Abstract: Xianshuihe Fault, a main strong earthquake activity belt in southwest China, begins from Ganzi in the northwest, passes through Luhuo, Daofu, and Kangding, and then extents along the Dadu River valley. The fault is divided into two parts at Shimian, one part turns to south and converses to Anninghe Fault extending further to south, the other part, continuing to extend to southeast, cutting through Xiaoxiangling and then changing to Daliangshan Faults in the north of the Yuexi Basin, has the length of about 400km. Since 1700AD, there have happened 22 earthquakes larger than magnitude 6.0 and 8 earthquakes larger than magnitude 7.0. In this paper, we systematically collated and computed the gravity repetition measurement data along the Xianshuihe fault zone since 1988, and by referring to the anomaly index of gravity field of the predecessor achievements, analyzed the spatial-temporal variation of the regional gravity field and the relation to the occurrence of ≥ M_S5.0 earthquakes. The mechanism of the regional gravity changes is further studied, and also the implication of strong earthquake risk because of the dynamic variation of gravity field in the near future is discussed.The results show that:1)The mobile gravity observation has the ability to detect crustal activity and M_S ≥ 5.0 earthquake events. 2)There is definite correspondence between interannual gravitational field change and the 8 earthquakes among the 13 M_S ≥ 5.0 earthquakes occurring in the surveying area since 1988, which can be determined according to the change of interannual gravitational field. Three M ≥ 6.0 earthquakes occurred 3~4 years after the abnormal image was developed, 4 earthquakes that occurred in the region of no data available were not determined. 3)A significant feature of the spatial-temporal variation of the regional gravity is a north-south run-through image before 2004, and characterized by the alternatively positive or negative variation in different year, the earthquakes of M_S ≥ 5.0 occurring in this period were not distributed along the fault. Gravity variation magnitude indicates that there were two similar crustal material movement waves before 2004, corresponding to the course of earthquake space-time distribution from strong to weak in the study area. After 2010, the variation image shows that the local positive and negative zones are concurrent within a year, different from the image before 2004, and earthquakes of M_S ≥ 5.0 basically occurred on the fault. It is believed that the variation of gravity field since 1988 and the seismic distribution fit with the geodynamic mode of strong and weak stages of the northeast motion of Indian plate. According to the conclusion we can try to optimize gravity anomaly index. After the Kangding earthquake in 2014, the north segment of Moxi Fault was still subject to negative high value changes till 2017 and then the gravity variation was further developed to a four quadrant distribution image. Based on the analysis of this paper and the previous variation trend of gravity field, we believe that the north segment of Moxi Fault has the background of medium-long term, strong or large earthquake risk.

Key words: Xianshuihe fault zone, variation of gravity field, gravity anomaly index, geodynamic model, medium-long term strong earthquake risk

摘要： 文中系统整理并计算了1988年以来鲜水河断裂带地区流动重力复测资料，参照以往震例研究总结提出的重力场变化异常指标，分析了区域重力场时-空动态变化及其与测区内发生5级以上地震的关系，进一步研究了区域重力场变化的时-空分布特征及其机理，讨论了近期区域重力场动态变化的强震危险含义。结果表明：1）流动重力测量具备了检测地壳运动与5级以上地震事件的能力；2）1988年以来测区发生的13次5级以上地震中的8个地震与年际重力场变化有较为确定的对应关系，可依据年际重力场变化确定前兆异常，其中3个6级以上地震在异常图像发展3～4a后发生，4个发生在无资料区的地震事件不能确定前兆异常；3）测区重力场的1个明显特征是2004年以前区域重力场时空动态变化图像呈整体的南北贯通，且年度间为正、负值交替变化，其间所发生的5级以上地震不沿鲜水河断裂带分布。重力场变化值反映2004年以前存在2个相似的地壳物质移动波，对应了研究区地震时空分布由强到弱的过程。2010年以后的图像变化为年度内的局部正、负值区域同时存在，无年度间的正负交替变化现象，其间5级以上地震基本发生在断裂带上。1988年以来测区重力场动态变化与地震分布符合印度板块NE向运动强、弱分期的动力学模式，可据此优化重力异常指标。根据文中的分析总结和近期重力场变化趋势，提出磨西断裂北段区域具有中-长期强震危险性的初步结论。

关键词: 鲜水河断裂地区, 重力场变化, 重力异常指标, 动力学模式, 中-长期地震危险性