Secondary Faults Research Articles

Two-step classification of highly mixed oil in the Qikou Sag (Bohai Bay Basin, China): Implications for origin, maturity, and biodegradation

Mixing of crude oil leads to unpredictable changes in the property and composition of mixtures, bringing uncertainties to petroleum production. Therefore, it is crucial to reveal the origin, maturity, and biodegradation of mixed oil during the exploration stage. Here we address these issues using the highly-mixed oil from the Qikou Sag (Bohai Bay Basin, eastern China) after a two-step classification. Firstly, the highly-mixed oil was classified into two groups (groups I and II) according to their physical property (API gravity and sulfur content), molecular composition, and gas chromatograms of saturated hydrocarbons. Secondly, the group I oil was further divided into two sub-groups (groups I-1 and I-2) based on the principal component analysis (PCA) and hierarchical cluster analysis (HCA) of biomarkers (including n-alkanes, isoprenoids, terpenoids, and steroids). The group I oil is non-biodegraded, in which the group I-1 oil from deep reservoirs is a mixture of hydrocarbons from three sets of source rock, whereas the group I-2 oil from shallow reservoirs is mixed by hydrocarbons from five sets of source rock. Maturity parameters of biomarkers suggest that the group I oil is thermally mature. The group II oil was slightly (PM 1–2) to severely (PM 4+) biodegraded, as indicated by low API gravity, high percentages of asphaltenes and NSO, high ratios of Pr/n-C17 and Ph/n-C18, and the assignment of UCM and 25-norhopane. The burial depth of reservoirs and secondary faults (formed after the Neogene) are the primary controlling factors for the biodegradation. Our results demonstrate that the two-step classification facilitates the grouping of highly mixed oil, which provides a basis for indicating their origin, maturity, and biodegradation. The results also provide guidance for regional petroleum exploration and production.

Active detachment faulting controls folding and faulting in western Borneo, SE Asia

The origin of active deformation and structural evolution in large areas of Western Borneo has been highly debated, with two contrasting views involving gravitational tectonics and plate tectonics. The scarcity of field data on land has significantly hampered our understanding of the onshore structures and their relationship with those of the offshore regions. The Baram Delta province is one of the best examples in SE Asia, where debates on the origin and evolution of active tectonic versus gravitational tectonic structures are broadly associated with the complexity of faults and folds in Brunei and Sarawak (Malaysia). In this paper, we present the results of the first large-scale satellite-based structural mapping and the detailed outcrop-based structural mapping in Brunei Darussalam. The results of the satellite-based mapping reveal a major ∼NE-SW trending detachment fault in Brunei and adjacent regions, which is termed the Tutoh fault, with numerous associated secondary faults and folds. The fault acts as a detachment structure onto which several NW-SE trending faults ramp and have asymmetrical folds showing an en echelon fold-fault system. The topographic expression and the recent strike-slip faulting event on the Tutoh fault system suggest that the fault remains active, challenging the discourse that gravitational tectonics is the only cause of active deformation in Borneo.

Evaluation of Natural Gas Hydrate Fault System: A Case from a Sag in Deep-Water Slope Area of the Northern South China Sea

AbstractThe fault system is one of the structural carrier systems of gas hydrate accumulation, which plays a vital role in controlling the distribution of natural gas hydrate (NGH) accumulation. The previous studies mainly focus on summarizing the vertical migration mode of high flux fluid along the fault with obvious geophysical response characteristics on the seismic profile, such as “fault with gas chimney,” “fault with mud diapir,” and “fault with submarine collapse”, but lack of evaluation methods for the fault carrier system. We use the X sag in the deep-water continental margin slope area of the northern South China Sea as an example to study the fault systems closely related to NGH. This paper puts to use attribute technologies, such as coherence, curvature, and fusion, to analyze the characteristics and combination of the fault systems. We discussed migration patterns and evaluation methods of dominant fault carrier systems. This research proves that the strike-slip fault system in the platform area can directly connect the gas source bed with high-quality hydrocarbon generation to the gas hydrate stability zone (GHSZ). The activity of this fault system is more conducive to the accumulation of hydrocarbon in the GHSZ. This area has a good site for pore-filling gas hydrate prospecting and a preferential favorable fault carrier system. The composite fault system, consisting of a normal dip-slip fault system and a polygonal fault system, in the slope area can jointly communicate the biogenic gas-rich reservoir. Its activity and well-migration performance are the main reasons for the submarine gas leakage and collapse. It is a secondary favorable fault carrier system in the study area. There may be massive and vein natural gas hydrate formation in fractures in the leakage passage, and pore-filled gas hydrate may exist in the submarine nonleakage area. In this work, a three-factor evaluation method of the fault carrier system is proposed for the first time. This method is of great significance for the evaluation and exploration of NGH reservoirs in the continental margin slope area of the northern South China Sea.

