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SU Fang TIAN Wei (Department of Basic Sciences, University of Petroleum, Beijing, 102200, China)  
A new method about measurement of the precursory infrasonic waves of strong earthquake was introduced. A tripartite array was arranged in Changping, Beijing. The infrasonic signals (the P t curve, the direction and horizontal projection of downward velocity of the wave) stimulated by strong earthquakes during February April 2001 were observed. With fast Fourier transformation, the three dimensional dynamic frequency spectra were obtained. The results over a year are as follows∶ (1) 10 days or so before the occurrence of strong earthquake, the precursory infrasound of the earthquake with strong amplitude and measurable direction can often be measured. The characteristics of the spectrum are∶ the amplitudes of precursory infrasound increase from weak (10~20Pa) to strong (40~80Pa) or even stronger; the final periods become very wide (2~65min); their waveforms are distinctly different from those of meteor shower, strong wind and thunder. (2) The maximal amplitude of the spectrum of the precursory infrasound measured is related not only to the magnitude of earthquake, but also to the distance and the depth of the epicenter as well as some other factors. (3) The generating mechanism of the precursory infrasound of earthquake is that the long period deformational waves produced by pre slips before stick slips or crack propagation before rupture transmit to the atmosphere. The propagation of the precursory infrasound is because its attenuation is extremely small. Besides, sound waveguide of lower channel in the atmospheric layer plays an active role. (4) If the directions of two remote tripartite arrays intersect, the location of the epicenter can be predicted. Therefore the measure of the precursory infrasound could become a new and more effective method of the imminent prediction of strong earthquake.
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