Abstract: Northwest Guangxi is located in the Youjiang fold belt and the Hunan-Guangxi fold belt of secondary structure unit of South China fold system. The South China fold was miogeosyncline in the early Paleozoic, the Caledonian fold returned and transformed into the standard platform, and the Indosinian movement ended the Marine sedimentary history, which laid the basic structural framework of this area. Since the neotectonic period, large areas have been uplifted intermittently in the region and Quaternary denudation and planation planes and some faulted basins have been developed. Affected by the strong uplift of Yunnan-Guizhou plateau, the topography of the region subsides from northwest to southeast, with strong terrain cutting and deep valley incision. Paleozoic carbonate rocks and Mesozoic clastic rocks are mainly exposed on the earth's surface, and its geomorphology is dominated by corrosion and erosion landforms. The dating results show that most of the structures in northwest Guangxi are middle Pleistocene active faults, and the movement mode is mainly stick-slip. According to the seismogeological research results of the eastern part of the Chinese mainland, the active faults of the middle Pleistocene have the structural conditions for generating earthquakes of about magnitude 6. In the northwest Guangxi, the crustal dynamic environment and geological structure are closely related to Sichuan and Yunnan regions. Under the situation that magnitude 6 earthquakes occurred successively in Sichuan and Yunnan region and magnitude 7 earthquakes are poised to happen, the risk of moderately strong earthquakes in the northwest Guangxi region cannot be ignored. Based on the analysis of deep structure and geophysical field characteristics, it is concluded that the Tian'e-Nandan-Huanjiang area in the northwestern Guangxi is not only the area with strong variation of the Moho surface isobath, but also the M_L3.0 seismic gap since September 2015, and the abnormal low b value area along the main fault. Regions with these deep structural features often have the conditions for moderately strong earthquakes. The paper systematically analyzes the spatial and temporal distribution features and mechanism of regional gravitational field and horizontal crust movement and further studies and discusses the changes of regional gravitational field, crustal horizontal deformation and interaction between geologic structure and seismic activity based on 2014-2018 mobile gravity measurements and 2015-2017 GPS observation data in the northwestern Guangxi. The results show that:1)On July 15, 2017, a M_S4.0 earthquake in Nandan happened near the center of four quadrants of changes of gravity difference, and the center of abnormal area is located at the intersection of the Mulun-Donglang-Luolou Fault, the Hechi-Nandan Fault and the Hechi-Yizhou Fault. The dynamic graph of differential scale gravitational field reflects the gravity changes at the epicenter before and after the Nandan earthquake, which is a process of system evolution of "local gravity anomaly to abnormal four-quadrant distribution features → to earthquake occurring at the turning point of gravity gradient zone and the zero line to backward recovery variation after earthquake". Meanwhile, according to the interpretation of focal mechanism of the Nandan earthquake, seismogram and analysis of seismic survey results, the paper thinks that the four-quadrant distribution of positive and negative gravity, which is consistent with the effect of strike-slip type seismogenic fault before Nandan earthquake, demonstrates the existence of dextral strike-slip faulting; 2)The pattern of spatial distribution of gravitational field change in northwestern Guangxi is closely related to active fault. The isoline of cumulative gravity generally distributes along Nandan-Hechi Fault and Hechi-Yizhou Fault. The gravity on both sides of the fault zone is different greatly, and gradient zone has influences on a broad area; the spatial distribution of deformation field is generally featured by horizontal nonuniformity. Tian'e-Nandan-Huanjiang area is located at the high gradient zone of gravity changes and the horizontal deformation surface compressional transition zone, as well as near the intersection of Hechi-Yizhou Fault, Hechi-Nandan Fault and Du'an-Mashan Fault; 3)The geometric shape of gravitational field in northwestern Guangxi corresponds to the spatial distribution of horizontal crustal movement, which proves the exchange and dynamic action of material and energy in the region that cause the change and structural deformation of fault materials and the corresponding gravity change on earth's surface. The recent analysis of abnormal crustal deformation in northwestern Guangxi shows that Tian'e-Nandan-Huanjiang is a gradient zone of abnormal gravity change and also a horizontal deformation surface compressional transition zone. It locates at the section of significant change of Moho isobaths, the seismicity gap formed by M_L3.0 earthquakes and the abnormal low b-value zone. According to comprehensive analysis, the region has the risk of moderately strong earthquake.

Key words: northwestern Guangxi, gravity change, GPS, tectonic activity, seismic risk

摘要： 文中利用桂西北地区2014-2018年流动重力与2015-2017年GPS观测资料，结合区域地质构造环境和区内发生的南丹M_S4.0地震震例，系统分析了区域重力场变化与地壳水平运动的时空分布特征及其机理，进一步研究和探讨了区域重力场变化、地壳水平形变、地质构造和地震活动的相互关系。结果表明：1）2017年7月15日南丹M_S4.0地震发生在重力差异变化四象限中心附近，异常区中心位于木伦-东兰-逻楼断裂、河池-南丹断裂与河池-宜州断裂的交会处。差分尺度重力场动态图像较好地反映了南丹地震前后震中附近的重力变化，是一个局部重力异常、四象限分布特征性异常、重力变化梯度带及零值线拐弯区发震及震后反向恢复变化的系统演化过程。同时，综合南丹地震震源机制解、地震动图和地表调查结果分析认为，南丹地震前与走滑型发震断层效应一致的正、负重力变化四象限分布论证了右旋走滑兼具逆冲运动趋势的存在。2）桂西北地区重力场变化空间分布形态与活动断裂密切相关。累积重力变化等值线走向总体上沿南丹-河池断裂、河池-宜州断裂分布，断裂带两侧重力变化差异较大，且梯度带影响区域较宽。形变场空间分布总体上存在较明显的横向不均匀性，天峨-南丹-环江一带处于重力变化高梯度带、水平形变面压缩过渡带，且位于河池-宜州断裂、河池-南丹断裂与都安-马山断裂交接处附近，受断块差异运动影响的区域及活动断裂交会部位更易于积累应力。3）桂西北地区重力场的几何形态与地壳水平运动空间分布关系密切，证实了区域内存在物质与能量的交换和动力作用，引起活动断层物质变迁和构造变形，在地表产生相应的重力变化。近期，桂西北地区地壳形变异常分析表明：天峨-南丹-环江一带是重力异常变化梯度带，亦是水平形变面压缩过渡带，同时也是莫霍面等深线显著变化地段、M_L3.0地震围空区、异常低b值区。综合分析认为该地区仍存在发生中强地震的危险。

关键词: 桂西北地区, 重力变化, GPS, 构造活动, 地震危险性