Name : YAMASHINA, Ken'ichiro
Position : Associate Professor
Division/Center : Division of Earth Mechanics
Research Area : Physics of Earthquake and Volcano
Name : YAMASHINA, Ken'ichiro
Position : Associate Professor
Division/Center : Division of Earth Mechanics
Research Area : Physics of Earthquake and Volcano
Research :
Together with field observations of earthquakes and volcanic eruptions,
their physical processes and statistical or probabilistic
features are analy zed. Based
on them, we investigate techniques which are available for the p rediction
of seismic and volcanic
activities. For example, during
the activ e period of Unzen volcano, southern Japan, between 1991-95, marked
repetitio ns of inflation
and deflation with interval of
about 1-2 hours were found by a tiltmeter installed close to the crater.
Considering that such an oscilla
tion of tilt was likely to occur
by rhythmical supply of magma along the ven t, a method of real-time monitoring
of the supply rate of
magma was proposed. As a result
of this method, reactivation of magma was convinced before the extrusion
was actually witnessed
on the surface in 1993, and the
temporal change of inferred rate was referred to for the planning of disaster
prevent ion. According to
reanalysis of crustal deformation
data at Unzen volcano, a detailed process of ascending magma column and
occurrence of subsurface
la teral intrusion was proved
during the period just before the first appearenc e of a lava dome in 1991.
Physical processes are
investigated also at Unzen volcano
for various interesting phenomena such as saw-like repetition of tilt change
associated with a
strong earthquake swarm with magitudes
of 2-3 in 1993-94, peculiar oscillation of the mountain with period of
30-100 hours in
1994-95, and so on. As for the
observation of crustal deformations, a method named time-differential stereoscopy
was put to
practical use at Unzen volc ano
using photographs taken at exactly the same location, and it is exprimen
tally applied to different
fields, e.g. Iwate volcano. In
addition to these, in order to clarify the premonitory process and aftermath,
crustal deforma tion data and
others are reexamined with respect
of the great events such as the 1914 eruption of Sakurajima volcano, the
1923 Kanto and the 1946
Nankai earthquakes. A probability
prediction of seismic activity especially based on hypocentral catalog
data is another subject. With
special references to mechanical
correlation between various factors and events, practical criteri a to
distinguish foreshocks,
characteristic features before
a large earthqua ke, and techniques for predicting aftershock activity
are investigated.
Publications :
Yamashina, K. and H. Shimizu, Crustal deformation in the mid-May 1991 crisis preceding the extrusion of a dacite lava dome at Unzen volcano, Japan, J. Volcanol. Geotherm. Res., 89, 43--55, 1999.
Yamashina, K. and T. Matsushima, Ground temperature change observed at Unzen volcano associated with the 1990-1995 eruption, J. Volcanol. Geotherm. Res., 89, 65--71, 1999.
Yamashina, K., T. Matsushima and S. Ohmi, Volcanic deformation at Unzen, Japan, visualized by a time-differential stereoscopy, J. Volcanol. Geotherm. Res., 89, 73--80, 1999.
Yamashina, K., Experimental prediction of the number of aftershocks of the 1999 Chi-Chi, Taiwan earthquake, Bull. Earthq. Res. Inst., Univ. Tokyo, 75, 79--91, 2000.