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Shallow P-wave Velocity Structure of Shirane Pyroclastic Cone, Kusatsu-Shirane
Volcano Derived from a Controlled Seismic Experiment
Shin’ya ONIZAWAῌ, Takehiko MORIῌῌ, Tomoki TSUTSUIῌῌῌ, Jun’ichi HIRABAYASHIῌ, Kenji NOGAMIῌ,
Yasuo OGAWAῌ, Takeshi MATSUSHIMAῌῌῌῌ and Atsuo SUZUKIῌῌῌῌῌ
P-wave velocity structure at a summit area of Kusatsu-Shirane volcano is investigated by using ﬁrst arrival
time data of a controlled seismic experiment conducted in ,**-. The objectives of this study are to reveal ashallow P-wave velocity structure of Shirane pyroclastic cone and to make correlation of the resultant velocity
model to geological information. The ﬁrst arrival times for two dynamite shots recorded by /3 temporal seismic
stations are used for investigating a two-layered velocity model. The ﬁrst layer velocity is estimated by using
arrivals at stations close to a shot point. The second layer velocity and depths of the interface are inferred by thetime-term analysis. Estimated velocities of the ﬁrst and second layers are +.+ km/s and ,.3 km/s, respectively.
Thickness of the ﬁrst layer varies from almost * m at surface exposures of lavas to about +** m at the summit of
the pyroclastic cone. The ﬁrst layer is interpreted as pyroclastic deposits covering the surface of the Shirane
pyroclastic cone and the second is the uppermost subsurface lavas, by comparing to surface and boreholegeological information and P-wave velocity of borehole core samples.
Key words : Kusatsu-Shirane volcano, Shirane pyroclastic cone, controlled seismic experiment, P-wave velocity
structure,-11ῌ+1++ -./0123456734789:
0.+ῌ-0;<=>6?@ABCDEFGHI34JK@AL
MN
Kusatsu-Shirane Volcano Observatory, Volcanic Fluid
Research Center, Tokyo Institute of Technology, 0.+ῌ
-0 Takijirihara, Kusatsu, Agatsuma, Gunma -11ῌ
+1++, Japan.
,23+ῌ+.+3 OPQRSA5 +1,,ῌ+3<T6?UVDENWX@AYZDEFGHI[\ : ,203ῌ+.** ]^/_`2abc<T6?6?d*?DEeWXfgh?DEij@ADEFGHI
Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, +1,,ῌ
+3 Sakurajima-Yokoyama, Kagoshima 23+ῌ+.+3,
Japan.
Present Address : Institute for Geothermal Sciences,
Graduate School of Science, Kyoto University, Choyo,
Aso, Kumamoto 203ῌ+.**, Japan.

,*+*ῌ2/*, kl/klQmn?o5 +ῌ+kl6?=?pq?r
Faculty of Engineering and Resource Science, Akita
University, +ῌ+ Tegata-Gakuen, Akita *+*ῌ2/*,, Japan.
,2//ῌ*2.- as/:QtA ,ῌ/0.-ῌ,3
uv6?6?d*?DEdWXfw@ALMDEF
GHI
Institute of Seismology and Volcanology, Faculty of
Sciences, Kyushu University, ,ῌ/0.-ῌ,3 Shin’yama,
Shimabara, Nagasaki 2//ῌ*2.-, Japan.
,*/,ῌ*+*0 xyz{|2}~5 +.,xyz6?6?d*?DEeWXfw@ADELM
FGHI{|@ALMN
Usu Volcano Observatory, Institute of Seismology andVolcanology, Graduate School of Science, Hokkaido
University, +., Tatsuka, Usu, Hokkaido */,ῌ*+*0,
Japan.
Corresponding author : Shin’ya Onizawa
E-mail : onizawa@ksvo.titech.ac.jp

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11

(a)

(b)

Fig. +. (a) A geological map around Shirane pyroclastic cone, Kusatsu-Shirane volcano after Uto
et al. (+32-) and a conﬁguration of shot and station locations in the ,**- controlled seismic
experiment. Topographic contour interval is +* m. Stars : shot points. Solid circles : temporalseismic stations deployed for the experiment. Solid inverse triangles : borehole seismic stations
by Volcanic Fluid Research Center, Tokyo Institute of Technology. Ellipse : center of the low
Bouguer anomaly (Makino et al., ,**, ; ,**.). (b) Geological columnar sections (Uto et al.,,**.) and wet densities and P-wave velocities of core samples (Volcanic Fluid Research Center,
Tokyo Institute of Technology, unpublished data) at the borehole seismic stations.

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12

Fig. -. Travel time plots for S, and topography along
the survey lines. (a) Eastern N-S survey line. (b)Northern E-W survey line. Notations are same as
those in Fig. ,.

(Fig. - (b)).
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Error bar shows picking error. Star indicates theshot location. The travel times are reduced by .
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layer velocity adopted for the analysis. (b) RMS
of travel time residuals for the eastern N-S survey
line after the time-term analysis. Solid star indicates RMS for the least-squares velocity.

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line, in the case that the second layer velocity is ,.3
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Notations are same as those in Fig. / (b).

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topography and the interface, respectively. The
borehole geology shown in Fig. + (b) is overlaid. (a)Eastern N-S survey line. (b) Yugama survey line.
(b)

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