Abstract: The pre-seismic anomalous daily variation of geomagnetic field in the Z-component is mainly expressed as phase change as well as amplitude variation. On the basis of the data recorded for many years by china geomagnetic network, Mani M_S7.5 earthquake on Dec. 8, 1997 and Jiujiang M_S5.7 earthquake on Dec. 26, 2005 have been especially studied. We find that the spatial distribution of the appearance time of the minima (the low-point time) of the geomagnetic vertical component appears anomalous (i.e., phenomenon of low-point displacement) features before Mani earthquake, Jiujiang earthquake and most strong earthquakes. It has been found that most earthquakes occur in the vicinity of the boundary line of sudden change of the low-point displacement and generally within four days before and after the 27th or 41st day counting from the day of appearance of the anomaly. The anomalies of diurnal-variation amplitude near the epicentral area have been also studied before Mani M_S7.5 earthquake and Jiujiang M_S5.7 earthquake.

Key words: Daily variation of geomagnetic field, Geomagnetic Z component, The low-point displacement, Strong earthquake