3 - 2 Precursory phenomena of phreatic eruption
(Part 1)

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On 21 and 22 November 1989, the earthquake swarm began beneath Tachibana Bay (Chijiwa Bay) at the western foot of Unzen Volcano under which the magma chamber seems to hide itself. The area of the hypocenters moved and broadened with time towards the east, and in July, 1990 an earthquake swarm occurred just under the Mt. Fugen area, including an earthquake with the magnitude of M4.9 (the seismic intensity of 3). Besides, volcanic tremors were detected, showing the sign of movement of lava. Thereafter, the tremor continued to occur more frequently. In addition, there were observed a damping of the seismic wave propagating through the area just under the summit of Mt. Fugen. This seismic activity in the Shimabara Peninsula at this time showed a sharp increase of the underground pressure.
It was seen from these phenomena that the underground activity for eruption of Unzen Volcano was strengthened at this time, but we could not predict the date and time of the eruption.
On 17 November 1990, the morning edition of "Kumamoto Nichi-nichi Shinbun" newspaper reported this tense atmosphere with a headline "Unzen Volcano, eruption starts?". The eruption took place just on this day.

Daily frequency of earthquakes and volcanic tremors

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