改进的地磁低点位移法与原方法对比分析

采用数学语言及MATLAB语言，对地磁低点位移法与其搭载软件进行改进：1)利用地磁突变界线自动成图技术代替人工画线以消除随机误差，思路为通过建立突变界线与台网地磁低点时间之间的几何解析关系来实现自动成图。首先对地磁台网进行Voronoi剖分，将突变界线的走向限定在地磁台网的Dirichlet镶嵌线上；然后通过最近邻插值法来计算低点时间等值线；最后将低点时间等值线图转化为二阶等值线图，从而提取出突变界线。2)利用地磁台站当地时间代替原有的格林威治时间(Greenwich time, GMT)，以消除由经度效应引起的低点时间偏差，思路为建立台站的格林威治时间、台站经度与台站当地时间的函数关系，输入台站格林威治时间、台站经度，输出台站当地时间。并以2017年九寨沟MS7.0地震前07-20地磁低点位移现象为例，对改进后的新方法进行评价。

Comparative Analysis of the Improved Geomagnetic Low-Point Displacement Method and the Original Method

Based on mathematical or MATLAB language, this paper improves the geomagnetic low-point displacement method and software. This paper replaces the manual drawing with the automatic drawing technology of geomagnetic abrupt boundary to eliminate the random error. The idea is to realize automatic mapping by establishing the geometric analytical relationship between the abrupt boundary and the geomagnetic low-point time of the station network. Voronoi subdivision is done for the geomagnetic network, and the trend of the abrupt boundary is limited to the Dirichlet tessellation line of the geomagnetic network. Then the low-point time isoline is calculated by nearest neighbor interpolation. Finally, the low-point time contour map is transformed into the second order contour map, so as to extract the abrupt boundary. We replace the original GMT with the local time of geomagnetic station to eliminate the low point time deviation caused by longitude effect. The idea is to establish the functional relationship between station GMT, station longitude and local time, then input station GMT and station longitude to output the station local time. This paper takes a geomagnetic low-point displacement phenomenon on July 20, 2017 related to the Jiuzhaigou MS7.0 earthquake as an example to evaluate the improved method.