Vertical Fault Research Articles

Dynamic off-fault failure and tsunamigenesis at strike-slip restraining bends: Fully-coupled models of dynamic rupture, ocean acoustic waves, and tsunami in a shallow bay

Inelastic off-fault deformation can lead to large tsunamigenesis in different tectonic settings. Here a mechanism for tsunami generation by strike-slip earthquakes that involves dynamic off-fault failure at restraining bends is presented. Dynamic rupture on a vertical strike-slip fault is modeled with undrained inelastic off-fault response incorporated by the Drucker-Prager yield criterion. I show that in a local transpressive stress regime dynamic off-fault failure at restraining bends produces significantly larger and more localized surface uplift than is produced by purely elastic dislocation models, resulting in a positive flower and coseismic pop-up structure. The larger uplift is due to frictional sliding with a thrust component on conjugate microfractures, modeled by inelastic deformation. The short-wavelength inelastic uplift, largely controlled by bend size, is shown to generate localized tsunami efficiently in a shallow bay by fully coupling dynamic rupture, ocean acoustic waves, and tsunami. Fully-coupled models also indicate that supershear rupture on a vertical planar strike-slip fault does not generate large tsunami as the large kinetic energy of supershear rupture is carried away by ocean acoustic waves. Dynamic off-fault failure at fault complexities, such as restraining bends and compressional stepovers, may need be urgently incorporated in the current tsunami hazard assessments in strike-slip environments.

The structure and role of faulting and fracturing system in spatial oil saturation of rocks and in occurrence of oil reservoirs within Chezhen sag of Jiyang depression in the Bohai Bay Basin in East China

The article reports the integrated analysis of the history, geodynamics, modern faulting system, internal structure and spatial occurrence of Paleozoic and Paleogene (Eocene) accumulations of oil in the promising Chezhen block of the Chezhen sag of the Jiyang depression in the Bohai Bay Basin in East China. After review of the new geological, geophysical and field studies, the authors find out that in this structurally complex system of faults and fractures of different age, orientation and types, the present-day spatial occurrence of oil reservoirs is governed by the neofaults which blanket the whole geological section, including the Paleozoic period. The studies (analysis of polished sections) into internal structure (micro fracturing) and oil saturation of rocks produce some new data of interest for the theory and practice. The studies of samples from the reservoir and non-reservoir rocks in exploration drilling in the Shahejie pay horizon show that oil saturation is connected with the presence of the younger age system of micro fractures (permeable) of horizontal and vertical (faults) orientation. The further exploration in the promising Chezhen block of the Shenli oil field is recommended to continue in the zones of the Cenozoic faults as any rocks (irrespective of their lithology, including low-porosity rocks) can be saturated with oil in these zones which contain the best-type reservoirs (fractured) and feature active fluid neodynamics.

Source Characteristics of the Shallow 2019 Ms 6.0 Changning, China, Earthquake Sequence in the Salt Mining Area

AbstractThe 17 June 2019 Ms 6.0 Changning earthquake occurred on the southern margin of the Sichuan basin in China, which breaks the historical record for the largest earthquake in the Sichuan basin. Based on the abundant local seismic, strong motion, and Interferometric Synthetic Aperture Radar line of sight displacement data, we investigate the detailed source characteristics of this earthquake sequence. We determine focal mechanisms of 68 ML≥2.0 aftershocks with P wave first-motion polarities and S/P amplitude ratios. The triangle diagram of focal mechanisms shows that 82% of the aftershocks have thrust faulting mechanisms. The spatial distribution of aftershocks together with the determined focal mechanisms indicates that this earthquake sequence was mainly controlled by the southeast–northwest-extended faults. In addition, the revealed diversity of aftershock focal mechanisms implies that some small subsidiary faults with different geometries and motion features have likely been ruptured. The kinematic finite-fault joint inversion results reveal a complex rupture process of the mainshock on two fault segments with different geometries. The rupture initiated on the southeastern fault segment with a gentle dip angle and then jumped to the vertical northwestern fault segment. The main rupture length and duration are approximately 11 km and 8 s, respectively. The released total scalar seismic moment during the rupture process is 4.6×1017 N·m, corresponding to a moment magnitude of Mw 5.7. Our results suggest that the nucleation and the rupture initiation and propagation of the 2019 Ms 6.0 Changning earthquake sequence were likely controlled by the intrinsic structure and stress heterogeneities of the involved seismogenic faults, as well as the variation in pore-fluid pressure caused by the long-term water injection in the Changning salt mining area and adjacent areas.

