since the Late Paleozoic, controlled the development, reconstruction and preservation of several sets of high-quality source rocks, represented by the Late Paleozoic Carboniferous–Permian coal-measure source rocks and the Paleogene world-class extra-high-quality lacustrine source rocks, which provided an important guarantee for the hydrocarbon accumulation in the super oil-rich basin.

Surface and Deep Structure of the Hanshan–Wuwei Basins in the Lower Yangtze Region: Implications for Mesozoic Tectonic Evolution of the South China Block

AbstractIndicating the tectonic features of the Hanshan–Wuwei basin can reconstruct the framework of the basins formed in Mesozoic and further understand the Mesozoic tectonic evolution of the South China Block. Studies on surface structure, regional stress field and deep geophysical characteristics of the Mesozoic Hanshan–Wuwei basin in Lower Yangtze region were carried out. NE–NNE trending folds and faults developed in the northern margin of the basins. The reconstruction of tectonic stress fields indicates four stress stages dominating the basins' evolution including NW–SE compression, N–S compression, NW–SE extension and NWW–SEE compression. 2D seismic profiles reveal coexistence of thrust, strike‐slip and normal faults in the basin. Combined with regional geological studies, the geodynamic processes for the formation of the Hanshan–Wuwei basin can be divided into five stages: 1) During the Late Triassic, EW trending foreland basin was formed by N–S compression; 2) From Mid‐Jurassic to Late Jurassic, continuous compression strengthened the foreland deformation and formed thrust nappes. In this stage, the integrated foreland basin was compartmentalized or fragmented, and transferred to the broken foreland basin; 3) NE‐trending sinistral strike‐slip movement at the beginning of the Early Cretaceous; 4) Regional extension resulted in normal faults and rift basins developing in the Late Cretaceous; 5) The NWW–SEE compression at the end of the Late Cretaceous caused NW sinistral strike‐slip faults to form, which partly transformed the rift basin.

Present-day incipient fault coalescence at a relay zone (Jiloca extensional basin, Spain): Evidence from instrumental seismicity

The relay zones between NW-SE to NNW-SSE striking faults of the Jiloca graben (Iberian Chain) mostly show distributed along-strike fault and fracture patterns. The latter are chiefly controlled by the Late Pliocene-Quaternary regional stress field, and secondarily respond to local controls from inherited structures. Such fracture patterns contrast with the classical models of transverse connecting faults controlled by relay kinematics. North of the Concud fault trace, at the relay zone with the Sierra Palomera fault, an unusually high seismic activity has been noticed since 2014, with magnitudes up to M = 3.5. Upgrading of the National Seismic Network allowed obtaining such new detailed records, while the installation of a new seismometer by the IGN within the study area has improved the reliability of focal depth data since 2017. A high-precision absolute relocation of seismicity from 01/01/2000 to 30/05/2022 has been carried out. The results show that (i) the epicentres are significantly clustered along a nearly N-S trending band, and (ii) the focal depths range from 0 to 14 km, in good agreement with the thickness of the brittle crust. This 3D spatial distribution of seismicity is interpreted as a consequence of activation of either a single fault or a fault zone, nearly vertical and N-S striking. Such structural setting is consistent with the surficial fracture patterns observed at both map and outcrop scale: NNW-SSE and NNE-SSW oriented faults and fractures, orthogonal to the ENE-WSW to ESE-WNW regional σ3 trajectories, together with NW-SE trending ones controlled by inherited contractive faults. The present-day seismic activity suggests that along-strike, incipient fault propagation at the relay zone between the Concud and Sierra Palomera faults is currently operating under the control of the remote stress field.

