Infrasound signals detected by the observation system in Isumi, Japan

An infrasound observation system is installed in Isumi, Japan (35.3N, 140.3E) as the component of the International Monitoring System for CTBT. The observation system was deployed on November 2004. It comprises a six-element array. Five elements of the array are deployed as a pentagon with sides approximately 1.2 km long. The sixth element is located within the pentagon-shaped array. Until now, some interesting infrasound signals were observed. One of them was the infrasound wave produced by the large earthquake that occurred off the Pacific coast of North-East Japan on 16 August 2005. The infrasound wave-train, which had a spindle-shaped envelope, was detected following the arrival of the seismic wave. The arrival direction of the infrasound wave, which had maximum amplitude, indicated the direction of the land area, which had maximum intensity. The time difference between the on-set time of that infrasound wave and the origin-time of the earthquake coincided with the travel time for propagation in the atmosphere between the area that had maximum intensity and the observation site. Therefore, it was estimated that the observed infrasound waves were produced by shaking ground. In this paper, some remarkable infrasound signals including the above are discussed.