The geological structure in the Red River fault zone (RRF) and adjacent areas is complex. Due to the lack of high-precision gravity data in the study area, it is difficult to obtain the distribution of materials within the Earth’s crust. In this study, a gravity data-fused method is proposed. The Moho depth model data are utilized to construct the gravity anomaly trend, and the mapping relation between the gravity field model data and the measured gravity data is established. Using 934 high-precision measured gravity data as control points, the bilinear interpolation method is used to calculate high-precision grid data of the RRF. Finally, the apparent density inversion method is used to obtain clear crustal density images across the RRF. The experimental results show that the fuses data not only reflect the regional anomaly trend but also maintain the local anomaly information; the root-mean-square error of the fused data is less than 5% and the correlation coefficient is greater than 90%. Through an in-depth comparative analysis of density images, it is found that the low-density anomalous zones, with depths of ~20 km in the northern and southern sections of the RRF, are shallower than those in the middle. The data-fused method provides a new way to process geophysical data more efficiently.

High-precision repeated absolute gravity observations conducted at the Luzhou observatory provide valuable insights into the processes of mass redistribution in the station's vicinity. In this study, we analyze four campaigns absolute measurements from two absolute gravimeters, FG5X-255 and FG5X-259, observed at the Luzhou gravity observatory and find a decrease in the gravity value of (− 93.3 ± 3.1) × 10 –8 m·s −2 from October 2020 to July 2022. By subtracting the contributions of vertical deformation, hydrological change, earthquake, and offset between instruments from the observation results, we derive a residual gravity change of (− 92.7 ± 4.1) × 10 –8 m·s −2 . Further analysis of field site photos and satellite images reveals that excavation related to the construction of a building near the Luzhou station is responsible for the observed gravity decrease. We use the load theory to calculate the gravity change at the Luzhou station due to the mass removal in the construction area and find that this factor could produce a gravity decrease consistent with the magnitude of the residual gravity change. Our results demonstrate that localized sources of mass redistribution, such as excavation at construction sites, can cause gravity variations exceeding 90 × 10 –8 m·s −2 nearby. Overall, our study highlights the importance of considering local mass redistribution when interpreting gravity variations.

The gravity inversion results of three-dimensional density interface are often not unique,which brings some difficulties to further scientific research.The classical particle swarm optimization algorithm has a higher global extremum search ability,faster inversion speed in computing highdimensional nonlinear inversion problems,and the final solution is independent of the initial model compared with traditional inversion density interface algorithms such as L-M,Tikhonov regularization,Gauss-Newton method,etc.However,in classical particle swarm optimization,the initial model setting and parameter selection are not perfect.Therefore,this paper further enhances the algorithm based on the classical particle swarm optimization algorithm,referring to the previous optimization ideas.The test results of various models show that the optimized particle swarm optimization algorithm has a stable ability to search for the optimal global solution,and the depth error is smaller.In addition,if we adopt parallel computing,the inversion speed can be effectively improved.We obtained the Indosinian density interface depth model of the Changning area by inversion using multiple measured high-density gravity profile data based on the improved algorithm.The overall scope of the survey area is small and diamond-shaped,including the complete Changning-Shuanghe anticline and some surrounding synclines.The inversion results show that the Indosinian density interface generally presents the characteristics of uplift in the middle and depressions around it,and the depth range is 0.3～3.3km,which is basically consistent with the inversion results of the drilling data and previous gravity data,and the details are more prominent.It can better express its structural characteristics.The depression degree of the interface on the right side is significantly larger than that on the left side.The uplift part corresponds to the Changning-Shuanghe complex large anticline,and the depth varies from 0.3km to 1.9km.The core of the anticline is exposed to the surface by uplifting and erosion of the tectonic movement.The inversion result provides essential information for studying the seismotectonic environment and is also a vital reference for studying the multi-layer density interface model.Density interface fluctuation is the product and sign of a specific area under the action of multistage tectonic movement,which plays an essential role in studying basin basement,regional structure,and deep structural fluctuation.It provides critical information for the analysis of the origin of earthquakes.Therefore,we analyzed the structural characteristics of this area and its relationship with earthquakes combined with the undulating morphology of the Indosinian surface.Earthquakes in the Changning area are concentrated on the north and south sides of the large anticline.The seismic distribution pattern and focal parameters on both sides are obviously different.The main reason for this phenomenon is that there are significant differences in the causes of earthquakes.The Indosinian surface in the north wing of the anticline is steeper than that in the south wing.The location of the strip distributed shallow earthquakes in the north wing is highly related to the fluctuation of the Indosinian surface,and they mainly occur at the places where the Indosinian surface fluctuates violently.The local density changes drastically,and the earthquakes’occurrence is greatly affected by hidden faults.The clumped distributed shallow earthquakes in the south wing occur at locations where there is an apparent depression on the Indosinian surface,which may be caused by shale gas exploitation,and the earthquakes are more affected by local stress changes.Deep earthquakes may be closely related to the revival of basement faults.There may still be seismic risk in the northeast wing of the large anticline in the future.In general,the optimized particle swarm algorithm has achieved good results in both model testing and practical applications.In order to further improve the accuracy of the inversion results,we will focus on improving the applicability of the algorithm in various situations and the ways of adding multiple constraint information.More detailed geophysical research should be carried out in this area,which will help to better understand its crustal structure,earthquake mechanism,geological structure,and the development of earthquake prevention and disaster reduction.

