Abstract: In this paper, from the point of view of the action of horizontal tectonic stress, the author has investigated the stress, deformation and tilt fields of the earthquake sources and derived the formulas to evaluate the seismic moment and magnitude from the vertical deformation data. The results agree with the observational data rather well.Finally, the seismo-tectonic condition for the occurrence of strong earthquakes and the credibility and generality of the elastic rebound theory has also been discussed.