Abstract: The question of the cause of the anomalous deviation of the results of the astronomical time and latitude determinations before a strong earthquake has been analysed in this paper. It is believed that variation of the ground water level resulting in the deviation of the vertical chiefly gives rise such anomalous phenomena. The deviation of the vertical consists of the change of the horizontal and vertical components of the gravity field, the former can cause anomalies in the astronomical determinations of time and latitude.Migration of ground water level is mainly in the vertical direction. Such migration can be transmmitted to a certain extent in the horizontal direction. From the anomalous time and latitude determinations and the variation of ground water level, one may notice some regularities of the change of the tectonic stress field. The variation of the horizontal gravity component caused by ground water level migration is much larger than that of the vertical component, they are also quite different in appearance.