13
56 (2003) 2 267279 膸膗臧臤腎腨腣腗腼至臞膮臬腴腮腸腳腺膆腵腹腄腱腟 膓膵腍腧腢臏膸臜膼 膥膠臖臶膇膫膄膬膲膢膶腽臋臱臲膍 膇膫膄膬膥膠臃膟膕膏膰῍῍ 臐臕臷膩 膇膫膄膬臋膢臃膟膕膏膰῍῍῍ 臦臘臣膙 The Shape and Seismic Characteristics of the Philippine Sea Plate around the Iyo-nada, Shikoku, Japan, Judging from the Hypocenter Distribution Shinichi M6IHJH6@> and Yuki O=CD Shikoku Electric Power Co., Inc., Marunouchi 25, Takamatsu 7608573, Japan Michiharu I@:96 Shikoku Research Institute, Inc., Yashimanishi-machi 21098, Takamatsu 7610192, Japan Yoshimitsu FJ@JH=>B6 Ohsaki Research Institute, Inc., Uchisaiwai-cho 222, Chiyoda-ku, Tokyo 1000011, Japan (Received November 29, 2002; Accepted November 5, 2003) Using hypocentral data of JMA from 1997 to 2000, we determined upper surface shape of the Philippine Sea plate around Iyo-nada. The contour lines indicate that the Philippine Sea plate is spreading northward from Iyo-nada to Aki-nada in west Setonai-kai area. From 3-D bird’s eye view, slight dent of the plate-upper surface, possibly due to buckling, can be seen at bound of the plate subducting around Iyo-nada area. It is considered that in addition to the plate bending, change of local tectonics from buckling to spreading may contribute to seismicity of the Iyo-nada area events. As well as this change may contribute to the source mechanism of small events and cyclic Geiyo earthquake. Key words : Philippine Sea plate, hypocenter distribution, plate-upper surface, 2001 Geiyo earth- quake 1. ῎ῌ῏῍ 膑臟腴腮腸腳腺膆腵腹腄腱腟膛臄腭臮腨腎腞腔腧腋 腛腕腪膕膏腏膾臶臔腞膝腬腫腙腊腪腖腫腨腞腠致膭臭腏腉腩臢膱臏膸腟膸膗臧臤腎腨腵腹腄腱膓膵腭膺 臓腔腧腋腛腕腪膨腣 臸腌腡臑臇(1997)腆腀 臋膌臯 臏膸臎膡腞腧腩臅臗膛臄腭膺臓腕腪膨腣 臸腌腡 Taka- hashi et al. (2002)腆腀 腒腫腨膭臭腞腧腪膔膃腭臚膟腔腗 膕膏 臸腌腡臡膴 臍膁 (2001)腆腀 臏膸臠腱腶腯腷 腴腮腄 臸腌腡 Zhao et al. (2000)腆腀 膘臆臠腦自膉臠腭 臵腊腗膅臀 臸腌腡 Ohkura (2000), Nakanishi (1980)腆腀 腹腰腄腲膊膻膅臀 臸腌腡臹臍 (2001)膪膖 (2002)腝腜腏腉腑腨腫腪腴腮腸腳腺膆腵腹腄腱腟膓膵腭臢膱臏膸腟膸膗臧臤腎 腨膺臓腔腧腋腛腕腪膨腣腠膜膳 (1977) 腞腧腘腙膦腤 腨腫腗腼至臞膮臬腞腍腊腙腥臉腐腟膕膏膽膃腏臛腨腫 腙腍腩臫膤腻臑 (1981)膣腾(1991)臰臇(1994)臑臇(1997) 腨腠膸膗腟臨膐膷腓腝腜 腎腨腵腹腄腱腟臙膷臂膹腭膎腤腙腊腪腖腔腙腒腫腨腟 膕膏腞腧腩腴腮腸腳腺膆腵腹腄腱腏膥膠臑膀臥腚腠 7608573 膞膲膧膋腟臝 25 ῍῍ 7610192 膞膲膧膂臘膿臒 21098 ῍῍῍ 1000011 臁臊臕膒臝膚臒 222

˘ˇˆ˙˝merfc.chu.jp/briefcase/paper/p267.pdfRepresentative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 and NS15, respectively in Fig. 4. Circles