Magmatic intrusion impacts on source-to-sink system of rift basins: Eocene Lufeng-22 sub-sag, Pearl River Mouth Basin, South China Sea

• Magmatic intrusion is classified based on deformation and position. • Magmatic intrusion changes the figuration of border faults and basin structure. • Intrusion-induced deformation separates original unified source-to-sink system into minor system. • Intrusion-induced deformation transforms distal source-to-sink system into proximal system. Magmatism plays an important role in the rifting of the continental margins. The intrusion effects on topography and petroleum systems within sedimentary basins have been well reported. However, the way in which the source-to-sink (S2S) system responds to intrusions is less well studied. In order to ascertain the effects of intrusions, different types of intrusion and their relationship with the S2S system are investigated. Three-dimensional seismic data calibrated with wireline logs covering Eocene Lufeng-22 sub-sag in the Northern Pearl River Mouth Basin provide support for the construction of a stratigraphic framework and for the recognition and timing of magma emplacement, and the restoration of the S2S system. Six types of magmatic intrusion are classified based on various intrusion-induced deformation in different tectonic units. The geomorphology shows that the original half-graben basin structure is complicated by intrusion-induced forced folds and secondary faults. Accompanied with deformation along border faults, weak-deformation intrusions generate low-throw secondary faults and uplift with little denudation, whereas intense intrusions result in the large-offset faults and high-amplitude denudation. The intense uplift and denudation occur both in the erosional and depositional zones of the pre-emplacement S2S system, which transform distal source system into proximal system. Moreover, forced folds and faults in the sink area separate original unified subsidence center into several minor centers. In addition, the S2S system of the syn-emplacement period exhibits complex basin structure and a reduced storage area. The analysis of intrusion-induced deformation is important in basin analysis and quantitative restoration of the S2S system.

Research on a Secondary Active Limiting DC Fault Current Limiter Topology

The current-limiting performance of the smoothing reactor is weak. The traditional saturated core-type fault current limiter (TFCL) is better, but will cause the dc circuit breaker (DCCB) to endure high overvoltage, energy absorption and also increase the current fall time. When DCCB recloses on the permanent fault, the whole dc system will face a harmful second shock that cannot be solved effectively by TFCL. To solve the above problems, a secondary active limiting fault current limiter (SAFCL) topology is proposed. It can reduce the DCCB overvoltage and energy absorption greatly. Meanwhile, it can store fault energy and uses it to active limit the current when the DCCB recloses on the permanent fault. The secondary active current limiting effect is better than the first and the DCCB breaking electric stress is lower than the first. First of all, the topology and principle of the SAFCL are introduced. The main electromagnet parameters are then analyzed. The performance of different FCLs are simulated comparatively to verify the superiority of the SAFCL. Final, the experiment is carried out, the comparison with simulations verifies the feasibility of the SAFCL.

2019 Mw 6.0 Mesetas (Colombia) Earthquake Sequence: Insights From Integrating Seismic and Morphostructural Observations

AbstractThe boundary between the Northern Andean block and the South American plate is a first‐order tectonic structure in Colombia with historically M > 7 earthquakes. However, details about individual sections of the system remain unknown. We illuminate the seismotectonic of the Algeciras fault by investigating an earthquake sequence that started on 24 December 2019. Using seismic networks at region, we estimate foreshocks and aftershocks focal mechanisms, local stress field, kinematic slip models of the largest events, and Coulomb stress changes. We integrate seismological and morphostructural observations to characterize source properties and reinterpret local faults. Two mainshocks (doublet of Mw 6.0 and 5.8) occurred within 16 min, rupturing just a few kilometers from each other. Discrimination of causative faults among the centroid moment‐tensor nodal planes is difficult because the focal zone is a complex tectonic environment. The focal mechanisms and the local stress field obtained are consistent with a regional NE trending dextral transpressive shear. The relocated aftershocks show a cluster with an L‐shaped pattern concentrated in a ∼7 × 7 km area. Our model defines the Algeciras fault with two structural styles for its northern termination. The NW part is characterized by a duplex‐style of right‐lateral strike‐slip with inner secondary faults of the same sense of movement, and the SE zone by a domino‐style system with inner minor faults of sinistral kinematics. The earthquake doublet is in the zone characterized by the duplex style. In contrast, the southern part of the aftershocks is located in the zone characterized by the domino style.

