人工地震探査より推定した草津白根火山白根火砕丘表層のP波速度構造

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(5) ! " #,**. $ 1 % ,+ &'() ,**. $ +, % +* &'*+. 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 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 ,-11ῌ+1++ -./0123456734789: ,*+*ῌ2/*, kl/klQmn?o5 +ῌ+ 0.+ῌ-0 kl6?=?pq?r ;<=>6?@ABCDEFGHI34JK@AL Faculty of Engineering and Resource Science, Akita MN University, +ῌ+ Tegata-Gakuen, Akita *+*ῌ2/*,, Japan. Kusatsu-Shirane Volcano Observatory, Volcanic Fluid ,2//ῌ*2.- as/:QtA ,ῌ/0.-ῌ,3 Research Center, Tokyo Institute of Technology, 0.+ῌ uv6?6?d*?DEdWXfw@ALMDEF -0 Takijirihara, Kusatsu, Agatsuma, Gunma -11ῌ GHI +1++, Japan. Institute of Seismology and Volcanology, Faculty of ,23+ῌ+.+3 OPQRSA5 +1,,ῌ+3 Sciences, Kyushu University, ,ῌ/0.-ῌ,3 Shin’yama, <T6?UVDENWX@AYZDEFGHI Shimabara, Nagasaki 2//ῌ*2.-, Japan. ,*/,ῌ*+*0 xyz{|2}~5 +., [\ : ,203ῌ+.** ]^/_`2abc <T6?6?d*?DEeWXfgh?DEij@ xyz6?6?d*?DEeWXfw@ADELM ADEFGHI FGHI{|@ALMN Sakurajima Volcano Research Center, Disaster PreUsu Volcano Observatory, Institute of Seismology and vention Research Institute, Kyoto University, +1,,ῌ Volcanology, Graduate School of Science, Hokkaido +3 Sakurajima-Yokoyama, Kagoshima 23+ῌ+.+3, University, +., Tatsuka, Usu, Hokkaido */,ῌ*+*0, Japan. Japan. Present Address : Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Choyo, Corresponding author : Shin’ya Onizawa Aso, Kumamoto 203ῌ+.**, Japan. E-mail : [email protected].

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