Abstract:

On May 21, 2021, an earthquake with a magnitude of 6.4 occurred in Yangbi, Yunnan, China, which broke the calm of earthquakes above M_S6.0 in Yunnan Province for nearly seven years since 2015. This paper collects the relative gravity observation data of Yunnan from 2015 to 2021, and adopts the parameters estimation method based on Bayesian principle to obtain the station-value series of multi-year time-varying gravity field. Through years of time-varying gravity data, and according to the guiding idea of “Seeking the source from the field and combining the field and the source”, the series data are divided into different “time-space domains”. The “time domain” takes the epicenter as the center to analyze the one-year difference changes of the measuring points within a radius of 200km since 2015 and the multi-year cumulative change deduced backward from the origin time of earthquakes. The “space domain” takes the epicenter as the center, collects the point value change sequence of the measuring points with the radius of 200km, 100km, 50km and 25km respectively, and uses the method of “layered processing and step-by-step analysis” to analyze the characteristics of gravity change before the earthquake. The research results show that the geological active period of this earthquake is 2017-2018, the geological activity is relatively stable in 2018-2020 and there was the phenomenon of “escape” of underground materials in 2020 to 2021, and the earthquake occurred in the “escape” period. The abnormal gravity near the epicenter is characterized by a four-quadrant distribution, the positive and negative changes of gravity in the three years before the earthquake were more significant, reaching 80×10^-8m/s², the change of gravity in the two years before the earthquake was small, but the four-quadrant distribution characteristics were still maintained. After the earthquake, the gravity field in the study area changed inversely, and the magnitude of change reached the cumulative change of 3 years before the earthquake. This earthquake occurred near the center of the four quadrants of the gravity anomaly and coincided with the northern segment of the Honghe fault zone, so it is inferred that the Honghe Fault is the seismic structure of the Yangbi earthquake. The research methods and results of this paper can provide a reference for earthquake examples for the study of time-varying gravity field variation characteristics and the analysis and interpretation of pre-earthquake gravity signals, as well as for exploring and predicting the areas of future mid-strong earthquakes.
This paper systematically analyzes the characteristics of gravity changes in different “time-space domains” of the 2021-05-21 Yangbi M_S6.4 earthquake, and attempts to analyze its seismogenic meaning. The following conclusions can be drawn:
(1)The gravity anomaly appeared before the Yangbi earthquake and lasted for a long time. The phenomenon of energy accumulation appeared four years before the earthquake.
(2)The variation of gravity point value within the radius of 200km is basically consistent, and the variation of gravity point value within the radius of 100km is highly consistent, indicating that the range of gravity anomaly during the earthquake preparation period reached more than 200km, which is consistent with the research results of previous empirical statistics.
(3)The geological activity associated with this earthquake started at about 4 years before the earthquake, then there was a calm period of up to two years. Half a year before the earthquake, there was an overall decline in the value of gravity points near the epicenter, which is in line with the common sense in seismology that “seismic energy gathers first and then diffuses, and the earthquake occurs in the diffusion period”, but there is a calm period before this earthquake.
(4)The “four quadrant” phenomenon appeared and gradually increased in the variation trend diagram of gravity field in one year, two years and three years before the earthquake.
(5)The “four quadrants” of the variation trend of the gravity field before the earthquake are generally consistent with the focal mechanism solution and the variation of the coseismic displacement field issued by the USGS and Seismology research group, indicating that the phenomenon of “four quadrants” variation in the gravity field will appear before earthquake of strike-slip fault geological structure, and the gravity variation can reveal to a certain extent the earthquake-generating mechanism before earthquake.
(6)The change trend of gravity after the earthquake is completely opposite to that before the earthquake, which is the release and recovery of compression and accumulation of underground materials before the earthquake. Therefore, it is considered that this earthquake can effectively release the earthquake preparation energy in this area, and it is unlikely that earthquakes of M_S≥6.0 will occur again in a short time.

Key words: time-varying gravity field, Yangbi earthquake, Bayesian adjustment method, time-space domains

摘要：

2021年5月21日云南省漾濞县发生 M_S6.4 地震, 该地震打破了云南区域自2014年以来近7a的M_S>6.0地震平静。文中采用基于贝叶斯原理的平差方法解算了2015-2021年的相对重力流动观测数据, 获取了研究区内多年的时变重力值, 分析了漾濞 M_S6.4 地震前后的重力变化特征。研究结果显示, 本次地震地质活动的活跃期为2017-2018年, 2018-2020年地质活动相对稳定, 2020-2021年出现地下物质“逃逸”现象, 地震发生于“逃逸”期; 以震中为中心的200km、 100km、 50km、 25km半径范围内, 重力异常值呈四象限分布特征; 震前3a重力正、 负变化更加显著, 达80×10^-8m/s², 震前2a重力变化较小, 但依然维持四象限分布特征; 震后研究区内的重力场均发生反向变化, 变化量级达震前3a的累计变化量; 本次地震发生于重力异常四象限中心附近, 且与红河断裂带北段重合, 故推断红河断裂为漾濞地震的孕震构造。文中的研究方法和成果可为时变重力场变化特征研究和震前重力信号分析与解释提供震例参考; 同时, 可为孕震区地质构造运动的孕震含义提供有效解释, 并对下一步可能发生中强震区域的探讨及预判提供参考。

关键词: 时变重力场, 漾濞地震, 贝叶斯平差, 时空域