ヘリボーン空中磁気探査から推定される雲仙西部地域の磁化強度分布

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(4). . ,**0 . +- ,**0 / +, . Magnetization Intensity Distribution in the Western Unzen Region, Kyushu, Japan, Inferred from a Helicopter-borne Aeromagnetic Survey Ayako OKUBO῍, Yoshikazu TANAKA῍῍, Tsuneomi KAGIYAMA῍῍, Mitsuru UTSUGI῍῍ and Wataru KANDA῍ A helicopter-borne, aeromagnetic survey was performed in October ,**. over the western Unzen region, Kyushu, Japan. The survey was flown at an altitude of about ++** m above sea level along north-south survey lines and east-west tie lines, spaced /** m and +*** m apart, respectively. The observed aeromagnetic anomalies were reduced onto a constant surface of +/** m above sea level. Then, a magnetization intensity distribution was estimated, assuming that it varies only laterally. The characteristics of the magnetization intensity distribution are summarized as follows ; (+) The hydrothermally altered areas of Unzen hot spring, Ii-Dake, Shimo-Dake, and Kami-Dake show magnetization lows, which indicates that high temperature fluids associated with hydrothermal activities reduce the rock magnetization. These fluids are likely to migrate through E-W trending faults developed inside the Unzen graben. (,) Magnetization lows lie outside of the Unzen graben, corresponding to debris flow deposits and pyroclastic-flow deposits of the Older Unzen (/**ῌ+/* ka). In contrast, magnetization highs are distributed in the northern part region of Azuma-Dake and Hachimaki-Yama and around TakaiwaYama, where the lava of the Older Unzen (/**ῌ-** ka) is exposed. (-) Magnetization lows, corresponding to fan deposits of the Older Unzen (/**ῌ-** ka), are distributed along the Chijiwa fault. Key words : aeromagnetic survey, western Unzen region, magnetization intensity distribution, hydrothermal alterations. +ῌ ῐ ῎ ῑ ῏. p.- Yellowstone )qE&\. !/rs[E). !"#$%&'( )*+,-./$%. * &]*tu,-./23:%J. 0",1"23 456789, :;< =>?@. vwM PxyP z{/|}2 vw)~". ABCDEFGHI3:%J KL Okuma et al.. LI%/$% FI {. (+33.) M NOPEQ R< =>S98.-=TU. D23:%J 3 vwM )~E . VWX,YZ [E&\]* ^_^

(5) `/a%&. !/a%"/a%,RA-2. b=Ecde,fg2;J hi/j:3kl; Finn. 3;F%J 0E;Z vw /*ῌ-* . and Morgan (,**,) AmWnd< =>S98no n. EM)~Ei/lFWX23:%A . 23+ῌ+.+3 ¡¢P£¤P¥ +1,,ῌ+3 ¦§¨©ª«¬® !ª«¯°89 Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, Sakurajima-Yokoyama, Kagoshima 23+ῌ+.+3, Japan.. 203ῌ+.*. ±²³´µ¶³´· ¦§¨¨¸¨ª«¹®)º&¨ª«»¼ ª«¯°89. Aso Volcanological Laboratory, Graduate School of Science, Kyoto University, Kumamoto 203ῌ+.*., Japan. Corresponding author : Ayako Okubo e-mail : [email protected].

(6) 176. \³!´µ!¶:"#!·7¸¹!º!»¶ ¼. Fig. +. Topographic map of the western Unzen region. The x and y axes indicate the Easting and Northing coordinates of the UTM zone /, (central meridian²+,3 degrees E), respectively. Contour interval is /* m. The DEM data with /* m mesh (Geographical Survey Institute, +333) was employed to produce this map. Bold lines indicate track-line paths of the aeromagnetic survey. Solid circles labeled as UZ-+ to -1 show the locations of drilling sites (NEDO, +322 ; Fig. .). I-D : Ii-Dake ; S-D : Shimo-Dake ; K-D : Kami-Dake ; UZ-HS : Unzen hot spring ; OB-HS : Obama hot spring ; FG : Fugendake ; MI : Maidake ; KN : Kunimidake ; MY : Myokendake ; NO : Nodake ; SA : Saruba-Yama. See also Fig. ... -*ῌ+/

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(70) . 179. Fig. .. (a) Total magnetic intensity anomalies (IGRF residuals) of the western Unzen region upward continued to the altitude of +/** m ASL. Contour interval is ,* nT. (b) Terraincorrected magnetic anomalies of the western Unzen region at the altitude of +/** m ASL. Contour interval is ,* nT. Solid lines indicate normal faults, and Unzen hot spring is shown as a broken line after Hoshizumi et al. (,**,) and the Research Group for Active Faults of Japan (+33+). Illumination indicates topographic undulations. TK : Torikabuto-Yama ; KU : Kusenbudake ; YA : Yadake ; TI : Takaiwa-Yama ; AZ : Azumadake ; HA : Hachimaki-Yama ; CH FT : Chijiwa fault ; TA FT : Take fault ; KA FT : Kanahama fault ; TI FT : Takaiwayama fault. See also Fig. +.. Fig. /. Result of the magnetization intensity mapping. The terrain-corrected anomalies (Fig. . (b)) were used as the input data of inversion. The mean magnetization of ..* A/ m calculated from the terrain correction was added to the result. Illumination indicates topographic undulations. Contour interval is *./ A/m. See also Fig. ...

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(76) Finn, A. C. and Morgan, L. A. (,**,) High-Resolution aeromagnetic mapping of volcanic terrain, Yellowstone National Park. J. Volcanol. Geotherm. Res., ++/, ,*1ῌ ,-+. Grauch, V. J. S. (+321) A new variable-magnetization terrain correction method for aeromagnetic data. Geophysics, /,, 3.ῌ+*1. IJ·± r¡²i³r´ µ¸A o.

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