Abstract: In order to study the frequency non-stationary characteristics in seismic process, the frequency spectra were analyzed for the two major earthquakes occurred in Longmenshan fault in recent years. The near fault seismic records of Wenchuan and Lushan earthquakes show that compared to Lushan earthquake, Wenchuan earthquake has large strike-slip components. Most stations located in front of fault slip of Wenchuan earthquake received higher frequency components, while stations located behind fault slip received lower frequency components. Although Lushan seismic fault has the smaller strike-slip component than that of Wenchuan earthquake, the conclusions that are the same as Wenchuan earthquake can still be drawn. The difference is that although the dip-slip component of Lushan earthquake is large than that of Wenchuan earthquake, the amplitude of the three-direction components recorded in the same station match each other in magnitude. We transformed the three-component records of Lushan earthquake into the directon of strike-slip and upward direction along the fault plane, it is proved that the high-frequency components in the upward direction along the fault plane are more abundant. Higher seismic wave frequency is received in front of the fault slip and lower frequency is received behind the fault slip, which is the result of Doppler effect. Due to the objective existence of Doppler effect, its influence on frequency non-stationary characteristics is as universal as source, propagation path and site effect. Therefore, the seismic wave frequency received in engineering site depends not only on the source, propagation path, site effect, but also on the fault sliding velocity (Doppler effect).

Key words: earthquakes, non-stationary of frequency, Doppler Effect, Longmenshan fault