Crustal Density Research Articles

Dynamic changes of gravity field before the Luding MS6.8 earthquake and its crustal material migration characteristics

On September 5, 2022, an earthquake of magnitude MS6.8 occurred in Luding County, Sichuan Province. This earthquake occurred at the key part of the southeast-clockwise extrusion of material on the eastern margin of the Qinghai–Xizang Plateau, the Y-shaped confluence of the Xianshuihe, Longmenshan and Anninghe fault zones. In this study, the three-dimensional dynamic crustal density changes in the earthquake area are obtained by the typical gravity change data from 2019 to 2022 before the earthquake and gravity inversion by growing bodies. The results indicate that gravity changes presented an obvious four-quadrant and gradient belt distribution in the Luding area before the earthquake. The three-dimensional density horizontal slices show that small density changes occurred at the epicenter in the mid-to-upper crust between 2019.9 - 2020.9 and 2019.9–2021.9. At the same time, the surrounding areas exhibited a positive and negative quadrant distribution. These observations indicate that the source region was likely in a stable locked state, with locking-in shear forces oriented in the NW and NE directions. From 2021.9 to 2022.8, the epicentral region showed negative density changes, indicating that the source region was in the expansion stage, approaching a near-seismic state. The three-dimensional density vertical slices reveal a southeastward migration of positive and negative densities near the epicenter and on the western of the Xianshuihe Fault Zone, indicating that the material is flowing out to the southeast. The observed local negative density changes at the epicenter along the Longmenshan Fault Zone are likely associated with the NE-oriented extensional stress shown by the seismic source mechanism. The above results can provide a basis for interpreting pre-earthquake gravity and density changes, thereby contributing to the advancement of earthquake precursor theory.

Строение центральной части палеопротерозойского Лосевского террейна по данным плотностного моделирования

The results of detailed three-dimensional density modeling of the upper crust of the central part of the Losevsky terrane located in the northern part of the Voronezh crystal massif are considered. The distribution of absolute crustal density values of to a depth of 16 km was obtained as a result of the inversion of local anomalies of the gravitational field in the Bouguer reduction. The solution of the inverse problem of gravimetry was implemented within the framework of the initial model, which was formed using digital geological and geophysical data from the study area: the regional density model of the East European Platform lithosphere and its corresponding regional gravity field; the field of local gravity anomalies, representing the difference between the observed and regional fields of the lithosphere model; generalized information on the density of sedimentary cover rocks and the crystal basement; the thickness scheme of the «gravity-active» layer, obtained on the basis of statistical analysis of the anomalous field; geological and topographic information. Quantitative interpretation of the gravimetry data allowed us to obtain fundamentally new information about the geological structure of the upper crust of the study area. The reliability of the obtained results is confirmed by the consistency of a priori data and the correspondence of the total model (local plus regional) and observed gravitational fields.

The Stability of a Dense Crust Situated on Small Bodies

Small planetary bodies in the solar system, including Io, Ganymede, and Callisto, may have a crust denser than their underlying mantle. Despite the inherent gravitational instability of such structures, we show that the growth timescale of the Rayleigh–Taylor (RT) instability can be as long as the age of the solar system, owing to the strong temperature dependence of viscosity. Even in cases where the instability timescale is shorter, the instability is confined to a thin layer at the base of the crust, making the foundering of the entire crust improbable in many scenarios. This study delineates the onset and aftermath of the RT instability, applying a quantitative framework to assess the stability of (i) rock-contaminated crust on icy satellites, and (ii) silicate crust floating on top of a subsurface magma ocean on Io. Notably, for Io the RT instability peels off only 10–100 m from the crust’s base, and thermal diffusion rapidly recovers the crustal thickness through solidification of a magma ocean. Despite recurrent delamination of the crustal base, the initial crustal thickness is maintained by thermal diffusion, virtually stabilizing a floating dense crust. Cracking of the crust also is unlikely to result in the foundering of the crust. A dense crust on a small body is therefore difficult to be overturned, suggesting the potential ubiquity of dense surface layers throughout the solar system.

