3D Geomagnetic Structure of Miyake-jima Volcano before the Eruption in 2000 : an Application of 3D-GeomagneticTomography Method

Abstract

Three dimensional magnetic structure of Miyake-jima volcano before the eruption in June 2000, was derived from the airborne magnetic survey data by applying a 2D-FFT inversion method and a 3D-geomagnetic tomography method. This study revealed : (1) Bulk magnetization of Miyake-ji ma volcano is around 10.3 Alm, (2) Terrain connected magnetic anomalies show a positive residual amounting to 200nT to the south of the summit cone of Oyama, and relative low residuals of about -200nT in the western and eastern flank of the volcano, (3) 3D geomagnetic structure of Miyakejima volcano derived from geomagnetic tomography method shows a dome-like structure of high magnetization intensity instead of an comcal stratovolcamc structure: the magnetic somce with magnetization intensity higher than 5A/m is confined within the central part. On the other hand, the surrounding flank mainly composed of pyroclastic materials shows relatively lower magnetization. (4)The 3D-geomagnetic structure shows a strongly magnetized source ainounting to 16A/m trending N-S direction beneath the summit of Oyama‘(5) The result also shows a weekly magnetized zone whose magnetization intensity 1s less than 7A lm below the sea level The weekly magnetized zone in the western flank may correspond to the demagnetized zone due to enhanced thermal activity presaging the fissure eruption in June, 2000. The derived results also show availability of 3D-geomagnetic tomography method to detect the 3D magnetic strncture of the volcano.