Tectonic Stress Regime Research Articles

Uncertainties in geomechanical models – exhaustive vs. feasible approach

Abstract. The stress state is a key component for the safety and stability of deep geological repositories for the storage of nuclear waste. For the stability assessment and prediction over the repository lifetime, the stress state is put in relation to the rock strength. This assessment requires knowledge of both the future stress changes and the current in situ stress state. Due to the limited number of in situ stress data records, 3D geomechanical models are used to obtain continuous stress field prediction. However, meaningful interpretation of the stress state model requires quantification of the associated uncertainties that result from the geological, stress and rock-property data. This would require thousands of simulations which in a high-resolution model is called an exhaustive approach. Here we present a feasible approach to reduce computation time significantly. The exhaustive approach quantifies uncertainties that are due to variabilities in stress data records. Therefore, all available data records within a model volume are used individually in separate simulations. Due to the inherent variability in the available data, each simulation represents one of many possible stress states supported by data. A combination of these simulations allows estimation of an individual probability density function for each component of the stress tensor represented by an average value and a standard deviation. If weighting of the data records can be performed, the standard deviation can usually be reduced and the significance of the model result is improved. Alternatively, a range of different stress states supported by the data can be provided with the benefit that no outliers are disregarded, but this comes at the cost of a loss in precision. Both approaches are only feasible since the number of stress data records is limited. However, it is indicated that large uncertainties are also introduced by variabilities in rock properties due to natural intra-lithological lateral variations that are not represented in the geomechanical model or due to measurement errors. Quantification of these uncertainties would result in an exhaustive approach with a high number of simulations, and we use an alternative, feasible approach. We use a generic model to quantify the stress state uncertainties from the model due to rock property variabilities. The main contributor is the Young's module, followed by the density and the Poisson ratio. They affect primarily the σxx and σyy components of the stress tensor, except for the density, which mainly affects the σzz component. Furthermore, a relative influence of the stress magnitudes, the tectonic stress regime and the absolute magnitude of rock properties is observed. We propose to use this information in a post-computation assignment of uncertainties to the individual components of the stress tensor. A range of lookup tables need to be generated that compile information on the effect of different variabilities in the rock properties on the components of the stress tensor in different tectonic settings. This allows feasible quantification of uncertainties in a geomechanical model and increases the significance of the model results significantly.

Tectonic Stress Distribution in the Song Tranh 2 Hydropower Reservoir: Implication for Induced Earthquake

The Song Tranh 2 hydropower reservoir was built in Tra My area, Quang Nam province, composing magmatic and high-grade metamorphic rocks of the northern part of the Kon Tum massif. Since the reservoir was put into operation, induced earthquakes have occurred in the Song Tranh 2 hydropower reservoir and its vicinity. Tectonically, the northwest-southeast to east-west striking faults developed strongly. Detailed analysis of slickensides and attitude of faults occurring in the studied area have shown that the northwest-southeast striking faults are reactivated as dextral ones during the Pliocene-Quaternary up to the present day. Based on the geometric distribution of the fault network, kinematic characteristics, and the youngest tectonic stress regime, we computed the distribution of tectonic stress in the studied area. Computation results show two positive anomalies of stress directly related to the northwest-southeast faults numbered 2, 10, 11a, 11b and sub-latitude striking fault numbered 1. These faults run in line with the local river channels and are likely to reactivate and generate induced earthquakes.

Tectonic stress regime and stress patterns from the inversion of earthquake focal mechanisms in NW Himalaya and surrounding regions

