Abstract:We derive the formula of spherical harmonic expansion for surface gravity changes. We analyze the dynamic evolution characteristics of gravity field around the focal area before the 2017 Jiuzhaigou MS7.0 earthquake based on spherical harmonic method. Analysis of gravity data leads to two main conclusions: 1) The changes of gravity field around the Jiuzhaigou earthquake focal area are obvious from April 2015 to April 2017. There are transition zones of positive-negative gravity changes in the study area during three time periods, which are from October 2015 to April 2016, April 2016 to October 2016 and October 2016 to April 2017. Jiuzhaigou earthquake focal area is located in the intersection area of the transition zone in three periods. Compared with the results of the previous two periods, the spatial trend of the transition zone of positive-negative gravity changes in the last period near the earthquake source rotates about 90°. The Jiuzhaigou earthquake occurs after the direction of the transition zone rotates. The mobile gravity observation data well reflects the precursory phenomenon of Jiuzhaigou earthquake. 2) The spherical harmonic analysis of gravity changes can effectively highlight the dynamic evolution law of large-scale gravity field in the research area and restrains local detail features at the same time. This is really useful to identify the main features of gravity field changes.
LI Zhongya,HU Minzhang,HAO Hongtao et al. Resesrch on Gravity Field Variations Relative to 2017 Jiuzhaigou MS7.0 Earthquake Based on Spherical Harmonic Analysis Method[J]. jgg, 2021, 41(9): 949-953.