Normal fault transmissibility characteristics under the transition condition of fault conduction and sealing observed in simulation experiments

In rift basins, normal faults conducting and sealing fluids are the main mechanisms for evaluating fault-related reservoirs, and the defined threshold value of fault conduction or sealing is based on statistics. However, fault transmissibility characteristics under a transition condition of fault conduction and sealing have not been clarified. This study used structural deformation and hydrocarbon charging as a foundation to quantitatively characterize fault transmissibility under the transition condition of fault conduction and sealing. This study examined faults occurring in sandstone–mudstone interbeds and used the fault throw and mudstone content as the main variables. The physical simulation results revealed that despite all models exhibiting simple “one-layer” fault architecture with only a fault core, their internal composition undergoes a sequential change from the mudstone drag, mudstone drag and sliding surface to the mudstone breccia and sliding surface. Similarly, when the fault is in the transition condition of fault conduction and sealing under the constraints of mudstone, a normal distribution is observed in terms of the number of oil-bearing layers, oil-bearing length, and oil-bearing area of the lateral fault conduction. However, when the shale smear factor (SSF) is too high, the “one-layer” fault architecture mainly comprises the mudstone breccia and sliding surface, which increases the vertical fault transmission and reduces the lateral fault conduction. Conversely, an excessively small SSF, where the fault zone is a “one-layer” fault architecture comprising mainly the mudstone drag or mudstone drag and sliding surface, will restrict the vertical fault transmission and reduce the lateral fault conduction. When the lateral fault conduction near the inflection point of the normal distribution reaches its threshold, the superposition of mudstone along the fault and the presence of mudstone breccia in the fault zone become critical for decelerating the vertical fault transmission and accelerating the lateral fault conduction. The study of normal fault transmissibility characteristics under the transition condition of fault conduction and sealing provides a solid theoretical foundation for the risk assessment of fault-related hydrocarbon accumulation.

The Whole-Space Modeling of the Hazardous Geological Body ahead of the Tunnel Face by the Transient Electromagnetic Method

Karst caves are widely distributed in southwest China, causing difficulties and disasters for tunnel construction. To better detect the krast caves in front of the tunnels under construction using the transient electromagnetic method, in this paper we propose a 3-D finite element method to simulate the multi-parameter transient electromagnetic response of unfavorable geological bodies in a whole-space. First, the models of vertical water-filled faults, water-filled caves and complex geological bodies in front of the tunnel face are established. The horizontal electric field component and the vertical magnetic field component at different time in the whole-space are researched. Secondly, the electromagnetic response features of the caves with different resistivity, buried depths and scales are studied. We found that the resistivity of the target body is 10 times larger than that of the surrounding rocks, and the anomaly amplitude increases obviously with the growing distance from the target body. The deeper the buried depth, the later the anomaly appears and the smaller the anomaly amplitude. The larger the target size, the longer the transient electromagnetic response delay and the larger the anomaly amplitude. We arranged a measuring line on the tunnel face. The full-time apparent resistivity section shows the position and characteristics of the low-resistivity anomalous body, indicating that the transient electromagnetic method (TEM) has obvious advantages in detecting the low-resistivity body in front of the tunnel face. Finally, the TEM is successfully applied to the advanced detection of a karst tunnel to get the electrical distribution of the surrounding rocks in front of the tunnel face. According to the geological conditions of the excavated tunnel, the validity of the TEM in the tunnel advanced prediction is verified.