The lithospheric structure of Greenland from a stepwise forward and inverse modelling approach

Summary Greenland’s tectonic history is complex, and the resulting lithospheric structure is, although extensively researched, not well constrained. In this study, we model the lithospheric structure of Greenland in a consistent, integrated framework with three steps. First, we build a lithospheric background model by forward modelling, adjusted to gravity gradient data and shear wave velocities from a regional tomography model. Subsequently, we jointly invert for the upper crustal density and susceptibility structure by minimizing the gravity residuals and magnetic total field anomaly misfit. The last modelling step searches for upper crustal thermal parameters to fit our model to the most recent geothermal heat flow predictions for Greenland. Finally, we present 3D models of the density, temperature and velocity structure for the lithosphere as well as thermal parameters and susceptibilities for the upper crust. Our model also includes the depth of the Moho and LAB in Greenland. A comparison between inverted crustal parameters and surface geology shows a clear correlation. The novelty of our model is that all these results are consistent with each other and simultaneously explain a wide range of observed data.

Three‐Dimensional Density Distribution and Seismic Activity along the Guxiang–Tongmai Segment of the Jiali Fault, Tibet

AbstractThe Guxiang–Tongmai segment of the Jiali fault is situated northeast of the Namche Barwa Syntaxis in northeastern Tibet. It is one of the most active strike‐slip faults near the syntaxis and plays a pivotal role in the examination of seismic activity within the eastern Himalayan Syntaxis. New study in the research region has yielded a 1:200000 gravity dataset covering an area 1500 km2. Using wavelet transform multiscale decomposition, scratch analysis techniques, and 3D gravity inversion methods, gravity anomalies, fault distributions, and density structures were determined across various scales. Through the integration of our new gravity data with other geophysical and geological information, our findings demonstrate substantial variations in the overall crustal density within the region, with the fault distribution closely linked to these density fluctuations. Disparities in stratigraphic density are important causes of variations in the capacity of geological formations to endure regional tectonic stress. Earthquakes are predominantly concentrated within the density transition zone and are primarily situated in regions of elevated density. The hanging wall stress within the Guxiang–Tongmai segment of the Jiali fault exhibits a notable concentration, marked by pronounced anisotropy, and is positioned within the density differential zone, which is prone to earthquakes.

Large variations in the lithospheric thickness of northwestern India: Imprints of collisional and thermal reworking

The fold belts, which border the cratons, are the building blocks for understanding the origin and modification of older continents. The Aravalli-Delhi Fold Belt (ADFB) in northwestern India provides evidence of crustal block accretion due to continental collision, whereas Malani Igneous Suite (MIS) modified with widespread magmatism. Imaging of upper mantle structure and lithospheric modifications, if any, of such regions has been intricate owing to reworking by subsequent superimposed tectono-magmatic processes. We applied a 2D modelling approach to model the lithospheric architecture along a 1000-km long WNW-SSE geotransect across northwestern India, which has been deformed in the past. Our modelling technique combines terrestrial gravity anomaly, heat flow data, satellite-based geoid, and topographic datasets using the basic premise of thermal steady-state and local isostasy. The overall 38 to 40 km thick crustal geometry underneath the Marwar Block had the maximum lithological heterogeneity. The region surrounding the MIS is characterised by 8–10 km thick high-density (2.78 g/cm3) sills deposited in the upper crust down to 9 km depth and another 10–15 km thick high-density (3.05 g/cm3) mafic mantle material near the Moho. About 42 km thick crust, including an 8 to 10-km thick high-density (3.05 g/cm3) underplated layer at its bottom, characterises the high-relief ADFB. The Vindhyan region of Bundelkhand craton is defined by a ∼ 1 km thick trap, having Moho extending at a depth of ∼40 km. The lithospheric thickness varies substantially from ∼143–168 km underneath the Marwar block, which thins to ∼135 km under the ADFB and thickens gradually to ∼150–165 km beneath the Vindhyan region. Substantial crustal density differences in distinct crustal domains, when integrated with the thin lithosphere, reinforce the concept that tectono-magmatic processes might have modified the lithosphere in NW India.

