Abstract: The Hutubi M_S6.2 earthquake of December 8, 2016 is a pure thrust event in the northern Tianshan thrust fold belt. The earthquake occurred between the Qigu Fault and the Junggar southern margin fault, which are both thrust faults. Based on mobile gravity measurements from 2013 to 2018 in the northern Tianshan, the gravity net adjustment was accomplished using Urumqi absolute gravity observation point as the datum, and the absolute gravity value of surface observation points were obtained. In order to eliminate the seasonal effect on gravity variation, the paper uses the observation data in May per annual as studying objects and obtains the temporal-spatial dynamic evolution images of gravity field differences in the northern Tianshan at different time scales as well as the time series of gravity variation of some points in the adjacent area of the epicenter. The characteristics of regional gravity variation before and after the Hutubi M_S6.2 earthquake on December 8, 2016 and their relations are analyzed systematically. The results show that: 1)The gravity variations in the study area are dramatic in generally, and the contours of gravity variation are consistent with the main faults basically. There was a four-quadrant distribution near the epicenter before the earthquake, and the Hutubi M_S6.2 earthquake occurred near the center of the four-quadrant distribution and at the turn of the gravity variation contour. The three years' cumulative gravity variation before the earthquake and the gravity variation after the earthquake are inversed, and the variation amplitudes are equivalent, suggesting that the M_S6.2 earthquake has released the stress and the energy accumulated before the earthquake. 2)This paper focuses on the analysis of gravity variation at the observation points on both sides of the Junggar southern margin fault near the epicenter. Regional gravity variation and gravity time series show that gravity variations at the same side of the Junggar southern margin fault are basically consistent, however, gravity variations at the different sides of the Junggar southern margin fault are different from each other obviously, indicating the difference of material migration laws in different structural regions. In addition, the strain energy accumulated in the epicenter is basically released after the earthquake, and the area nearby the epicenter tends to be stable. 3)The Hutubi M_S6.2 earthquake occurred near the center of the four-quadrant and at the turn of the high-gradient zone of gravity variation, reflecting the location of strong earthquake is related to the distribution of four-quadrant of regional gravity variation, the high-gradient zone of regional gravity variation and its turn. It has a unique advantage in determining the location of strong earthquake using gravity variation results. The regional spatial-temporal gravity variation before the earthquake is manifested as a systematic evolution process of “steady state→regional gravity anomaly→four-quadrant distribution→earthquake occurring in the reverse process”. Studying the temporal-spatial evolution characteristics of gravity field before and after Hutubi M_S6.2 earthquake has important practical significance for understanding the occurrence law of large earthquakes and capturing the precursory information of earthquakes.

Key words: northern Tianshan, Hutubi M_S6.2 earthquake, gravity variation, earthquake precursor

摘要： 文中基于北天山地区2013—2018年期间的流动重力观测资料, 详细分析了2016年12月8日呼图壁M_S6.2地震前、 后区域重力场的时空分布特征及其与地震发生的关系, 结果表明: 1)重力变化等值线与断裂的走向基本一致, 震前震中附近存在四象限分布, 呼图壁M_S6.2地震发生在四象限分布的中心附近; 2)区域重力变化及点值时间序列显示震中区域积蓄的能量在地震后得到了释放, 震中附近区域震后逐渐趋于稳态; 3)震前区域重力场的时空演化图像表现为 “稳态—区域性重力异常—四象限分布特征性异常—反向变化发震”的系统演化过程。

关键词: 北天山, 呼图壁M_S6.2地震, 重力变化, 地震前兆