Tectonic implication in the evolution of lake and Quaternary landforms in the Lohawati river basin, Kumaun outer Lesser Himalaya

The evolution of the Quaternary landforms in the Champawat area of the outer Kumaun Lesser Himalaya is reconstructed from the paleolake sections and the geometry of the landscape. Conjugate sets of normal faults are observed in both the bedrocks and Quaternary deposits at Banlek. The most prominent normal faults are those striking NE-SW and NW-SE and most of them are steeply dipping faults while some are vertical faults. Sediments akin to the lacustrine deposits are observed at four sites; while the contact between the bedrock and the lake sediments is observed at only one site in the peripheral side. The exposed sediments thicknesses vary from 8.30 m to 4.8 m and consist of an alternation of black carbonaceous mud and sandy horizons indicating different depositional regimes. The total thickness of the lake sediments appears to be more than 75 m as deduced from elevational differences between the exposed sites. From Site 1, two OSL samples collected from 2.5 m and 8 m from the present ground surface, were analyzed which give OSL ages of 16 ka and 17 ka, respectively; while one sample collected from Site 4 gives an OSL age of 13 ka. A total of 3653 lineaments were mapped encompassing 2,277.8 sq km from the eastern Kumaun Himalaya. Maximum of the lineaments trend ENE-WSW or almost E-W (∼46.5%); the major principal stress is deduced to be in NE-SW direction. Lake, mature topography and gentle stream gradient in the Champawat area are the result of highly weathered bedrocks and development of the normal faults.

36Cl exposure dating of glacial features to constrain the slip rate along the Mt. Vettore Fault (Central Apennines, Italy)

The Central Apennines are among the most seismically active regions in Italy. This region is affected by an extension accommodated by active normal faults. Those faults with high seismogenic potential hosted Mw > 6 shallow damaging earthquakes, such as the Mw 6.5 event of Oct. 30, 2016. Although fault slip rates are crucial to seismic hazard assessments, in the Central Apennines, slip rate assessments encompassing several seismic cycles (> 10 ka) are sparse. This is particularly true for the Mt. Vettore-Mt. Bove Fault system that ruptured during the mainshocks of the 2016–2017 seismic sequence. In this study, we present new geochronological constraints of offset geomorphological markers along the northern portion of the fault system on the Mt. Porche Fault segment using in-situ produced 36Cl cosmogenic nuclides. Offset measurements were made using a 5-cm resolution DEM obtained through a drone survey and constrained a fault scarp height of 15.5 ± 1.4 m and a maximal cumulative offset estimated between 32 and 40.5 m. Samples were collected from the Valle Lunga terminal moraine at 1710 m asl and yielded 36Cl exposure ages of 12.7 + 2.2/−1.9 ka, while the abraded surface, located on top of the tectonic scarp yielded 36Cl exposure ages of 23.4 + 5.3/−4.3 ka. Using the fault scarp height and the exposure age of this abraded surface, we constrained a minimum vertical fault slip rate of 0.7 + 0.2/−0.1 mm/yr. Assuming the offset started to accumulate when climate conditions allowed the scarp preservation, we constrained a maximal vertical slip rate of 1.2 ± 0.2 mm/yr along the main fault of the Mt. Vettore-Mt. Bove Fault system. This rate is higher than those previously obtained from trenches along secondary branches of the Mt. Vettore-Mt. Bove Fault system. Besides, the yielded chronology for the last glacial maximum in that area at ~23 ka is in good agreement with the timing previously proposed for the LGM in the Apennines.

Recognition of Petroleum Origin in the Northern Piedmont Belt of Turpan-Hami Basin and Its Significance

The Turpan-Hami Basin has been considered as the most typical coal-derived-oil basin in China for a long time. However, with the deepening exploration, this theory has caused increasing controversy and some scholars doubt that Permian or Jurassic dark mudstone may be the main source rocks. The origin uncertainty even restricts deciding the exploration direction. Taking the reservoirs of the northern piedmont belt as an example, this paper analyzed oil sources by the geochemical characteristic comparison of inclusions, crude oil, and source rocks. Combined with the previous studies, the hydrocarbon accumulation model is established. The results show that the geochemical characteristics of the inclusions and oil in most areas are obviously different. Most reservoirs are mixed-source reservoirs. The oil sources varied in different regions. In the area near Bogeda, C19+20TT and C28 sterane content in oil are high, indicating that coal and Permian lacustrine mudstone are main contributors. Far away from Bogeda, bounded by the thick coal seam of Xishanyao Formation, oil sources are quite different. Above it, C19+20TT is high and gammacerane and pregnane are low, C29>>C28>C27sterane, indicating that coal and carbonaceous mudstone are main sources. Under that, C21TT and C23TT content in the inclusions are high, C29≥C27>C28sterane, while C19+20TT and pregnane content in the oil are also high, indicating that it is a mixed oil system mainly derived from Jurassic lacustrine mudstone and coal. Lacustrine mudstone has higher contribution. This phenomenon is mainly related to different fault styles in different zones. Near Bogeda, strong stress caused vertical large faults developed, which connected Jurassic reservoirs and Permian source rocks. Far from Bogeda, the faults sliped along the thick coal seam, It is difficult for deep hydrocarbon to enter the shallow layer. In the future, it should be considered to look for the traps with relatively high-quality reservoir-cap combinations for exploration in the hydrocarbon supply window of effected hydrocarbon kitchens.

