4 - 7 GPS observation

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GPS (Global Positioning System) is a device that gives, for example, a traveling motorcar or airplane its exact position on the Earth by receiving radio waves simultaneously from four satellites out of twenty-four which launched by United States.
By interference of the waves received simultaneously at two different stations, we can measure the distance between these stations exactly to one centimeter. Accordingly, GPS is widely employed for geodetic survey and also for estimation of a rate of plate movement.
At Unzen Volcano we set GPS receivers on and around the lava dome and kept watching of the movement of the lava dome and the surrounding ground. During explosive periods, we frequently detected inflation of the volcano due to the ascent of a large mass of magma through the vent.
Furthermore, we observed that the lava dome was shrinking by cooling and sliding down on the east slope of the volcano even after the cessation of explosive activity.

The conceptual illustration of the GPS observation

GPS receiver on the lava dome
A red arrow indicates the GPS receiver. A rocky summit at the back of this picture is a volcanic spine extruded through the lava dome at the final stage of dome growth.

Deformation of the lava dome detected by GPS observation
(Deformation of the lava dome in two years until May 20, 1997)

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