Zencho(water)-3 (English)

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Precursory phenomena of phreatic eruption
(Part 3)

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Damping of seismic wave detected in the second half of the period of precursory earthquake activity

A seismic wave propagates

under the ground in all directions

around its hypocenter.

If there is an area

through which

the propagation is not easy,

damping of the wave

occurs.

Before the eruption

of Mt. Fugen, i.e., in the second half

of the period

of precursory earthquake

activity (Nov. 1989 - Nov. 1990),

a damping phenomena of

seismic wave

which propagated

just under the summit of Mt. Fugen

and also through the west side

of it detected.

This suggests that

there existed a domain

of liquid magma

which the seismic

wave does not easily go through.

The following two figures

(left and right)

show the amplitudes

of the waves

started from different hypocenters

(red asterisks)

observed at

Chijiwa Seismic Station (C) and

Suwanoike Seismic Station (S).

In the left figure,

the wave arrived at

the point S

propagated through

the west side of Mt. Fugen,

and, in the right figure,

that arrived at the point

propagated just under

the summit of Mt. Fugen.

These wave show clear dampings.

Namely, this suggests that

there existed liquid

magma just under Mt. Fugen

and on the west side

of it at this time.

Movement of hypocenters before and after the eruption (M >= 3)

The hypocenters of the

precursory earthquakes before

the eruption risen

from the deep area under

Tachibana Bay

toward the shallower domain

beneath Mt. Fugen.

The movement of the hypocenters

was just along the

path expected before.

This accorded with

the above-mentioned

damping of seismic wave.

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