Abstract: We discussed variation of seismic force scale in time and space before moderately strong earthquakes in North China. We first introduced the physical meaning of seismic force scale (σ value) and its calculation method has been derived by the traditional empirical formula. Based on seismic wave energy, σ value is a physical quantities of average driving force in the seismic source at the fault, which is between earthquake frequency and energy. It is derived that the seismic force scale σ value and magnitude can be described as the relation of lgσ=1.14M_L+6.22. Combined with the seismic activity characteristics in north China (34°~42°N; 110°~124°E), using more than 2500 3,0 earthquakes, and σ value variations before M_S≥5.0 earthquakes then are studied. It is found that this indicator can objectively reflect the enhanced and calm of seismic activity, with high σ value before moderately strong earthquakes. Spatial characteristics are related to geological structure, seismic zone and strong earthquake preparation process, earthquake force scale σ value has the characteristics of extensive increasing process.

Key words: Earthquake force scale, value, Earthquake prediction, North China