// (,*+*), 1/ 21 - P,**1 +, /,*+* - + Three-dimensional P-Wave Velocity Structure and the Explosion Field at the Shallow Part of Suwanosejima Volcano Hiroshi Y AKIWARA, Masato I GUCHI, Takeshi T AMEGURI, Tomoki T SUTSUI, Jun O IKAWA, Takahiro OHKURAand Hiroki MIYAMACHI Three-dimensional P-wave velocity model beneath Suwanosejima Volcano, southwest Japan, was determined by inversion of 10 rays from 3 artificial shots recorded on 31 seismic stations We constructed -/,/*/ grid models in order to perform the inversion with the possible grid spaces As a result of all inversions, the calculations of the,3, 3+2 grid models converged We also averaged the P-wave velocities calculated with the selected -** models with AIC (Akaike s Information Criteria) smallness from all the resolved models to obtain the final P-wave velocity model does not depend on the grid configurations The final P-wave velocity model is well resolved from the surface to about *- km below sea level A region with high P-wave velocity up to - km/s exists below sea level at + km west of the collapse caldera, which may represent silicic intrusive rocks Besides, another high P-wave velocity, mountain-like, body that gradually extends from sea level to the vicinity of the active crater of the volcano is revealed The peak of the body, however, shifts southeastward about */ km from the active crater We, therefore, suppose that the high velocity body is a result of the accumulation of the dense volcanic blocks and/or agglutinates in the vicinity of past eruptive vents through the growth process of the volcano edifice On the other hand, very-long period pulses lasting before and after explosive eruptions and the explosion earthquakes occur at the inside of the high P-wave velocity, -* -2 km/s, area beneath the crater It is, therefore, concluded that the explosion field of Suwanosejima Volcano is located in the high P-wave velocity area Key words : Suwanosejima volcano, three-dimensional P-wave velocity model, inversion, explosive eruption, explosion earthquake 23, *21+ +*20+ ++- **-, + + + Earthquake Research Institute, University of Tokyo, Nansei-Toko Observatory for Earthquakes and Vol- + + + Yayoi, Bunkyo-ku, Tokyo ++- **-,, Japan canoes, Graduate School of Science and Engineering, 230 +* /,2* Kagoshima University, +*20+ Yoshino, Kagoshima 23, *21+, Japan Aso Volcanological Laboratory, Kyoto University, /,2* 23+ ++3 +1,, +3 Kawayo, Minamiaso, Kumamoto 203 +*, Japan 23* **0/ +,+ -/ Sakurajima Volcano Research Center, Disaster Pre- vention Research Institute, Kyoto University, +1,, +3 Earth and Environmental Science, Graduate School Sakurajima-Yokoyama, Kagoshima 23+ ++3, Japan of Science and Engineering, Kagoshima University, *+* 2/*, + + +,+ -/ Korimoto, Kagoshima 23* **0/, Japan Faculty of Engineering and Resource science, Akita Corresponding author: Hiroshi Yakiwara University, + + Tegata-Gakuen, Akita *+* 2/*,, Japan e-mail : yakiwara@scikagoshima-uacjp
76 +,* km (Fig + ) 2km 0 km Fig + Tn, Ot, Ng, Su +32- ;,**+ / 1 +33,** +* Fig + Cc ++ / 1 +3,**/ +2, +2+- +33/ +331,**+,**,,**,**+ +22 +22/,**/,**-,** +3/ ; +3+2 +3/1,*** ;,**/ Iguchi, +33+ ;,*** P (Iguchi, +33+ ) +323,*** +, P,,+,**/ +*, ++, Fig + Iguchi et al (,**2) (,**0) +** -* m 2 (S+ S 2 ) +2+- (S 3) +** m S+ S2,0-- m - +/* m (Iguchi et al,,**2) S 3 + / m S+ S2, kg S3,* kg,**0 +* +* m (Ishihara, +33* ) - 0-22 (,**) 3 Fig + *, *-, Hz,** -** m Mark Products L,, -D ** 0** m / ms +0 bit DATAMARK LS2*** SH
