Shallow P-wave Velocity Structure of Shirane Pyroclastic Cone, Kusatsu-Shirane Volcano Derived from a Controlled Seismic Experiment

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1 /* (,**/) 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 Kusatsu-Shirane Volcano Observatory, Volcanic Fluid Research Center, Tokyo Institute of Technology, Takijirihara, Kusatsu, Agatsuma, Gunma , Japan ,, +3 : ** Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, +1,, *+* 2/*, + + Faculty of Engineering and Resource Science, Akita University, + + Tegata-Gakuen, Akita *+* 2/*,, Japan. 2// *2.-, /0.-,3 Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University,, /0.-,3 Shin yama, Shimabara, Nagasaki 2// *2.-, Japan. */, *+*0 +., Usu Volcano Observatory, Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido +3 Sakurajima-Yokoyama, Kagoshima , University, +., Tatsuka, Usu, Hokkaido */, *+*0, Japan. Japan. Present Address : Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Choyo, Aso, Kumamoto **, Japan. Corresponding author: Shin ya Onizawa onizawa@ksvo.titech.ac.jp

2 */,**- P P P P (,**. a, b), +3/1 ; +32- ; ; +33+ (+32-) Fig. + (a) -,**, ;,**.,** -** m (KSE, KSW),**. Fig. + (b) P KSE / m /* m +./ m fall-back,**. KSW 1/ m 0* m P -, (S+, S,) /3 (Fig. + (a)).. (,**, ;,**.) KSW +,* +/* m +** m Mark Products, Hz LS-2*** SH,/* Hz S+, S, / kg P S+ S, S+ S+

3 P 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 : 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. Fig., (a) (c) S, Fig. - (a), (b). km s S+ +**,** m. km s (Fig., (a)) 2** m. km s (Fig., (b))

4 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)). Fig.,. Travel time plots for S+ and topography along the 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., (c)) S, (Fig. - (a)) S+ 1** m. km s. km s S, S+. Fig., -, +,, S, time-term (Scheidegger and Willmore, +3/1)

5 P 13, time-term +) S+, S, (Figs., (a) and - (a)),,) S, time-term time-term +, + S+ S+ -.** m Fig.. (a) +** m. km s +** m km s time-term,,** m time-term,, time-term,,.* km s /.* km s *., km s RMS Fig.. (b),,,.3 km s RMS *.**0 s Fig., (a) Fig. - (a). km s,. km s RMS *.*+3 s, Fig. / (a),,.3 km s time-term, time-term S+ *., km +.* km time-term, time-term * s, time-term, time-term Fig. / (b) km s time-term,.* /* 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 indicates 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,.3 km/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.

6 14., S+, S, KSW,,, time-term,, Fig. 0,,.3 km s timeterm km s, (Fig., (b)) time-term + / / + Fig. 0. (a) Time-terms along the Yugama surveyline, in the case that the second layer velocity is,.3 km/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). Fig. 1 (a) time-term KSE,.* /* m, S+,** m Fig. + (a), +.+ km s + mantle bedding S+ + km + +,, + (,**, ;,**.),** -** m,.,** m KSE / m /* m (Fig. + (b)). (,**.) KSE, P,.3 km s..-1 km s

7 P 15 (a) Fig. 1. The surface topography and the interface of twolayered P-wave velocity structure and its interpretation. 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. (b) /, Fig. 1 (b), KSW S+, S+,** -** m,,, + +** m KSW +, P,.1* km s time-term (Fig. + (b)). KSW,**. +,, / - P +** m P,.3 km s, P (+323)

8 16 P S +.1- P,./ km s, + km,./ km s (+323) P, +.+ km s + KSE KSW 0 P time-term +, +.+ km s,,.3 km s + * m +** m +,,,**- (+32-) 23 /++ /,/. (+323), (+323) ,/ -./. (+33+) K-Ar 00,33 --,. (,**,) -3 /, /0. (,**.). 03 2*. (+3/1) / + Scheidegger, A. E. and Willmore, P. L. (+3/1) The use of a least squares method for the interpretation of data from seismic surveys. Geophysics,,,, 3,,. (,**. a). +*/ ++3. (,**. b),** *. (+32-) - (,**.). /3 02.