РОЛЬ РЕГИОНАЛЬНЫХ ТЕКТОНИЧЕСКИХ НАПРЯЖЕНИЙ В ГРЯЗЕВОМ ВУЛКАНИЗМЕ ЮЖНОГО САХАЛИНА

The article is devoted to the study of the tectonic stress field of Southern Sakhalin on the basis of seismological, geological, tectonophysical, GPS and remote (satellite) methods. The most characteristic type of stress state for the South of Sakhalin is latitudinal — sublatitudinal horizontal compression. The field forming the morphology of the Pugachev and Yuzhno-Sakhalinsk mud volcanoes, as well as the shape of their lithoclastite fields, fully corresponds to the nature of this state, which is confirmed by the analysis of satellite images.The images were used to supplement the results of previous researchers on the study of the directions of the gryphon bands and to measure deformation of volcano lithoclastite fields. The Pugachev volcano lithoclastite fields bend strikes 25°, and that of the Yuzhno-Sakhalinsk mud volcano strikes 30°, which well corresponds to the regional tectonic stress field. The expediency of using remote (satellite) methods as an supplement to geological and geophysical methods, in particular borehole caliper, is shown.

Association between 82Rb positron emission tomography-derived regional myocardial blood flow, severity of angiographic coronary artery stenosis, and mortality in patients with chest pain

Impairment in global myocardial flow reserve (MFR) on rubidium-82 (82Rb) positron emission tomography (PET) stress testing predicts cardiovascular events; however, the relationship between regional coronary artery territory myocardial blood flow (MBF) and invasive coronary angiography is unknown. In this study, patients with acute chest pain who were referred for coronary angiography after abnormal PET stress testing were evaluated. Both global and regional coronary territory stress and rest MBF were derived using 82Rb PET. Coronary artery stenosis severity was assessed using quantitative coronary angiography (QCA) performed within 3 months of PET. A total of 189 patients were followed for a median of 4.1 years. The results showed a weak correlation between regional MFR impairment (<1.7) and stenosis severity in the left descending artery (r = −0.20, P = 0.005), left circumflex artery (r = −0.15, P = 0.042), and right coronary artery (r = −0.26, P < 0.001). In addition, a weak correlation was observed between global MFR and stenosis in any vessel, in both binary and continuous analyses. However, impairment in MFR within any territory was associated with increased all-cause mortality in both unadjusted and adjusted analyses. In conclusion, this is the first large-scale study to examine the relationship between regional coronary territory MBF, coronary artery stenosis severity as assessed using QCA, and mortality. Although coronary territory MFR demonstrated a weak correlation with coronary stenosis severity, impairment in per-territory MFR was significantly associated with increased all-cause mortality, suggesting that mechanisms such as diffuse atherosclerosis and/or microvascular disease may be contributing factors.

Distribution Characteristics and Genesis Mechanism of Ground Fissures in Three Northern Counties of the North China Plain

The North China Plain is among the regions most afflicted by ground fissure disasters in China. Recent urbanization has accelerated ground fissure activity in the three counties of the northern North China Plain, posing significant threats to both the natural environment and socioeconomic sustainability. Despite the increased attention, a lack of comprehensive understanding persists due to delayed recognition and limited research. This study conducted field visits and geological surveys across 43 villages and 80 sites to elucidate the spatial distribution patterns of ground fissures in the aforementioned counties. By integrating these findings with regional geological data, we formulated a causative model to explain ground fissure formation. Our analysis reveals a concentration of ground fissures near the Niuxi and Rongxi faults, with the former exhibiting the most extensive distribution. The primary manifestations of ground fissures include linear cracks and patch-shaped collapse pits, predominantly oriented in east-west and north-south directions, indicating tensile failure with minimal vertical displacement. Various factors contribute to ground fissure development, including fault activity, ancient river channel distribution, bedrock undulations, rainfall, and ground settlement. Fault activity establishes a concealed fracture system in shallow geotechnical layers, laying the groundwork for ground fissure formation. Additionally, the distribution of ancient river channels and bedrock undulations modifies regional stress fields, further facilitating ground fissure emergence. Rainfall and differential ground settlement serve as triggering mechanisms, exposing ground fissures at the surface. This research offers new insights into the causes of ground fissures in the northern North China Plain, providing crucial scientific evidence for sustaining both the natural environment and the socio-economic stability of the region.