Because the impact of earthquake effects can extend over great distances from their origin, stress field inversion was performed for 440 earthquake focal mechanisms from the northwest Himalayas and surrounding regions compiled from the data bulletins of international seismological institutions. Earthquakes between November 1976 and April 2019 in the depth range of 10–258 km with moment magnitudes between 4.6 and 7.9 were selected. High-quality solutions were inverted to locate the best fitting stress tensor. Most of the earthquake fault plane solutions indicated thrust faulting, confirming northward underthrusting of the Indian plate along the Main Boundary Thrust and the Main Central Thrust systems and eastward underthrusting along the Burmese Arc. The focal mechanisms indicated right-lateral motion along the Karakoram Fault and left-lateral motion along the Kirthar–Sulaiman Range, which agrees with the expected sense of the lateral mass movement of the continental collision model. The present-day stress regimes obtained from the earthquake focal mechanism inversions indicated a predominantly compressional stress regime represented by NNE–SSW trending normal fault mechanisms in northwest India and Nepal and NNW–SSE trending normal fault mechanisms in Pakistan and Hindukush. These are consistent with the direction of the ongoing India–Eurasia plate collision and the extensional stress of WNW–ESE trending thrust faulting in the Xizang and Kashmir regions. These tectonic regimes connected with the major tectonic affecting the Arabian Peninsula. Accordingly, it is highly recommended to assess the earthquake hazards of the major cities in the eastern countries of Arabian Peninsula as Kuwait, Saudi Arabia, United Arab Emirates and Oman.

Automatic Extraction and Geospatial Analysis of Lineaments and their Tectonic Significance in South Cameroon Area using Remote Sensing Techniques and GIS

This study investigates the dominant orientations of lineament features and the relationship between these trends and the spatial orientation of tectonic structures in the transition zone of the Congo craton and the Pan African belt in South Cameroon area. Landsat 8 OLI/TIRS and hill-shaded images, constructed from 30 m-resolution SRTM-DEM data, were used for automatically extracting and mapping geological lineaments. Lineament features were analyzed by means of azimuth frequency and length density distributions. Three major sets of lineaments trending W–E, ENE-WSW and WNW-ESE are identified in the South Cameroon area. These trends are probably related to repeated reactivation of pre-existing crustal structures during Eburnean and Pan-African tectonic episodes. The lineaments were formed under the compressional tectonic stress regimes generated during these tectonic events.

Geophysical implications for the exploration of concealed orogenic gold deposits: A case study in the Sandy Lake and Favourable Lake Archean greenstone belts, Superior Province, Ontario, Canada

Since 2000, gold discovery rates have not reflected the record levels of exploration expenditure, with those discoveries that are made being predominantly in areas of post-mineral cover. Hence, gold exploration has been increasingly focused on concealed exploration targets where geophysics takes on an increasingly important role in the exploration process.Archean orogenic gold deposits are attractive exploration targets that represent a major proportion of global gold resources. Orogenic gold deposits consistently form late during the tectonic and structural evolution of their host terranes, normally during a transition from compressional to transpressional tectonic stress regimes. Thus, for target generation, the late timing of gold mineralization within the structural history of a prospective orogenic belt is crucial, as it means that current geological maps and sections reflect the geometry of the gold deposits at the time of formation unless there has been subsequent orogeny. Lithological contacts between granitic intrusions and volcano-sedimentary greenstone sequences, as well as between volcanic and sedimentary sequences, are important rheological and geochemical trap sites. Lithosphere-scale faults and dilational jogs in second-order faults act as ore-fluid conduits with anticlinal folds as structural traps for ore-fluid infiltration and gold deposition given suitable host rocks and fluid seals.Geophysical exploration for concealed orogenic gold deposits is challenging due to normally subtle petrophysical contrasts between the gold lodes and their host rocks. Most greenstone-hosted lode-gold deposits do not contain strong conductors as they normally contain <5 vol% of disseminated arsenopyrite and pyrite, which are poor conductors, and rarely contain significant quantities of strong conductors such as pyrrhotite. However, pyrrhotite may be a component of rock sequences such as sulfidic shales or cherts adjacent to the orogenic gold lodes. It is proposed that the combination of HTEM and magnetic surveys be used to explore for concealed orogenic gold deposits, using the Favourable Lake and Sandy Lake greenstone belts in the Superior Province, Canada, as a case study. AIP Cole-Cole time-constants, combined with magnetic and EM data, can efficiently detect disseminated pyrite/magnetite at shallow depths, indicating their potential use as vectoring tools for concealed Archean orogenic gold deposits.