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Page 1: ˘ˇˆ˙˝merfc.chu.jp/briefcase/paper/p267.pdfRepresentative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 and NS15, respectively in Fig. 4. Circles

� �� 56� (2003)

� 2 �267�279�

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%&'()*+,� - . / 012 3 4 5)*+,%&6789:�� ; < = >)*+,2.6789:��� ? @ A B

The Shape and Seismic Characteristics of the Philippine Sea

Plate around the Iyo-nada, Shikoku, Japan, Judging from the

Hypocenter Distribution

Shinichi M6IHJH6@> and Yuki O=CD

Shikoku Electric Power Co., Inc., Marunouchi 2�5, Takamatsu 760�8573, Japan

Michiharu I@:96

Shikoku Research Institute, Inc., Yashimanishi-machi 2109�8, Takamatsu 761�0192, Japan

Yoshimitsu FJ@JH=>B6

Ohsaki Research Institute, Inc., Uchisaiwai-cho 2�2�2, Chiyoda-ku, Tokyo 100�0011, Japan

(Received November 29, 2002; Accepted November 5, 2003)

Using hypocentral data of JMA from 1997 to 2000, we determined upper surface shape of thePhilippine Sea plate around Iyo-nada.

The contour lines indicate that the Philippine Sea plate is spreading northward from Iyo-nada toAki-nada in west Setonai-kai area. From 3-D bird’s eye view, slight dent of the plate-upper surface,possibly due to buckling, can be seen at bound of the plate subducting around Iyo-nada area.

It is considered that in addition to the plate bending, change of local tectonics from buckling tospreading may contribute to seismicity of the Iyo-nada area events. As well as this change maycontribute to the source mechanism of small events and cyclic Geiyo earthquake.

Key words : Philippine Sea plate, hypocenter distribution, plate-upper surface, 2001 Geiyo earth-quake

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Page 2: ˘ˇˆ˙˝merfc.chu.jp/briefcase/paper/p267.pdfRepresentative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 and NS15, respectively in Fig. 4. Circles

N11�W ������ 9.8� ���� (1994)� � ���� ���������������� 5���������� �!"#�$ 34.5� %&'�()�*+�,-. (1991)� /"� 0�12��34�����5�� 40 km��1�1 (N75�W)�67���89)�:����� 34�2��5�� 140 km�(;+�,-. (1991)� /":�����<=> ���&?��@A�BCDEF��GH�I*� J��KL�M@A�NO�<=PQRSTU����T�#�MVW:XY�<+Z[���� \]^_`aPQRSTU�����12�b*��cd�ec\f`S&!�'�()�*+ ��1. (2001)� /"� 34��ghi1��b*��5� 180 km%&'�()�*+�jk (2000)� /":�����<=>l�m��\]^_`aPQRS�no:��<� /�=p��. (1997)�� 40 km�H5q:/��r

�s�t"�u�*+> v�� ,wjx (2002)��1997?y2001?�z{|M}~R�8 *���)=��� 34"0��������b*�� �!"#�MVW�G�� 45 km�����t)�*+"��)�*+> '=��. (2000)#M}~R�")�Nankai net (1985�1992) ���. (1997)� 8� )�5� 40 km'��t��H5q�8�)�*+>�/�����b*��� l�m�����8�����)=Z[�E�#��)�\]^_`aPQRSG����NO8��=> �)�� �MVW�� � ��2001?¡��M�¢£¤¥¦�§*���¨#�©�ªu=>

� 2. � � ������� �M£�f«")�z{|v¬­M}~R�8� )=> /�[® JMA2001¯°± �G�

Fig. 1. Hypocentral distribution map of JMA data from October 1997 to December 2000. (a) all eventsof depth from 0 km to 200 km, (b) slab events of depth from 20 km to 200 km. (c) and (d) are bird’seye view of slab events from N30�W and N85�W.