The CO2CRC Otway shallow CO2 controlled release experiment: Fault characterization and geophysical monitoring design

CO2CRC has made a significant investment into establishing the feasibility of conducting a CO2 injection experiment into a shallow fault at the CO2CRC Otway International Test Centre. Two appraisal wells drilled and cored through Brumbys Fault indicate the fault extends to the base of the upper 2 m thick Hesse Clay layer, which forms a seal to the underlying Port Campbell Limestone aquifer. The fault does not have a defined core; rather, it is expressed by an approximately 6-10 m wide cataclastic zone. Permeability within the Port Campbell Limestone is variable, ranging from tens to thousands of millidarcies (10−14 to 10−12 m2). The rock strength is low and it is recommended to conduct the experiment at approximately 80 m depth rather than the 40 m originally proposed. This provides more confining pressure and will ensure that the injection pressure does not exceed the fracture pressure. A deeper injection also provides better spatial and timing conditions for geophysical monitoring and tracking of the CO2 plume. Simulations indicate that a 10 t CO2 injection experiment would be sufficient to monitor CO2 migration using geophysical techniques and the planned deployment of reverse 4D vertical seismic profiling would be able to track this small quantity of CO2 up a fault. In addition to providing an opportunity to demonstrate semi-continuous, near real-time monitoring of CO2 migration up a fault, the planned CO2 injection experiment presents a unique opportunity to obtain field measurements on vertical fault permeability at a shallow strike-slip fault.

Large-scale, crustal-block vertical extrusion between the Hines Creek and Denali faults coeval with slip localization on the Denali fault since ca. 45 Ma, Hayes Range, Alaska, USA

AbstractOblique convergence along strike-slip faults can lead to both distributed and localized deformation. How focused transpressive deformation is both localized and maintained along sub-vertical wrench structures to create high topography and deep exhumation warrants further investigation. The high peak region of the Hayes Range, central Alaska, USA, is bound by two lithospheric scale vertical faults: the Denali fault to the south and Hines Creek fault to the north. The high topography area has peaks over 4000 m and locally has experienced more than 14 km of Neogene exhumation, yet the mountain range is located on the convex side of the Denali fault Mount Hayes restraining bend, where slip partitioning alone cannot account for this zone of extreme exhumation. Through the application of U-Pb zircon, 40Ar/39Ar (hornblende, muscovite, biotite, and K-feldspar), apatite fission-track, and (U-Th)/He geo-thermochronology, we test whether these two parallel, reactivated suture zone structures are working in tandem to vertically extrude the Between the Hines Creek and Denali faults block on the convex side of the Mount Hayes restraining bend. We document that since at least 45 Ma, the Denali fault has been bent and localized in a narrow fault zone (<160 m) with a significant dip-slip component, the Mount Hayes restraining bend has been fixed to the north side of the Denali fault, and that the Between the Hines Creek and Denali faults block has been undergoing vertical extrusion as a relatively coherent block along the displacement “free faces” of two lithospheric scale suture zone faults. A bent Denali fault by ca. 45 Ma supports the long-standing Alaska orocline hypothesis that has Alaska bent by ca. 44 Ma. Southern Alaska is currently converging at ~4 mm/yr to the north against the Denali fault and driving vertical extrusion of the Between the Hines Creek and Denali faults block and deformation north of the Hines Creek fault. We apply insights ascertained from the Between the Hines Creek and Denali faults block to another region in southern Alaska, the Fairweather Range, where extreme topography and persistent exhumation is also located between two sub-parallel faults, and propose that this region has likely undergone vertical extrusion along the free faces of those faults.

