13
59 (2007) 2 209221 2004 12 14 膎腆膐腺腄腅膢膘腯膇膍膔腇 膅腸 (M JMA 6.1) 腇膠腸腖膋 膙腓膌膂腔膂腔腏膡腔腢腝腏膓膁膅腸腒腬腢腝腙膀腊腌腋腁腎膟腳腛腽膉腚腨 腬腤腵腘腩腟腨腧 腕腣 膙腓膌膂腔膂腔腏膡腔腢腝腏腰腾腭腑腔腼腦῍῍ 腪膄 腍膒膂腔膂腔腏膡腥腔腢腝腑腹膡膕腱腑腔腼腦膅腜腷腐腔腈腁腉῍῍῍ 腬膛膝膑 Aftershock Distribution of the December 14, 2004 Rumoi-nanbu Earthquake (M 6.1) in the Northern Part of Hokkaido, Japan Masayoshi I8=>N6C6<>, Takahiro M6:96, Teruhiro Y6B6<J8=>, Hiroaki T6@6=6H=> and Minoru K6H6=6G6 Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo 0600810, Japan Tsutomu S6H6I6C> Department of Natural History Sciences, Faculty of Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo 0600810, Japan Akihiko Y6B6BDID Department of Earth’s Evolution and Environment, Division of Mathematics, Physics, and Earth Sciences, Graduate School of Science and Engineering, Ehime University, Bunkyo-cho 25, Matsuyama 7908577, Japan (Received August 28, 2006; Accepted November 22, 2006) On December 14, 2004, an M 6.1 earthquake occurred in the northwestern part of Hokkaido, Japan. We installed nine temporal seismic stations around the source area immediately after the occurrence and had continued the observation for about two months after the main shock. We determined 823 hypocenters of aftershocks and one dimensional P-wave velocity structure model from the travel time data. It is found that aftershocks are clearly distributed on an eastward dipping plane with a dip angle of about 25 degrees. This plane agrees well with one of the nodal planes of the focal mechanism determined by P-wave first motions in this study. Next, we investigated a position of the main shock relative to the aftershock distribution. The station correction values for permanent stations were determined from the data during the period of the temporary observation. Estimated main shock hypocenter was confirmed to be situated on the aftershock plane. Finally, we estimated three-dimensional P-wave velocity structure based on the temporary travel time data. Relocated aftershocks using the three dimensional velocity structure were mainly distributed along the bound- ary between the high and low velocity zones. Key words : Aftershock distribution, P-wave velocity structure, Reverse fault , ῍῍ 0600810 腫膚腮膙腠膙 10 腶腻 8 膆膞 ῍῍῍ 7908577 腴腬腮膖腞膈 25

2004 12 14 6.1) · 1. 2004 12 14 14 56 ˘ˇm jma 6.1 ˆ˙˝˛˚˜ ! "#ˇ$%˙&’( 44 )4.46 *+,141 ) 42.2 *-.9km /%!0 ˆ˙ *12 3 ˙)5 4*56 3 7 ˙)5 8*56 9:*;< = *>?@abc*de@

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Page 1: 2004 12 14 6.1) · 1. 2004 12 14 14 56 ˘ˇm jma 6.1 ˆ˙˝˛˚˜ ! "#ˇ$%˙&’( 44 )4.46 *+,141 ) 42.2 *-.9km /%!0 ˆ˙ *12 3 ˙)5 4*56 3 7 ˙)5 8*56 9:*;< = *>?@abc*de@

� �� 59� (2007)

� 2 �209�221�

2004� 12� 14���� �������� (MJMA 6.1)�����

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���������� !�>?@A�BC�� D E FGH������I� !AJ�KLA�BC�MNO�P+Q��� % R S T

Aftershock Distribution of the December 14, 2004 Rumoi-nanbu

Earthquake (M 6.1) in the Northern Part of Hokkaido, Japan

Masayoshi I8=>N6C6<>, Takahiro M6:96, Teruhiro Y6B6<J8=>,

Hiroaki T6@6=6H=> and Minoru K6H6=6G6

Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University,Kita 10, Nishi 8, Kita-ku, Sapporo 060�0810, Japan

Tsutomu S6H6I6C>

Department of Natural History Sciences, Faculty of Science, Hokkaido University,Kita 10, Nishi 8, Kita-ku, Sapporo 060�0810, Japan

