Search

Due to its unique geographical location and complex geological evolution processes, the South China Sea has been a focus of extensive research. Previous studies on the density structure of the South China Sea mostly focused on 2D density structures, with relatively limited research on 3D density structures. A comprehensive study is still needed to refine the expansion mechanism and tectonic evolution of the South China Sea. In this study, we utilized wavelet multi-scale analysis of gravity data to obtain a 3D density model of the South China Sea and discussed its tectonic evolution from the pattern of density anomalies. The inversion results show that (1) the expansion of the South China Sea caused the typical thin oceanic crust and parts of the continent–ocean transition zone may fracture due to the expansion; (2) the low-density anomaly in the upper mantle of Luzon Island may indicate partial melting or the upwelling of asthenosphere materials; and (3) the expansion of the South China Sea is influenced by multiple plate forces and uneven forces affect the distribution of high-density anomalies in the upper mantle.

The Parker-Oldenburg method, as a classical frequency-domain algorithm, has been widely used in Moho topographic inversion. The method has two indispensable hyperparameters, which are the Moho density contrast and the average Moho depth. Accurate hyperparameters are important prerequisites for inversion of fine Moho topography. However, limited by the nonlinear terms, the hyperparameters estimated by previous methods have obvious deviations. For this reason, this paper proposes a new method to improve the existing Parker-Oldenburg method by taking advantage of the invasive weed optimization algorithm in estimating hyperparameters. The synthetic test results of the new method show that, compared with the trial and error method and the linear regression method, the new method estimates the hyperparameters more accurately, and the computational efficiency performs excellently, which lays the foundation for the inversion of more accurate Moho topography. In practice, the method is applied to the Moho topographic inversion in the South China Sea. With the constraints of available seismic data, the crust-mantle density contrast and the average Moho depth in the South China Sea are determined to be 0.535 g/cm3 and 21.63 km, respectively, and the Moho topography of the South China Sea is inverted based on this. The results of the Moho topography show that the Moho depth in the study area ranges from 5.7 km to 32.3 km, with more obvious undulations. Among them, the shallowest part of the Moho topography is mainly located in the southern part of the Southwestern sub-basin and the southern part of the Manila Trench, with a depth of about 6 km. Compared with the CRUST 1.0 model and the model calculated by the improved Bott’s method, the RMS between the Moho model and the seismic point difference in this paper is smaller, which proves that the method in this paper has some advantages in Moho topographic inversion.

The Tibetan Plateau is a region where the Indian and Asian-European plates collide. A high-precision Moho topography model is important for the study of plate motion and internal tectonics of the Tibetan Plateau. However, previous gravity-based models failed in representing adequately the Moho topography because crustal effects and biases in the inversion parameters were not fully considered. To address these issues, we extracted the gravity effect caused by the crust using wavelet multi-scale analysis, and used an adaptive linear inversion method with available gravity and seismic data to estimate more accurate inversion parameters. With these two improvements, we inverted for a high-precision Moho topography model of the Tibetan Plateau. The results show that the majority of the Tibetan Plateau has a Moho depth of 60–70 km. The Moho depth in the center of the Qaidam Basin is about 50 km, and the Moho topography between the east and west of the basin has obvious undulations. The Moho depth of the Tarim Basin ranges from 38–50 km. While the undulation of Moho is limited in the center of the basin, obvious uplifts are shown in northern and southern basin. The plate driving force between the Tibetan Plateau and the Tarim Basin may be the primary cause of this phenomenon. By comparing the difference with the seismic data, we found that the RMS of our model is 2.8 km smaller than that of the CRUST 1.0 model, which shows that our model is more accurate.

It is significant for revealing the formation mechanism of the lunar Mascon to invert the refined 3-D lunar crust and upper mantle structure of the Mare Serenitatis. As the development of space exploration technology, lunar gravity data has advantages of high accuracy and resolution, which can be used to invert the lunar crust and upper mantle structure. However, gravity anomaly reflects all anomalous material during the whole Moon’s interior, and its vertical structure recognition capability is poor. Thus, this paper adopts wavelet multi-scale analysis method to decompose the gravity anomaly in the Mare Serenitatis for enhancing vertical resolution, and the corresponding field source depths of the decomposed gravity anomalies are further estimated by power spectrum method. Subsequently, the layered densities and the crust-mantle interface depth of the Mare Serenitatis are inverted. The research results show that the 3-D morphological character of two large high-density materials in the Mare Serenitatis is clearly depicted. The southwest high-density material with its bottom center at approximately (15°E, 25°N) has the depth range of 50 km–80 km and the maximum diameter of approximately 150 km. As for the southeast high-density material, its bottom center is located at approximately (23°E, 25°N), the depth range is 30 km–60 km and the maximum diameter is approximately 100 km. Another new finding is that the crust-mantle interface uplift has obviously fallen back in the center of the Mare Serenitatis. The high-density materials and crust-mantle interface uplift may together promote the formation of the Mascon in the Mare Serenitatis.