Aseismic Slip and Cascade Triggering Process of Foreshocks Leading to the 2021 Mw 6.1 Yangbi Earthquake

AbstractUnderstanding the nature of foreshock evolution is important for earthquake nucleation and hazard evaluation. Aseismic slip and cascade triggering processes are considered to be two end-member precursors in earthquake nucleation processes. However, to perceive the physical mechanisms of these precursors leading to the occurrence of large events is challenging. In this study, the relocated 2021 Yangbi earthquake sequences are observed to be aligned along the northwest–southeast direction and exhibit spatial migration fronts toward the hypocenters of large events including the mainshock. An apparent static Coulomb stress increase on the mainshock hypocenter was detected, owing to the precursors. This suggests that the foreshocks are manifestations of aseismic transients that promote the cascade triggering of both the foreshocks and the eventual mainshock. By jointly inverting both Interferometric Synthetic Aperture Radar and Global Navigation Satellite Systems data, we observe that the mainshock ruptured a blind vertical fault with a peak slip of 0.8 m. Our results demonstrate that the lateral crustal extrusion and lower crustal flow are probably the major driving mechanisms of mainshock. In addition, the potential seismic hazards on the Weixi–Weishan and Red River faults deserve further attention.

Control of mechanical stratigraphy on the stratified style of strike-slip faults in the central Tarim Craton, NW China

The large-scale (tens to hundreds of kilometers long) but small-displacement (<2 km) strike-slip faults in the Shunbei region of the Tarim Basin, NW China, provide an opportunity to document the role of mechanical stratigraphy in an intracratonic strike-slip situation. Uniaxial compression test results and dipole acoustic logging data illustrate that the lower Paleozoic mechanical profile in the Shunbei region consists of 2 incompetent layers (middle Cambrian and Upper Ordovician) and 3 competent layers (Sinian–lower Cambrian, upper Cambrian–Middle Ordovician, and Silurian). Borehole and high-resolution 3D seismic data illustrate that the 3 competent layers are characterized by low-amplitude folds and isolated brittle faults. The discrete element method (DEM) modeling results indicate that mechanical stratigraphy is an important factor controlling the soft-linked faulting style that is independent of multiphase deformation. A pre-existing structure promotes the formation of narrow and localized fault zones. The early–middle Silurian evolution of the Shunbei No. 1 fault zone reflects a classic stratigraphically decoupled deformation model: the middle fault arrays (upper Cambrian–Middle Ordovician layers) and en echelon normal faults (Silurian layer) synchronously experienced left-lateral slip. These two styles of faulting have different geometries (fault dips, slips, and arrangements) but are kinematically coupled and genetically related. This proposed deformation model provides mechanical support to document deeply buried structures by characterizing genetically related shallow structures, which may be widely employed as multiphase inheritance structures with mechanical stratigraphy. We also point out that vertical fault connectivity is a key research aspect of hydrocarbon exploration in the Shunbei oilfield.

Stresses and displacements for two imperfectly joined half-spaces induced by vertical tensile fault

In the present paper, we have obtained analytic expressions for displacements and stresses for two imperfectly (dislocation-like) joined half-spaces caused by vertical tensile fault. It has been observed that deformation field is significantly affected by imperfection at the interface. Using dimensionless approach, results are discussed numerically with the help of graphs and mesh grids with contours for continental crust model.

Aeromagnetic-Imaged Basement Fault Structure of the Eastern Tarim Basin and Its Tectonic Implication