The orientation of intra-arc crustal fault systems influences the copper budget of magmatic-hydrothermal fluids

Some of the largest magmatic-hydrothermal copper ore deposits and deposit clusters are associated with arc-oblique fault systems. Whether this structural context impacts the geochemistry of hydrothermal fluids, including their copper contents, remains unknown. Here, we investigate the copper concentration and helium isotope signature of geothermal fluids as modern analogs of hydrothermal ore deposits in the Andes of central-southern Chile. We show that fault systems broadly parallel to the regional stress field facilitate the early release of fluids from deep primitive magmas. By contrast, fault systems oblique to the regional stress field prevent the early escape of fluids and promote magmatic enrichment in copper, volatiles, and ligands, enhancing the potential to form copper deposits. We conclude that the orientation of fault systems actively influences the copper budget of ascending hydrothermal fluids, explaining the contrasting distribution of metals along distinct structures often observed in porphyry-epithermal systems and other types of magmatic-hydrothermal deposits.

Catalog of focal mechanism solutions for the Sichuan and Yunnan region from 2012 to 2022 using the community velocity model of Southwest China

The focal mechanism solution is one of the important focal parameters for exploring fault activity and studying regional stress distribution and it has a wide range of applications. The geological structure of the Sichuan-Yunnan region in China is complex, with frequent earthquakes and abundant historical observation data, making it one of the popular areas of concern for scholars. This study utilizes the high-precision community velocity model v2.0 of southwest China, obtained through joint inversion based on multiple data methods. The Cut-And-Paste (CAP) method was employed to fit and invert the observed waveforms of 1475 events with ML ​≥ ​3.5 in the Sichuan-Yunnan region from January 2012 to December 2022, thereby constructing a catalog of double-couple focal mechanisms. By comparing the focal mechanism inversion results of small earthquakes with those from multiple one-dimensional velocity models and conducting comparative statistical analysis on events below magnitude 4, it has been demonstrated that the model used in this study provides a better fit than one-dimensional models. This contributes to establishing the lower magnitude limit for producing deeper focal mechanism solutions. This study compares the results of larger magnitude earthquakes in the catalog with those published by the Global Centroid-Moment Tensor (GCMT) project and smaller magnitude earthquakes with the catalog released by the Institute of Earthquake Forecasting, China Earthquake Administration. These comparisons serve to validate the accuracy of the catalog results. Leveraging the high-resolution velocity model, this catalog has re-examined the historical earthquake focal mechanism catalog of the Sichuan-Yunnan region. The inversion has yielded reliable results for smaller magnitudes and a greater number of events, providing additional data and support for understanding the regional stress field, active faults, the mechanisms of large earthquake genesis, and earthquake prediction efforts. Consequently, this enhances the depth of scientific research in the Sichuan-Yunnan region.

Abnormal low-magnitude seismicity preceding large-magnitude earthquakes

Unraveling the precursory signals of potentially destructive earthquakes is crucial to understand the Earth’s crust dynamics and to provide reliable seismic warnings. Earthquake precursors are ambiguous, but recent experimental studies suggest that robust warning signs may precede large seismic events in the short (day-to-months) term. Here, we show that the M6.4-M7.1 2019 Ridgecrest sequence (California) and the M7.1 2018 Anchorage earthquake (Alaska) were preceded by up to ~3 months of tectonic unrest on regional scales, as evidenced by abnormal low-magnitude seismicity spreading over the ~15-25% of Southern California and Southcentral Alaska. This precursory unrest has been discovered with an algorithm that integrates an innovative random forest machine learning approach and statistical features built from earthquake catalogs. Supported by a novel suite of finite element solid mechanics models, we propose that precursory, abnormal, low-magnitude seismicity arises if the pore fluid pressure within large fault segments escalates significantly as they approach failure, which leads to major uneven changes in the regional stress field. Our findings and method may open up new perspectives for surveillance agencies to anticipate when a region approaches an earthquake of great magnitude weeks to months before it occurs.

