Abstract:According to the definition for the spectral magnitude advanced by Duda and Nortmann,using the digital vertical displacement data recorded by 5 telecontrol stations in Jiangsu area and 13 groups of 0.5-octave band-pass filters of which central period are 1.2 s,1.0 s,0.67 s,0.4 s,0.25 s,0.147 s,0.119 s,0.1 s,0.078 s,0.065 s,0.054 s,0.047 s,0.041s respectively,the average velocity spectra of P and S wave of 54 earthquakes has been determined.And the synthetical fact f of each earthquake can be gotten.Change feature of corner periods of the velocity spectra of P and S wave and the f values with time are studied.The results show that the synthetical fact f of velocity spectrum of medium-minor earthquakes changed from high to low with time during one year or shorter period before medium-strong earthquakes which occurred in the duration of low f-value.