The Formation of the Jiaodong Gold Province

AbstractGold deposits in Jiaodong, termed Jiaodong‐type, are tectonically located in the southeastern margin of the North China Craton. Their major features are reviewed in this paper to highlight the differences between Jiaodong deposits and other genetic types of gold deposits. The mineralization was synchronized with asthenosphere upwelling indicated by syn‐ore OIB‐like mafic dike and large‐scale crustal thinning suggested by decrease of Sr/Y from pre‐ore to syn‐ore granites. Asthenosphere upwelling induced by the roll‐back of Paleo‐Pacific Plate drove partial melting of lithospheric mantle and devolatilization, which induced the release of the ore‐forming fluids. In concomitant with magmatic records, mineralization migrated from the western Jiaobei terrane (133–127 Ma) to the eastern Sulu orogenic belt (114–108 Ma), corresponding to the eastward roll‐back of Paleo‐Pacific Plate. Gold mineralization in Jiaodong formed in the transitions of ductile to brittle deformation, rapid to slow crustal uplift, and regional compression to extension. In the regional‐scale, the gold deposits in the Jiaobei terrane are mostly situated at intersections between NE‐trending faults and EW‐trending basement faults, and gold orebodies dominantly controlled by the lithologic contacts between Precambrian metamorphic rocks and Mesozoic granites. The mineralization was dominated by the disseminated‐veinlet ores related to quartz–sericite alteration in strong cataclasite‐breccia zone, with subsidiary thick quartz‐sulfide veins developed in secondary fault zones. The ore‐forming fluids belong to a H2O–CO2–NaCl±CH4system and show minor variations in salinity among different types of ore. Structure‐fluid feedback involving fluid‐rock reaction and hydrofracturing triggered the fluid phase separation and resultant gold deposition. The Jiaodong gold deposits are distinct from orogenic and intrusion‐related gold deposits in terms of tectonic setting, origin of ore‐forming fluids, and mechanism of gold deposition.

DESIGNING AN EFFICIENT OBSERVER FOR THE NON-LINEAR LIPSCHITZ SYSTEM TO TROUBLESHOOT AND DETECT SECONDARY FAULTS CONSIDERING LINEARIZING THE DYNAMIC ERROR

The presence of faults in a system leads to a lower value for efficiency, accuracy and speed, and, in some cases, even a complete breakdown. Thus, early fault detection is a major factor in efficiency and productivity of the procedure. In recent decades, many research studies have been conducted on troubleshooting and secondary fault detection. The current work presents an efficient and novel observer design capable of stabilizing the residue and dynamic error for the nonlinear Lipschitz systems with faults as well as a troubleshooting analysis and determining the formation of secondary faults in defective systems. The observer is designed based on linearizing dynamic error considering uncertainty, disturbance, and defects by employing non-linear gain factors instead of using state transformation. The dynamic error and residue stabilization of a non-linear faulty system have been discussed as well as the likelihood of secondary fault generation. The results indicate that the observer is able to determine fault-emergence, fault-disappearance and secondary fault formation well and quite fast.

Fast 1-D Velocity Optimization Inversion to 3D Velocity Imaging: A Case Study of Sichuan Maerkang Earthquake Swarm in 2022

To obtain an accurate one-dimensional velocity model, we developed the EA_VELEST method based on the evolutionary algorithm and the VELEST program. This method can quickly generate a suitable 1D velocity model and finally input it into the 3D velocity inversion process using the TomoDD method. We adopt TomoDD methods to inverse the high-resolution three-dimension velocity structure and relative earthquake hypocenters for this sequence. This system processing flow was applied to the Sichuan Maerkang earthquake swarm in 2022. By collecting the seismic phase data of the Maerkang area between 1 January 2009 and 15 June 2022, we relocated the historical earthquakes in the area and obtained accurate 3D velocity imaging results. The relocated hypocenters reveal a SE-trending secondary fault, which is located ~5 km NW of the Songgang fault. In the first ten-hour of the sequence, events clearly down-dip migrated toward the SE direction. The inverted velocity structure indicates that the majority of earthquakes during the sequence occurred along the boundaries of the high and low-velocity zones or high and low-VP/VS anomalies. Especially both the two largest earthquakes, MS 5.8 and MS 6.0, occurred at the discontinuities of high and low-velocity zones. The EA_VELEST method proposed in this paper is a novel method that has played a very good enlightenment role in the optimization of the one-dimensional velocity model in geophysics and has certain reference significance. The 3D velocity results obtained in this paper and the analysis of tectonic significance provide a reference for the seismogenic environment of this Maerkang earthquake and the deep 3D velocity of the Ganzi block.