Akihiko Y6B6BDID

Department of Earth’s Evolution and Environment, Division of Mathematics,Physics, and Earth Sciences, Graduate School of Science and Engineering,

Ehime University, Bunkyo-cho 2�5, Matsuyama 790�8577, Japan

(Received August 28, 2006; Accepted November 22, 2006)

On December 14, 2004, an M 6.1 earthquake occurred in the northwestern part of Hokkaido,Japan. We installed nine temporal seismic stations around the source area immediately after theoccurrence and had continued the observation for about two months after the main shock. Wedetermined 823 hypocenters of aftershocks and one dimensional P-wave velocity structure modelfrom the travel time data. It is found that aftershocks are clearly distributed on an eastward dippingplane with a dip angle of about 25 degrees. This plane agrees well with one of the nodal planes of thefocal mechanism determined by P-wave first motions in this study. Next, we investigated a positionof the main shock relative to the aftershock distribution. The station correction values for permanentstations were determined from the data during the period of the temporary observation. Estimatedmain shock hypocenter was confirmed to be situated on the aftershock plane. Finally, we estimatedthree-dimensional P-wave velocity structure based on the temporary travel time data. Relocatedaftershocks using the three dimensional velocity structure were mainly distributed along the bound-ary between the high and low velocity zones.

Key words : Aftershock distribution, P-wave velocity structure, Reverse fault

�, �� U060�0810 VWX�Y� 10Z[ 8\]��� U790�8577 ^%X_`a 2�5

Page 2: 2004 12 14 6.1) · 1. 2004 12 14 14 56 ˘ˇm jma 6.1 ˆ˙˝˛˚˜ ! "#ˇ$%˙&’( 44 )4.46 *+,141 ) 42.2 *-.9km /%!0 ˆ˙ *12 3 ˙)5 4*56 3 7 ˙)5 8*56 9:*;< = *>?@abc*de@

� 1. � � � ����������� ����� 2004� 12

� 14� 14� 56���MJMA 6.1������ !"#��$%�&'�( 44) 4.46�* +, 141)42.2�* -. 9 km�/%! 0���* 12 3��) 54* 56 � 37��) 58* 56 9:* ;< = * >?@ABC* DE@ FG��) 4HIJ� ! K * �&��L�� ���MN�O/%PQRSTUVWX K-NETY 4�Z[\ (HKD020)�'* ]^�) 68HZ[� ! 0���$_`ab�8c* defgh� 165i�jklm��n ! =

���o 24�pqr���� Bs��'�) 3� 3

t* �) 2� 2t* �) 1� 5t�/u vw6xy"#z (2005){!

Fig. 1�'* M 2.0q|��}~���/% 1991

����t��� ��K��p�* "#���^.� ���&��H��OL% � �� 1910��1918����� ��������* 1986����� (M 5.3)H��O/%�! 0���' 1910��1918����������M 5.3�M 6.0�� 1986

����� (M 5.3), 1995������� (M 5.7)

vw6xy"#z (2000){ ��M 5.0�6.0�����

Fig. 1. Location map showing seismic activity of earthquakes greater than M 2.0 in northern part ofHokkaido determined by Japan Meteorological Agency (JMA) from January 1, 1991 to December14, 2004 (date of the main shock). A solid star indicates the epicenter of main shock determined byJMA. The inset map shows the study area and epicenters of earthquakes with M 5.0 or greater.

f� ¡¢2�£¤¢?¥¦§¢¨©¤ª¢«¬ ­¢®¯ °¢?=C±210

Page 3: 2004 12 14 6.1) · 1. 2004 12 14 14 56 ˘ˇm jma 6.1 ˆ˙˝˛˚˜ ! "#ˇ$%˙&’( 44 )4.46 *+,141 ) 42.2 *-.9km /%!0 ˆ˙ *12 3 ˙)5 4*56 3 7 ˙)5 8*56 9:*;< = *>?@abc*de@

���������� ���������������������� � !"������ ��#$%&'�()*+ GPS�,��-.�/0�1234 5����6789:;8< =>8?@6:;8<ABCD8EF:;8<=�G:;8<A�HI���JKL�MN5B�OPQRN� S�TU Heki

et al. (1999), Takahashi et al. (1999)V� MWXYZ(2005)�[RU ����*������Z\+6789:;8<BCD8EF:;8<B�HI�5�]^�&'�34��*�_�`� abBN�� 5R4�5B#c�T�B !"�� ���d���#ef gh`�5B+ij����

