This paper applies the non-stationary phenomenon coming out within the system before the sudden change of phase state in modern statistical physics to the study of earthquake prediction.It is obtained that the ratio between T,the preparation time of large earthquakes,T₁,the interval of elastic strain and T₂,the non-linear strain interval,which is respectively set around 5 and 4.This ratio has nothing to do with the way of focus fracture,the difference of medium strength and the difference of tectonic movement rate.So it can be used as the critical index of shallow earthquakes,and it is of the properties of universal constant,available for the earthquakes in different areas.By means of this ratio and T₁,T₂,determined by seismometry,the mid long-term forecasting of earthquakes can be done.In addition,two formulas between T,the preparation time of earthquake and magnitude are also obtained,which respectively correspond to the different areas with the different rate of tectonic motion.
M₁=4.2+1.5logT (1)
M₂=5.8+1.2logT (2)
Formula (1) can be used in the areas where the rate of tectonic motion is high,and (2) used in the common areas.In the above formulas,T is in the unit of year.These two formulas can contribute to the preliminary long-term prediction of earthquakes.