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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.
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