3��4 5�����k�]^�lm��/0n (Fig. 2a)+ ���opopqrpghqst��uvgh/0wxy8=z{ ISVA�|}/0n (TOI)

B~��p��gh��Hi-net��� (ORWH)B��( (OREH)� 3/0n3�3� � ��12�a4R ISV�98�x/0�[��k���������k+ '��d�������� d����(�����`�����4R � 3��4 �k��Q���4�N������� � �5�� + d���#¡¢[��l`�£¤�¥�¦�§��¨��/0#©ª � '���« 1¬­�/0®8y#¯N °±¤�²³12+ }´Xµ (2005)�¶QRN��

Fig. 2. (a) Permanent seismic stations used in this study. Open squares, triangles and circles arestations of Hokkaido University, JMA and Hi-net (National Research Institute for Earth Science andDisaster Prevention), respectively. (b) Temporary seismic stations (solid squares). Open squaresand circles indicate permanent stations. A solid star is the epicenter of the main shock determinedby JMA.

2004· 12¸ 14&��� ¹º»��*��� (MJMA 6.1)�d��� 211

Page 4: 2004 12 14 6.1) · 1. 2004 12 14 14 56 ˘ˇm jma 6.1 ˆ˙˝˛˚˜ ! "#ˇ$%˙&’( 44 )4.46 *+,141 ) 42.2 *-.9km /%!0 ˆ˙ *12 3 ˙)5 4*56 3 7 ˙)5 8*56 9:*;< = *>?@abc*de@

����� ��� 2���� ��������� ������������� ��������� � � �������� ���!"��#$%&��'() *�%� �����������+�,-.%&���/0�123)

� 2. ��������Fig. 2a%� 4��%��"�5� ����67�8����������'() �9:� �;� 7�8�<:����=� ISV Hi-net� 4�������>3) ? Fig. 2b� Fig. 2a�9:�@?� ���A�� ��� BCDEF��� ��������G9:�'() ���� 6��� 14� 21�%*H����������6%I�� �J%��� KFig. 2b

� OTD���L) ���� MN����!��� ����� 24�O"%����� 6����� ) ? 12

16�%�5�% 1� (HNG)�#P� ) ����QR%$S�ST���%��/&� F� U'�VW%��� (H�����6�)*%�����+%��(3X F�YT�Z ) ���� ,-[.\] K/^,0123_`a7�8b45L �c6%��78%d'�eZ�f��� 12 22��� 20059 1 12�?��: 2g%7���� ����;6 )*% 2����� (HTM, SKB)���(3X F�Y ) Table 1

%h������iF? j�>3)��klm.�On��S%�<� ) =o���F��(3X �>��� p���?b��?�@�A�3X T�qB(3X FrCTstuvwxyz{|�}��~�P��p�� JEP-6A3 Kw����DL���� ) �BE�%�� tuvwxyz{| 24 bit

� LS7000XT K���`�DL ����� 100 Hz�{��{�������B� ) �BE��F�%�G��5�/&�������xm�� (12 V, 24 Ah)� 2

&HI%J�� ) ���K�� GPS������ 6�� %eZ ) ���� 1 Gbyte���xk�+������%qB5�� 2�3g% 1�_�%LY���C��eZ ) X��S%�� 20049 12 14

��� 20059 2 9�?��: 2�%� ������eZ )C�5� ������M������ ��������� N���WINklm. �O��P (1991)  ����¡��eZ ) ��������P�qBT��Q�� �M��¢%��¡��� PMRF 5���OS�� �& SMRf 2���OS�qB5���3��%&��hR�T£¤��eZ ) X�U¥�V (��� 823¦�>Z )

� 3. P�� �������3.1 1�� P�� �������X���� PM����/^,/����� �����"���§e¨©W%ªj XY (1986)� P

M��� KFig. 3�GZ�R�«[L� 7�8� /^,/�%\¬� ������������� PM����"�� ]^�P (2003)�#$F>3 KFig. 3

�­Z�R�«[L) XY (1986)®]^�P (2003)���� ISVF_������3 PM��� KOnISV��¯ Fig. 3�­Z�`[L %a2b5 2 km��fc�����T�d���>3X F��3) X�

Table 1. Station corrections used in the calcu-lation of hypocenters. The 4th columnshows the operation period.

