8
῍῏ /* (,**/) + 3+0 腎腇腕腍腔腊腃腄腏腗腀腁腑腖腚腉腅腌腚腉腅腋腆腛腐腂 P 腙腓腘腈腒 腗臀膊膶 腢膫 腅腅 膢腋膚腾 腅腅腅 膯膾膂腌 膷膆腢腹 膄膎腪膹 膅膠 腅腅腅腅 膿膵膥膌 腅腅腅腅腅 ,**. 1 ,+ 膦腽膬,**. +, +* 膦腽膻Shallow P-wave Velocity Structure of Shirane Pyroclastic Cone, Kusatsu-Shirane Volcano Derived from a Controlled Seismic Experiment Shin’ya ONIZAWA , Takehiko MORI ῌῌ , Tomoki TSUTSUI ῌῌῌ , Jun’ichi HIRABAYASHI , Kenji NOGAMI , Yasuo OGAWA , Takeshi MATSUSHIMA ῌῌῌῌ and Atsuo SUZUKI ῌῌῌῌῌ 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 腅腅 -11+1++ 腞膩腤腨腮腟膑膝膛膖腸膑膝腸膘膇腥 0.+-0 膡腚腩腛膖腓腑腳膽膕腣腙腇腉腈腂膑膝膪腭腑腳腖 膒膃 Kusatsu-Shirane Volcano Observatory, Volcanic Fluid Research Center, Tokyo Institute of Technology, 0.+-0 Takijirihara, Kusatsu, Agatsuma, Gunma -11+1++, Japan. 23++.+3 腺腷膠腴腲膠腏腳膛 +1,,+3 腚膟膖腓膲腯腣腙膃膭膓腑腳腔膣腣腙腇腉腈腂 腧腰 : 203+.** 腝膴腤腊膏腟膜膺膔 腚膟膖腓膖腓腍膻腓腣腙腐膭膓膙腘膧腓腣腙腵膍腑 腳腣腙腇腉腈腂 Sakurajima 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. 腅腅腅 腅腅腅腅 腅腅腅腅腅 *+*2/*, 膁膞腤膁膞腴腻腠腓腎膛 ++ 膁膞膖腓腩腓腶腦腓膮 Faculty of Engineering and Resource Science, Akita University, ++ Tegata-Gakuen, Akita *+*2/*,, Japan. 2//*2.- 膜腱腤膠腥腴膈腳 ,/0.-,3 腜膀膖腓膖腓腍膻腓腣腙腍膭膓膙膋腑腳腖膒腣腙腇 腉腈腂 Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, ,/0.-,3 Shin’yama, Shimabara, Nagasaki 2//*2.-, Japan. */,*+*0 膳腒膤膸腼腟膐膱膛膼腫 +., 膳腒膤膖腓膖腓腍膻腓腣腙腐膭膓膙膋腑腳腣腙腖膒 腇腉腈腂膸腼腑腳腖膒膃 Usu 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]

PP-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

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Page 1: PP-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

� �� ��� � /*� (,**/)

� +� 3�+0�

���������� �������������P�����

� � � �� � � � � �� � � �� � � �� � � �� � �� ! "

#,**.$ 1% ,+&'() ,**.$ +,% +*&'*+

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]

Page 2: PP-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

+� � � � ���������������������� ����� � �������� ������������������������ ���� �!"�� ���#$�%&'� +2*/�(���)���� �*�+�'& � ,��������-�� !�.�"/01'� �2�#$"%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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~~ e��_� ,**, ; ,**.g �Fo��"��� ���,����+ ,**�-**m���TU">?4'& ��#$ps%8d�s�p�Q��B���/]^��\[�3=45>? (KSE, KSW) ��@A��'�56B7*C efg�_� ,**.g �#$DE ep�Q��B���/]^��\[� � ���g �FG�<�Fig. + (b) �'�(n�@AD����()%1 �T%8d��R PRST�"DL+�'��4<�KSE����M2 / mM�; /* m��+�w�u5��9� n�(�����#���#�u5���W�}'& � ���M� +./m(�����"��<fall-back�>?+� u���5������ �,��9[�}+b~~��-�� ���+�< efg�_� ,**.g� @A#$X4'���,���5&"K�4'�W�w�u5����T��ST��9� n�892���89}�T�}ST��}'& � 6-�KSW>?�'"� ��W; 1/ m�����#$��#� &�n�����#M��9� n�2�W�; 0* m�u5���� � +�n�2�����"�+���������x{�� tOyz��5��01� � PRST"� ��������u5�2��tOyz��5����Mn�89��&�0� �

-� �� ��=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�_&

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Page 3: PP-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

� 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

Page 4: PP-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

������ ������� ������������������� �!"�#$�% �&��'� ()*+�, (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

Page 5: PP-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

� ,���� 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:\

Etu,-� �PNvwx��4& abc.rs $-�>?6�-TUV ,**mW&����O���)P� �tu �1 y���� time-termzdR� ,

����{�\P|}�~B1 � ,��� ?time-term�i�{�\P|}pP� A� �� ,

��� ,.* 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

Page 6: PP-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

���������.�, ����������� ����������� ������������ S+, S,� � ������� !"#�� $� %&�'KSW����()*���+ ��,()-����� ./��0�����1� ,23������4����� ����� 5678 9#:6���+ � � , !"�#$ �%;$&*'�� <=!� , !"��:��()���>*?@*A+�+ B���� time-term��*C��=� D#:������ , E,F"-.*#$G�=� ��� =�H+ � �� , */I��J01!"��K$���

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��

/� � ef!gh�?@�� ,()�i'jk;&����NDOIl�mn No��p1!"#$?@����*q��

/�+ �� ��

Fig. 1 (a) ��()��� time-term<>*?@�<�>r+NDOKSE()��i=7s*PG�� , E,�:�c��t�u�> �W�:6��+ <�v?�ND< .*�/*m"��t�v?D�5\6� � , �w@�� �i���W1�(�� S+1�A ,** mx!�By�CDEz+ =�D�)������f �{�:6�|y�}FEz!"��~���� �Fig.+ (a) ���� ��B�GH�Ez���I!�=�� ,

�����,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

Page 7: PP-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

�����/�, ������Fig. 1 (b) ����� � ,���������

�� ���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�

/�- ���� � P����������� �/�f><q +** m¡¢ £W¤� P��

� ,.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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