Three-dimensional VP, VS, and VP/VS imaging based on AI microseismic detection reveals the mechanism of induced earthquakes in the Xiluodu Reservoir Area, China

The Xiluodu Reservoir (XLD), the third hydropower station developed downstream of the Jinsha River, is currently the fourth largest hydropower station worldwide. The reservoir has been impounded for 10 years, and numerous earthquakes have occurred, thus indicating an increased seismic risk. In existing studies, the deep tectonics and fine-scale structure in the reservoir area have not been sufficiently explained. Thus, the mechanism underlying reservoir-induced seismicity in this area remains unclear. In this study, we utilized a microseismic catalogue compiled based on the LOC-FLOW workflow (Li et al., 2023), alongside long-term observation data from a dense seismic array in the Jinsha River downstream, to obtain high-precision 3D VP, VS and VP/VS structures, as well as refined seismic relocations in the XLD Reservoir area. Additionally, we incorporated petrological, geomagnetic, gravity, crustal density, geodetic observation and numerical simulation data, which demonstrate that seismicity in the reservoir head area and near the Manao Fault is influenced by fluids. Moreover, the two M4 earthquakes that occurred east of the Lianfeng Fault are indicative of tectonic activity. Of these earthquakes, the Tianba earthquake cluster in the reservoir head area is located in a high-VP/VS zone, while a low-VS anomaly channel is observed here, which suggests that large amounts of fluid infiltrated along the fault, increasing the pore pressure and reducing the effective stress on the fault. The Wuji earthquake cluster in the head area is also located in a low-VP/VS zone, which indicates a low porosity and possibly undrained conditions. Thus, the earthquakes in this area are the result of coupled elastic stress-pore water pressure interactions.

Mesozoic structural evolution of the Northern South China Sea margin using potential field modelling

This study conducts a comprehensive geophysical inspection of the Northern South China Sea Margin (NSCSM), a complex and pivotal area for unraveling the tectonic history of Southeast China. We employ structural interpretation of the gravity and magnetic data by applying processing techniques and 2D forward modelling to delineate and subdivide the primary subsurface structures, and also to identify the sedimentary basins. Our findings reveal that the NSCSM exhibits a succession of extensional forearc basins above a heterogeneous basement, including a Mesozoic Andean-type magmatic arc and a Western Pacific-type marginal oceanic basin. It has substantial variations in crustal densities and susceptibilities across the NSCSM, reflecting its intricate geological history. The integrated interpretation of this study highlights the substantial impact of the Indosinian and Yanshanian thrusting and orogenies. These geological processes have played a significant role in deformation partitioning, leading to the development of elongated troughs and basins intersected by numerous prominent structures trending in the NE, ENE, and EW directions. The Indosinian faults provide insights into the boundary between these structures, which formed due to the Dian-Qiong Suture and the Paleo-Pacific Plate subduction. These tectonic processes have major structural trends, particularly impacting the offshore forearc region in the NE and EW directions, characterized by deep basins overlying thin crust. Our study also identifies multiple minor trends, including horse-tails and secondary fault systems of the Yanshanian origin, which inherited from the regional tectonics. These trends suggest that post-orogenic stresses and shearing have played a role in rotational and ridge jump processes during the opening and closing phases of the South China Sea (SCS). Furthermore, an uneven relationship exists between the calculated magnetic anomaly and the thickness of Cenozoic sediments, with a significant increase in sediment depth observed in the southern NSCSM compared to both crustal thickness and its heterogeneity.

Deep structural characteristics and dynamic significance of the Southeastern margin of the North China Craton: Insights from gravity/GNSS/seismic observations

The North China Craton is a natural laboratory for studying thinning, transformation, and replacement of ancient lithospheres. Its density structure and isostatic state of the crust provide key information for a better understanding of the deep tectonic characteristics and dynamic mechanism. In this study, we analyze observed seismic and gravity/GNSS co-located data, and explore the tectonic characteristics and dynamic significance in the southeastern margin of the North China Craton, using isostatic gravity analysis based on Airy theory and crustal density structure inversions. The results indicate that depth of the Moho interface in the research area ranges between 32 km and 41 km. The crust gradually thins from west to east, and the changing trend of the upper, middle, and lower crust is similar to the Moho interface. There has been significant replacement of the deep mantle and lower crust in the research area, especially where fault zones are located. In addition, the overall area is in a positive isostatic state, indicating that the crust is on an upward trend. The lithosphere on the east side is basically in the state of isostasy, but with a slight tendency for Moho subsidence, indicating a potential thickening of the lithosphere in this region.

