Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally...

10
ῒΐ /, (,**1) . ,++,,* 腉腏腎腊腍腁腌腇腃腂腈腀 - 腆腄腇腃腅腋 臑臰膙 臁膪膅 腊腊 膤臝膰膃 腊腊 臋膿 腊腊 ,**0 +, +. 臸臇,**1 1 . 臸臇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 .*to +., 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 (*./ 腅腈+./ 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 (*./腅腈+./ 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 (Nansei- Shoto), airborne geophysics +ῐ῎ῑ῏ 臺臖至腠膽至腟臷臚,** km 腞膀臨腖腫膑膋膿 (Fig. +)膬膻腛腦臲腰腳腻腼腷腹臃膋腧腶腺腮 腴臃膋腭膥腪腕腚腍腫膿臡腠臆腞 . 腙腟膋膿腑臉 臜腕腚腛腒腘膶膋膿腛腽膿膖臅腟膖腜膋膿膺膣腐腩膧臘腔腬腚腍腫 +32- ; +30. ; Tiba, +323腇腁 膿臡腟膹膴腠膮膐腟臷133 m 腖腫腟膎臮臧膧腭膾臍腖腫腜臓臺臖至腟膗 臮腠臕臒 1** m 膠腞腌腪* m 膠腠膎膈腟臧 膧腭膱腖腫腜腢腣 / 腞臞臵腖腫臺臖至腞腙腍腚腟臧臂膘腞膔腕腘臫膸腠腛腟臧臂膘膓臟膨膊腜腗腟膍臛膨膊腑臠臶 (+3/.) 腞腨 膷腔腬腚腍腫腗腬腞腨腬腡膿臡腟膟臂膉膞臲 腜腕腚2.* Am 腟臦腑膷腔腬腚腍腫腕腐腕腓腟 膨膊腠臎腛腟膭腩腬腘膓臟膨膊腞腨腫臔臭臦腛腌腫腘 臙膏腝臂膉膞臲腟臔臭腟腘腥腞腠膎膂腭膕腥腘膋膿 臧膧腭腮腵腄腖腫膲膢臂膘臟腲腄腱腑臼腛腌腫膃臻腞臂膘膁臏腟膫膄腜腝腫臂臗膳腠腯腸腄腹腄臯 膇臲腾臎腞臥腖腫腜臂臗腭臄腎腑臺臖至腟腨腎腝 腽膿膖臅腛膯臘腔腬腫膋膿腛腦臂臗膳腑膕腤腬腚腏 腊腊 //2***. 臢膼臀臊膚膡臬膛致 ,+0//*0 膝膌 +*.***. 致膜臱自膆膡臩臧 /-+ 腌腃 Correspnding author : Ueda Yoshio e-mail : yueda+*@hotmail.com

Transcript of Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally...

Page 1: Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally small in magnetization intensity (ˆ*./˝ˆ+./A/m) except the high magnetization

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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 .*� to

+.�, 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 (Nansei-

Shoto), airborne geophysics

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Correspnding author : Ueda Yoshio

e-mail : yueda+*@hotmail.com

Page 2: Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally small in magnetization intensity (ˆ*./˝ˆ+./A/m) except the high magnetization

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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.

Fig. +. Location of Suwanose-jima island

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Page 3: Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally small in magnetization intensity (ˆ*./˝ˆ+./A/m) except the high magnetization

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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 by

solid and dotted lines, respectively.

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Page 4: Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally small in magnetization intensity (ˆ*./˝ˆ+./A/m) except the high magnetization

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�, Case-+6"��#���²9 Z[#���²³ª

Table +. Results of uniform magnetization analyses

of Suwanose-jima volcano.

Dr : degree from North to East, Ir : degree from hori-

zontal 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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Page 5: Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally small in magnetization intensity (ˆ*./˝ˆ+./A/m) except the high magnetization

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A�m-./.��� !���01 ������23*�04�*+���*,536789����:;7<���� !�$"1 =�����>��,?� @ABCD1 +32-E F'G�1 �������H�,IJ67�*K��� !"��LM��N��OPQ*, +#. A�m, ��RST�U�� ����*, -#1 A�m, �7�VW��X�YZ��*, +.0#,.0 A�m�$���1 [\]^0_`9a���b�cdef����'�� !�gghi>��,?� @S�j��1 ,**-E

Fig. .�����kd7��l������mn�," +/* nT�op�����0qfd7� ���l'����rs"��0�t'�b2ucdv789��f�wx�a��:;d7� ���l'y�t'�b2u�rs��8"1 ���23���+��� !�z9{��|��}~�,����'(0:;d7� ���;<1 ��|���b2u�y�t���N,`7<1 ���l'���|��2u�������0���'�

.�, ����������,"1 Fig. .�������$����.���1 �����������G�i,U��1 � +�c

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Fig. /"���$����$��cdef����� (s)� CG�����V����� ��¡¢���� ���� s"£¤�¥¦,"§¨�©W��01?�� cd" s"ª«t��$�'� ��,"1 s

0ª«t��¬��­®,�¯ @�V����� 0+*E �£°¯��� ���l��8ef�y�t��±�²�¯³��� Table ,�´�f� 'g1 ¯³����;�µ¶� @� .�E �·¸��¹���f1 µ

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 are

shown every +** nT.