The Tangshan M(S)7. 8 earthquake in 1976 was located at the transitional part between the Yanshan fold and the North China Plain. The research on the existing crustal structure in the earthquake area is still not fine enough, and there are some controversies in the understanding of its seismogenic tectonic conditions. Combining the high-resolution Bouguer gravity anomaly data in Tangshan earthquake area with the latest global gravity field model WGM2012, based on wavelet multi-scale signal separation technology and spatial domain iterative inversion algorithm, the Moho surface fluctuation and the inner crust density disturbance distribution images in the earthquake area are obtained. The results show that the Tangshan earthquake area is located on the east side of Taiyuan-Yanqing Buge gravity anomaly gradient belt (that is, the middle part of the famous gravity gradient belt in eastern China) and the Moho steep belt, its isoline curves from NNE to NE, and the Moho depth is between 32 similar to 37 km. The epicenter of Tangshan is located in the transition between Moho steep belt and uplift area. The internal density distribution of the crust in the earthquake area presents the characteristics of high in the south and low in the north, and the scattered high-density bodies in the shallow part gradually aggregate with increasing depth. The former reflects the difference of tectonic movement between the North China Plain and Yanshan fold, while the latter reflects that the shallow high-density bodies mainly originate from deep upper mantle magmatism. From the density structure profile of the vertical and parallel Tangshan fault, it can be found that the Tangshan fault runs through the Moho surface, and its deep (about >7 km) pre-existing fracture surface is gradually opened by the intrusion of upper mantle magma under the action of horizontal tension and uplift compression of Moho surface, and the magma expands more and more as it goes up. The relatively uniform shallow part (within a depth of about 7 km) may be due to the continuous deposition of new strata and diagenesis on the surface, which prevents magma from invading, thus forming a locking area where stress and strain are easy to accumulate at the top of high-density body. This may be an important reason why the Tangshan earthquake occurred on the top of the high-density body and showed the characteristics of flower-like rupture in the shallow. Combining with the existing achievements, the seismogenic model of the Tangshan earthquake by the combined action of magmatic upwelling and faults is put forward from the point of view of material migration movement, and at the same time, it provides new evidence for the seismic sounding speculation of the seismogenic structural conditions of the Tangshan earthquake.

介绍了 2021年中国大陆构造环境监测网络(简称陆态网络)绝对重力比对观测的工作情况.基于各观测单位提交的数据,采用加权最小二乘平差方法,进行了统一的处理和分析,获得了各比测点位的标准参考值和各观测仪器的等效度.比测结果表明:8台绝对重力仪的等效度在-6.1～3.0 μGal之间,标准差RMS为2.8 μGal,基于归一化偏差进行判断,可见所有参加此次比测的仪器都是等效的,均可满足陆态网络项目要求.