Effects of supercritical CO2 exposure on diffusion and adsorption kinetics of CH4, CO2 and water vapor in various rank coals

The physico-chemical effects caused by supercritical CO2 (ScCO2) exposure is one of the leading problems for CO2 storage in deep coal seams as it will significantly alter the flow behaviors of gases. The main objective of this study was to investigate the effects of ScCO2 injection on diffusion and adsorption kinetics of CH4, CO2 and water vapor in various rank coals. The powdered coal samples were immersed in ScCO2 for 30 days using a high-pressure sealed reactor. Then, the diffusion and adsorption kinetics of CH4, CO2 and water vapor in the coals both before and after exposure were examined. Results indicate that the diffusivities of CH4 and CO2 are significantly increased due to the combined matrix swelling and solvent effect caused by ScCO2 exposure, which may induce secondary faults and remove some volatile matters that block the pore throats. On the other hand, the diffusivities of water vapor are reduced due to the elimination of surface functional groups with ScCO2 exposure. It is concluded that density of the surface function groups is the controlling factor for water vapor diffusion rather than the pore properties. The unipore model and pseudo-first-order equation can simulate the diffusion and adsorption kinetics of CH4 and CO2 very well, but the unipore model is not capable of well describing water vapor diffusion. The effective diffusivity (De), diffusion coefficient (D) and adsorption rates (k1) of CH4 and CO2 are significantly increased after ScCO2 exposure, while the values of water vapor are decreased notably. Thus, the injection of ScCO2 will efficiently improve the transport properties of CH4 and CO2 but hinder the movement of water molecules in coal seams.

Stress change generated by the 2019 İstanbul–Silivri earthquakes along the complex structure of the North Anatolian Fault in the Marmara Sea

The North Anatolian Fault Zone is a dextral system operating between the Eurasian and Anatolian plates in northern Turkey. Across the Marmara Sea south of İstanbul, it deforms at ~18 mm/year slip rate, where the statistics of historical earthquakes suggest that a few M7+ earthquakes are generated every ~250 years. Currently, M7+ earthquakes are overdue as this section of the fault has not been reactivated since the 1766 and 1754 earthquakes. In this frame, identifying the stress change by the 2019 M5.8 İstanbul–Silivri earthquake is essential to characterize its influence on the critically stressed Marmara Sea segments of the North Anatolian Fault Zone. To do this, we mapped not only the main segments but also the secondary faults in the vicinity of the 2019 M5.8 İstanbul–Silivri earthquake to locate the fault patch it reactivated, as well as to accurately model the fault segments, where it significantly changed the stress. The joint analysis of relocated hypocenters and the Coulomb stress modeling shows that the mainshock failed a SW–NE striking splay fault, generating a stress increase of up to three bars on the fault that has the potential to generate M7+ earthquake.Graphical

Mineralization Based on CSAMT and SIP Sounding Data: A Case Study on the Hadamengou Gold Deposit in Inner Mongolia

The Hadamengou deposit is the largest gold deposit in Inner Mongolia. However, given that the sources of ore-forming alkaline magmatic hydrothermal solutions and ore-controlling structures are still controversial, the theories behind the genesis of the deposit have been controversial. In this study, four controlled-source audio magnetotellurics (CSAMT) and spectral induced polarization (SIP) profiles in the mining area were used to obtain the underground resistivity model and the pseudo section map of the apparent frequency dispersivity based on fine inversion. In the resistivity model, there are two high-resistivity blocks with resistivity greater than 3000 Ω m and three low-resistivity channels with resistivity less than 50 Ω m. Combined with the regional geological and drilling data, it is inferred that the high-resistance bodies, R4 and R5, may be alkaline magmatic intrusions related to multiple stages of magmatic hydrothermal activities, ranging from the Precambrian to Yanshanian periods. The highly conductive channels, C3, C5, and C4, may represent the Baotou-Hohhot fault, secondary faults, and ductile shear zone, respectively, which were formed in the Precambrian era and underwent multiple activations during the Hercynian to Yanshanian period. According to the spatial relationship, it is inferred that the ductile shear zone is an important ore-controlling and ore-hosting structure. However, the Baotou–Hohhot fault may be a pre-metallogenic fault rather than an ore-controlling fault. By comparing the resistivity model with the pseudo section of the apparent frequency dispersivity, it was found that all the known gold veins are located in the superimposed area of low resistivity and high-frequency dispersivity. It is speculated that the ductile shear zone outside the alkaline magmatic rock with the superimposed characteristics of low resistivity and high-frequency dispersivity is the favorable area for mineralization.