Fig. 3. One dimendional (1-D) P-wave velocitystructure model inverted from the initialmodel by Crosson’s method (1976). We usethe smoothed inverted model for hypo-center determination. Also shown are themodels estimated by Moriya (1986) andTamura et al. (2003), and that routinelyused by ISV (Institute of Seismology andVolcanology, Hokkaido University).

Re°±�f]²³��´gµ�¶·³¸�¹º h�»Y i���jk212

Page 5: 2004 12 14 6.1) · 1. 2004 12 14 14 56 ˘ˇm jma 6.1 ˆ˙˝˛˚˜ ! "#ˇ$%˙&’( 44 )4.46 *+,141 ) 42.2 *-.9km /%!0 ˆ˙ *12 3 ˙)5 4*56 3 7 ˙)5 8*56 9:*;< = *>?@abc*de@

����������� �������������� Crosson (1976)����� 1�� P������������ Hirata and Matsu’ura (1978)� !"#$%&'( HYPOMH�)�� ISV������*+ !�,��� *+ !�,-./ 0� Vp/Vs�12� (1983)���� 1.75����� *+ !�345� P� 6 9789:.� 47775���;�44707� P���������� � ���<=�>? 8 km��@����A�� BCD�EFC5� P���������� GH)��� Crosson

(1976)�������"#$%&'(I1J��*+��5�K��A���LM6�N���O#�P� "#?;���QRS�H�QRS�R6 0.001���SM��STU6V���S�?;.� W�� BCD�EFC�XY���;. Resolution

matrix��Z[6 1\]^_` a�U�bc6d�aS�e���.�I �fgI1 a���h@1\�@��hiUS��j�-.� 2����SISV��� k�*+��S�� � <=�>? 8

km�I� 2 kmlS 4��BCD�EFC�P��� �m� 2����I1 n!�� 0o2 km6 2�p�;��.6 GH����5�1Wa�I�U"6M�SL 2�� 1o2 km��� 0o2

km��S��j���� W�XY���;��Z[1(Table 2) 2������#6 ISV��5������q� 1\^ U�bc6d��I GH1 2�����*+���S��j���XY�hiUS��� a�U���.��N���"#� 4HP���a�hiUI1$%���QR1*+���� 0.06�5� 0.04�rK��� 89�XY���;� P������ Fig. 3�st�&'Ie���.� a���1 ISV���q.S $%()���M���.6 2� (1986)u*+v, (2003)�����w�;.���-��1 GH�Ux5�1���;M5���*+v, (2003)1 .y/.��zI10{6|^�����.�� ��-���1p.236:.S}q��.� �5� a;1GH�� 64~������@5 20 km89.�6z �.� 44� 30

�8.����S��UxI:.� 7�������.SGH�� 64~���zI1W�81�7��69����� 2� (1986)u*+v, (2003)6��S���..:��z�q�0{6|^1M^ W���-��6w�;M5��SL�;.�

Crosson (1976)���I1 ����S !�;���P���.6 P� �������.�I ����M !���.�� S� @��� HYPOMH

�. !��,�P��� Vp/Vs�1<_`�*+ !,�STS;= 1.75S��� HYPOMHI1���>�Ap� P������?L�"#-. BCD�EFCI���;� P�����1��I:.�I 0�|?�@��A.�3���>�Ap.aSIBC(M������������ �Fig. 3��'�� �� a������� !�,�P��� � a��,I��;�0�DE� P�S S���D��SFG���R�H����DEI�� (Table

1)S�������?L�3LI @3n� !,���hi !S���

2004J 12� 15K5� 2005J 2� 10K�I�+�A�� Fig. 2b Ie��L��DES 3A���DE����.hi !S� � P�*M����(U� Fig. 4ae��� � z�NT?15 12 km