Structural control on badland slope evolution: A case study from the southern Apennines (Italy)

This work focuses on a multi-method study of badlands exposed in two different areas of southern Italy. Results of both geomorphological and structural geological studies are obtained by combining field and laboratory analyses of the badlands affecting Plio-Quaternary silty-clays of southern Italy. Results are discussed in terms of deformation mechanisms and badland formation and evolution. Results highlight the role played by pre-existing structural heterogeneities, such as high-angle faults and joints, on both gravitational processes and water circulation. The aforementioned heterogeneities are due to specific tectonic stress regimes, and are interpreted in light of their genetic nature, kinematics, attitude, geometry, relative timing of formation, and aerial distribution. Results of this work are summarized in a four-stage conceptual model characterized by specific genetic mechanisms. The conceptual model includes representative sketches aimed at assessing the time-dependant evolution of the badlands. A profound link between structural geology and geormophological processes is therefore envisioned for the badlands of southern Italy.

In-situ stresses, abnormal pore pressures and their impacts on the Triassic Xujiahe reservoirs in tectonically active western Sichuan basin

Pore pressures in the Xujiahe gas-bearing formations are highly overpressured in the western Sichuan basin. The abnormally high overpressure and strong tectonic stress cause very high horizontal stresses. The high in-situ stresses, tectonic stress regimes, and tight formations severely impact development and production of the Xujiahe tight sandstone reservoirs. Overpressure generation mechanisms are analyzed based on measured data in several dozen wells in the studied gas fields. It is found that the sonic or seismic transit time can be used to predict pore pressure. Two major reservoirs (Xu2 and Xu4) belong to different fluid compartments with distinct pore pressure gradients. The Xu4 has a much higher pore pressure gradient (20 MPa/km) than the Xu2 (15.6 MPa/km).Measured data from hydraulic fracturing tests and borehole image logs in vertical wells are analyzed. The results show that the minimum horizontal stress gradient in the Xu4 reservoir reaches as high as 24 MPa/km, close to the overburden gradient. The Xu2 reservoir has a slightly lower minimum horizontal stress gradient. The high minimum horizontal stress and high rock strength cause abnormally high formation breakdown pressure gradient (24.8–34 MPa/km). The high breakdown pressure, high propagation pressure, and unfavorable stress regime result in a great difficulty for the reservoir stimulation. One of the major impacts is that hydraulic fracturing operations can only create very short hydraulic fractures, which markedly limits reservoir productivity. Based on field measurements, in-situ stress determination and assessment are given, and a new method for horizontal stress estimation is proposed. Accordingly, reliable prediction of formation breakdown and propagation pressures are proposed, which are more suitable for the tight reservoirs. Recommendations are given for effective development of the tight reservoirs with impacts of high in-situ stresses.

Structural Control, Evolution, and Accumulation Rates of Massive Sulfides in the TAG Hydrothermal Field

AbstractThe Trans‐Atlantic Geotraverse (TAG) hydrothermal field on the Mid‐Atlantic Ridge is one of the best‐studied hydrothermal systems to date. However, high‐resolution bathymetric data obtained in 2016 by an autonomous underwater vehicle (AUV) reveal new information about the distribution of active and inactive hydrothermal deposits, and their relation to structural features. The discovery of previously undocumented inactive vent sites contributes to a better understanding of the accumulation rates and the resource potential of seafloor massive sulfide deposits at slow‐spreading ridges. The interpretation of ship‐based and high‐resolution AUV‐based data sets allowed for the determination of the main tectonic stress regimes that have a first‐order control on the location and distribution of past and present hydrothermal activity. The data reveal the importance of cross‐cutting lineament populations and temporal variations in the prevalent stress regime. A dozen sulfide mounds contribute to a substantial accumulation of hydrothermal material (~29 Mt). The accumulation rate of ~1,500 t/yr is comparable to those of other modern seafloor vent fields. However, our observations suggest that the TAG segment is different from many other slow‐spreading ridge segments in its tectonic complexity, which confines sulfide formation into a relatively small area and is responsible for the longevity of the hydrothermal system and substantial mineral accumulation. The inactive and weakly active mounds contain almost 10 times the amount of material as the active high‐temperature mound, providing an important indication of the global resource potential for inactive seafloor massive sulfide deposits.