A framework for evaluating the combined effects of water transfer and storage strategies on water stress alleviation

Water transfer and storage are two effective anthropogenic management strategies to alleviate the contradictions between water supply and demand. However, the trade-off and synergistic impacts of management strategies in alleviating water stress remain unclear at the national level. Therefore, this study proposes a framework that integrates the fraction of runoff being withdrawn (scarcity coefficient) and the variation of runoff weighted by reservoir (variability coefficient) to evaluate the multifaceted impacts of management strategies on water stress mitigation. The proposed framework evaluates the changes in both the water supply–demand balance and the historical variability of runoff by considering physical water transfers, virtual water flows, and reservoir operations. This study applied the framework to evaluate the spatiotemporal patterns of water stress and used principal component analysis to estimate the relative contributions of management strategies across ten first-order basins in China for the period of 2014–2018. Results show that water-resource scarcity coefficient varied between −37.15% and 13.28% at the basin scale (the national average varied −5%) and water-resource variability coefficient varied between −100% and −19.26% at the basin scale (the national average varied −61.11%). Management strategies, incorporating water transfer and storage strategies, shifted the distribution patterns of national water stress. The attribution analysis revealed that reservoir storage capacity was the largest contributor to the first principal component representing infrastructure element, whereas the second component representing economic element was affected by the net virtual water inflows. Overall, through exploring the outcomes of combined effects among management strategies, this proposed framework provides a comprehensive perspective for investigating how human activity alleviates regional water stress.

A model for the hydrothermal tremor source of the Mefite d’Ansanto (Italy) CO2 non-volcanic emissions in the intermediate frequency band (1–15 Hz)

In the Summer 2021 a seismic passive survey was carried out at Mefite d’Ansanto (Italy), well-known for the cold non-volcanic and lethal CO2 emissions. Mefite is located close to that part of Irpinia region hosting the large historical earthquakes’ faults, that generated the destructive magnitude 6.8 earthquake of 1980 and have been related to the CO2 leakage by the scientific community. The survey was conducted by installing a small short-period seismic array with the purpose of determining the wavefield features and detecting the possible signature of the regional stress variations. By analyzing the acquired data, we have determined the properties of the seismic wavefield associated with the emission vents, in the intermediate frequency band, 1–15 Hz, which was found to be composed of a stationary background component and an intermittent higher energy one. Basing on our results, considerations on the medium properties and the deep source available information, we have depicted a schematic model of the shallow seismic source: the background components are linked to the shallower activity of the hydrothermal system, e.g. the bubbling, while the intermittent ones are likely generated by the passage of the overpressured gas, trapped in the first-tens-meters’ layers, after overcoming the internal cohesion charge. The characteristics of the wavefield that we have defined, refer to a condition in which no regional earthquakes occurred and could be considered as a basis to identify possible significant variations linked to CO2 emissions and to the regional stress changes.

Prediction of Structural Fracture Distribution and Analysis of Controlling Factors in a Passive Continental Margin Basin—An Example of a Clastic Reservoir in Basin A, South America

