Abstract: A tripartite array in Changping, Beijing was arranged. The infrasonic signals (P-t curve, velocity and direction of wave) resulted from the earthquake on 26 September 2003 in Japan were observed. With fast Fourier transformation, the three-dimensional dynamic frequency spectra were obtained. The results of the spectrum analyses are as follows: 1. 2~7 days or so before occurrence of strong earthquake, the precursory infrasound of earthquake with strong amplitude can often be observed. The spectrum characteristics are: the amplitudes of precursory infrasound vary from weak (10~15 Pa) to strong (50~80 Pa) or even stronger; their periods are in 10~30 minutes and 30~50 minutes respectively at first, and then become 2~62 minutes. Their waveforms are distinctly different from that of strong wind and thunderstorm. 2. The recording and recognizing of local and epicenter infrasound waves is not so easy as that of precursory infrasound. 3. If the directions of two tripartite arrays intersect, the location of epicenter will be obtained. So the measurement of the precursory infrasound could become a new and more effective method of the imminent earthquake prediction.

Key words: Infrasonic wave, M_S8.0 earthquake in Hokkaido, Japan, Earthquake precursory, Changping, Beijing