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Shallow P-wave Velocity Structure of Shirane Pyroclastic Cone, Kusatsu-Shirane
Volcano Derived from a Controlled Seismic Experiment
Shin’ya ONIZAWA�, Takehiko MORI��, Tomoki TSUTSUI���, Jun’ichi HIRABAYASHI�, Kenji NOGAMI�,
Yasuo OGAWA�, Takeshi MATSUSHIMA����and Atsuo SUZUKI�����
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 a
shallow 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 the
time-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 borehole
geological information and P-wave velocity of borehole core samples.
Key words : Kusatsu-Shirane volcano, Shirane pyroclastic cone, controlled seismic experiment, P-wave velocity
structure
,-11�+1++ -./0123456734789:0.+�-0;<=>6?@ABCDEFGHI34JK@ALMNKusatsu-Shirane Volcano Observatory, Volcanic Fluid
Research Center, Tokyo Institute of Technology, 0.+�-0 Takijirihara, Kusatsu, Agatsuma, Gunma -11�+1++, Japan.
,23+�+.+3 OP�QR�SA5 +1,,�+3<T6?UVDENWX@AYZDEFGHI[\ : ,203�+.** ]^/_`2abc<T6?6?d*?DEeWXfgh?DEij@ADEFGHISakurajima Volcano Research Center, Disaster Pre-
vention Research Institute, Kyoto University, +1,,�+3 Sakurajima-Yokoyama, Kagoshima 23+�+.+3,
Japan.
Present Address : Institute for Geothermal Sciences,
Graduate School of Science, Kyoto University, Choyo,
Aso, Kumamoto 203�+.**, Japan.
,*+*�2/*, kl/klQmn?o5 +�+kl6?=?pq?rFaculty of Engineering and Resource Science, Akita
University, +�+ Tegata-Gakuen, Akita *+*�2/*,, Japan.
,2//�*2.- as/�:QtA ,�/0.-�,3uv6?6?d*?DEdWXfw@ALMDEFGHIInstitute of Seismology and Volcanology, Faculty of
Sciences, Kyushu University, ,�/0.-�,3 Shin’yama,
Shimabara, Nagasaki 2//�*2.-, Japan.
,*/,�*+*0 xyz{|2}~5�� +.,xyz6?6?d*?DEeWXfw@ADELMFGHI{|@ALMNUsu Volcano Observatory, Institute of Seismology and
Volcanology, Graduate School of Science, Hokkaido
University, +., Tatsuka, Usu, Hokkaido */,�*+*0,
Japan.
Corresponding author : Shin’ya Onizawa
E-mail : [email protected]
+� � � � ���������������������� ����� � �������� ������������������������ ���� �!"�� ���#$�%&'� +2*/�(���)���� �*�+�'& � ,��������-�� !�.�"/01'� �2�#$"%345&�5'�#"()��"/*4'&),��+,�� �6-� ���������'.789�'5/01�%3���:�/*�2���'& � ;<���#$��3=45>?��� @A��'�56B7*C�#$DE�FG�<� !������� �8""/� 9�H��::;"<I 9�H� =789>?+�<#$�,����'@A�"BJ4� #$�C/�4'KDL� M""/4�1�N���&�
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-� �� ��=7���4< ���=4��VW� J]^�������#$lm� ,uw� � (S+, S,) � /3uw�=4�Z[\"5&< (Fig. + (a)). =4���!"�� .J�p��q�4�9�qp���-0��!��4�9��4'& � psq�4����_ (,**, ;
,**.) 8 }+b~~"� �!��4��KSW>?"� 8V��4<� �!��4�� +,*�+/*m��� n�_�4�� +**m��"P¡��4<� �`'�=4��Mark Products¢£92��3� e��lR� , Hzg %8d��Q�¢£Z[\j�[ LS-2***
SH" 54� ,/*Hz¡�¢£�¤��3�"*�4'& � S+, S, ���n�v�yzu5� w�u59��4� ¥ / kg�¦§¨©§ª" 54<�
PRSTTU>?01'«=4��*�+�<�3R¬M�« �D� ¤�¥¦®"§¯¨}<� S+
X4'�l§°±²³��M}<�!�p���x"©���4�ª´�«µ�Z[\�N��<� S,X4'H S+�� ��¶�Z[\�N��'& � ¬Z[\��"_M�<�4�·��¥¦®"=4�Q��DB+����¢j¸ª4<� S+ �D� psq�4�� �!��4�� qsp�4�_&
¹�®¯�° º±�^?²³�´µ¶6�9º»�·f¼¸�¹º º�»¼H10
� Fig. , (a)�(c) �� S,�������� �� �� ����� Fig. - (a), (b) ����� ������� . km�s��� !"��#���$��%& S+�'()*+�,�� -�./0� ��1
��23�4567 +**�,**m89�:;<=>?��@3AB9C . km�s�<DEF=GH� ��2I�JKL�B9�B�MDNO���=D�PQ��� R�
SHT.�� ��SU�VGU���WX!YDZ+���� �� � (Fig. , (a)) �[���\]67C 2** m%�@3AB9C . km�s�^?�DR�_T�EF�`�DZ+���� ab��cd��efGghij�Z+��&� ��`��k 2�lmn�S�1���G�=opQ��� q.� rstu � (Fig. ,
(b)) ����vwxy�WX!Y�gh=�z��{
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 : temporal
seismic 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.
