ῒ῔  /+  (,**0) -  +1/ῌ+2+ ΐ ῕,**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 surveylines 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 wasestimated, 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, andKami-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 todebris 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, hydrothermalalterations+ῌ ῐ ῎ ῑ ῏p.- Yellowstone )qE&\ !"#$%&'( )*+,-./$%* &]*tu,-./23:%J0",1"23 456789, :;< =>?@ABCDEFGHI3:%JKL Okuma et al.

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3;F%J 0E;Z vw /*ῌ-*and Morgan (,**,) AmWnd< =>S98no n EM)~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.
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Aso Volcanological Laboratory, Graduate School ofScience, Kyoto University, Kumamoto 203ῌ+.*., Japan.
Corresponding author : Ayako Okuboe-mail : ayako@svo.dpri.kyoto-u.ac.jp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 thelocations 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. ..

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Major faults are taken from the Research Groupfor Active Faults of Japan (+33+). The areabounded by broken lines indicates the studyarea.

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Table +. Outline of the helicopter-borne aeromagnetic survey in the western Unzen region, Kyushu,Japan.

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utilized in this helicopter-borne aeromagneticsurvey.

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179

Fig. .. (a) Total magnetic intensity anomalies(IGRF residuals) of the western Unzen region
upward continued to the altitude of +/** mASL. Contour interval is ,* nT. (b) Terraincorrected magnetic anomalies of the westernUnzen region at the altitude of +/** m ASL.
Contour interval is ,* nT. Solid lines indicate
normal faults, and Unzen hot spring is shown asa broken line after Hoshizumi et al. (,**,) andthe 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 theinput data of inversion. The mean magnetization of ..* A/m calculated from the terrain correction was added to theresult. Illumination indicates topographic undulations. Contour interval is *./ A/m. See also Fig. ..

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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.
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Nakatsuka, T. (+33.) Aeromagnetic anomalies over thearea of Unzendake volcano. J. Geomag. Geoelectr., .0,181/,3ῌ/.*.
Nakatsuka, T. (,**-) Library software for geophysicaldata processing and representation. Geological Survey
Japan Openﬁle Report, no. .**, 20 p.
NEDO : i /  ¡G¢ (+322)
H¡G£¤I "ST No. +/ ¥% +*0* p.
Okubo, A., Tanaka, Y., Utsugi, M., Kitada, N., Shimizu,
H. and Matsushima T. (,**/) Magnetization intensity
mapping on Unzen Volcano, Japan, determined fromhigh-resolution, low-altitude helicopter-borne aeromagnetic survey. Earth Planets Space, /1, 1.-ῌ1/-.
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Earth Planets Space, /2, ,-ῌ-+.
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(,**.) Applications of paleomagnetism in the volcanic
ﬁeld : a case study of the Unzen Volcano, Japan. EarthPlanets Space, /0, 0-/ῌ0.1.
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