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,**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 thealtitude of +*00 m by a proton magnetometer installed on a craft of YS-++. Magnetic anomalies of total force arecalculated 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 recognizedover 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 themagnetization 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 ismagnetized in the same direction as in the uniform model with a variable magnetization intensity, is conducted toderive an internal magnetic structure of the ediﬁce. This result makes it apparent that the volume weighted meanmagnetization 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 ῏

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212

Fig. +.

Location of Suwanose-jima island  -  

Fig. ,. Topography of Suwanose-jima island and its
adjacent sea areas. Contour interval is +** m.
T : Mt.Tondati-dake, O : Mt.On-take, N : Mt.
Negami-dake ; M : Makodai plateau.

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213

Fig. -. Track lines and total intensity magnetic anomalies at the altitude of +*00 m. Left : Track lines. Right :Magnetic anomalies of total force. Contour interval is /* nT. Positive and negative anomalies are shown bysolid and dotted lines, respectively.

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Table +. Results of uniform magnetization analysesof Suwanose-jima volcano.

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Dr : degree from North to East, Ir : degree from horizontal to downward. Target area of analysis isrestricted near Suwanose-jima island enclosed by therectangular zone whose coordinates of left-down andrigh-up corners are (0., 0/) (1., 11), respectively.
The coordinate values are shown in Fig. ..

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Fig. .. Observed data, calculated data and residuals in the uniform magnetization analysis. Contour interval is,* nT. Positive and negative anomalies are shown by solid and dotted lines, respectively. Thick lines areshown every +** nT.

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Fig. . Fig. /. Standard deviations (s) and GFR (goodnessof-ﬁtting ratio) between the observed and calculated anomalies againt iteration numbers in CG
solution.

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216

Fig. 0. Observed, calculated and residuals in the non-uniform multi-block model. Contour interval is ,* nT.
Positive and negative anomalies are shown by solid and dotted lines, respectively. Thick lines are shown every+** nT.

Table ,.

Results of analyses by multi-block models for the residuals in case +.

Target area of analysis is the same zone in Table +.
*+ : total summation of the product of magnetization intensity (J) and volume ofeach block.
*, : a percent value of the total summation of the absolute value of the product ofmagnetization deviation and volume of each block against a bulk magnetization.

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217

Fig. 1. A -D magnetic structure of Suwanose-jima volcano derived from aeromagnetic anomalies. The upper left
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are (0,, 0-) (10, 13), respectively.

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218

Table -. Results of analyses by multi-block models
for the zone including the base ﬂoor of Suwanosejima island.

Fig. 2. A EW cross sectional view of -D magnetic
structure in Fig. 1. The cross section from west
to east at the N-S coordinate of 1+.* is shown.
The estimated error bars of magnetizations
along the proﬁle are shown in the upper part,from the ﬁrst to the third layers, respectively.

Target area of analysis is the zone enclosed by therectangular whose coordinates of left-down and righup corners are (0+, 0-) (2,, 2*), respectively. The
coordinate values are shown in Fig. ..
*+ : total summation of the product of magnetization
intensity (J) and volume of each block.
*, : a percent value of the total summation of theabsolute value of the product of magnetization
deviation and volume of each block against a bulk
magnetization.

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219

Fig. 3. A -D magnetic structure of Suwanose-jima volcano derived from aeromagnetic anomalies. The captionsof ﬁgures are the same as described in Fig. 1. The analyzed region is the rectangular zone whose left-down
and right-up corners ore (/3, 0+) and (2., 2,), respectively.

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220

ῌ ῑ ῐ ῍Fig. +*. Horizontal distribution of magnetization intensities of Suwanose-jima volcano. Block models
with horizontal length of /** m in north andeast directions above the depth of 1** m areassumed to be magnetized in the present ﬁld
direction. A calculated GFR is +*.2 and thestandard deviation of misﬁts is +1., nT, respectively. Contour interval is + A/m.

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Bjorck, A. and Elfving, T. (+313) Accelerated projection
methods for computing pseudo inverse solutions of systems of linear equations, BIT, +3, +./ῌ+0-.
8fghi9 1:ij;<- (+321) =k>lm?
@nABoC@ nDpEpqrst 2, -1*ῌ-2+.
FGu:ivHI (+32-) wxyz{|>
}  ,2 +*+ῌ++/.
0#F$ (,**-) )C !$~J iKi8h (+333) 
N8rOC$ .,TLMB-+ ++ῌ+2.
Macmillan, S. and Maus, S. (,**/) International geomagnetic reference ﬁeld - the tenth generation. Earth, Planets
and Space, /1, ++-/ῌ++.*.
Menke, W. (+323) Geophysical data analysis : discrete inverse theory, revised edition. International Geophysics
Series, ./, Academic Press, Inc.
vHI (+30.) wxyz{|
 +30* C+E  3 /1ῌ0,.
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T+E ,**- C ++ X?YZ[ N8rOC$ .1B, 11-ῌ112.
Smith, W.H.F. and Wessel, P. (+33*) Gridding with continuous curvature splines in tension. Geophysics, //, ,3-ῌ-*/.
\/ i] i ^ (,**+) _`@A
B7+E a¡b¢c0d ,**+ C
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Tiba, T. (+323) Petrochemistry of the volcanic rocks from
Kuchino-shima, Nakano-shima and Suwanose-jima,
Tokara Islands. Mem. Natn, Sci. Mus., Tokyo, ,,, 1ῌ+3.
¨©- (,**0) -g - ª«)¬h ,*** CYU ­i¡  /+ +0+ῌ+1..
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Uyeda, S. and Richards, M. (+300) Magnetization of four
Paciﬁc seamounts near the Japanese Islands, Bull.Earthq.
Res.Inst., .., +13ῌ,+-.
Wessel, P. (+323) XOVER : a cross-over error detector for
track data, Computers & Geosciences, +/, ---ῌ-.0.
Wessel, P. and Smith, W.H.F. (+33+) Free software helps
map and display data. EOS. Trans., AGU, 1,, ..+ῌ..0.
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