(a)
(b)
|}g\~�SH���������������#M� P<B9�� 11
������ ������� ������������������� �!"�#$�% �&��'� ()*+�, (Fig. , (c)) -�./012��3456�7�4"48�9�%S,�:8�;� *)(��, (Fig. - (a)) <; S+ =�� >?@�� 1**mA<&"BCD� . km�s<E#F� G�HI<� ����J��KFLM&"BCD�� . km�s 9N���% O;F� J�<PQ956�R;9�% S�T� S,<; S+UV� � �<;9�+�WX;YZ[F\D��]����� P^T9�_`��!� J� ��<;a <Q9�^bT9cd�e9N���7!<K�
(Fig. - (b)).
.� �����Fig. ,f7M --�./01�g,�h�9�� ;CD�ijkl�m�8���% An���o�<p�@� q�&rBCD�_`8���� "�� s�9��t��CD�_`'�� �YZ[��% u�� vwx��n���"�;yzCDOG��{|��K��;p��WCD�R9V�}~'�� ;��<K�% G�<���<;�j� ,z��<��8� � +zCD� � ,zCD� � ,z��sD����'�� �8n%�������8n�,!]*)(��,; S,�
������018���4������<��<fF� �����<�4����������% G�<A�*)(��,CD������!% ��������� ¡89��¢£¤¥�,�¦§����¨��%
.�+ ������<;A�� +zg���� 9� ©�������o����� �� +zCD�ª��% ���time-term! (Scheidegger and Willmore, +3/1) � ��
Fig. ,. Travel time plots for S+ and topography alongthe survey lines. (a) Eastern N-S survey line. (b)
Yugama survey line. (c) Southern E-W survey line.
Error bar shows picking error. Star indicates the
shot location. The travel times are reduced by .km/s.
Fig. -. Travel time plots for S, and topography alongthe survey lines. (a) Eastern N-S survey line. (b)
Northern E-W survey line. Notations are same as
those in Fig. ,.
«"¬#® ¯$°±²³�%´u&�¯µ¶·¸'¹º ¯()»¼12
� ,���� time-term����� �� +) S+, S,
������������ (Figs. , (a) and - (a))�������� �!��"# $%� ,�&�'��() *+,- �.�/0� $- �1 ,) S,23���456�789��.$- �1 :;56 time-term
��<�.<= $-�>?6�-56 $-� @ABtime-term56� +����CD�)E� ,�&�'�BFG,-�� +������B�23�H� ��:IJK���FL.MN6�- S+�O���)P� S+56QR- -�S��4& TUV .**mWX����BY)E��� Fig. . (a) BZ[\E����\P� TUV +** m]?P� ^56IJK��. . km�sB_`\ ��abc $-�>?6�-� 0 �d%e23�BH)TUV +**mf �O��� +
�ghc�i:\1 �6�)@jkl� � +�����\P� �mn +.+ km�s�ofpP�q�1 time-term� �� ,�abcrs�i:\
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��� ,.* km�s56 /.* km�sf *., km�s�i ?P� �6������ RMS�� Fig. . (b) B�,� �� �\Pe�.� ,���e�{�\P���mn 1 � ,����@<��\E ,.3 km�s.�6�1RMS. *.**0 s $pP� Fig. , (a) zdR Fig. - (a) � . km�s�)�[�)IJK��.MN6�E)P.1 � ,��� . km�s�\P��� RMS� *.*+3 s
�[��1 [�:IJK���� ,�������Bd-e��t�/�-�Fig. / (a) �� ,���. ,.3 km�s���y���
� time-term�\E)-� �4& �� ,�&������,- time-term���.MN6�-� S+56�TUV. *., kmzdR +.* km�H ���B[�)time-term���%1 � ,�.') �.��/�-�q�1 �4��� time-term� ¡ * s�:%1 ��H �� ,�. �¢;£*Bh\E)- �.��/�-� :z1 |}mn�q�B�¤� time-terme�6�� ,�'�BFG,�¥¦�B§- �B:-.1 �� time-term��¨��©ªX $-� Fig. / (b)��+����\E +.+ km�s«�\ time-term56FG\P� ,�&�'��� $%1 zd0 .*�/*m�¬�
Fig. .. (a) Travel time plot for the stations of which
horizontal distance from S+ is less than .**m.
Slope of the line indicates the inverse of the first
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 indi-
cates RMS for the least-squares velocity.