Predicting bathymetry based on vertical gravity gradient anomaly and analyses for various influential factors

The prediction of bathymetry has advanced significantly with the development of satellite altimetry. However, the majority of its data originate from marine gravity anomaly. In this study, based on the expression of vertical gravity gradient (VGG) of a rectangular prism, the governing equations for determining sea depths to invert bathymetry. The governing equation is solved by linearization through an iterative process, and numerical simulations verify its algorithm and its stability. We also study the processing methods of different interference errors. The regularization method improves the stability of the inversion process for errors. A piecewise bilinear interpolation function roughly replaces the low-frequency error, and numerical simulations show that the accuracy can be improved by 41.2 % after this treatment. For variable ocean crust density, simulation simulations verify that the root-mean-square (RMS) error of prediction is approximately 5 m for the sea depth of 6 km if density is chosen as the average one. Finally, two test regions in the South China Sea are predicted and compared with ship soundings data, RMS errors of predictions are 71.1 m and 91.4 m, respectively.

Gravity-Seismic Joint Inversion of Lithospheric Density Structure in the Qiongdongnan Basin, Northwest South China Sea

Abstract Qiongdongnan Basin (QDNB), located at the northwestern corner of the South China Sea (SCS), is a key juncture between the extensional tectonic regime in the northern continental margin and the shear tectonic regime in the western continental margin. Analyzing the crustal density structure and tracking the thermodynamic controlling factors are effective approaches to reveal the nonuniform breakup process of the northwestern SCS. Herein, focusing on the obvious tectonic deformation with distinct eastern and western parts in the QDNB, we present the crustal density structures of five profiles and identify the high-density anomaly related to the synrifting mantle underplating and postrifting magmatic intrusions. The crustal density model was constructed from the Bouguer gravity anomaly, ocean bottom seismic profiles, and multichannel seismic reflection profiles. The northern part of QDNB, with normal crustal density, lower surface heat flow of <55 mW/m2, and limited extension factor of 1.25–1.70, is recognized as the initial nonuniform extension continental crust. The mantle underplating beneath the QDNB is identified as a high mantle density of 3.30–3.40 g/cm3 and a high lower crustal density of 2.92–2.96 g/cm3, which is usually recognized by the high-velocity layers in the northeastern margin of SCS. The magmatic intrusions are identified as the high-density bodies ranging from 3.26 g/cm3 at the base to 2.64 g/cm3 at the top, which become stronger from the west to east. The central part of Xisha Trough is featured by the cooling of the heavily thinned lower crust in the final continental rifting stage, which is close to the cold and rigid oceanic crust. Lateral variations in the deep magmatic anomaly should be the crucial factor for the nonuniform breakup process in the northwestern margin of SCS.

Crustal density structure investigation of the East China Sea and adjacent regions using wavenumber domain 3D density imaging method

The East China Sea, situated at the intersection of the Eurasian, Philippine Sea, and Pacific plates, is characterized by complex geology influenced by tectonic phenomena such as plate movements, volcanism, faults, and uplifts. Crustal density structure inversion provides a thorough understanding of the region's geological history as well as Earth's dynamical evolution, providing critical insights into seismic disaster mitigation, resource exploration, marine environmental protection, and maritime safety. The inversion process, on the other hand, presents challenges in data quality, quantity, model complexity, uncertainty, and computational resources. With the advancement of next-generation satellite gravity measurements and developing inversion techniques, the inversion of marine crustal density structures promises to be more precise and comprehensive. We explored the density distribution in the East China Sea and surrounding areas using an innovative wavenumber domain three-dimensional density imaging method along with high-precision global satellite gravity data. By overcoming data quality and computing resource constraints, wavenumber domain three-dimensional density imaging has transformed the discipline of marine geophysics, successfully delivering accurate density distributions in the study area. We were able to get a more precise and comprehensive characterization of the crustal density structure by combining high-precision satellite gravity data and cutting-edge imaging methods. Our investigation has unveiled previously unknown details about density distribution in the East China Sea and its environs. The East China Sea shelf displays smooth low-density perturbations at 18 km depth, whereas the trench–arc–basin region exhibits increasing density perturbations. Notably, the Okinawa Trough, which is surrounded by the Tokara Volcanic Ridge and the Ryukyu Trench, displays strong positive anomalies with a north–northeastern to northeastern orientation. In contrast, the Ryukyu Ridge and the Philippine Sea Basin exhibit smaller negative values and substantial northwestward positive density trends, respectively. These findings indicate diverse material distribution, which provides important insights into the area’s geological evolution and tectonic processes. This study adds new insights into density distribution in the East China Sea and adjacent regions, offering information on the geological complexity of the region. The research lays the groundwork for future research on crustal dynamics and enhances the field of marine geophysics and related disciplines.Graphical abstract