Fault imaging at Mt Pollino (Italy) from relative location of microearthquakes

SUMMARYRelative location of microearthquakes that occurred at Mt Pollino (Italy) from 2011 to 2013 have been analyzed with the aim of a detailed imaging of the geometry of active faults. We identified 27 clusters composed of a number of earthquakes from 9 to 33, with local magnitude in the range 0.6–2.7. The relative location shows that the distribution of hypocentres in each cluster is characterized by extension from few tens of meters to at most 350 m. For each cluster the hypocentre distribution was fitted by a plane to infer the fault orientation, and results were compared with the fault plane solutions corresponding to the focal mechanism of earthquakes of the same cluster. The comparison shows a good agreement in most of the cases. The relative location analysis, generally applied to earthquakes with similar waveform, has been improved to permit also the relative location of earthquakes characterized by not similar signals. To achieve this purpose a modified procedure that overcome the condition of very similar waveforms has been applied to estimate the time delay between first pulses of the master events. The relative location of master events of all clusters shows a precise imaging of the relative position of all analysed sources and allows also to follow with high accuracy the evolution in time of the seismic swarm within the selected periods. The hypocentre position of master events and the nearly parallel fitting planes of any clusters suggest that most of the analyzed earthquakes were produced by different patches of the same fault. The final results depict a main fault plane characterized by NW–SE strike, dip of about 35–45° and depth between 4.5 and 6.5 km b.s.l. Focal mechanisms, used also to evaluate the local stress field, are mostly of normal type with few strike slip solutions for the shallowest events. This result is in good agreement with the local tectonic stress regime that is characterized by predominant NE–SW transtension, as inferred from structural, seismological and geophysical data.

Evaluation of the Plio-Quaternary tectonic stress regime from fault kinematic analysis in the lake Van Basin (Eastern Anatolia)

We focus on the Neogene–Quaternary tectonic evolution of the Lake Van Basin located within the Turkish-Iranian Plateau. To better understand the complex tectonic history of the region and determine the paleostress patterns, we investigate and report on the geometric, structural, and kinematic characteristics of the Basin based on field observations of fault-slip orientations which are classified according to radiometric ages of the basin stratigraphy.The analysis of large-scale structures and fault kinematics indicate that three different deformation phases prevailed in the Lake Van Basin during the Neogene-Quaternary periods. Phase 1 is characterized by NW-SE extension and NE-SW contraction that gave rise to the development of strike-slip faults with thrust or normal components during the late Miocene, deforming the fluvial sediments which expose at east/northeast of the Basin. Phase 2 is characterized in fluvial and lake deposits of the Middle Pleistocene, deformed by dominant contraction stress regime effective along NW-SE direction. The late Pleistocene tectonic regime (Phase 3) consists of transpressional deformation that develops under NNW-SSE compression and ENE-WSW extension. According to our analysis, the present-day deformation pattern of the Lake Van Basin is dominated by compression at east, while at the northern part is transtensional.

Seismicity induced by massive wastewater injection near Puerto Gaitán, Colombia

SUMMARY Seven years after the beginning of a massive wastewater injection project in eastern Colombia, local earthquake activity increased significantly. The field operator and the Colombian Geological Survey immediately reinforced the monitoring of the area. Our analysis of the temporal evolution of the seismic and injection data together with our knowledge of the geological parameters of the region indicate that the surge of seismicity is being induced by the re-injection of produced water into the same three producing reservoirs. Earthquake activity began on known faults once disposal rates had reached a threshold of ∼2 × 106 m3 of water per month. The average reservoir pressure had remained constant at 7.6 MPa after several years of production, sustained by a large, active aquifer. Surface injection pressures in the seismically active areas remain below 8.3 MPa, a value large enough to activate some of the faults. Since faults are mapped throughout the region and many do not have seismicity on them, we conclude that the existence of known faults is not the only control on whether earthquakes are generated. Stress conditions of these faults are open to future studies. Earthquakes are primarily found in four clusters, located near faults mapped by the operator. The hypocentres reveal vertical planes with orientations consistent with focal mechanisms of these events. Stress inversion of the focal mechanisms gives a maximum compression in the direction ENE-WSW, which is in agreement with borehole breakout measurements. Since the focal mechanisms of the earthquakes are consistent with the tectonic stress regime, we can conclude that the seismicity is resulting from the activation of critically stressed faults. Slip was progressive and seismic activity reached a peak before declining to few events per month. The decline in seismicity suggests that most of the stress has been relieved on the main faults. The magnitude of a large majority of the recorded earthquakes was lower than 4, as the pore pressure disturbance did not reach the mapped large faults whose activation might have resulted in larger magnitude earthquakes. Our study shows that a good knowledge of the local fault network and conditions of stress is of paramount importance when planning a massive water disposal program. These earthquakes indicate that while faults provide an opportunity to dispose produced water at an economically attractive volume–pressure ratio, the possibility of induced seismicity must also be considered.