Fig. /. Standard deviations (s) and GFR (goodness-

of-fitting ratio) between the observed and cal-

culated anomalies againt iteration numbers in CG

solution.

������º�b�� -»¼�b2u 215

Page 6: Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally small in magnetization intensity (ˆ*./˝ˆ+./A/m) except the high magnetization

���������� ����������������� ����� ! "#$%&�'()*�)*+,-./0 ,.211-.+, A�m 2)*0 -.** A�m3 �4�,5+6*7/08����9���:;0 .*�! 8�

�����:;�0 +.���<� =>��?@AB��CD0E��@! ���0GFRF@GH�8�����IJK���L�MN��� 8�����IO�����0! 8P+�0! Q .��,-./RS@

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 of

each block.

*, : a percent value of the total summation of the absolute value of the product of

magnetization deviation and volume of each block against a bulk magnetization.

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Page 7: Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally small in magnetization intensity (ˆ*./˝ˆ+./A/m) except the high magnetization

� -����������Fig. 0 ������������ ������ ������ !"#� ������$�%&'��(����)*�� +,-./'012� ��3���45 ��678�9:;<=>�45�?0@��ABC�� D#� Fig. 1� EFGHI�J��)KLM45$ � Fig. 2� EFGHI NO�P�)*QRST U�V�W!"#�

Fig. 1�� +�1��� ,� XYZ�)KLM�45 !"#V�[\�� ]^Z�_`C&ab`cd ef3� + A�mgM�)KLM�hiYAjk&'����h)KLM�Z4 � Fig. 2�ST$[Vl&C��(�EFGHI�mYZ�� -��V�n[@�� ���(�ef3�)KLM�hiY�o@0 � ��'&�Z4 Qp��q]rst�u@� �vMw xy"#z{)QR xy"#V�� @( ,o�|}~A

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 (-** m�h�* m), the lower left is the

third layer (* m�h��1** m), and the lower right is the fouth layer (�1** m�h��+,** 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.

EFGHI��)*�� -��)*QR 217

Page 8: Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally small in magnetization intensity (ˆ*./˝ˆ+./A/m) except the high magnetization

������ ��� �� ����������������������������� !�"#�$% &'()* +333+ �,-.�/0��1��2���345�$%�6���7 �8�9#:;�<=>?@�:��,-AB�CD%�������E>F� ��GH�HI J���KL�M��1�NO��"P; &QR)* ,**.+ �,-�/0��SST�"#��UV��WX��,-.�� %Y�V�E�� Z -[�-\]�� ��^��_����`������a��b"c�-\]�d.��O���� $�:Fefa�g��Z .[���O��"P; ^��_���$����hiV�jgk�lm��<�n�8�Am!�"#�%� !���^��_��M.��opVe-qVAB6NO�rO stiu�v6wx="st=ryz6 Table -�{=r� |r Fig. 3�}�yz�:�-\]� ~�E�� p����-\]� ,.12 A�m%e; Table

,�styz%��"���!�e�"#�� $�CD�M.��Z -[�-\]�^��_���.��

��"�!#rO op���V-\]%="��=rrO�E�� Fig. 3�-\]� ��Z -[�^��_�����������b"�hiV�c�-\].�� 5�� }���!�������/���g��%�e�:F�E�� }�*�M.��-\]�hiV���-\]�d6{5%$>��� �.����" +.*�+.* A�m�-\]��!#� $�:Fe���g�%�����Z .[���O����M.���$�:Fe��6:;K�V�{5rO M�L 1** m6g�U%= }����� 6���a�������� /** m�������np61�=������-\]6K� CG!�NOryz6 Fig.

+*�{=r� $�~��� M.��� +�, A�m��-\]�E; �� ^��_��������.&M.�� <+ �� . A�m�¡�-\]6{5$%�¢����¡�£�r:F� ^��_��M.�6Am5�Z -[�P#"�^��_���.��¤"!��¥n¦§��n�8:;�SS¨©§��<�n§�Am!�"#�ª#k�� !��� $��«#"�W¬�¦­®¯��:�K�V°±�:�²³�$%�E>F�

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.

Table -. Results of analyses by multi-block models

for the zone including the base floor of Suwanose-

jima island.

Target area of analysis is the zone enclosed by the

rectangular whose coordinates of left-down and righ-

up 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 the

absolute value of the product of magnetization

deviation and volume of each block against a bulk

magnetization.

