三宅島の3次元磁気構造と2000年噴火によるその変化

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(4) ++ . -D Magnetic Structure of Miyakejima Volcano before and after the Eruption in ,*** Yoshio UEDA῍ A magnetic inversion algorism for constructing a -D-magnetic structure of a volcanic edifice was developed and applied to the airborne magnetic anomalies of Miyakejima Volcano surveyed in +321, +333, and ,**+. The method consists of two steps. First, a uniformly magnetized terrain model is assumed for calculating the mean magnetization intensity, and the residual field was calculated by subtracting the terrain e#ect. In the second step, the magnetization deviations (J) from the mean value are calculated for each prismatic block constituting the three dimensional volcanic edifice, by the refined Conjugate Gradient iteration method under the condition of +* A/mJ+* A/m. Characteristics of the derived -D-magnetic structures are summarized as following four features. (+) Miyakejima Volcano has rather uniform magnetization, whose non-uniform magnetization is less than +*,*ῌ of total bulk-magnetization(total summation of the product of block volumes and magnetization intensities). (,) A nearly N-S trending magnetic basement is estimated at the deeper part of the volcano. (-) Relatively weak magnetization zones are estimated beneath the eastern coastal zone and its o#shore area, and beneath the western flank of the edifice. (.) A relatively high magnetization zone is estimated beneath the northern o#shore area. Comparison of the -D-magnetic structures derived from surveys in +321 and +333 made apparent that the zone of relatively weak magnetization intensity had spread wider from +321 to +333, with the most demagnetized zone at the base (* m-** m b.s.l.) in about 1/* m southeast from the center of the crater. The -D-magnetic structure in ,**+ indicates the demagnetized zone is considerably restricted in comparison with that in +333. This feature may suggest that the thermal demagnetization has been depressed after the eruption in ,***. These results suggest that the detection of demagnetization process associated with volcanic activity might be promising by conducting repeated airborne magnetic surveys. Key words : -D magnetic inversion, magnetic tomography, Miyakejima, magnetic anomaly, airborne geophysics. +ῌ ῐ ῎ ῑ ῏. *`v 2

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(143) ,*** . 167. Fig. -. -D magnetic structure of Miyakejima Volcano derived from +333 survey data. The left is the first layer (h-** m a.s.l.), the middle is the second layer(-** mh* m), and the right is the third layer (* mh -** m), respectively. Contour interval is + A/m. Dark area means negative deviations (J) of magnetization intensity. (a) The magnetic structure at the first iteration of CG method. (b) The magnetic structure at the .th iteration. This result is cited in Table -. (c) The magnetic structure under the minimum standard deviation of misfit at the 2th iteration..

(144) 168. . Fig. .. Final residuals by the two-step analysis of +333 data. (a) Input anomalies are residuals shown in Fig. ,. (b) Calculated anomalies. (c) Final residuals. Contour interval is Contour interval is /* nT. Positive values are shown by solid lines and negative ones by broken lines.. Fig. /. -D magnetic structure derived from direct analysis of total intensity magnetic anomalies in +333. This shows the the minimum standard deviation of misfit solution under the condition of *.* A/mJ (magnetization intensity),*.* A/m. The GFR and standard deviation of misfit are ..*1 and 1,.* nT, respectively..

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(163) bcd - " ,*** , m(*~A. 171. Fig. 1. The profiles of magnetization intensity deviations( J) along the section shown in Fig. 0. The upper most : contribution factors of Jk given by ATAk in equation (0). Unit is (nT),/(A/m),. The values of +321 are shown by asterisks, of +333 by broken line, and of ,**+ by solid line, respectively. The middle : comparison between the results in +321(solid line) and +333(broken line). The error bars are calculated by equation (2). The lower most : comparison between the results in +333 (broken line) and ,**+ (solid line). The error bars are calculated by equation (2)..