针对中国大陆重力站主要装备的gPhone相对重力仪(约62套)由于缺乏标定,限制其高精度重力数据的科学应用的问题,收集了中国大陆12站同址FG5绝对重力观测与同时段gPhone观测的资料,提出了基于DDW-NHi全球潮汐模型约束的FG5绝对比测标定gPhone重力仪的方法,并与gPhone重力仪出厂格值法、理论固体潮标定法进行比较,讨论未来绝对重力标定的策略.研究结果表明:利用同址绝对观测时长26-64h资料标定gPhone重力仪,格值系数的相对误差为±0.001 3～±0.003 8;标定后各站重力残余振幅≤1×10-8m/s2,达到gPhone重力仪标称精度,优于出厂格值和理论固体潮标定结果;标定后经固体潮、气压负荷潮改正的重力残差振幅与中国大陆海潮负荷规律相符.

The Huangling Anticline, which formed during the Jinning movement, is the structural core of the Yangtze River Three Gorges area. In this study, we investigated the geological structure of the Huangling Anticline through correlation analysis among magnetic, Bouguer gravity, and crustal susceptibility anomalies of the Three Gorges and adjacent areas. Aeromagnetic anomalies, which were processed by reducing to the pole, reveal four main aeromagnetic anomaly regions. Three-dimensional (3D) magnetic susceptibility constrained inversion was used to obtain the susceptibility distribution from 0 to 30 km depth. We focused on the spatial distribution of four major rock masses: Sandouping, Huanglingmiao, Dalaoling, and Xiaofeng. The low susceptibility of the Huanglingmiao rock mass is tentatively explained by the combined effect of weathering and stripping of the upper body and a tectonic ductile shear zone at the bottom.

长期的重力变化和地壳形变观测是研究地下物质运动的重要手段.基于武汉九峰地震台2013-2020年的绝对重力和全球卫星导航系统(global navigation satellite system,GNSS)观测数据,计算了长期的重力与地壳垂直形变的变化趋势,并以1年、2年和3年的时间间隔分段,进行线性趋势拟合,分别获得不同时间段的绝对重力年变化率、地壳垂直形变速率以及对应的比值.研究结果表明,武汉九峰地震台长期的重力年变化率为0.479 9μGal/a,地壳垂直形变速率为-1.2 mm/a,两者的比值为-0.399 9 μGal/mm,与理论值存在一定的偏差,可能与该区域的地下水活动有关.将不同时间段的重力变化与地壳垂直形变的数据展布在一张图中,发现数据点离散分布在不同区域,由此可初步判断地下物质运动过程,为区域动力学机制解释提供参考.

The Yinchuan Basin is located along the transition belt between the Alxa Massif and Ordos Basin, and is characterised by a complex geological structure. However, several important scientific problems remain unresolved, such as, the causative fault of the 1739 Pingluo Ms 8.0 earthquake. In order to provide new insight into the geodynamic processes, the normalized full gradient (NFG) method and 3-D gravity inversion are utilised to image the crustal structure beneath the Yinchuan Basin in the study. The NFG results indicate that the upper crustal structure is complex, particularly at depths shallower than 20 km. The inversion results infer that the locations of large faults are consistent with the high-low gradient zones in horizontal direction. The integrating NFG and density contrasts imaging results reveal that the south-western boundary of the Ordos Basin is the Yellow River Fault which is divided into two segments with the boundary at Yongning; the East Helanshan and Qingtongxia-Guyuan faults are the south-eastern boundary of the Alxa Massif; the Yinchuan fault is the causative fault of the 1739 event and the focal depth of the event is ~15 km. These findings provide a scientific basis for studying the crustal dynamics and seismogenic mechanisms of strong earthquakes.

We applied a 2-dimensional, non-spectral technique to investigate the spatial variations of the lithospheric effective elastic thickness (T-e) in the eastern Bayan Har block and its adjacent areas. A program was designed to calculate the Moho flexure induced by topography loading, as well as the Bouguer gravity anomalies caused by that Moho flexure. The T-e values were determined by minimizing the RMS differences between the observed and calculated Bouguer gravity anomalies. The results suggested that T-e varies significantly from eastern Tibet to the Sichuan Basin. The eastern Bayan Har block and northern Sichuan-Yunnan block have low T-e values (0 < T-e < 20 km), indicating an easily deformable lithosphere. The Sichuan Basin has high T-e values (40 km < T-e < 100 km) acts as a rigid block that resists the eastward extrusion of plateau materials. The moderate T-e values (30 km < T-e < 40 km) found under the Longmen Shan fault belt indicate that the lithosphere of the Sichuan Basin plays an important role in supporting the Longmen Shan topography. The extremely low T-e (T-e < 10 km) observed in the seismogenic zone of the Ms7.0 Jiuzhaigou earthquake indicated that the topography is compensated locally, which is significantly different from the Longmen Shan topography. A banded region of relatively low T-e values (< 40 km) stretching from the northeastern Bayan Har block to the southwestern Ordos block sketches out an escape channel for plateau materials, and challenges the existing of crustal flow on the north side of the Sichuan Basin.

