Abstract: Based on seismic wave energy, using digital seismic waveform data, we present a quantitative predicttion index called seismic force scale σ value. We first introduced the method of calculating the σ value in detail, and then studied the variation of σ values in time and space before strong earthquakes. The results show: ① The relationship between σ values and magnitude M_L follows lgσ=0.98M_L+6.93, and σ is a physical quantity that characterizes the average driving force of fault in earthquake source, with size between the earthquake frequency and energy; ② In the Yunnan area, σ values appeared cyclic similar to the sine wave. Earthquake occurred in the vicinity of peak most occurred in the form of repeating, but mainly in the form of single earthquake around the trough. Taking the point 1.5 months from the bottom up in the σ value curve as forecast time,then the possibility of strong earthquakes is 78% within 3 months; ③ High-value anomaly in σ values appeared 2~4 months before the main shock in the larger region. With the approach of the main shock, high σ values will move, concentrate and contract to the area of future main shock and the surrounding area, and σ values will be rapidly getting smaller a short period of time before the main shock (within 1 month), with obvious quiecence of seismicity.

Key words: Seismic force scale, Seismic wave energy, Filtering, Short-term Eartquake prediction, Yunnan region