- P 77 Fig + Right : Location map of Suwanosejima Volcano Left : Map of seismic stations of the active seismic survey Solid stars and crosses represent artificial shots and seismic stations, respectively Triangles are the summits of the peaks of the volcanic edifice (Tn : Tondachi-dake, Ot : O-take, Ng : Negami-dake, Su : Su-saki) The center position of the active crater(cc), O-take Crater, is also shown with an arrow The black contours (contour interval +** m) represent surface topography +, / Hz Geospace GS-++ D ms, bit (,**0) DATAMARK LS2,** SD A, B, C, L + km +33/ - G Streck- eisen AG STS-, / ms, bit DATA- MARK LS1*** XT 10 - Guralp CMG-- T +* ms P +0 bit DATAMARK LZ2*** - + - Mark Products L P C-- D / ms, bit DATAMARK LS 1*** - P P Thurber GPS ( +32-),, (Thurber, +32-) (,**0)
78 -, + P Thurber ( +32-) + P + P - P - - P P AIC + P +310 ; +32+ + - P AIC Pseudo-Bending Thurber ( +32-) -+ / + P - + P Downhill Simplex AIC + (Press et al, +322) P +* /* P kms *, kms P, - - P P + - P Pseudo-Bending (Um and Thurber, +321) A +* B Down-hill Simplex + P */ C *,/ L *+ + P -+ n n+, */ +- km *2 +, km +, *2 ** km *+ km kms *, kms n+ Down-hill Simplex + P n+ / /, *+ *+ km -- - P Thurber ( +32-) -/,/*/ + Pseudo- - P bending Root Mean Squares WRMS
- P 79 Table + Weighted root mean squares (WRMS), and Akaike s Information Criteria (AIC) on each grid configuration 3 WRMS ***+ s - P WRMS AIC AIC + +* *1 *2 km *1 *2 km (Menke, +323 ) **+ WRMS *+ *, km * **+ */ km AIC 2 AIC + +* 1 * 1 * 2 km *1 *2 km P Table + WRMS AIC WRMS **3,1 s **,/3 s 1, AIC AIC +***, WRMS Table + - P **3,0 s **-0 s 0-,3, 3+2 Fig, (a) AIC + - P 2 AIC + +* -* /* +** -** *, km P /** +,***, Resolution * 0 + P WRMS
80 Fig, Plan view of the obtained P-wave velocity distributions at a depth of *, km (a) is the velocity distribution calculated from a model with minimum AIC (Akaike s Information Criteria) Dots show the positions of the nodes arranged by the grid model (b), (c), (d), and (e) are the P-wave velocity distributions smoothed with +**, -**, /** and +,*** models, respectively Velocities are contoured every *,/ km/s White lines represent the shoreline The regions with low resolution are masked by gray Fig, (a) AIC Fig, (a) AIC Fig, (b) (e) P +** -** /** +,*** *, km P -** (Fig, c) /** - - P (Fig, d) +,*** (Fig, e) - P AIC + *+ *, km/s P P, -** -** Resolution - P +** AIC + +** P Resolution Resolution *0 P (Menke, +323; Wiggins, P +31, ) P Resolution *0 P - /* P P +
- P 81 Fig - Results of the checkerboard test on six grid spaces The velocity patterns are depicted at the three depths that are arranged for the grid nodes on each grid space AIC No is the number countered in the order of the AIC smallness for the grid space The parameters providing the grid space are also shown Top : Depth of upper surface of the grid space V Int : Vertical interval between the grid nodes H Int : Horizontal interval between the grid nodes Disp N : Northward displacement of the whole grid space Disp E : Eastward displacement of the whole grid space
