Abstract:

The long-term variation of gravity field can provide an important reference for studying the regional dynamic background. In 1980s, the Western Yunnan Earthquake Prediction Study Area was established by the State Seismological Bureau for the purpose of monitoring and predicting earthquakes. Since 1984, gravity monitoring in the Western Yunnan Earthquake Prediction Study Area has been carried out continuously by Yunnan Earthquake Agency and Institute of Seismology of China Earthquake Administration. In this research, long-term gravity variations in the Western Yunnan Earthquake Prediction Study Area are obtained by using 62 campaigns relative gravity data and absolute gravity data of Xiaguan station, Lijiang station and Eryuan station from 1986 to 2014 in this area. On this basis, we first analyze the relationship between gravity variation and tectonic activity background in combination with fault distribution and historical seismicity. Then, the mechanism of gravity variation is discussed combined with the vertical crustal deformation, crustal structure and dynamic background. The main results are as follows.

1)After a fine processing of the gravity data, gravity variation rates of 87 gravity stations are obtained, among which 77 stations show negative change. This indicates that the long-term gravity variation background in the study area is dominated by negative changes. The annual average rate of 87 gravity stations is about -1.24×10^-8m/s², which is consistent with the mean gravity variation rate in the Qinghai-Tibet Plateau and its adjacent areas by absolute gravity observation from the existing research result.

2)In terms of spatial distribution, the intensity of gravity variation is closely related to the distribution of fault zones and historical strong earthquakes. In the area along the north section of Honghe Fault and Longpan-Qiaohou Fault, gravity variation shows strongly negative anomalies, and the frequency of historical strong earthquake activity is the highest. This indicates that the gravity variation clearly reflects the strong activity background of the fault zone on the west boundary of the central Yunnan secondary block. In the east area of Chenghai Fault, the spatial distribution of gravity variation is symmetrically positive and negative in a four-quadrant pattern. In 2003, the Dayao M_S6.2 and M_S6.1 earthquakes occurred in the central area of the four-quadrant, and their coseismic gravity variations caused by strike-slip dislocation are consistent with observed four-quadrant characteristics. Therefore, the observed gravity variations reflect the shear stress background in this region. While in the southwest of Yunnan, which is located in the west of the Honghe Fault, the magnitude of gravity variations and frequency of seismic activity decreased significantly in comparison with that of the Honghe fault zone and its eastern region. This indicates that the Honghe fault zone, which is the boundary of Sichuan and Yunnan rhombus block, has an obvious boundary control effect on the gravity field variations and tectonic activity in the study area.

3)Vertical displacement velocities of GNSS stations in the study area are collected, and then the vertical displacement gravity effect and observed gravity variations of 13 GNSS stations are analyzed. The result shows that ground surface vertical movement is dominated by uplift, and the gravity effect of surface vertical displacement is basically consistent with the observed gravity variation in the direction. This indicates that observed gravity variation reflects the uplift background of crustal vertical movement in this area. The magnitude of the average observed gravity variation is about 0.784×10^-8m·s^-2/a after removing the denudation-caused gravity change rate, while the average gravity effect of vertical displacement is about -0.252×10^-8m·s^-2/a, accounting for just about 30% of the observed gravity variation. Therefore, observed gravity variation cannot be explained completely by vertical displacement, and mass loss in the study area is also an important factor causing gravity variation.

4)The Western Yunnan Earthquake Prediction Study Area is located in the middle and lower crustal flow channel in the Qinghai-Tibet Plateau. The overall negative trend of gravity variation may be caused by Moho surface subsidence and surface uplift induced by the crustal flow. The spatial distribution details of gravity variation have a good correlation with the density distribution of the middle and upper crust. Therefore, we speculated that spatial distribution details of gravity variation are caused by activity of specific fault and local material distribution changes.

Key words: Western Yunnan Earthquake Prediction Study Area, mobile gravimetry, long-term gravity variation, vertical crustal deformation

摘要：

文中基于滇西地震实验场1986-2014年间近30a的流动重力观测资料, 研究了该地区重力场的长期变化背景。结果表明, 重力场长期变化背景以负变化为主, 年平均变化率约为-1.24×10^-8m/s²; 空间分布上, 重力场变化的剧烈程度与断裂带分布和历史强震活动存在密切关联, 红河断裂北段、 龙蟠-乔后断裂对本地区的重力场变化和地震活动分布具有明显的边界作用。结合地壳垂直形变、 地壳结构和区域动力学背景对重力场变化机理进行分析, 重力场整体负变化趋势可能反映下地壳物质流引起的地表隆升和地壳增厚, 而重力场变化空间分布的细节则与区域动力学背景下具体断裂带的活动特性以及相关的局部性物质分布变化有关。

关键词: 滇西地震预报实验场, 流动重力观测, 重力长期变化, 地表垂直位移