諏訪之瀬火山の地磁気異常と３次元磁気構造

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(3) . . ,**0 +, +. ,**1 1 . . Aeromagnetic Anomalies and a -D magnetic structure of Suwanose-jima Island Yoshio UEDA῍, Ken-ei ONODERA῍῍, Kouiti KUMAGAWA῍῍ and Kaoru KOYAMA῍῍ Airborne magnetic surveys of Suwanose-jima Island were conducted on August, ,. and ,/, +333 at the altitude of +*00 m by a proton magnetometer installed on a craft of YS-++. Magnetic anomalies of total force are calculated by subtracting IGRF-+*. The negative peak (-,0 nT) is observed over the north to northeastern coastal zone and a positive peak (,., nT) is over the southeastern slope. Another positive peak is also recognized over the southern tip of Island. The volcanic edifice is approximated by the assemblage of the prismatic bodies for the -D magnetic analyses. A -D model analysis based on an uniform magnetization assumption shows the magnetization intensity of -.+. A/m corresponding to a magnetization direction polarized in the direction defined by declination of * and inclination of .,./. A -D non-uniform magnetization analysis, where each block is magnetized in the same direction as in the uniform model with a variable magnetization intensity, is conducted to derive an internal magnetic structure of the edifice. This result makes it apparent that the volume weighted mean magnetization intensity is ranging from ,.21 to -.+, A/m, and a non-uniformity of magnetization from .*ῌ to +.ῌ, by taking into consideration of the base layer underlain by the volcanic edifice. Besides, the following features are also made apparent. (+) Relatively low magnetization zones (*./ +./ A/m) are recognized in the southwest of Mt Ontake. This low magnetization zone extends to the base layer between *m and 1**m below sea level. This zone is thought to be linked with the conduit of Suwanose-jima Volcano. (,) Relatively strong magnetization zones (*./+./ A/m) are recognized in the southeast of the summit area and they further extend northeastward to the east o$ng of Suwanose-jima Island. (-) The base layer between * m and 1** m b.s.l. is generally small in magnetization intensity (*./+./ A/m) except the high magnetization zone denoted in ,). The above features may imply that the base layer is composed of more siliceous rocks than pyroxene andesites forming the present volcanic edifice. Key words : Suwanose-jima, magnetic anomalies, -D magnetic analysis, active volcano, Ryukyu-Islands (NanseiShoto), airborne geophysics. +ῌ ῐ ῎ ῑ ῏. . !". &HDC#drs&tBMuv! w #$% ,** km &'()*+,. -#drsxyz{R|#}~z{I (+3/.) &. - (Fig. +) ./-0123456789,:;<=. AYZCD*E |Z&Z F# r 1. >9,?@A BCD*E F!G& . H#,IJ. RBC 2.* Aῌm #IYZCD*E BUB #. KBC-LM N,- OPQ# PR,ST

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(39) "!" , D6{|'( E ῎ ῌ+ FῌA , ῍, AJk,῏ i j9k. Table +. Results of uniform magnetization analyses of Suwanose-jima volcano.. ,. (-). &*20" ,B@ CD~a>' 69 (+) D]1 !"#$%"} *Z[c"} dfi " , ,!"#$%" }" , ,D6{|'( dfi,FῌA,ik · dAJk,. (.). k. (.) DlY) . !"#$%"}. d (AJk) +') h6'( ))6,)|"} Z[ 9 #$%"!"h#8*"$. Dr : degree from North to East, Ir : degree from horizontal to downward. Target area of analysis is restricted near Suwanose-jima island enclosed by the rectangular zone whose coordinates of left-down and righ-up corners are (0., 0/) (1., 11), respectively. The coordinate values are shown in Fig. ...

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(63) EFGHI)* - )*QR. 217. Fig. 1. A -D magnetic structure of Suwanose-jima volcano derived from aeromagnetic anomalies. The upper left is the first layer (h-** m a.s.l.), the upper right is the second layer (-** mh* m), the lower left is the third layer (* mh1** m), and the lower right is the fouth layer (1** mh+,** m b.s.l.), respectively. Magnetization devations from mean value (-.+. A/m) are shown in contour lines with interval of *./ A/m for the first to third layers. For the fouth layer, the magnetization contrast in the layer is shown in the same contour intervals. The analyzed region is the rectangular zone whose left-down and right-up corners are (0,, 0-) (10, 13), respectively.. -

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