Structural fracture distribution is essential in oil and gas transportation and development in passive continental margin basins. In this paper, taking as an example the clastic reservoirs in the A-Basin, a passive continental margin in northeastern South America, the paleotectonic stress field of the Late Cretaceous Maastrichtian formation in Basin A was numerically simulated by finite element technique through the integrated interpretation of seismic total data, logging data and core data, and the distribution of tectonic fractures was later predicted based on rock fracture criterion. The results of the study show that: (1) The distribution of tectonic stress and fractures during the Late Cretaceous Maastrichtian formation of Basin A is affected by the fracture zone, mechanical properties of rocks and tectonic stress, regions with extensive fracture development are susceptible to stress concentrations, resulting in significant stress gradients. (2) The development of structural fractures in the study area was predicted using the Griffiths criterion, and the tensile rupture coefficient T was introduced to quantitatively characterise the intensity of fracture development, with larger values reflecting a higher degree of fracture development. The well-developed and relatively well-developed fractures are mainly located in the fracture zones and the interior of submarine fans. (3) Fracture zones and sedimentary phases mainly control structural fractures in Basin A; within 5 km outside the fracture zones, the development of fractures is controlled by the fracture zones, beyond which the regional tectonic stress field controls them; inside the sedimentary fan, the development of fractures is controlled by the sedimentary subphase, which decreases in the order of the upper fan, the middle fan, and the lower fan; inside the subphase, they are controlled by the regional tectonic stress field, and the fractures show the increasing trend in the direction of NW-NE.

Evaluation of the Lower Cretaceous Alam El Bueib Sandstone reservoirs in Shushan Basin, Egypt – Implications for tight hydrocarbon reservoir potential

This study presents the evaluation of the potential reservoir intervals in the early Cretaceous Alam El Bueib Formation of the Shushan Basin, Western Desert. Seismic 2D lines, and wireline logs (including image logs) were assessed to characterize the potential intervals. The study area is characterized by E-W to ENE-WSW striking parallel sets of steeply dipping normal faults. Based on the breakouts on image log, the regional maximum horizontal stress orientation is inferred as NE-SW. The AEB Formation, as observed on the image log, consists of massive sandstones, planar laminated siltstones, sandstone-siltstone heteroliths and laminated shales, deposited in a fluvial depositional environment. The bedding planes are WNW-ESE striking with a mean true dip of NNE (around 10°). Wireline log based quantitative petrophysical assessment identified multiple promising hydrocarbon-bearing reservoir intervals within the AEB Formation. The potential reservoir intervals are clean with shale content <10% with water saturation <50%. However, all these intervals are tight with effective porosity between 4 and 12%, dominantly ∼5%. Such tight effective porosity can be contributed by extensive silica cementation in the AEB Formation, as seen from the nearby fields in Western Desert. High porosity zones are observed to be water-bearing. The wells drilled in the north and northeastern area exhibit a cumulative net pay thickness between 70 and 150 ft, while south-southeastern region exhibits a very low cumulative net pay of 10–30 ft. Based on the breakout son image log, the regional minimum horizontal stress orientation is inferred as NW-SE, which can be preferred azimuth for placing highly deviated or horizontal wells to exploit such tight clastic reservoirs by optimizing hydraulic fracture propagation. The formation evaluation presented in this work shed critical insights into the tight hydrocarbon reservoir potential of the early Cretaceous AEB.

Super-shear ruptures steered by pre-stress heterogeneities during the 2023 Kahramanmaraş earthquake doublet

The 2023 M7.8 and M7.5 earthquake doublet near Kahramanmaraş, Turkey, provides insight regarding how large earthquakes rupture complex faults. Here we determine the faults geometry using surface ruptures and Synthetic Aperture Radar measurements, and the rupture kinematics from the joint inversion of high-rate Global Navigation Satellite System (GNSS), strong-motion waveforms, and GNSS static displacement. The M7.8 event initiated on a splay fault and subsequently propagated along the main East Anatolian Fault with an average rupture velocity between 3.0 and 4.0 km/s. In contrast, the M7.5 event demonstrated a bilateral supershear rupture of about 5.0–6.0 km/s over an 80 km length. Despite varying strike and dip angles, the sub-faults involved in the mainshock are nearly optimally oriented relative to the local stress tensor. The second event ruptured a fault misaligned with respect to the regional stress, also hinting at the effect of local stress heterogeneity in addition to a possible free surface effect.