´(µ&)�'¶·¸)¹º»�)�n ¼218

Page 9: Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally small in magnetization intensity (ˆ*./˝ˆ+./A/m) except the high magnetization

/� � ����������� �����������

��������� -.+. A�m �� �� !�"#�$%�&���������'(��" #��)*+,-����.��������/ -.** A�m ���" 01��23�45�6�7�4�89:;<=%�>?�@��4A&��� (non-uniformity of magnet-

ization) �B + CDEB - C!�������F��G�'(�� .*��@�H" ��IJ�@�IKC LB .

CM NOP� +.� �� IKC'(<QGRS�T �'(U�VW�XYZ��H" [\S����

H]�[D�" ^_�`8a:bc<d�efHgh�@ij ������IJk�#J 1** m lm<@n" #J

$%�01���.� +,, km- �� #J������������op#�qrs +.*t,.* A�m u��" ����vw� (-.*t/.* A�m) xyz����H{[ T�T DE" �����,|��}~01�����dn����<��01��� ��01���@��T Hv��S� ��_" ������01���op#���<��dn��I�<��01��H��" op#�����A<����

Fig. 3. A -D magnetic structure of Suwanose-jima volcano derived from aeromagnetic anomalies. The captions

of 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.

������+�4�� - ���4�� 219

Page 10: Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally small in magnetization intensity (ˆ*./˝ˆ+./A/m) except the high magnetization

������������ ���������������������������������� ��!"#"������$�� %&��

� �������'(�)���*�+�,+��-./0��1�2�'(3�� LA1*+45�5�678�9:��;<�=���>?� >�� '(�)��@AB�C� !"D*��E+�,+�0#F$'%��G�HIJ�KLE�&M��N�� �'(�O)�*+&�P�,-Q� R./SQTU70V*+�WX120VY9��� 3Z[Z�K4�\�]^_$=`� ab�+�,+�� c�E;<�=���>?� $T� �'(�56�d���GMT (Wessel and

Smith, +33+)�e7�>���

� � � �

Bjorck, A. and Elfving, T. (+313) Accelerated projection

methods for computing pseudo inverse solutions of sys-

tems of linear equations, BIT, +3, +./�+0-.

8fghi9 1:ij;<- (+321) =k>�lm?@��nA�BoC@�nDpEpqrst� 2,� -1*�-2+.

FGu:iv�HI (+32-) wx�yz{|>����������}� ��� ,2� +*+�++/.

0#F$ (,**-) �������)C� !$~��J� �i��K�i8�h� (+333) ��������T���LM��� �N8rO����C$� .,B-+� ++�+2.

Macmillan, S. and Maus, S. (,**/) International geomag-

netic reference field - the tenth generation. Earth, Planets

and Space, /1, ++-/�++.*.

Menke, W. (+323) Geophysical data analysis : discrete in-

verse theory, revised edition. International Geophysics

Series, ./, Academic Press, Inc.

v�HI (+30.) wx�yz{|������������ +30*C����+E� ��� 3� /1�0,.

PQRS (+3/.) ���������kE!rT����:$�� �U�V�$� +� +-�,3.

�� �i��12iW/[� (,**.) ��������T+E ,**-C ++��X?��Y��Z����[� �N8rO����C$� .1B, 11-�112.

Smith, W.H.F. and Wessel, P. (+33*) Gridding with con-

tinuous curvature splines in tension. Geophysics, //, ,3-�-*/.

\/ �i]� �i�� ^ (,**+) �_�`�@AB�7+E a¡��b�¢c�0d��� ,**+C�0#eO£¤rs¥¦f§V�

Tiba, T. (+323) Petrochemistry of the volcanic rocks from

Kuchino-shima, Nakano-shima and Suwanose-jima,

Tokara Islands. Mem. Natn, Sci. Mus., Tokyo, ,,, 1�+3.

¨�©- (,**0)-g�� -ª«�)¬h� ,***CY��U�­�i¡� ��� /+� +0+�+1..

¨�©-ij®k�iPl¯�°i±²³mi8�´:(+32-) '(�)���µ�¶L��n�f·�����)¸¹��)¬h�`o��$~� +2� -1�0..

Uyeda, S. and Richards, M. (+300) Magnetization of four

Pacific seamounts near the Japanese Islands, Bull.Earthq.

Res.Inst., .., +13�,+-.

Wessel, P. (+323) XOVER: a cross-over error detector for

track data, Computers & Geosciences, +/, ---�-.0.

Wessel, P. and Smith, W.H.F. (+33+) Free software helps

map and display data. EOS. Trans., AGU, 1,, ..+�..0.

�0V*+ WX12�

Fig. +*. Horizontal distribution of magnetization in-

tensities of Suwanose-jima volcano. Block models

with horizontal length of /** m in north and

east directions above the depth of 1** m are

assumed to be magnetized in the present fild

direction. A calculated GFR is +*.2 and the

standard deviation of misfits is +1., nT, respec-

tively. Contour interval is + A/m.

¨�©-iPl¯�°iºp»:iP� ¼220