The property of the magnetic basement and the faults in the basement is significant for structural evolution, the Phanerozoic deposition, and oil resource exploration of the Tarim Basin. Based on the newly acquired aeromagnetic and industry seismic data, we mapped the distribution of basement faults by applying magnetic gradient-processing methods such as the horizontal gradient derivative, the first vertical derivative, the tilt derivative, and the upward continuation method. The dips of basement faults were confirmed and the susceptibilities of basement blocks were obtained by forward modeling of five profiles using the constraint of sedimentary strata depth and Moho topography. On the basis of comprehensive analysis of the magnetic anomalies, the distribution and inclination of basement faults, and susceptibilities differentiation obtained by forward modeling and field measurement, the property of the basement faults and their implication were discussed and interpreted. Our results show that the origin of the Central Highly Magnetic Anomaly Belt is highly magnetic Archean metamorphic rocks. The weakly magnetic Southeastern Domain and highly magnetic Central Tadong Domain assembled along the Tadong South Fault during the Paleoproterozoic. The Paleozoic Cherchen Fault is just an interior fault in the weakly magnetic Southeastern Domain although it presents a large vertical fault displacement. Considering the prominent variation of strikes of the Tadong North Fault system, and the moderately magnetic anomalies in the Northeastern Mangal Domain corresponding to the center of Neoproterozoic deposition, it is likely that the basement of the Northeastern Mangal Domain modified by the Neoproterozoic rifting could be originally the same as the basement of Central Highly Magnetic Anomaly Belt.

Orthogonal Dual Polarization GPR Measurement for Detection of Buried Vertical Fault

This short letter proposes a ground-penetrating radar (GPR) measurement technique to better detect vertical fault that occurs in a base rock under a surface layer. The technique exploits polarization-dependent radar response of the linear feature of the fault end that appears on the featureless base rock surface. A set of GPR measurements of which polarizations are orthogonal to each other are carried out in a fashion that one polarization is parallel to the putative target fault line, while another is normal to it. Taking the difference of instantaneous power A-scope data of the measurements cancels contribution from the featureless base rock surface while the contribution from the fault fracture end is elicited, thanks to its strong polarization-dependent radar response. The technique was described by an analytic model, its feasibility was demonstrated by finite difference time domain (FDTD) simulation, and was verified by a field measurement experiment.

Marine Paleoseismic Evidence for Seismic and Aseismic Slip Along the Hayward‐Rodgers Creek Fault System in Northern San Pablo Bay

AbstractDistinguishing between seismic and aseismic fault slip in the geologic record is difficult, yet fundamental to estimating the seismic potential of faults and the likelihood of multi‐fault ruptures. We integrated chirp sub‐bottom imaging with targeted cross‐fault coring and core analyses of sedimentary proxy data to characterize vertical deformation and slip behavior within an extensional fault bend along the Hayward‐Rodgers Creek fault system in northern San Pablo Bay. We identified and traced four key seismic horizons (R1–R4), all younger than approximately 1400 CE, that cross the fault and extend throughout the basin. A stratigraphic age model was developed using detailed down‐core radiocarbon and radioisotope dating combined with measurements of anthropogenic metal concentrations. The onset of hydraulic mining within the Sierra Nevada in 1852 CE left a clear geochemical and magnetic signature within core samples. This key time horizon was used to calculate a local reservoir correction and reduce uncertainty in radiocarbon age calibration and models. Vertical fault offset of strata younger than the most recent surface‐rupturing earthquake on the Hayward fault in 1868 CE suggest near‐surface vertical creep is occurring along the fault in northern San Pablo Bay at a rate of approximately 0.4 mm/yr. In addition, we present evidence of at least one and possibly two coseismic events associated with growth strata above horizons R1 and R2, with median event ages estimated to be 1400 CE and 1800 CE, respectively. The timing of both these events overlaps with paleoseismic events on adjacent fault sections, suggesting the possibility of multi‐fault rupture.

Analytical solutions for analysing pumping tests near an infinite vertical and anisotropic fault zone based upon unconventional application of well-image theory

We present a new set of analytical solutions for analysing pumping tests in a well near a vertical fault zone, dyke or vein. We consider a fault zone with finite width, storativity and hydraulic conductivity, which can be anisotropic, and without limitation of the diffusivity contrast between the three aquifer domains (pumped aquifer, fault zone and opposite-side aquifer). This configuration, not described before, also considers flow transience within the fault zone. Drawdown solutions were developed for the three-aquifer domains based on an unconventional application of the well-image theory. Their comparison with numerical modelling results was very satisfactory. Drawdown behaviour at the pumping well is discussed for contrasted properties between the three-aquifer domains. Original analytical solutions for transient flow along both sides of the fault zone, and net transient flow from the fault itself, are proposed and discussed, with particular attention to flow reversal, from the pumped compartment to the fault zone, which occurs where the fault zone is the most transmissive. Finally, we extend the solutions to the case of a well intersecting and pumping a vertical fracture of finite length located near the same vertical geological discontinuity.