Dynamic Fault Interaction during a Fluid-Injection-Induced Earthquake: The 2017 Mw 5.5 Pohang Event

ABSTRACT The 15 November 2017 Mw 5.5 Pohang, South Korea, earthquake has been linked to hydraulic stimulation and fluid injections, making it the largest induced seismic event associated with an enhanced geothermal system. To understand its source dynamics and fault interactions, we conduct the first 3D high-resolution spontaneous dynamic rupture simulations of an induced earthquake. We account for topography, off-fault plastic deformation under depth-dependent bulk cohesion, rapid velocity weakening friction, and 1D subsurface structure. A guided fault reconstruction approach that clusters spatiotemporal aftershock locations (including their uncertainties) is used to identify a main and a secondary fault plane that intersect under a shallow angle of 15°. Based on simple Mohr–Coulomb failure analysis and 180 dynamic rupture experiments in which we vary local stress loading conditions, fluid pressure, and relative fault strength, we identify a preferred two-fault-plane scenario that well reproduces observations. We find that the regional far-field tectonic stress regime promotes pure strike-slip faulting, whereas local stress conditions constrained by borehole logging generate the observed thrust-faulting component. Our preferred model is characterized by overpressurized pore fluids, nonoptimally oriented but dynamically weak faults and a close-to-critical local stress state. In our model, earthquake rupture “jumps” to the secondary fault by dynamic triggering, generating a measurable non-double-couple component. Our simulations suggest that complex dynamic fault interaction may occur during fluid-injection-induced earthquakes and that local stress perturbations dominate over regional stress conditions. Therefore, our findings have important implications for seismic hazard in active georeservoir.

Geomechanical characterization of sedimentary basins using tectonic stress and strain

We examine whether the combination of tectonic stress and strain is useful in characterizing tectonic condition and geomechanics of sedimentary basins. We compile measured stress, pore pressure, and mechanical rock property data from six major sedimentary basins worldwide. We find that the measured horizontal principal stresses are always higher than the theoretically calculated stresses based on a simple uniaxial compaction poroelastic equation, known as Eaton’s model. From this we conclude that there are always external tectonic stresses in seemingly stable basins. We extract these external tectonic stresses by subtracting the theoretically calculated horizontal stresses from the measured horizontal stresses, and relate them to the tectonic strains estimated by the linear elastic constitutive model. Given the external tectonic stresses and associated strains, we interpret the tectonic conditions of individual basins from the tectonic differential stress and strain conditions. The six basins exhibit distinct tectonic stress and strain regimes, which can be related generally to geological processes such as brittle and ductile deformation. The results show that combining tectonic stress and strain is useful in characterizing sedimentary basins.

Sismicidad somera de intraplaca en la provincia de Buenos Aires (Argentina) y áreas adyacentes: ¿están relacionadas con la Fosa de Quilmes?

In the light of the November 30th, 2018 (N30) earthquake activity, some neighborhoods of the city of Buenos Aires were shaken by a 3.8 mb earthquake (4.53 km estimated depth). We examined the historical and recent seismic records in order to analyze possible mechanisms related to the distribution of tectonic stresses as responsible for such unusual earthquakes in a region where only very little seismic activity is reported. According to this, at list one historical event occurred on June 5th, 1888 and other small magnitude earthquakes are mentioned since 1848 interpreted as being associated with the Rio de la Plata faulting. But there is, still no consensus about the role of this structure compared to other structures with orientation SW-NE. The lack of evidence to support one over the other structures makes it difficult to analyze these earthquakes. The presence of the Quilmes Trough connecting the Santa Lucía Basin in Uruguay and the Salado Basin in Argentina was recently proposed to play a tectonic role by a system of ENE-WSW trending controlled by extensional faulting related to the beginning of the Gondwana breakup. This depocenter with a thickness of almost 2,000 m of Mesozoic and Tertiary sequences could be acting as a zone of weakness in the crust and therefore responsible for the mentioned earthquake activity. The orientation of this structure correlates well with the present convergence vector between the Nazca and the South American plates and could therefore be propitious for strain release triggering shallow intraplate seismicity. We propose that most of the epicenters from historical and recent earthquakes might be aligned sub-parallel to the principal axis of the Quilmes Trough. Nevertheless, more data is needed to produce a reliable earthquake monitoring system in order to elucidate the tectonic stress regime and the existence of such structures at depth

