关键词: 重力剖面, 布格重力异常, 剩余密度相关成像, 地壳密度结构, 地震活动

Abstract:

We examine the density structure, tectonic features, and seismic activity along the Yichuan-Tai’an profile. To do this, we use regional Bouguer gravity anomaly data and analyze the correlation between the Bouguer gravity anomaly, residual density, and density structure distribution along the profile. We also incorporate seismic detection and geological tectonic results to conduct density inversion and residual density correlation imaging studies on the profile. The results show that:

(1)The Bouguer anomaly in the profile ranges from-(161.7~5.5)×10^-5m/s² and shows an overall upward trend from west to east. The Yichuan-Linfen-Changzhi section has a low background with a gradual upward trend, but the Bouguer anomaly is noticeably “depressed” between the Luoyunshan Fault and the Huoshan Mountain Frontal Fault. This depression could be due to the presence of low-density structures in the upper crust of the Linfen Basin and the density “loss” between the enclosing rocks.

The Changzhi-Taihongshan-Linxian section changes from a gentle rise in a low background to a rising trend in a high background. The Bouguer gravity anomaly suddenly drops and then rises again, indicating the existence of pre-Taihang Mountains fractures and the undulations of the Moho interface. The Anyang-Nanle-Tai’an section shows a gradually rising trend of high background, with many local anomalies and small-scale gradient zones distributed in the background. These anomalies and zones are related to smaller internal depressions and uplifts.

The background anomaly responds to the change in crustal thickness, showing a pattern of deep crust in the west and shallow crust in the east. However, the crustal uplift in the Linfen Basin and North China Fault Basin may be due to the extrusion and expansion uplift of the crust when the upper mantle material invades upward. In the uplifted areas of Luliang Mountains and Taihang Mountains, the crust is depressed, likely due to the splitting between the upper and lower crust, making the uplifted massif thicker than the basin crust.

We observed changes in the gradient of the Bouguer gravity anomaly at the major fault zones spanned by the profile. The magnitude of the gradient change indicates the relative depth of the fractures, and the Bouguer gravity anomaly change may be controlled by the development of the fractures.

(2)The model of profile density distribution reveals differences in the distribution of crustal density in both horizontal and vertical directions. Horizontally, the crust is divided into four blocks from west to east: Ordos fault block, Shanxi fault zone, Taihang Mountain fault block, and North China Plain fault block. The density of each block varies significantly, with the Linfen Basin showing low density and the Taihang Mountains showing high density. The low density in the Linfen Basin may be related to Cenozoic material deposition, while the high density in the Taihang Mountains may be related to contraction, extrusion, and folding movements, and may also be the remnant of refractory, high-density lithosphere of the Taiyuan continental plate remaining after demolition and sinking.

Longitudinally, the crust can be divided into four structures: Basal, upper, middle, and lower crust. As depth increases, the density difference becomes smaller. There are clear low-density anomalies in the crust on the eastern side of the Taihang Mountains fault block and the North China Fracture Basin. These anomalies may be caused by the process of mantle uplift, the upwelling of deeper plastic material along the fracture in the crust, or the rising of deep magma along the fracture. These events can cause partial melting of material in the crust, resulting in phase change metamorphosis, and expansion. The physical properties of the medium on both sides of this low-density body are different, and historically strong earthquakes have occurred in this area, which is known as the “earthquake-prone layer.”

Moreover, the low-density medium of the North China Plain fault block basin is connected to the eastern part of the Taihang Mountains fault block, indicating that the evolutionary tectonic movements are closer, and the deep background tends to be consistent.

(3)Around 70% of earthquakes with a magnitude of 5 or above happen in the transition zones between high and low densities. These transition zones correspond to active fractures, which suggest that the heterogeneity of crustal material density is a factor in earthquakes.

In this study, we investigate the relationship between crustal density structure and seismic activity by analyzing the gravity and density structure of the profile lattice. The subduction of the Pacific plate is a significant factor in the study area, and the inhomogeneity of the crustal medium is the primary cause of seismic activity. The Ordos fault block has relatively homogeneous crustal medium and low seismic activity, while the Shanxi fault zone and the North China Plain fault block have significant inhomogeneity and frequent seismic activity. Fracture zones influence the development of density anomalies, and there is a close connection between fracture activity and earthquake occurrence. Earthquakes are more likely to happen in the transition zones between high and low densities corresponding to active fractures.

Key words: gravity profile, bouguer gravity anomaly, residual density correlation imaging, structure of the crustal density, seismic activity