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Page 3: ˘ˇˆ˙˝merfc.chu.jp/briefcase/paper/p267.pdfRepresentative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 and NS15, respectively in Fig. 4. Circles

� (2002)� �������� ���������� � �� 2000� (CD-ROM)�������1997� 10��� 2000� 12���� �� !����" ��#�$%&'� ��(�)��*�depth slice �����+��,��" ���-./01 2�345�678�9�":;<=3�> [email protected]?� AB [email protected]? ���" ��CD����EFGH��� I3 8526J

�89� (Fig. 1a)"�����K�L�)���M3�NOPQRSTU V�W7�� XY�Z[&TU V\� � )8]^_`a\� �bc���Wd� e4_`a\� �f)g �h�" ���3� ijk�#& 20 kmCd#�� �_`a\� �bc������ 20 kmel�� �m,��+�� Fig. 1(b) �n)" Fig. 1(c)3��_`a\� ���o�p (N30?W) ��q��r�hd� s� Fig. 1(d) 3o�(N85?W)���r�h�" _`a\� �tI3 3913

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3(a)3}� !�y���f��r�hd� Fig. 3(b)3_`a\� �N�Tr�h�" _`a\� �5��������W��� ��I���8���k��3���8�"

� 3. � � �����TU V����)���3� ���|�������" ���3�  ��¡�¢��£�¤¥��¢�¦�� �� §#��¨���©#ª�«|��� #��¬ _`­_��d©#ª�®¯�¦*���� °±�_`­_��²��� ��³��5�´µ¶9�©#ª�·|��8[&h�&� ��

Fig. 2. Relation between magnitude andfrequency.

Fig. 3. Number of events per month in thisarea. Magnitude is larger than or equal2.0. (a) for all events, (b) for slab events.

Fig. 4. Divided into 30 and 28 sections for EWand NS. Upper surface of seismogenic zonewas estimated at individual sections.

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Page 4: ˘ˇˆ˙˝merfc.chu.jp/briefcase/paper/p267.pdfRepresentative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 and NS15, respectively in Fig. 4. Circles

���������������� ���������� ������� ��� ��������� ����!"�#��$�%&'()*+�%&�,�� �-�./01�2345���6&789�:;<=�, (Fig. 1) �>?�@�2ABCDE�&����� Fig. 4 �F%�, GH32.0IJ35.0I� KL 130.5IJ134.0IM �NO��' ���P�KQ�RS� 30TUVW/ (EW01JEW30)

�XY3� Z�[�� 28�\� (NS01JNS28)� KQ]��2^[]��_ `@;ab]������ �����;cad�#@;&2A����efU(Fig. 5)���� �:��g9�h GKQ 30i60� [28i56M ��-�./0 Gjk 10lfm�nfU�opqrs�M �t��� �����uv�wx��'@�� yKjzQ�S�{Z:�|} 180 km�~�&

� ;��%&��;�a:��&' Fig. 5(b)�F%|} 180 km���� �� 1999� 6� 26��� G|} 180.74 kmM S�&' �� (2000);��%&��3�����;B����&�������a:��������E���� ;��%&���a:&' �d����sTf ������ ������/ �;C@���� ����S����O������'

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Fig. 6(a) (b) � S15IW da������� Fig. 6(c) �N75IWda� Fig. 6(d)� N30IWda �����S�&' Fig. 6(a)S�� ¡¢£¤¥¦§¨uv;©ªS�&' Fig. 6(b)�� � �����;«E¦�&�Rda�F��¬S�3� ���R (N15IE)� ­®¯°UVW

Fig. 5. Representative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 andNS15, respectively in Fig. 4. Circles and stars denote hypocenter and the upper surface.

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Page 5: ˘ˇˆ˙˝merfc.chu.jp/briefcase/paper/p267.pdfRepresentative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 and NS15, respectively in Fig. 4. Circles

�������N15�E�� ������������������ � �� ��� ! (1989)" #$�% (1991) �&'�()� * Fig. 6(c) (d) ��+,-.�/01� 2�34 33534� 67��89:;��<= >??@ABCDEFG HIJ KLM�� * �N�" Fig. 5(a) (b)���O��/0P1�Q�2�RST� 354 km�UV�BW�NWX �" 3-D�YBW= �Z[�NW�C���\ * ��HI�]^K_`� �" �a�bcdefDg��89:;��h=iQde�jk�P * Fig.