Effect of variable crustal density on the surface magnetic field of radio pulsars

ABSTRACT We study the surface magnetic field fluctuations due to radial oscillations as a viable cause for the microstructures of the radio pulsar pulse patterns. The electrical conductivity of matter in the outer layer of the crust of a neutron star (NS) plays a crucial role in the resulting surface magnetic field if we assume that the magnetic field is confined to this layer. This outer layer has a rapidly varying matter density that changes the microphysics of the material, affecting the electrical conductivity at every stage of the density change. In this study, the varying electrical conductivity in this rapidly varying density regime of the outer layer of the NS crust – from ∼1011 to about 104 g cm−3 – has been used to calculate the surface magnetic field using the induction equation. A finite effect of the strong gravitational field at the NS surface has also been taken into account. The equations have been solved in MATLAB using the method of lines. Any minor radial fluctuation due to stellar oscillation, in particular the radial oscillations, leads to a fluctuation of the electrical conductivity in the outer layer of the crust. This leads to fluctuations in the surface magnetic field with a frequency equal to the frequency of the stellar oscillation. We find that not only is the variation of the surface magnetic field substantial, but also it does not remain constant throughout the lifetime of the NS.

Coconut oil as an alternative to butter and shortening in bread making.

The characteristics of bread prepared with coconut oil were investigated to determine whether it can be used as an alternative to butter and shortening. Loaf height of the bread increased by adding butter and shortening water content of bread containing oils and fats was lower than that without oils and fats, and baking loss increased with decreasing water content. The addition of oils and fats influenced the baking color of bread and hindered the hardening of bread samples over time. Moreover, the addition and type of oils and fats influenced the crust density of bread samples and dough expansion. Furthermore, numerous fine bubbles were present in bread samples without oils and fats, whereas the size and number of bubbles increased and decreased in bread samples containing oils and fats, respectively. The band concentrations of insoluble proteins at approximately 39, 41, and 48kDa in freeze-dried bread samples without oils and fats were significantly lower than those containing oils and fats. Thirty volatile compounds were detected in all bread samples tested, and the number was high in the following order: bread samples with butter, shortening, and coconut oil, and without oils and fats. However, sensory evaluation showed no significant differences among all bread samples tested. Therefore, it was suggested that bread containing coconut oil had the same characteristics as that containing butter and shortening. PRACTICAL APPLICATION: Butter and shortening are usually used in bread making, although bread prepared with coconut oil can possess the same characteristics as that containing them. Therefore, this study evaluated the characteristics of bread prepared with coconut oil and revealed that use of coconut oil enabled a vegan bread with reduced environmental impact because coconut oil is a vegetable-derived oil that does not require the cutting of tropical rainforests.