The South China Sea (SCS) is the result of several major tectonic plates’ interaction, and a more detailed density structure is essential to study the tectonic evolution of the marginal seas. In the past decades, density studies in the SCS have focused more on profiles, and the lateral density structure has been less mentioned by scholars. Previously, gravity data have been used many times to study the SCS; this can satisfy the demands of large-scale lateral studies, but a point that cannot be ignored is that the gravity method has certain deficiencies in vertical resolution. Therefore, this paper applies a gravity multiscale analysis method to isolate the Bouguer gravity anomaly signal in the SCS, and then uses the power spectrum method to estimate the mean field source depth, which corresponds to the decomposed gravity anomaly signal. On this basis, we modeled stratified crustal and upper mantle structures in the SCS to determine the lateral density distribution at different depths. The results show the following: (1) There are two large high-density materials in the SCS. The first one is located near the Yinggehai Basin, where the depth ranges from 42.4 km to 71.2 km, with a diameter of nearly 220 km; the second one is located near the northwest sub-basin, where the depth ranges from 106.8 km to 128.8 km, which is probably part of the Paleo-Pacific remnant in the SCS. (2) The proto South China Sea subduction zone is located in the northwestern part of the Sulu Sea and has a northeast–southwest trend.

It is significant for identifying mass movement patterns to invert horizontal tectonic stresses at different depths underneath Tibet. In recent years, a large number of achievements focusing on two-dimensional tectonic stresses have been obtained from gravity data. However, three-dimensional tectonic stresses in Tibet are still unknown or debatable. Therefore, in the present study an improved method to multilayer horizontal tectonic stresses using gravity observations is developed. The inverted multilayer horizontal tectonic stresses are in agreement with those from previous studies. In addition, rich tectonic structure and development can be revealed from the inverted multilayer horizontal tectonic stresses: (1) the distribution of horizontal tectonic stresses at various depths shows strong correlation with that of the tectonic elements, where major faults and earthquake epicenters are corresponding with stress highs and the stable basins are consistent with stress lows. (2) the mass movement patterns of whole Tibet present clockwise, and the material movement directions in the west and east are approximately south-north and east-west, respectively. (3) in eastern Tibet, the eastward materials caused by the south-north extrusion between Indian and Eurasian plates are divided into two parts by the stable Sichuan Block, one flowing nearly southeast and the other moving almost northeast. The inverted multilayer horizontal tectonic stresses may provide direct evidences for mass movement patterns in Tibet.

The complex frequencies of the low-degree toroidal modes are helpful in constraining the mantle structure. In the early years, the traditional method to detect the low-degree toroidal modes is seismic approach, which is primarily limited by the horizontal noise of seismometers. Recently, it has been proved that gravity method is competitive. Therefore, we re-estimated or newly estimated the complex frequencies of the toroidal modes below 1.5 mHz excited by the 2004 Sumatra-Andaman event using gravity method in this paper. The results show that (1) The quality factors of 0T5–0T9 and 1T2–1T3 are estimated from superconducting gravity data for the first time; (2) The estimated frequencies and quality factors for 0T2–0T9 and 1T2–1T3 are consistent with those provided by previous publications, which verifies the correctness and effectiveness of our estimations; (3) The maximum difference of frequencies between our results and those from CPREM is on the order of 0.01 mHz and the maximum difference of quality factors is at the level of 10. Keywords: Low-degree toroidal modes, Superconducting gravity data, Complex frequency, Quality factor

Abstract Background Previous studies have shown a positive correlation between the non-high-density lipoprotein cholesterol/high-density lipoprotein cholesterol(non-HDLc/HDLc ratio) and metabolism-associated steatohepatitis (MASLD), suggesting it is a superior predictor of MASLD.The aim of this study is to assess the predictive value of the non-HDLc/HDLc ratio for MASLD. Methods In this study, we retrospectively analyzed data from 4,498 adult patients at the Health Management Centre of Guangdong Provincial Hospital of Integrative Medicine, regardless of gender, and the diagnostic criteria for MASLD were derived from guidelines. We used univariate and multivariate logistic regression to verify the relationship between the non-HDLc/HDLc ratio and MASLD and assessed the stability of the results through interaction tests. ROC curve analysis was then employed to compare the diagnostic efficacy of several lipid indices, including the non-HDLc/HDLc ratio, high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), total cholesterol (TC), and the composite lipid profile in diagnosing MASLD. Results These data suggested that non-HDLc/HDLc, LDL-C, TG, and TC were positively correlated with MASLD, while HDL-C was inversely correlated, even after adjusting for confounders such as gender, age, body mass index, ALT, AST, uric acid, blood glucose, and hypertension. Among them, non-HDLc/HDLc ratio(OR: 2.709, 95% CI: 2.316–3.169, p 0.05). ROC analysis confirmed the validity of the non-HDL cholesterol/HDL cholesterol ratio in predicting MASLD incidence, with an AUC of 0.801. Moreover, the composite lipid indices demonstrated greater predictive power for MASLD compared to individual indices (AUC: 0.857), and validation set results were consistent with those of the training set. Conclusion The present study revealed that the non-HDLc/HDLc ratio was positively correlated with the development of MASLD, and the ratio was also effective in predicting MASLD.