5(a) (b) KF �" RST�fl mnop����a�qr 5 kmse�tuvBWX * ��qr�wxyfl mnop����auv�qr�z{W�� 455 km� �� |}�~� (1994)�&'�����\Q" ��-���.��abcde��X��KY�����G�N * �C;��h=iQde�X

W�" �auv�� 5 kmse��X�������������89:;���HK���� se�X���N C{" h=iQde�L�rNWX ��G *����89:;�HI����rNC���O�

Fig. 7�\ * 34 33534� fl ;��<=�" �r 35 km����1�Q�BW�NWX * ��HI�" �a�-��BW Nankai net (1985�1992) K��BC��� ~ (2001)�¡¢��£�P���NWX * �����" �¤¥�LMrNC<=��-���.?�abcde�jk�PX��KYBWX *Pf¦§�xyt�����XW�" �r 40 km���P{��P¨©ªKLM�� *r�" ���������89:�ST«K¬{W=C* Fig. 8Y� 4���� (section A5D)K�Q";��STK­®BC* ��������89:;��

Fig. 6. 3-D view of the upper surface. (a) and (b) are viewing from S15�W, and (b) is lower view than(a). (c) and (d) are viewing from N75�W and N30�W.

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Page 6: ˘ˇˆ˙˝merfc.chu.jp/briefcase/paper/p267.pdfRepresentative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 and NS15, respectively in Fig. 4. Circles

���������N75W��� ���� 34N,

132E ���� (section A), 33.5N, 132E ���� (section B), 33N, 132E ���� (section C),

32.5N, 132E���� (section D)� 4�������� 132������� 0 km�� �!"�#$�� Fig. 8 %&'�()�*+�,�� 4����-./01232456789:;<2=>?67%@A-�B��� CDE*�!"�F��'�GHIJ KLI3()��MN?�� OPQR2./%12S�T()U2 5V8� ./0123��W-X35� ��W67�Y?�Z%[\%()�Y�� 67100 km]6�- 65V70�0^�' 5�OPQR

�./0123�_`�ab�b� section AVD %&'�()�c1d�e��� KL- Fig. 8���#%f?IJ%./0123�SW%&ghi�jd���e��� �����L��� ��ik%lmn ]o�-K�pqr�1sturR�v?K��?

Fig. 7%-Mj2.0] �wpxy9:�z{|}~B�r�Q�f��� KL- Fig. 4%f?�_���� ��� 6�� �� 7.5�� ��3�z{9:~�������r�Q�� KL���� 1sturR%�^����9:2��|}��'K�2��

Fig. 7. Upper surface of the seismogenic zone is indicated by blue contour lines. Red contour linesindicate distribution of number of slab events per grid in one year [Mj�2.0]. Green broken lineindicates truncated area due to quiet seismicity. Violet broken line indicates where theseismogenic zone is deformed greatly in the perpendicular direction. Black line is the seismicsurvey line and solid circles are observation points after Takahashi (2002). Black broken lineindicates the fault of slow thrust slip in 1997 after Hirose et al. (1999). 3 stars indicate majorevents for the period from October 1997 to December 2001. From north to south, 2001/03/24 theGeiyo earthquake Mj 6.7, 1998/05/23 Mj 5.3 and 2001/04/25 Mj 5.6, respectively.

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Page 7: ˘ˇˆ˙˝merfc.chu.jp/briefcase/paper/p267.pdfRepresentative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 and NS15, respectively in Fig. 4. Circles

� 5. � �5.1 ��������������������� ���� ��������������������� ����� !"���#� !"�$��%�&�' !"()�*��%�&�' [Seno et al. (2001)]��&+,�&-&������� ./01 ����������2345�67, �8��9�23:;*��<��� 9�23:;*�Takahashi et al. (2002)����� ����45=>� Fig. 7�?�� P 9�23:;*� Fig.