绝对重力仪在长期使用过程中,同一台仪器完成的多期观测数据,会因为激光波长和原子钟频率波动等自身因素导致的性能改变,造成观测数据出现轻微偏差;此外,同一测点各期使用不同仪器测量,不同仪器之间的互差也会导致多期观测数据之间存在一定程度偏差,当后期计算重力变化趋势时,必须通过统一重力基准处理,将这种因测量仪器性能改变引起的基准偏差予以剔除.通过对2010至2020年云南区域内的陆态网络9个重力基准点的历期绝对重力观测数据分析,提出了一种统一重力基准的方法,对比了重力基准统一前后,短期(期与期之间)和长期的重力变化趋势,虽然整体变化趋势基本保持一致,改变量也小,但也存在部分测点的短期变化趋势反转、长期变化速率改变量大的现象,重力基准的统一对于研究重力场变化速率十分必要.

Sichuan-Yunnan and its adjacent area are the main places where the eastward material flow from the Tibet Plateau escapes and spins to SE-SSE with active geological formations and frequent seismic activity. The study of the crustal structure and material transport characteristics of this region is important to reveal the mechanism of earthquake occurrence here. It can also deepen the understanding of the tectonic activity mechanism of the southeastern margin of the Tibet Plateau. This paper summarizes recent achievements and important advances in the study of regional Bouguer gravity anomaly and crustal density structure in Sichuan-Yunnan and its adjacent areas. (1) Bouguer gravity anomaly information of eight gravity profiles was obtained by using gravity and GNSS positioning techniques, and the basic framework of crustal density structure of the southeastern margin of the Qinghai-Tibet Plateau and its adjacent areas was constructed by using gravity inversion techniques constrained by various geophysical observations. (2) New gravitational evidence of clockwise rotation of the Sichuan-Yunnan rhomboid massif was found through the separation and extraction of gravity signals by wavelet decomposition and normalized gradient. (3) Through the scientific research of the Yushu MS7.1, Lushan MS7.0 and Ludian MS6.5 earthquakes, we have deepened our understanding of the seismogenic tectonic environment in the southeastern margin of the Tibet Plateau. There are significant differences in density distribution on sides of the left-slip shear fault zone (Ganzi-Yushu, Xianshuihe and Xiaojiang) and the large thrust nappe structure (Longmen Shan), both of which are regional seismicity-controlled faults. The seismicity of the Ludian earthquake zone may be related to the eastward migration of the Xiaojiang fault zone. The nappe characteristics and detachment structure of the Longmenshan fault zone may be an important factor in the occurrence of the Lushan earthquake at the Houshan fault.

文中利用国家重大科学工程"大陆构造环境监测网络"2010—2020年云南境内及周边区域10个基准站的绝对重力观测资料,初步获得了各基准站的重力基准及其动态变化.其中,9个测站的3个不同时间尺度和时段的重力变化趋势结果显示重力变化先增后减,转折发生于2014年前后,在重力变化增大至转折点时,先后发生了2014年鲁甸MS6.5、盈江MS6.1和景谷MS6.6地震,随后至2021年漾濞MS6.4地震发生之前重力一直呈减小趋势,昆明站的情况则正好相反.文中研究表明,云南及邻近区域内重力场变化幅度大、速度快、升降转换周期短,且变化趋势一致.其重力场变化机制可能是由青藏高原受印度板块推挤向NE运移,后受四川盆地阻挡,青藏高原下的地壳物质转向SE的云南及邻近区域的运动所主导.