�O.#�� 11 km �P¡#�� I ¢Q (2001)6e��R£(M� �ST{ SS¤&�¥¦�§M�¨©ª

log S«M¬4.0

���� U­®¯°±fg� F-net�� ����C²¤&�¥¦�§�8<Mw� (Mw 5.7)5����ST{ S (7.0 7.0 km)��@uuNT^����� �5�M6� Mw1� �&³�NT?�8���� hi(M� z1nV(� �&³6z��@NT^M.SL�;.�I L��D5������ z�NT?6Mw5����ST{��@NT^M.aS1:�3.� ��� 1 ¡5�P�5��>^M.�3��� � �����(U�nWT�´T�#�SXY(I:.� �� Fig. 4b1hi !S;µ����)�� ISV��I"#�� !S��¶�e��� �m� -q��� ��e-.Sw]�^M.�� aaI1M 2.089�� A����e���Fig. 4b�w.S ISV��I,���· !6¸@,?;.�·6:��6 hi !I1W��3M !1M^M���.� a;1 ISV��1$%(d��I:.��I:� ��^����������.aS��¹�Z^ !,6IT�SL�;

Table 2. Diagonal elements of the resolutionmatrix.

2004J 12� 14K4~��[\º]O��� (MJMA 6.1)�� »M 213

Page 6: 2004 12 14 6.1) · 1. 2004 12 14 14 56 ˘ˇm jma 6.1 ˆ˙˝˛˚˜ ! "#ˇ$%˙&’( 44 )4.46 *+,141 ) 42.2 *-.9km /%!0 ˆ˙ *12 3 ˙)5 4*56 3 7 ˙)5 8*56 9:*;< = *>?@abc*de@

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�� ����� ��� ���� 12� 22����� 2���� 2���������� !"#�� ��$ ���%&'()�*+ ,-�*+.#,-�/#"� 0��M 2.012�3&�/#"45$ 6%Fig. 4c�7$ ���6%8���9:�;<=>?$ 3&�@AB�C��;:D�3&�EFG>?$"#.#� $�$� 2���$ H�1I�3&J?��K-L%�� M� Fig. 4a, b� Fig. 5�6�/#"�� 2�����%N#"OP ! &'%7$"#��

3.2 �������������Q&�RST�UV! M� W���X��Y��Q&�&'Z[%\M�����<.#� $�$.��� �����X��P]����X�%^Y-V_����� Q&�&'�/#"�`aCbc�.�� Sakai et al. (2005)�� 2004d 10� 23��RS$ efghi3& (M 6.8)�j$"� ������P]���%^Y-V_"\M�! ���klm%�#"� P]���no%pq"Q&r�s�&�tOP%Uq"#������� Sakai et al. (2005)�uv"Q&�&'tOP%wY������H�h�RS$ 3& 823x�j$� Fig.

2b�7$ B�����klm (Table 1)�\M�!"#�������&'y�z# 3��P]���(TOI, ORWH, OREH)�`�m�� W�{�P]���(Fig. 2a)��|Q&�}Cz#����`�m%*+&'()%U�� W$"� *+ P]����~���%W��������klm�B�� �� 2���z#����`�m%*+"&'()%U#� 0��~���%���klm�B�� n$� B��\M�! klm�/#"���_��()�p�B�� �����$" Fig. 2a�7$ 10���P]�������klm%\M� (Table 3)� }T������%U��C�

M"� Q&RST�H������j$��klm%�#"P]���no�&'()%Uq �������G\M�! &'D�% Fig. 5a�7$ � F � B�"�3&%67B���.q"8��=.� M3&��%�q"� M 2.512�3&�j$"���klm (Table 3)%�# ,-�W��.#,-�&'�45% Fig. 5b�7$ � Fig. 5b%8��� ���klm%�#��()$ &'�4�� ���klm%�#"()$ &'�� ���� ��j���=�F �h��#"# 3&�����OP !����.q � F � � Z[��¡��¢£¢�¤�>?$"#�� Fig. 5c������¥¦������OP$ &'OP¥¦%� ����%Uq H��M 2.012�C��/#"45$"7$ � ��6��� �������¥¦�� ������q"\M�! &'D��G�j��§=� ¨©¨�¤�ªª>?B�C��� �«��.Gz#D�%7B���.q ���D���������� ������ ¬�E��­+�!��Q&�/#"C����%®�$"tOP%Uq"#��� ��l$#&'Z[%¯�� M�� 2�����&'OP$ C���|� ����H�h�RS$ 3&�h�� Q&�z#,��RS$ 3/�3&�/#"� Fig. 5d��������\M &'����������OP$ &'�°±�C�9���Z[%45$ � ��6��� #�!�3&C�����\M &'�2�����4�"� �¤��¢£¢�¤��§ �¤���#�¤�� 1 km>?$"#����D���$ �q"�0v,��RS$ Q&�/#"C�������Eq .�²0v���>?$ Z[�&'�E��­+�!���� 3/��&� ³�>?´DnoQ&%>? _ �Fig. 5d��±�µ¶��°±�µ¶·�� F � \M�! &'Z[% Fig. 4a�������&D��h�µ¶�7B� Q&�� ¸�

