ANALYSIS OF GRAVITY VARIATION CHARACTERISTICS AND ITS SEIMOGENIC MEANING BEFORE AND AFTER YANGBI Ms6. 4 EARTHQUAKE

On May 21, 2021, an earthquake with a magnitude of 6. 4 occurred in Yangbi, Yunnan, China, which broke the calm of earthquakes above Ms6. 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 80x10 m/s, 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 Ms6. 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 Ms≥ 6. 0 will occur again in a short time. © 2022 State Seismology Administration. All rights reserved.