Abstract: Based on the concept of information field of earthquake precursors, using the method of the spatial linearity to calculate the geomagnetism data of Z direction we can find the precursory characteristic value-spatial linearity short-term abnormal characteristic value (α), which describes the earthquake source region in the process of earthquake preparation. By analyzing the geomagnetism data of Z direction before seven strong earthquakes with M>5 previously happened in North China, we can conclude that the spatial linearity (α) of geomagnetism data of Z direction will decline obviously nearby the earthquake source region before half a year of the strong earthquake.

Key words: Short-term anomaly of strong earthquake, Vertical component of geomagnetism, Spatial linearity