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(184) (,**,) Aeromagnetic survey data rept. of hydrographic observations. Series of astronomy and geodesy, -0. Kodama, K. and Uyeda, S. (+312) Magnetization of Izu Island with special reference to Oshima volcano. J. Volcanol. Geotherm., Res., 0, -/-ῌ-1-. (+33+) !"#$%&'# () ..) /-ῌ0,. Menke, W. (+323) Geophysical data analysis : discrete inverse theory, revised edition. International Geophysics Series, ./, Academic Press, Inc. *+ (+32.) ,- !. ./) ,3) .01) S+ῌS+/. 234 25!672"#$28/ %2 & '2()* (+32.) +32- + +* 9. :,;<=>-? ./) .01) ,3) S+*+-S++,. . (+32.) /.@0ABC1DE ./) ,3) .01) S+0-S,- !23244F,2! 5267GH2I7J82 &7K2LMNO2PO9:2;4<Q (,**+) ./ ,*** +.RST/=U>@AV T W) ++*) +02ῌ+2*. Nakatsuka, T. (+33/) Minimum norm inversion of magnetic anomalies with application to aeromagnetic data in the Tanna area, Central Japan. J. Geomag. Geoelectr., .1, ,3/ῌ-++. Okuma, S., Makino, M. and Nakatsuka, T. (+33.) Magnetization intensity mapping in and around Izu-Oshima Volcano,Japan. J. Geomag. Geoelectr., .0, /.+ῌ//0. 34 2 6 X 2 5!67 2 Y/Z[ 2Zlotnicki, Jacques2\?]2^\@] (,**+) =>2A_ àBbcdCef!./ ,*** +gD W) ++*) ,,0ῌ,... Smith, W.H.F. and Wessel, P. (+33*) Gridding with continuous curvature splines in tension. Geophysics, //, ,3-ῌ -*/. EF h2ij24klm2n!o8 (+323) =>pq r) 1) .3ῌ//. EF s2G! t2u! H (,**+) ÎJvw% xyKz{|L?;<M}NO~ ,**+ +J rP0 Talwani, M. (+30/) Computation with the help of a digital computer of magnetic anomaly caused by bodies of arbitrary shape. Geophysics, -*, 131ῌ2+1. Q64F]2EFR (+332) ./ 1*** + ., ./) .-) +.3ῌ+00.. 173. n!o82S+9]2872OT2& (+32-) =>BUbc<VW./ =>pqA=>X ) +2) -1ῌ0.. n!o8 2 6Y 2 87 2 EF 2 ij 26

(185) !9Z (,**+) ,*** +.[ =>XT=> %¡¢T ) -1) +3ῌ-0. Utashiro, S., Takigawa, U., Oshima, S. and Kondo, T. (+31,) Aeromagnetic and marine magnetic survey around Japan. In Researches in Hydrography and Oceanography (Commemoration publication of the centenary of the Hydrographic Department of Japan), Tokyo, 1/ῌ+++) M£ Uyeda, S. and Richards, M. (+300) Magnetization of four Pacific seamounts near the Japanese Islands. Bull. Earthq. Res. Inst., .., +13ῌ,+-. ῌ ῍ Fig. A+ ¤.@\]vJ * m bcÎ. -** m ¥W - _¦ §! + km, ¨`©= ;{z!A;{ª«;<=>pq¬§! ®®¬) ./ a=?¯J¤ +*.* Aῌm, =?b°¤c± .0 J) d± * JA;{ª«;{z! Fig. A, ¤ Fig. A+ =>pqw%xe²d` /* nT. ³´µ"¶yf·;<w%xyg1w%xA;{) h ij ¬<¸_=?¯Jy¹;<º. Fig. A+. Test data of magnetic anomalies of Miyakejima Volcano used for verification of the method. The anomalies are calculated based on the following assumptions. Magnetization intensity is +*.* A/m, declination and inclination of magnetization are *.* and .0.* in degree unit respectively. The block located to the southeast of the crater, whose horizontal dimension is + km by + km and top and bottom are * m, and -** m b.s.l. is assumed to be demagnetized. Contour interval is /* nT and thick lines are plotted every /** nT..

(186) 174. n o p q. Fig. A,. -D magnetic structure derived from a direct analysis of the anomalies shown in Fig. A+. intensities are shown by contour lines in + A/m interval.. Magnetization. Fig. A-. -D magnetic structure derived from the two-step analysis of the anomalies shown in Fig. A+. Deviations of magnetization intensities from +*.*A/m are shown by contour lines in +A/m interval..

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