5 - 7 Underground structure of Kyushu Island
observed with seismic wave

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As we investigate the inside of a human body with CT (computer tomography), we can do so the inside of the earth with the seismic wave. This method is called "Seismic Wave Tomography".
Fig. 1 shows the velocity distribution of P wave (longitudinal wave) around the depths of about 70km under Kyushu Island. Looking at this, we can see that there are low-velocity regions (red) around Unzen and the western or southern Kyushu. And there is a high-velocity zone (blue or dark blue) along the eastern seashore of Kyushu Island.
Fig. 3 shows a velocity cross section along B-B' line that goes through Unzen Volcano (Fig. 2). It is seen from this cross section that there are two low-velocity regions; one is 0 - 20km deep region in the crust and the other is 70 - 120km deep in the upper mantle. These two regions seem to be "hot". Contrary, the high-velocity zone in the eastern Kyushu denotes the cold Philippine Sea Plate sliding down under Kyushu. The upper red part in Fig. 3 seems to show a possibility of existence of low-velocity soft layer.

Fig. 1: Seismic wave velocity distribution around 70km deep region under Kyushu Island

Fig. 2: Position of B - B' section

Fig. 3: Velocity distribution of P wave along B - B' section

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