利用球谐分析方法研究2017年九寨沟MS7.0地震前重力场变化

推导地表重力变化数据球谐展开计算公式，并根据该方法分析2017年九寨沟M_S7.0地震前震源周边区域重力场动态演化特征。结果表明：1） 2015-04～2017-04九寨沟地震震源周边区域重力场动态变化明显。2015-10～2016-04、2016-04～10和2016-10～2017-04这3个时段内，研究区内出现重力正负变化过渡带，九寨沟地震震源位于这3个时段过渡带的交会区域内部。震源附近2016-10～2017-04正负重力变化过渡带的空间走向与2015-10～2016-04和2016-04～10时段结果相比发生了约90° 旋转，九寨沟地震发生时间在过渡带空间走向旋转之后。流动重力观测数据较好地反映了九寨沟地震的前兆现象。2）采用球谐方法分析重力变化数据可以有效突出研究范围内大尺度重力场动态演化规律，抑制局部细节特征，有利于识别主要重力场变化特征。

Resesrch on Gravity Field Variations Relative to 2017 Jiuzhaigou Ms7.0 Earthquake Based on Spherical Harmonic Analysis Method

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 M_S7.0 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.