Abstract: Based on the definition of the spectrum magnitude,using the digital data of micro events recorded by Beijing seismic network the synthetic factor f-value for every event is calculated.In this calculation the0.5octave filter group is adopted,in which each central period is0.04s,0.06s,0.08s,0.12s,0.18s,0.28s,0.40s,0.60s,0.96s,1.28s, 2.40s,3.80s,6.20s,10.00s,respectively.The result indicates that the corner frequency of velocity spectrum for 3components of micro earthquakes is deviated from the high frequency direction in the stage before Zhangbei-Shangyi strong earthquake and synthetic factor f-value is increased with time.Epicenters of those blue earthquakes show a tendency to approach to the epicenter of the strong earthquake gradually.It is considered that the definition of blue earthquake could be quantified by synthetic factor f-value.

Key words: Blue earthquake, Synthetic factor, Velocity spectrum, Corner frequency