Failures statistics of on-line monitoring devices for gases dissolved in oil

The on-line monitoring device of dissolved gas in oil is an important starting point to understand the internal operation condition of large oil filled equipment such as transformer. However, the device has no unified maintenance strategy. At present, it basically adopts the strategy of “repair only when it is broken”, which seriously affects the real-time perception ability of the device to the health state of primary equipment. According to the components of the device, the device faults were divided into four categories: Status information access controller (CAC) fault, carrier gas fault, communication fault and monitoring device body fault. In addition, they were subdivided into eight secondary faults according to the detection principle and process. Through the big data analysis of the operation and maintenance records of a province in recent three years, it was found that the main causes of the device faults were the under voltage of the carrier gas and the fault of the monitoring device body. It was suggested to popularize the device using carrier gas maintenance free type or photoacoustic spectrum principle to reduce the frequency of device failure. At the same time, it was suggested that the device design should be transformed to modularization to facilitate the fault operation and maintenance of the device body. The degassing module and intelligent monitoring module of the unit should be focused when formulating the maintenance strategy of the unit in the future.

Earthquake Cycle Deformation Associated With the 2021 MW 7.4 Maduo (Eastern Tibet) Earthquake: An Intrablock Rupture Event on a Slow‐Slipping Fault From Sentinel‐1 InSAR and Teleseismic Data

AbstractIn the continents, the importance of earthquakes that occur away from major block‐bounding faults is still debated. The 21 May 2021 MW ∼ 7.4 Maduo earthquake occurred on a secondary fault away from previously‐identified major block boundaries. Here we use 7 years of Sentinel‐1 Interferometric Synthetic Aperture Radar (InSAR) time series (between October 2014 and November 2021) to determine the distribution of coseismic slip and early postseismic afterslip following the Maduo earthquake, and the preceding interseismic strain accumulation. We devised a 13‐segment 3‐D fault geometry constrained by the SAR range offsets and the distribution of relocated aftershocks and used a Bayesian method incorporating von Karman regularization to solve for coseismic slip and afterslip models. We also used teleseismic waveforms as a standalone inversion to show the rupture evolution in space and time during the earthquake, finding that it propagates bilaterally with three notable rupture episodes. Our preferred coseismic self‐similar slip model shows a moderate shallow slip deficit, with the majority of moment release occurring in the depth interval of 1–10 km. The coseismic slip deficit is taken up in part by afterslip at shallow (<4 km) depths that grows linearly with time during the first ∼6 months, and at >10 km depths where afterslip grows logarithmically with time. We suggest that this heterogeneity is likely controlled by spatial variations in fault friction related to lithology. We discuss the implications for seismic hazard away from major tectonic block boundaries in light of our observations of the earthquake cycle on this intrablock fault.

Seismic and Tsunamigenic Characteristics of a Multimodal Rupture of Rapid and Slow Stages: The Example of the Complex 12 August 2021 South Sandwich Earthquake

AbstractOn 12 August 2021, a >220 s lasting complex earthquake with Mw > 8.2 hit the South Sandwich Trench. Due to its remote location and short interevent times, reported earthquake parameters varied significantly between different international agencies. We studied the complex rupture by combining different seismic source characterization techniques sensitive to different frequency ranges based on teleseismic broadband recordings from 0.001 to 2 Hz, including point and finite fault inversions and the back‐projection of high‐frequency signals. We also determined moment tensor solutions for 88 aftershocks. The rupture initiated simultaneously with a rupture equivalent to a Mw 7.6 thrust earthquake in the deep part of the seismogenic zone in the central subduction interface and a shallow megathrust rupture, which propagated unilaterally to the south with a very slow rupture velocity of 1.2 km/s and varying strike following the curvature of the trench. The slow rupture covered nearly two‐thirds of the entire subduction zone length, and with Mw 8.2 released the bulk of the total moment of the whole earthquake. Tsunami modeling indicates the inferred shallow rupture can explain the tsunami records. The southern segment of the shallow rupture overlaps with another activation of the deeper part of the megathrust equivalent to Mw 7.6. The aftershock distribution confirms the extent and curvature of the rupture. Some mechanisms are consistent with the mainshocks, but many indicate also activation of secondary faults. Rupture velocities and radiated frequencies varied strongly between different stages of the rupture, which might explain the variability of published source parameters.