In this paper, the depth of the Moho surface in the South China Sea is investigated by the gravity inversion method under the spatial-domain spherical coordinate system based on the global gravity field model WGM2012. The gravity data are corrected for the lithospheric thermal gravity anomaly due to the large negative residual thermal anomaly caused by lithospheric extension in the ocean and continental margins. The calculation results show that the thermal anomaly is mainly concentrated near the residual oceanic ridges in the South China Sea and that the density anomalies caused by the anomaly show a "fusiform" variation with depth; i.e., the anomaly is larger in the middle of the lithosphere and smaller at the top and bottom. The Moho inversion results indicate that the depth of the Moho surface in the South China Sea ranges from 7 to 32 km with an overall northeastward wedge-shaped distribution and is characterized by a shallow basin and a deep perimeter. In addition, a depression of the Moho surface remains under the remnant oceanic ridges, with the depth increasing by 2-5 km, and the depth of the Moho surface on both sides of the basin changes rapidly. Further, we determined the location of the OCT in the South China Sea based on the Moho depth mapped by gravity inversion and combined it with the results of seismic reflection profiles and other relevant geophysical data; its width is approximately 200 km in the east and 70 km in the west.

Previous studies show that the calculated loading effects from global ocean tide models do not match actual measurements of gravity attraction and loading effects in Southeast Asia. In this paper, taking advantage of a unique network of gravity tidal stations all over the Chinese mainland, we compare the observed and modeled tidal loading effects on the basis of the most recent global ocean tide models. The results show that the average efficiencies of the ocean tidal loading correction for O1, K1, M2 are 77%, 73% and 59%, respectively. The loading correction efficiencies using recent ocean tidal models are better than the 40 years old Schwiderskis model at coastal stations, but relative worse at stations far from ocean.

对中国大陆7次强震前出现的典型重力场变化图像(梯度带和四象限)及其量化参数进行总结.结果表明,目前采用的地震预测指标具有一定的不确定性,与测网观测时段、测点空间分布、变化幅值空间差异等具有密切关系;芦山地震前潮汐因子异常空间范围是流动重力典型变化范围的近10倍,可能与连续重力观测站的精度比流动重力联测高1个数量级有关.基于闭锁剪力模式提出利用典型重力变化图像进行地震数值预测的方法,强震前在孕震源存在的双力偶闭锁剪力的持续作用下,走滑型力偶会在地表引起四象限重力变化,倾滑型力偶会在地表引起二象限(梯度带)重力变化.预测回溯算例实验结果初步表明,该方法对走滑型和倾滑型地震的震级预测较为有效,同时可给出未来地震的地点和类型.

文中基于矩形位错理论及USGS发布的断层模型,结合研究区地壳—上地幔平均波速分层结构,模拟计算了弹性-黏弹分层半空间中2021年玛多MS7.4地震产生的同震及震后地表形变和重力变化.经分析发现,同震形变和重力变化显示发震断层具有左旋走滑兼正断错动的综合特征,其变化主要发生于断层在地表投影周边50km的范围内,向断层两侧快速衰减,向E最大水平位移量>1000mm,向N最大位移量达570mm,垂直位移近750mm,重力变化达150μGal;远震区(与断层的距离>150km)的水平位移量值一般<10mm,向外衰减较慢;而垂直位移和重力变化图像呈现一定的负相关,呈蝴蝶状的正负四象限对称分布,向外衰减的速率明显强于水平形变,变化量值一般<2mm和<1μGal.震后效应随时间的推移逐步显现并持续增强,其图像变化形态与同震类似,表现出明显的继承性增强趋势;震后黏弹性松弛效应的影响范围远大于同震,震后400a间其影响量值在近场区一般≤同震的2倍,但远场区均>3倍;震后400a间黏弹性松弛对水平位移、垂直位移和重力变化的影响可达100mm、130mm和30μGal;同震效应的极值区域主要集中在断层两侧,且离断层越近量值越大,而震后黏弹性松弛效应的极值区分布于离断层两侧约50km处,两者并不重合;震后水平位移主要表现为持续单调增强,而垂直位移和重力震后的变化则相对复杂:近场区在震后5a内呈现相对同震的继承性增强,随后反向调整,而远场区则相反,先反向调整,后呈继承性增强;水平位移在100a后基本稳定不变,而黏弹性松弛效应对垂直位移和重力变化的影响会持续到震后300a.与GNSS实测结果对比后发现,两者在运动方向和量级大小上基本一致,远场符合更好,这可能与断层模型的分辨率有关.文中研究可为利用实际形变和重力资料解释此次地震的孕震过程研究提供理论依据.