The effects of a tectonic stress regime change on crustal-scale fluid flow at the Heyuan geothermal fault system, South China

The South China, Heyuan Fault Zone provides unique crustal exposure to study the meso- macroscale architecture of evolving fracture networks in response to a change in tectonic stress regime and associated changing fluid flow pathways. Multiple generations of fracturing across the fault zone reveal its structural evolution related to the change in stress regime from extensional (since the Mesozoic) to transpressive (in the Cenozoic), which coincides with generations of growth and fracturing on the microscale. Low angle, mode-I fractures, quartz veining, and a giant quartz reef (>75 m wide and 40 km long) were formed due to a combination of chemo-mechanical mechanisms, including a crucial phyllonite ‘cap rock’, during the extensional phase. This evolved through to a predominately sub-vertically aligned, shear fracture network, exhibiting only minor quartz precipitation. These open fractures are attributed to the transpressive regime experienced since the Cenozoic, in which strike-slip faults cross-cut the Heyuan Fault. We propose that this sub-vertical fracture network provides the fluid flow pathway supplying the present-day hot springs by penetrating through the phyllonite seal, which previously enabled SiO2 accumulation. Additionally, further quartz-reef building would likely be reduced due to (i) lack of seal integrity, and (ii) an unfavourable tectonic setting to channel the flow and provide accommodation space.

Paleostress field reconstruction in the western Dharwar craton, south India: Evidence from brittle faults and associated structures of younger granites

We present new fault slip data to decipher the tectonic stress regime in younger granites (2.61 Ga) of western Dharwar craton (WDC), south India. The study is based on paleostress reconstruction of the terrain using fault-slip data from the small scale oblique slip normal faults of Chitradurga granite. Paleostress analysis using Right Dihedron and Rotational Optimization methods reveal that these faults have developed under NNE-SSW directed extension. Similar results are also obtained when left-lateral and right-lateral oblique-slip normal faults are treated as separate entities. It is envisaged that all faults in Chitradurga granite have developed under a single tectonic event and are considered to be extensional shear fractures. We interpret that the oblique-slip normal faults in granite oriented along NNW-SSE to N-S (P shear), NW-SE (R shear), NE-SW (R′ shear), and WNW-ESE (T) are defined as the respective shear components of the regional riedel shear system. It developed due to sinistral movement along the Chitradurga Shear Zone (CSZ) during late D3 deformation at shallow crustal level. The fault planes act as primary or secondary shear planes, depending on their proximity to the CSZ and kinematically fit well with regional far-field compression. The present study addresses the formation of oblique slip normal faults in the Chitradurga granite and its implication in reconstructing the paleostress field in the Precambrian south Indian shield.

Seismic Moment Evolution During Hydraulic Stimulations

AbstractAnalysis of past and present stimulation projects reveals that the temporal evolution and growth of maximum observed moment magnitudes may be linked directly to the injected fluid volume and hydraulic energy. Overall evolution of seismic moment seems independent of the tectonic stress regime and is most likely governed by reservoir specific parameters, such as the preexisting structural inventory. Data suggest that magnitudes can grow either in a stable way, indicating the constant propagation of self‐arrested ruptures, or unbound, for which the maximum magnitude is only limited by the size of tectonic faults and fault connectivity. Transition between the two states may occur at any time during injection or not at all. Monitoring and traffic light systems used during stimulations need to account for the possibility of unstable rupture propagation from the very beginning of injection by observing the entire seismicity evolution in near‐real time and at high resolution for an immediate reaction in injection strategy.