Surface Deformation Surrounding the 2021 Mw 7.2 Haiti Earthquake Illuminated by InSAR Observations

ABSTRACT Earthquakes pose a major threat to the people of Haiti, as tragically shown by the catastrophic 2010 Mw 7.0 earthquake and more recently by the 2021 Mw 7.2 earthquake. Both events occurred within the transpressional Enriquillo–Plantain Garden fault zone (EPGFZ), which runs through the southern peninsula of Haiti and is a major source of seismic hazard for the region. Satellite-based Interferometric Synthetic Aperture Radar (InSAR) data are used to illuminate the ground deformation patterns associated with the 2021 event. The analysis of Sentinel-1 and Advanced Land Observation Satellite (ALOS)-2 InSAR data shows (1) the broad coseismic deformation field; (2) detailed secondary fault structures as far as 12 km from the main Enriquillo–Plantain Garden fault (EPGF), which are active during and after the earthquake; and (3) postseismic shallow slip, which migrates along an ∼40 km unruptured section of the EPGF for approximately two weeks following the earthquake. The involvement of secondary faults in this rupture requires adjustments to the representation of hazard that assumes a simple segmented strike-slip EPGF. This work presents the first successful use of phase gradient techniques to map postseismic deformation in a vegetated region, which opens the door to future studies of a larger number of events in a wider variety of climates.

Spatial distribution and tectonic significance of the landslides triggered by the 2021 Ms6.4 Yangbi earthquake, Yunnan, China

An Ms6.4 earthquake occurred on 21 May 2021 in Yangbi County, Yunnan Province, located in the strong earthquake hazard zone, the border of the Sichuan-Yunnan rhomb block, southeast Tibetan Plateau, causing severe loss of life and property. Adequate research on the distribution characteristics and seismotectonic mechanisms of seismic chain-generated hazards in the region is meaningful for mitigating seismic hazard risks. In this paper, based on the interpretation of remote sensing satellite images and the analysis on GeoScene platform, we compile a detailed inventory of landslides induced by the Yangbi earthquake and analyze the correlation of their spatial distribution with the influence factors. The results show that 95 landslides were interpreted, and their spatial distribution is correlated to the topographic, seismic and geological factors. Statistically, the landslide number and mobility increase with the slope angle; the south- and southeast-facing slopes and weaker metamorphic rocks are more prone to landslides; the higher the seismic intensity, the larger the density and scale of landslides. Furthermore, the development of the landslides is not only influenced by the Ms6.4 mainshock, but also by the Ms5.6 foreshock, which is significantly correlated with the size of landslides. Notably, the long axis of the landslide distribution area is in NW-SE direction, which is nearly parallel to the strike of the Weixi-Qiaohou-Weishan fault zone (WQWF). The landslides are mainly distributed in the southwest wall and southeast section of the fault. The landslide number and density unstably decrease with the vertical distance from the fault with many fluctuations. Combined with the previous studies, two possible seismogenic structure models are inferred. One model is a parallel secondary fault of the WQWF, Another one is a flower structure in the southwest wall of the WQWF. Both models are mainly characterized by right-lateral strike slip with a small normal-fault component. Additionally, the seismic rupture propagated mainly to the southeast along the seismogenic fault in the subsurface without surface rupture.