Fig. /. (a) Time-terms along the eastern N-S survey
line, in the case that the second layer velocity is ,.3km/s. (b) The interface between the first and
second layers translated from the time-terms by
assigning +.+ km/s as the first layer velocity. Solid
and broken lines indicate the surface topography
and the interface, respectively.
®¯£°±²d%��\P³´µ¶·¸µ¶·¹º*�� Pc��»¼ 13
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Fig. 0 � , !"* ,.3 km�s���LM� time-
termNDO� + !"* +.+ km�s���LM�� , E,F"-.*PG� 2QRSTU (Fig. , (b)) !2Q����3V���W������=�*4X�:+ 5Y6 Z��3V�V6�=[! time-term�7�5\]9�+ � + � ^�5\]9#:6�� $� 3V��_`�96819�ab D�c:�=� d9#:6��
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�����,x!J�:N�+ Y36 �=�H�Ez���*4X��.��~�����x�!" +.+ km�s�� + ����:����
*K�;�~�����+ CDEz�}FEz����G-�>G!^]mantle bedding�:696=�1����'=��Ez���!��$��L�MN;��O!"��~����� S+1��HP�QA + km�>�t�G-!.'��� + �^]9#:N�+ L�MN;��R&�~����� x�=�w@!��6S�� .����Ti�U���#:6��Ti D�� + "�6�� , EG�w�D�V�!W!"�9#:6��+ gh�D� , �X!���LM+ � + � ^�5\]��$���R&.~�����YZ,� (,**, ; ,**.) �H�Ez���[5*B��G�\�d]ab*���+ F$ ,**�-** m� ^�X*#$��� ��()���=� ^�X���B�*��#:6��+ ¡�[=�¢_!}FEz'H�Ez��� �WG�V!" �`�E�#�?@�a���� ,.!P���N�+ ��1�ND< ,**m� "�KSE()!���,f /m1�A /*m�^$!}FEz�"�+ <�eF!�� bMN;�Ez��£ 9#:6�� jk;&*Z��Vc",V!"9}FEzD�f¢!���zd*4X�:D�Wc",W!"�9#:6�=��¤1� (Fig. + (b)). ¥e,� (,**.) �¦f�/�+ Wd]ab�}FEzD�f¢�Wc"�G-*4X�:6�§[�� ¨g+KSE()�D� ��E!.©� *V!"9Ez�K#:6���1��<�eE.ª�«+ h�4�?@�=�Ez�-.*4X��.� 9#��~�����9N+ h�41��Ez �WG��~����� ,
� P1!"� ,.3 km�s�#$$��� ��+ }FEz�jk¬i�j1!"!� ..-1 km�s�5\6�19'ag&+ ®¯k�H�lm"9°�no."�+�p�4�jk¬i��!"����p±!�96�+jk¬i�}FEz�¨G-*4X�:6�� ��+�4?@�D��q��P²69!"*4X�:6��
Fig. 0. (a) Time-terms along the Yugama survey line,
in the case that the second layer velocity is ,.3km/s. (b) The interface between the first and
second layers translated from the time-terms by
assigning +.+ km/s as the first layer velocity.
Notations are same as those in Fig. / (b).
³r´s,µ ¶t,u)v·,Pw¸¨,ZE¶¹,ºx»y,¼z ¶,{|}~14
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�� ���KSW��� !"#$%&� '() S+*+,-. � ,�/()'0���12�3456�78&9�:;<=9� S+>< ,**�-**m ?�@ A��/B/C� ,�D�EFGHI9� J=�C0K)�/� ,�/LMNOP'()Q� �@�RSTU����V>GHWEFGH�C� XY56�9I/��NZ[�\]^HI9LM56�_`&9�ab=9� cdLM56 \]&9��NZ e�� ,�DRS�f�gh^HI9� ��NZ�C0K)�ij jEFGHI9�J��/NOk�78&9�ab=9� +� lmDn^� oplI�J��q +**m�h&9� m<���0K) KSW��� �rs&9�� � +�/LMNOPtuk�_`^� � ,���/RS��v o
�w56 ���xy^HI9� J 56�A z{|} ~�� P���/ ,.1* km�s� time-term���9����.I�$%^HI9 (Fig. + (b)). F�J KSW��v 56/� �c=9���k><�FE�p��NZ[ LM56�/�F9�m=9 ������ ,**.���� ���� ()'0��������� ���p�f����� ��E7rm=d� �� �����m=d� +�� � ,�/�=�=NOk� 56�78^� � ,�����/LMNOP���&9o�w56 o����$��^HI9�:;<=9�
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� ,.3 km�s � ,� ¥�$��^d� x¦� Y§LMN¨ P���/J=c���©�� (+323) DLMNOP�ª�*«^d¬�® ®¯°� ±���^H
Fig. 1. The surface topography and the interface of two-
layered P-wave velocity structure and its inter-
pretation. Solid and broken lines indicate the surface
topography and the interface, respectively. The
borehole geology shown in Fig. + (b) is overlaid. (a)
Eastern N-S survey line. (b) Yugama survey line.
(a)
(b)
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