Interpretation of Regional Geophysical Data Acquired at the Bathurst Mining Camp, Canada

The Bathurst Mining Camp is a major base-metal mining district in New Brunswick, Canada that was a focus area for the Geological Survey of Canada Targeted Geoscience Initiative Phase 3 project. As part of this project, more than 3500 gravimetric observations were acquired in the Bathurst Mining Camp, with an average spacing of 1-2 km, in 2006. The gravimetric data was reduced for an average crustal density value of 2.77 g/cm3 to obtain the Bouguer anomaly, which was analyzed and used to obtain 2.5D gravity models along five transects crossing the Bathurst Mining Camp. As a result of gravity modeling, a model of the general geometry of the main geological groups of the Bathurst Mining Camp (Fournier, California Lake, Tetagouche, and Miramichi) was obtained, which suggests that the Nine Mile Syncline is an open fold, and the Tetagouche Anticline is a gentle fold. Both folds are non-cylindrical and inclined, with their axial planes dipping in the 293° N direction: the angle of vergence of the folds increases towards the northeast of their axes. The structures under the Tetagouche Anticline axis gradually get deeper towards the Northeast reaching a maximum depth of 4 km. The depth of the structures modeled under the axis of the Nine Mile Syncline varies between 2 and 9 km, the deepest part being towards the center of the Bathurst Mining Camp; towards the northeast of its axis, the structures reach an intermediate depth of 4 km while towards the southwest they are 2 km deep on average.

Crustal Structures From Receiver Functions and Gravity Modeling in Central Mongolia

Abstract3D forward gravity modeling combined with receiver function (RF) analysis characterizes the crustal structures of the southern part of the Mongolian collage. The seismic signals of the 48 stations of the MOBAL2003 and the IRIS‐PASSCAL experiments were analyzed to get the RFs. This analysis revealed a significant difference between the crustal structures of the Hangay dome and the tectonic zones in the south. In addition, seismic stations south of the Hangay dome display significant signals related to the occurrence of a low‐velocity zone at lower crustal level confirmed by the gravity anomalies. Finally, these seismic analysis inputs, the boundaries, the lithologies, and the density values from rock samples of the different tectonic zones constitute the starting points from the 3D forward gravity modeling. The resulting crustal density model indicates: (a) the likely absence of a Precambrian basement block beneath the Hangay dome, (b) an alternation of two low‐velocity/low‐density zones (LVLDZs) with high‐density zones in the Baydrag microcontinent interpreted as fragments of early Tonian plutons, (c) the occurrence of an LVLDZ at the lower crustal level beneath the Lake zone, the Mongol‐Altai Accretionary Wedge, and the Trans‐Altai Zone. Therefore, the combination of the seismic RF with gravity analysis and modeling reveals new crustal structures of the Mongolian collage and enhances the occurrence and the extent of an LVLDZ at lower crustal level. These LVLDZ may demonstrate the existence of the relamination of a hydrous material in southern Mongolian collage.

Impact of batter solid-water ratio and frying time on meat-analog based coated fried food

In response to recent demand of animal-meat alternatives, plant derived composites based meat-analog is at the center of attention. This study investigated the impact of batters’ solid-to-water ratio (SWR) and frying time (FT) on the evolution of physicochemical, thermal and structural properties of meat-analog (MA) based batter coated fried products. Wheat and rice flour-based batter systems of different SWR (1:1.1, 1:1.3, 1:1.5) was used to coat a MA model, and were deep-fried at 180 °C for 2, 4 and 6 min in canola oil. Results revealed that SWR of batter coatings negatively correlated with batter-pickup (BP), water-retention-ability (WRA), cooking yield (CY), moisture and structural properties (thickness, densities) of the fried foods crust. Batters' SWR positively correlated with frying loss (FL), fat, textural attributes (hardness, brittleness, crispiness) and glass-transition-temperature (Tg) of the fried foods crust. The FL, crust thickness, fat content, Tg, and textural attributes positively correlated with frying time (FT); whereas CY, moisture content and crust densities were negatively correlated with FT. The Tg of MA-based batter coated fried foods crust were ranged between −23.51 °C and −20.16 °C. Higher SWR and FT diminishes post-fry mass redistribution, textural evolution, and color changes; whereas lower SWR and FT retained the juiciness of food-core substrate. Scanning electron microscopy analysis revealed that both the SWR and FT impacted surface microstructural properties (roughness, micro-opening) of MA-based batter coated fried products. Surface openings (SO) were negatively correlated with the moisture content, while positively correlated with the fat content of MA-based batter coated fried foods crust.