82 e, Fig g Resolution P -, + P +* /+ P /++ */ km */ km (Cc) -- - P (Fig a) (Cc) (Cc),2 kms Fig - - P 0 (Cc) AIC + -* /* +** -** /**,-, / kms (Cc) - (Cc) AIC AIC AIC /** (Fig d) AIC + (Ot),* kms (Fig + ) (Ot) */ km (Fig + ) ( *1 *2 km), P /+, *, km *, km (Cc) Resolu- -/ kms -* kms tion Resolution * + Fig - (Cc) */ km Resolution *, (Cc) Fig */ km (Fig a) - Resolution * 0 */ km *0 Resolution /+- *+ km *, km (Fig b) / P -1 kms (Cc) - P Fig (Fig c) Fig * / *, * + km (Cc) (Cc) (Fig ac) (Cc), Fig d, Fig f (Ot) Resolution Fig
- P 83 Fig Final three-dimensional P-wave velocity distributions (a), (b) and (c) are the plan view of the obtained P-wave velocity distributions contoured every *,/ km/s at depths of */ km, *, km, and *+ km, respectively P-wave velocity distributions on west-east (A-A ), and southwest-northeast (B-B ) cross-section are also shown in (d)-(g) These sections pass through the center of O-take Crater (Cc) (d) and (e) are the west-east (A-A ) cross-sections contoured every *,/ km/s, and every *+ of diagonal elements of the resolution matrix, respectively (f) and (g) are the southwest-northeast (B-B ) cross-sections contoured every *,/ km/s, and every *+ of resolution The region with low resolution is masked by gray +32- (Ng) -/ kms * + kms (Cc) */ km (Cc) + km (Cc) (Cc) -2-3 kms
84 /, P Fig d Fig e (Cc) (A-A ) P Fig f Fig g (Cc) (Ng) (B-B ) P Fig d Fig f P *,/ kms +* kms Fig e Fig g P Resolution * + P *- km (A-A ) (B-B ) (Cc) Fig / Comparison of P-wave velocity distribuena location on VLP, CEX and the EEX are (Cc) *- * km (Fig d) tion and the locations of geophysical phenom- (Cc) the source areas of very long period phases (Cc),2 -* kms,* -* kms (Iguchi et al,,**2), moderate contraction and sharp expansion of the explosion earthquake Fig d, Fig f (Tameguri et al,,**), respectively The (A-A ),* -* kms P-wave velocity is contoured every *,/ km/s The P-wave velocities of the gray area are equal to or higher than -* km/s A high velocity block (Vp * km/s) is hatched The (Fig d) (B-B ) demagnetized block (Ueda et al,,**1) nearly,* -* kms correspond to the hatched block The hypocenters of A-type earthquakes (Yakiwara et (Cc) al,,**/) are shown by circles (Fig f) Resolution *1 Fig e, Fig g 0 (B-B ) (Ot),* kms - P (Fig f) /++ (Ot), 0+ P (A-A ) (Cc) -1 */ *0 km - kms P - kms Fig d, Fig / * kms (Fig d) (B-B ) (Cc) -/ + kms (,**1) -, kms * kms (Fig f) Resolution *0 *1 Fig e, g *- km - ** km *1 km + Am P
- P 85 Volcano-Tectonic (,**1) P, A P (,**1) ( +333) 0, P +331 ; Tomatsu et al,,**+ +2+-, (Nishi,,**,) (Yamawaki et al,,**) ( +333 ),**- (Tanaka et al,,**,) (Onizawa et al,,**1) P (,**/) P (,**1), + + ( +331) P */ km P P P Chiarabba et al,,*** ; Di Stefano and Chiarabba,,**, P (,**1) P (Fig d) *1 *2 km Okuma et al (,**0) (Fig 0) (Okuma et al,,**0) P,**1 P *- kmp P (Fig /) (,**/) A Fig d P (Fig -2 kms +- -/ d) P km (Fig /) A P Fig / Tanaka et al (,**,) (VLP) (Iguchi et al,,**2)
86 P (-* -2 km/s) Fig 0 Photograph of the inside surface of Otake Crater The broken ellipsoid marks the sti# blocks (CEX) (EEX),,** P (-* -2 km/s) P (), *1 *2 km (No +*2*,*/ ) No +1*-+*+- 1 - P */ km */ km + km */ * 0 km - kms ( +310) AIC +/- /++ ( +32+ ) P,+- 1+*
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