Systematic Comparison of InSAR and Seismic Source Models for Moderate‐Size Earthquakes in Western China: Implication to the Seismogenic Capacity of the Shallow Crust

AbstractEarthquake source parameters are important for understanding earthquake physics and crustal fault properties. However, strong trade‐offs between parameters (e.g., depth and origin time) and a lack of accurate velocity models and near‐field seismic stations could cause large uncertainties of these parameters in seismic catalogs, particularly for shallow events. To further improve the resolution of earthquake source parameters, we use Interferometric Synthetic Aperture Radar (InSAR) images to derive source solutions of 33 moderate‐size (Mw 4.1–6.6) earthquakes that occurred at shallow depths (<20 km) from November 2014 to July 2020 in western China. After evaluating the uncertainties of the InSAR solutions, we systematically compare the location, centroid depth, focal mechanism and magnitude from InSAR models with that from seismic catalogs. We find that all seismic catalogs generally report deeper (4–10 km) hypocenters or centroid depths. The uncertainties of moment tensor solutions are partially related to the percentage of the non‐double‐couple components in the seismic catalogs. The InSAR solutions indicate that considerable seismic moments (i.e., ∼ Nm) were released in the uppermost crust (i.e., <5 km) in a period of ∼6 years, which is not resolvable in the seismic catalogs. The smooth seismic moment distribution along depth indicates a gradual change of the frictional properties from the surface to the middle crust. As most of the studied earthquakes are located on secondary and/or unmapped faults, these findings imply a considerable portion of velocity‐weakening friction in the uppermost crust along immature, secondary fault systems, which should be included in the seismic hazard evaluation.

Petrographic and Kinematic Analysis of the Itombwe Synclinorium Formations Exposed at Tshondo and Bugoy (South-Kivu region, Democratic Republic of the Congo)

The purpose of this study is to investigate the Itombwe synclinorium formations that were affected by the late Pan-African orogenesis in the nordeastern Congo. A variety of controversies surround Tshondo and Bugoy's tectonic evolution and associated metasedimentary formations. A field-based approach combined with paleostress inversion techniques and petrographic analyses were conducted on 190 structural measurements and 6 rock samples used in this study. The results reveal the presence of four major petrographic facies: conglomeratic facies (conglomerate and diamictite), carbonate facies (travertine), greenschist facies (graphitic black shale and pelite), as well as quatzitic facies (quartzite and sandstone). The mineral assemblages consisting of high contents (>70%) of muscovite/sericite and biotite albite, plagioclase, quartz, and some opaque minerals. With the assistance of Win-Tensor software, the kinematic analysis reveals two major deformation phases, (1) a ductile deformation phase (D1-2), which is associated with isoclinal folds and strike-slip faults, and (2) a submeridian brittle deformation phase (D2), which generated extensive faults trending NNW-SSE to NE-SW directions, while reactivating bedding surfaces (NE-SW) in a series of secondary faults. The findings of this research may assist geologists in conducting core logging operations and provide a baseline for understanding the relationship between rock, minerlization, and tectonics in mineral-rich areas.

The interplay between basement fabric, rifting, syn‐rift folding, and inversion in the Rio do Peixe Basin, NE Brazil

AbstractThe Rio do Peixe Basin is part of a series of aborted Cretaceous rifts formed within the Proterozoic Borborema Province, onshore NE Brazil in response to rifting between Africa and South America. The basin is remarkably well‐imaged and comprises fault‐bounded depocentres, the main ones being the NE‐oriented Brejo das Freiras and the E–W‐oriented Sousa half‐grabens. These grabens and their bounding faults are influenced by Neoproterozoic basement shear zones and present a complex framework of secondary normal faults and folds. Recent workers also interpret large reverse faults and regional post‐rift shortening driven by far‐field stresses from the Andes. For those reasons, the basin represents an ideal setting to investigate the multiphase history of rift basins. We thus combine borehole‐calibrated 2D and 3D seismic and magnetic data with section restoration and numerical modelling to investigate the architecture and evolution of this basin. We aim to understand: (i) the controls of the basement fabric in 3D fault architecture and kinematics and (ii) how syn‐rift faults controlled the geometry and development of fault‐related folds. By doing this, we also investigate the timing, kinematics, and magnitude of inversion in the basin to explore its multiphase history. We demonstrate that (i) the basement fabric controlled not only the strike of faults but also their geometry and polarity at depth, (ii) folds in the syn‐rift sequence are attributed simply to syn‐rift extension along stepped and/or curved faults, and (iii) inverted and/or reverse faults occur within the basin, but these are minor and appear to have formed during rifting. We explain this minor inversion by a change in plate kinematics related to the onset of the nearby transform margin to the north. These results have implications for understanding the 3D evolution of oblique grabens, the role of structural inheritance, and the recognition of inversion‐ versus extension‐related folds in rift basins worldwide.