Interpretation of Magnetic Anomalies over 2D Fault and Sheet-Type Mineralized Structures Using Very Fast Simulated Annealing Global Optimization: An Understanding of Uncertainty and Geological Implications

Abstract Identification of intraterrane dislocation zones and associated mineralized bodies is of immense importance in exploration geophysics. Understanding such structures from geophysical anomalies is challenging and cumbersome. In the present study, we present a fast and competent algorithm for interpreting magnetic anomalies from such dislocation and mineralized zones. Such dislocation and mineralized zones are well explained from 2D fault and sheet-type structures. The different parameters from 2D fault and sheet-type structures such as the intensity of magnetization (k), depth to the top (z1), depth to the bottom (z2), origin location (x0), and dip angle (θ) of the fault and sheet from magnetic anomalies are interpreted. The interpretation suggests that there is uncertainty in defining the model parameters z1 and z2 for the 2D inclined fault; k, z1, and z2 for the 2D vertical fault and finite sheet-type structure; and k and z for the infinite sheet-type structure. Here, it shows a wide range of solutions depicting an equivalent model with smaller misfits. However, the final interpreted mean model is close to the actual model with the least uncertainty. Histograms and crossplots for 2D fault and sheet-type structures also reveal the same. The present algorithm is demonstrated with four theoretical models, including the effect of noises. Furthermore, the investigation of magnetic data was also applied from three field examples from intraterrane dislocation zones (Australia), deep-seated dislocation zones (India) as a 2D fault plane, and mineralized zones (Canada) as sheet-type structures. The final estimated model parameters are in good agreement with the earlier methods applied for these field examples with a priori information wherever available in the literature. However, the present method can simultaneously interpret all model parameters without a priori information.

Fault parameters assessment from the gravity data profiles applying the global particle swarm optimization

This study focuses on interpreting gravity anomalies caused by fault structures. The faults types are the two-sided inclined fault, which represents mainly the normal and reverse faults, the two-sided vertical fault, the one-sided inclined fault, and the one-sided vertical fault. The study scheme is depends on the combination between the second moving average operator as a tool for separating the residual (desired) and regional (undesired) anomalies and then detecting the fault parameters applying the global particle swarm. The stability and efficiency of the proposed method has been applied to a synthetic example including the effect of regional background and to five real data sets from Iraq, USA, Egypt, and France. Available geologic and geophysical information supports our interpretation for the field examples from Iraq, USA, and Egypt. While, the forward model results from the detected parameters for the last field example (Pyrenees faults, France) was compared and matched well with measured gravity anomaly and found in a good agreement. So, this interpretation is open a sight-view for researchers around the world work in this area to use our results as a priori information for more investigations.

GPS determined coseismic slip of the 2021Mw7.4 Maduo, China, earthquake and its tectonic implication

SUMMARYThe 2021 Mw7.4 Maduo earthquake occurred on the Jiangcuo fault within the Bayan Har block in eastern Tibet. It is a rather unique event and attests that large earthquakes can occur in the interior of major tectonic blocks within the Tibetan plateau. By processing GPS data observed in the eastern Tibet region, we produce a data set documenting 3-D coseismic displacements of the Maduo earthquake. Using the data set to constrain a coseismic slip model, we find that the earthquake ruptured a nearly vertical fault about 170 km in length, with ∼90% of the moment released in the shallow layer above 20 km depth. The maximum slip of ∼3.6 m occurred near the surface around a bend in the east segment of the fault. The overall seismic moment release is 1.82 × 1020 N m and is equivalent to Mw7.4. Driven by the eastward extrusion of the Tibetan plateau, the deformation field in eastern Tibet is dominated by left-lateral shear, with the strikes of the tectonic faults rotating clockwise from west to east along with the shear stress orientation. This deformation pattern explains the mechanisms of earthquakes along block boundary faults, as well as the ones on faults within the blocks. The Jiangcuo fault is located ∼70 km south of the East Kunlun fault and could be connected to the Kunlun Mountain Pass fault to its WNW that ruptured during the 2001 Kokoxili earthquake, and a seismic gap of ∼240 km long between the two faults is worth special attention for its increased earthquake potential.