9 �?�� ���45=>��@��A 5 km/B��C�D.���E�FG�� ���H��� ��=>�I�,�E�JGK�L�$�������#M'��NO���P�I�, !"����K�L�$Q��()�*�����,�� R�STU� (1992a)� VQTW (2002)X #� NOY��P��K�L�$�����,��6Z�[�� ���� \HTW (2002)��

]�^�_`a�bc�� dNOY��P�Ie� !"�����������f9����K�L�$�g��� NO�����f9�����g���h ��Zi��j���� 8-�01 ���������k�l,�m�,n��� o8 20p30 kmq�I�,��������rs�9�23�rs�tu��+� �������K�L�$v 2��JGw�����x��o8 40 km�y�I�,�K�L�$��������#� �,+�#� 9�23:;*�����y�2345�!��[��z23�{|&"�#}�~,�&���M������ Ohkura (2000)�� �$%�&�����o8

60 km��� !"�����'()&�� #�=8�� ��H'e9� (Vp�6.7p6.8 km/s) #K�L�$��� 9��tu�����'-� ���P����K�L�$������,���i��,��� �'�� 60 km�o����B�,��� #H-�/� ��*+�� K�L�$��"��#��������,��K�L�$��� 9�#�����z� [�����#f9������.��()�*�����,���z���,��#� ����-#���'��� '-���~&���,��� W-��� �� #H-�&�*+�� �.���� !"()�*#/n���I�� ���� ��6��������[��z [Seno et al. (2001)] ���i�k[��8-�� {|&¡#��0��M/'&¢n�B�,�� �����������&�K�L�$#£¤����Z¥1Lk��-��� y2� /n�¦ !"��y���%����§3���K�L�$���4¨��67,�©,���i�8�,�� [Kirby

(1995)]� ª«��6Z�55�P�6�)&��%�kK�L�$���©,I�� ��4¨��67,�©,��� �8��� ¢n�o8# 3p4 km7��[��¢n����Ie��E��8# 5 km7��"��,�&�����9������ 55�P������8l&�K�L�$#£¤��¥1L�:+� w;�K�L�$���4¨��6���%���Z��¬ #�©,�����-��� ��#7,�«�6Z��������"�­®������"����-#¯°�[������'-� 01 ����������� ±&+�ko8 60 km���B�,�K�L�$v 2����<��������#�~� �Q�����bc�,���²z���x� 60 km�o�B�,��K�L�$����,���'()�*����,���'��=+� 0��³>´µ�����

Fig. 8. Sections of A, B, C and D of S75¶E areselected to see the upper surface slope.

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Page 8: ˘ˇˆ˙˝merfc.chu.jp/briefcase/paper/p267.pdfRepresentative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 and NS15, respectively in Fig. 4. Circles

5.2 ��������������� ��� ��������������������������� !�"#����$����%&'#(� )��*�+,-� ./0123����4���56�����7�� 8�9� �:,;�6��<=>?;�8 @��(2001)A !+�&BC��� ),-� )��:,;�8�����D��E�,���+,-����F-8�=�+GHC��� !�D�+,-4� �IJK (2002)

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Fig. 9. Comparison of upper surface with P wave velocity model by seismic survey. The velocitymodel is after Takahashi (2002) and this line is shown in Fig. 7. Black thick line is upper surfaceestimated in this study. Crosses are hypocenters locating 10 km width in the section. Open circlesare the observation points after Takahashi (2002).

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Page 9: ˘ˇˆ˙˝merfc.chu.jp/briefcase/paper/p267.pdfRepresentative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 and NS15, respectively in Fig. 4. Circles

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Fig. 10. P-axes and T-axes from NIED seismic moment tensor catalogue. The period is from January1997 to February 2003. (a) map showing distribution of P-axes. (b), (c) and (d) are cross sections ofband width from 33.9�N to 34.5�N, from 33.3�N to 33.9�N and from 32.5�N to 33.3�N respectively.From (e) to (h) are figures of T-axes.

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Fig. 12. Hypocenters for aftershock of the 2001Geiyo earthquake and the seismogeniczone upper surface viewing from N75»W.Circles indicate the hypocenters of theaftershocks occurred in one day after themainshock. Asterisks indicate the uppersurface of the seismogenic zone.

Fig. 13. Number of events per month in thefault area of slow thrust slip in 1997. Lowerlimit of magnitude is not considered.

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Page 12: ˘ˇˆ˙˝merfc.chu.jp/briefcase/paper/p267.pdfRepresentative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 and NS15, respectively in Fig. 4. Circles

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