基于形变与密度变化耦合运动理论,利用时变场内重力垂直梯度的计算方法,采用直立长方体模型,根据青藏高原平均降升速率,模拟计算在艾黎地壳均衡模式下,地表形变所引起重力及其垂直梯度的变化.结果显示,在山体抬升过程中,伴随着地表的隆升,重力值亦逐渐减小,导致其减小的原因为介质体密度减小与测点的高度增加.伴随着山体最高点抬升了 5 cm,在最高点处重力变化为-14 μGal,对应的重力梯度约为-2.6 E.重力垂直梯度与静态场重力梯度存在一定的差异,其原因为在重力梯度场中加入了时间效应.

The Barkam–Jiuzhaigou–Wuqi gravity profile extends across the Jiuzhaigou Ms7.0 earthquake (in 2017) zone and passes through several historical big earthquakes’ zones. We have obtained Bouguer gravity anomalies along the profile composed of 365 gravity observation stations with Global Positioning System (GPS) coordinates, analyzed the observed data and inverted subsurface density structure. The results show that the Moho depth has a big lateral variation from southwest to northeast, which shallows from 57 km to 43 km with maximum variation up to 14 km within 800 km. The most acute depth change of the Moho is in the boundary region between the Bayan Har block and West Qinling–Qilian block. According to our analysis, it is related to the eastward movement of the Bayan Har block. There are three main pieces of evidence that support it: (1) Density is higher in the east of the Bayan Har block and smaller in the west, which is the same as seismic activity; (2) Two thin low-density layers exist in the upper and middle crust of the Bayan Har block, which may promote inter-layer slip and the Jiuzhaigou Ms7.0 earthquake occurred in the boundary area of the two low-density layers, where the crustal density and Moho surface fluctuate sharply; (3) the GPS velocity field in the southwestern part gravity profile is significantly larger than that of the northeastern part, which is consistent with the density structure. Our studies also suggest that the large undulation of the Moho prevents the movement of the Bayan Har block, and strain is prone to accumulate here. The dynamic background analysis of the crust in this area indicates that the Moho surface uplifts in the West Qinling–Qilian block, which decelerates the eastern migration of material on the Qinghai–Tibet Plateau, and leads to the weak tectonic activity of the north part of the Bayan Har block.

In this paper, the flexural isostasy and gravity variations before the 2021 Madoi M(s)7. 4 earthquake were studied using the EIGEN6C4 Bouguer gravity anomalies, SIO V15. 1 topography model and the repeated gravimetry data. Firstly, based on the lithospheric flexural isostatic model, combining with Bouguer gravity anomalies and topography data, the effective elastic thickness (T-e) and the flexural isostatic gravity anomalies of the lithosphere in the epicenter and its surrounding area (northeastern Qinghai-Tibet Plateau) were calculated using the finite difference method. The results show that T-e ranges from 0 to 100 km in the northeastern Qinghai-Tibet Plateau, with obvious lateral changes and close relationship with block tectonic structures. To the north of the Bayan Har block, T-e of the Qaidam block is as high as 50 similar to 80 km, to the south, T-e of the Qiangtang block is larger than 20 km, with local high values larger than 30 km in the south of Wudaoliang and larger than 40 km around Yushu-Dege area. T-e of the Bayan Har block is 0 similar to 20 km, which is smaller than that of the north and south blocks, and it is more prone to deformation. Therefore, the mass in the Bayan Har block moves eastward under the north and south holding, and it is the primary area of eastward material flow in the central part of the Qinghai-Tibet Plateau. Earthquakes are prone to occur in the transition zones of lithospheric strength change (T, gradient zones) , as well as on the faults where T-e is low. The Madoi M(s)7. 4 earthquake occurred in the area with low T-e inside the Bayan Har block, with an effective elastic thickness of about 15 km near the epicenter. The analysis of repeated gravimetry data before the earthquake shows that the 3 similar to 5 years' accumulated gravity variations since 2015 present a regional change characteristic of negative-positive-negative from west to east, and a high gradient zone of gravity variation perpendicular to the fault zone is formed with the epicenter as the boundary, which mainly reflects the deep tectonic movement situation in the process of material flow from the Qinghai-Tibet Plateau to east before the earthquake. The gravity variations since 2018 is mainly characterized by a weak regional change from positive in the west to negative in the east around the epicenter, which shows that the epicenter area has been in the "solidification" state of high stress and strain, and the earthquake occurred at the turning point of the zero value line of gravity variations.