Detailed spatiotemporal analysis of the tectonic stress regime near the central Alpine Fault, New Zealand

We construct a new data set of 845 focal mechanisms derived from microseismicity recorded between late 2008 and early 2017, to investigate the state of stress near the central Alpine Fault, a major obliquely convergent plate boundary fault. We obtain an average maximum horizontal compressive stress orientation, SHmax, of 121±11°, which is uniform to first-order along the length of the central Southern Alps. We also make use of the local magnitudes of the earthquakes with focal mechanisms to examine the spatial patterns of seismic moment release and obtain a relatively uniform cumulative seismic moment release distribution adjacent to the central Alpine Fault, which appears to be independent of both the perpendicular and parallel distances to the fault. We observe an average angle between SHmax and the Alpine Fault of 66°, which is consistent with previous observations in the northern and southern sections of the Alpine Fault. This result implies that central Alpine Fault is not optimally oriented for reactivation. Detailed temporal investigations using two different approaches did not show any systematic change of the stress parameters with time. We also examine the distribution of the shear to normal stress ratio on the fault and do not observe any significant spatial variation along the Alpine Fault. This lack of variation combined with the high angle between the fault strike and SHmax implies that the fault is unfavourably oriented for slip.

Fracture analysis and in situ stress estimation of a gas condensate field in Persian Gulf using FMI and DSI image logs

A giant gas field (called Field X in this study) in the Persian Gulf is located in the northern part of Qatar’s North field was considered in this study. Since the reservoir subjected to a strong tectonic compression, an accurate estimation of in situ stresses is important for reservoir-related issues and stress-related geo-hazards, such as wellbore stability, well completion, drilling a horizontal well, and fracture conductivity in Kangan and Upper Dalan formations. The formation microimager (FMI) and dipole sonic imager (DSI), as effective borehole image techniques, were used for stress determination and borehole breakouts (BOs) and drilling-induced fractures (DIFs) detection in field X. Based on 10 DIFs and 65 BOs derived from FMI logs and measured velocities of P-wave and S-wave from DSI logs in 3 gas wells, in situ stresses were determined in different tectonic units. The orientations of the maximum and minimum horizontal stresses in well B and well C were identified to be NE–SW and NW–SE, respectively. Additionally, the value of minimum horizontal stress was varied from 6210 to 9664 psi, the maximum horizontal stress was ranged between 7124 and 9968 psi, and the vertical stress was gradually increased with the depth from 10,000 to 12,000 psi. Hence, according to the relationships between the in situ stresses ( $$\sigma_{v} > \sigma_{Hmax} > \sigma_{hmin}$$ ), the tectonic stress regime of the studied area is normal.

The focal depths of the 2008 Panzhihua earthquake sequence and the stress field in the source region

Focal depths of the 2008 MS6.1 Panzhihua earthquake sequence and tectonic stress field in the source area are investigated. Source depths of 24 earthquakes in Panzhihua earthquake sequence with a magnitude M≥3.0 were determined using the seismic depth phase sPL; additionally, the focal depths of 232 earthquakes were measured by fitting the three-component waveforms of the P and S waves. The source depth of the main shock is ~12 km. The majority of the aftershocks with magnitude M≥3.0 occurred in the brittle upper crust at the depths range of 12–18 km. Further, the Source mechanisms of the 232 events around the Panzhihua earthquake source area were determined, and the results show that the earthquakes have predominantly strike-slip mechanisms in the Dianzhong Block, but display complexity of the focal mechanisms outside and near the boundary of the Dianzhong block. The 232 earthquake mechanisms from this study are combined with the solutions from the Global Centroid Moment Tensor (GCMT) catalog to derive 2D stress field. The inversion results show that the Dianzhong block is predominantly under a strike slip faulting regime and the direction of the maximum principal compression σ1 is northwest-southeast (NW-SE)-trending. The distribution is coincide with GPS velocity field. However, orientations of principal stress axes as well as the faulting types change outside and near the Dianzhong block. The results show that the tectonic stress field in the study area is predominantly controlled by the southeast (SE)-trending horizontal movement and clockwise rotation of the Dianzhong block as a result of the eastward movement of eastern Tibetan meeting the old and rigid South China block (SCB). The Panzhihua earthquake ruptured at ~12 km depth where the tectonic stress regime is under the SE-direction horizontal compression and the NE-direction horizontal extension.