Fig. 4. (a) Aftershock distribution with 1s error bars determined by using the 1-D smoothed P-wavevelocity structure (Fig. 3) from December 15, 2004 to February 9, 2005. Gray squares indicateseismic stations used for the location. A solid star is the main shock determined by this study.Solid lines indicate active faults; � Rikibiru and � Hirotomi active fault. A dashed line is ananticline axis [Ikeda et al. (2002)]. A focal mechanism of the main shock determined from P wavepolarities is also shown.(b) Comparison between hypocenters determined by the final 1-D velocity structure model (opencircles) and ISV velocity model (the tip of the bar connected to a open circle). Hypocenters ofevents with a magnitude greater than 2 are plotted.(c) Comparison between hypocenters determined from two datasets (December 22, 2004�January12, 2005); the first dataset includes two stations (HTM and SKB) and the second dataset does notinclude the two stations. Hypocenters for the first dataset are shown by open circles, and those forthe second dataset are shown by the tip of the bar connected to an open circle. Hypocenters ofevents with a magnitude greater than 2 are plotted.

2004d 12� 14��RS$ �¹º»©:�3& (MJMA 6.1)��&J? 215

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���������� 4.96 km �������������������������� !"#$(2005) %� &�'()�*+�,-��./%��,01�2�3�456��78,������� 9:;<6����-��=>?�@A����B���

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Fig. 5. (a) Relocated hypocenter distribution determined by using only permanent stations with thestation correction. A solid star is the main shock.(b) Comparison between hypocenters with station correction (open circles) and ISV hypocenterswithout station correction (tip of the bar connected to an open circle). These hypocenters areshown for events with Mc 2.5.(c) Comparison between hypocenters determined by using temporary stations (open circles) andpermanent stations with station correction (tip of the bar connected to an open circle). An openstar is the main shock corrected based on the shift of two aftershocks near the main shock (see (d)).(d) The same as (c), but only for the main shock and three aftershocks, A dashed star indicates themain shock corrected by the shift of two aftershocks (small open circles).

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�� ������������ � P������������ 3�� P��������������� � Thurber et al. (1983)����� � 3�� P����� ������ !�� 3���"#�������$%��Um and Thurber (1987)��� pseu-

do-bending method������ �&'�()*+�� ���, -Y��. ����, -X��. �/�0�4.0 km12 63� ��, -Z��. �� 1 km, 3 km,

5 km, 7 km� 43�()*+����� -Fig. 6��4.� ���� ���� P��5 5678�9:;��8236 � P������� 72046 <��=��������� �������>!�"?��'�� #$@�()*+�A5B��"#�CD�%E����FG*H�I�+JK)LMNOPQRS [Humphreys and Clayton (1988), Inoue et al.

(1990)]�TU� Fig. 6�V@� ���&'5�(�)�����'�� *W+,5X�=- �Y�5�Z�.[;� ����<�� W��/��� 5 km

��0�1��*W � <�2��>!5<��\E;��� 7]�^3 �� =-�X�1��3���9�4_�@��

Fig. 7a, b�� � 1 km, 3 km, 5 km�567 ����� ���,����A�B`�a C�Db897 ����� ���&';��� � ����V@� �c�� Z�. �� resolution matrix�4d��5 0.3578����3��V����� :�e?��f�=-5<g�;hY��'� ��TU5<= !���>�W��(�*W�9 <�5� ��[;���,�,[��X*W -.�?@�*W.5� ����97�i����-A�B97.[;��-C�D97.�,[��X*W��5Aj@�k,�Y�����f�iE�� /��� >��Z�X*W�Bl��f�� �����i�Z�5 !�� CEm� C�D97�`���� X*W�8( >�5DhEF��`�� � 3 km �X*W�G( >�5E

