2017年南丹Ms4.0地震前后重力变化

利用广西及邻区2014~2017年流动重力观测资料，系统分析区域重力场变化及其与2017-07-15广西南丹MS4.0地震的关系，并结合GPS观测数据与地震地质调查成果，探讨区域重力场变化的时空分布特征及机理。结果表明：1）南丹M_S4.0地震前后，震区附近重力异常变化与主干断裂关系密切，反映沿控震断裂在2014~2017年间发生了引起地表重力变化效应的地壳变形和构造活动；2）差分重力图像表明，南丹M_S4.0地震前重力变化为“局部重力异常→四象限重力异常→重力反向变化发震”的过程；3）重力变化与GPS 观测反映的水平运动表明，南丹M_S4.0地震位于重力四象限分布中心部位及面压缩峰值附近，证明重力场和形变场动态变化对中强地震地点预测具有指示意义；4）天峨-南丹-环江一带是重力变化最剧烈地区，亦是水平形变面压缩过渡带，同时位于莫霍面等深线强烈变化地段、M_L3.0地震围空区、沿主断裂存在的低b值异常区。结合地震活动图像、定点前兆观测异常综合分析认为，该地区仍存在发生中强地震的可能。

Gravity Variation before and after the MS4.0 Earthquake in Nandan in 2017

This paper makes use of mobile gravity observation data of Guangxi and its neighboring regions from 2014 to 2017, systematically analyzing the change in the regional gravity field, as well as its relationship with the July 15, 2017 M_S4.0 earthquake in Nandan, Guangxi. Combined with GPS observation data and investigation results of seismology and geology, it explores the temporal and spatial distribution characteristics and the mechanism of the change in the regional gravity field. The results show that: (1) before and after M_S4.0 earthquake in Nandan, the abnormal change in the gravity near the earthquake area is closely related to the main fracture in space, which reflects the crustal deformation and tectonic activity along controlling earthquake fracture that caused the change in the earth's surface gravity from 2014 to 2017; (2) the dynamic image of the differential scale gravity field better reflects the gravity variation near the epicenter before and after Nandan M_S4.0 earthquake, and it is a system evolution process from “abnormity of local gravity→ characteristic abnormity of four-quadrant distribution→reverse change earthquake-triggering”; (3) gravity change and horizontal motion reflected from GPS observations suggest that Nandan M_S4.0 earthquake is located in the distribution center of gravity four-quadrant and near the peak of surface compression, and this observational fact further supports that the dynamic changes in gravity field and deformation field have implications for the prediction of locations of moderate-strong earthquakes; (4) the area of Tian'e-Nandan-Huanjiang is the most gravitationally-evolving area and it is also a surface compression transition zone of horizontal deformation. At the same time, it is located in Moho surface and other deep-line intensely changing sections, M_L3.0 earthquake encircling area, and abnormal b value area along main fracture. Combined with the images of seismic activities and the comprehensive analysis of observed anomaly of fixed-point precursors, it is considered that there still exist a danger of moderate-strong earthquakes in this area.