Abstract: The geomagnetic low-point displacement appeared on June 16th, 2013. In this paper, low-point time longitude effect is eliminated by the method of subtracting theoretical low-point time from measured low-point time. The isoline with low-point time of 0 h after eliminating the longitude effect is used as the boundary of low-point displacement. The epicenter of the M6.6 Minxian—Zhangxian earthquake is at the boundary of the geomagnetic low point displacement on June 16th. The diurnal variation of geomagnetic vertical components in mainland China during abnormal periods is statistically analyzed. There are significant zoning features. The diurnal variation pattern in the area with positive low-point time presents W type, while the diurnal variation pattern in the area with negative low-point time mainly presents V type characteristics. During the abnormal period, there are significant high gradient zones in the mainland of China. In this paper, the gradient value is used to quantitatively describe the spatial and temporal variation of low-point time. During the normal period of June 13th, there is no region in the mainland of China with a gradient value greater than 1.5 h /(°). During the abnormal period of June 16th, there is area with a gradient value greater than 1.5 h/(°). The epicenter of the 2013 M6.6 Minxian—Zhangxian Earthquake is located at the edge of the high gradient region.

Key words: the 2013 M6.6 Minxian—Zhangxian Earthquake, Geomagnetic vertical component, Daily variation, Geomagnetic low-point displacement, Low-point time