A New Lithospheric Density and Magnetic Susceptibility Model of Iran, Starting From High‐Resolution Seismic Tomography

AbstractWe propose a new model for the crust and upper mantle in Iran by joint inversion of gravity and magnetic fields, constrained with a seismic tomography model. We then calculate shear modulus from the Vs velocities and densities. The crust and mantle tomography model is first converted to a density cube through empirical and petrological velocity‐density relations. The starting susceptibility is assigned to a two‐layer homogeneous model, above a heat flow‐derived Curie depth. Considering the uncertainties in the density‐velocity relations, and the starting layered susceptibility variation, we refine the model by a constrained inversion of the gravity and magnetic fields with a Bayesian approach, producing the final 3D density and susceptibility model. The area is tectonically active with high seismicity and active faulting which are regulated by the crustal density and rigidity variations. Higher rigidity matches lower seismicity and extended deserts and basins, suggesting the control of their development. The Neo‐Tethys suture, extending ∼1,500 km long, as well as the Paleo‐Tethys suture match crustal scale density variations, defining characteristics of the lighter Arabian plate and denser Eurasian crust. The South Caspian Basin is enigmatic, due to focusing on the seismicity along all its borders, but with relatively low average rigidity, which is contrary to what is observed for Iran, where the reduced rigidity correlates with higher seismicity. The 3D density model will be useful for numerical geodynamic models and obtaining geologic inferences from the crustal‐scale units.

Bayesian inversion for modelling 3-D density structures in the eastern margin of Bayan Har block and its tectonic implications

SUMMARY Recently, the eastern margin of the Tibetan plateau has experienced three moderate earthquakes. Deep crustal structure is a critical factor for understanding the seismotectonic environment and deformations in this region. Although several multidisciplinary geophysical approaches have been used to develop crustal structure models for this area, substantial inconsistencies still persist in the results due to the intricate structural properties of crust. In this study, we introduce a novel approach to construct a 3-D crustal model with assimilated density structure (MADS) at a resolution of 0.2° × 0.2° × 5 km. This model is built by integrating data from various source, including multisource gravity anomalies and diverse available crustal structure reference models. We use a model-assimilated gravity inversion process with Bayesian parameter optimization. First, we combine the terrestrial gravity profiles data set with published global gravity field models to create a data set rich in reliable high-frequency anomaly information. Secondly, we incorporate three reference models derived from different seismological methods as prior constraints for our model. These models encompass seismic tomography, surface wave dispersion and receiver function data. We optimize the hyper-parameters of these constraints using the Bayesian criterion. The results demonstrate that the MADS not only captures significant changes in the crustal density but also discerns subtle variations in the upper and middle crust, thereby providing detailed insights into the morphologies of major faults. For instance, the central section of the Longmenshan fault is revealed as a high-angle deep thrust feature, while the frontal section of the Longmenshan fault appears as a low-angle mid-deep thrust feature, and the Xianshuihe fault exhibits a vertical deep subduction feature. Additionally, our findings indicate a correlation between the locations of moderate-to-large earthquakes in this region and the high density-gradient zones or asperities with high density within MADS. We believe that the insights into density characteristics offered by the new MADS model can shed light on the study of asperities associated with recent moderate earthquakes and enhance our understanding of deformation in this region.

New 2.75-D gravity modeling reveals the low-density nature of propagator wakes in the Juan de Fuca plate

Propagator wakes within the Juan de Fuca plate represent zones of oceanic crust affected by dynamic readjustments of the spreading centers. They are traditionally mapped from distortions of magnetic anomalies and have been previously interpreted as zones of denser than regular oceanic crust from gravity analysis along several seismic lines in 2-D approximation. In this paper, we utilized a 2.75-D modeling assumption that allowed us to incorporate the effects of the nearby seamounts that were not accounted for in the previous 2-D study. Our analysis reveals that the positive gravity effect of those geological objects was misinterpreted as a need for higher crustal density within propagator wake zones. When seamounts are included in the model, the crust of propagator wakes requires lower densities than the surrounding oceanic crust to explain the observed gravity field. We postulate that the lower densities of propagator wakes relate to faulting during the readjustment of the spreading centers, suggesting that they represent zones of weaker crust.