F�����Fig. 7c�nHop.�>�� ��Iq�V���>�WrJ�nHop�� �i��� rs�I?�XnHop�*W5���,[;K�tu !�`�� P

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Miyamachi et al. (1999)�� 1997Nz{O|��1P� (M 6.5)�`�� 3�� P����&'� Q*W >�5}R��EF����Z��~S����� g�� Kato et al. (2005), Korenaga et al. (2005),

Okada et al. (2005);�� 2004NT�|R�P�(M 6.8) �� X*W�Q*W���U��V�Dh>�5� ����Z��~S��� ����������5�(�)��`�� =-5<g��hY��'�WX �Y�5� Z�;�k,����P��`���i;���\E;�� >��Y5������������Z!�V��������\E;���

� 4. ���������P��>��� [ 25 ��,�k�@�78�� ����� Z�7�� \�� P��Y�H���= (Fig. 4a)��k� (36�)�]7� 11��d����<�5``�/"^_ <�� Z�759`7 <�f�\E;��� g�� �aPb�� GPS��c(GEONET)[;&'�9`��� ��aPb� (2004)��k��,����d�@����� k�d (42�)��[Y��!5<�� Z�9`7��@��'����� GPS���5 2��[Y��'� [Y�e !5<����\E;��� �¡f�1�� ��¢£¤��3¥9`¦�P�5EF@�Z�5D� �gh (1999)��g� GEONET�P§iY���TU[;� �¡f�1PW�� ����¢£�iY¨ <�Z�5V���� ��aPb� (1997)�� ���P����¢£�¥9`¦�P� <�� Z�PW�©WªH¨�j«�����\E;����9`¬­® (1991)���m� Z�P��EF��kl¯m�1��� H

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9`�©o9`�p°5~S���� (Fig. 4a)� Z�;�9`��q±�k��Y��`����P��9`7��²r7�Y@� st³u(2005)���m� Z�;�9`�´v9` �Yhw�_�EF��µ*¶R·RS <��Z!5X����`�� ���P���v�k��x°9`5´9` <��Z!5X������� 3g�� ���P��Z�PW���¸Y�¯&@��¹PR·RS8 EF�����=º !��

Table 3. Station corrections for the permanentstations.

2004N 12» 14y�EF��kl¯m�1�P� (MJMA 6.1)�>��Y 217

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���������� ��������������������� !"#$"%&��'�() *+,#- (2002)./��0�%&�1 30023456�789:;<9=�>?@ABCD����E�'�F��GH0�() *Tamaki and Honza (1985)IJK (2002)./ �����L�MN�O"��PQR ��!"S�TU�VFig. 4a�WXYI �����Z[\]^�F_E`���@A�a�b��Bc�(d

EI ef�������gh�ijk�l�mn�() Z[\o �GH0� /5pBq�r �I st��uvBCbwxI ISV

�yzuv{B|)�}�~��}������EI���$��>�$��A�I L�� 25��>O"��p\���() ����0���b/ qI ISV

�yzuv�� �}���������}\�0�����q0�)��FI >O"��������()

Fig. 6. Results of checkerboard resolution test for three dimensional (3-D) P wave velocity structure.We assigned �5� velocity perturbations side by side to every grid nodes in each layer. Theresults of the test are shown for depths of 1 km, 3 km, 5 km, and 7 km. Open circles are grid pointsused in 3-D inversion. Gray squares are used stations. A red star is the main shock.

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�������� ��� � �� �������GPS������������ !�"#$%&�'�� (��� )*��+,-���./0123456� 7829:,-� 6��(��;<= 1> ?@A�5�B-54�������� C�@ 3DEPFGHIJ2��� K LM�B�NOPQ�R"#

$%&���� SGHTU@(� VW�XY4���ZC[\%�GH]^_`%(��abVW-54���Yc�d��@Ye� (��B��fIJ ghi�jC�

Fig. 7. Three dimensional P wave velocity structure obtained by 3-D inversion. (a) Results fordepths of 1 km, 3 km, and 5 km. The reference P-wave velocity profile is shown in Fig. 3. (b)Vertical sections along the lines (A�B, C�D) shown in (a). Aftershocks are shown by solid circles.(c) Bouguer gravity anomaly map in the study area. Gray squares are temporary seismic stations.A red star is the main shock epicenter.

2004k 12l 14m@VW-�nopqrs �� (MJMA 6.1) (�tu 219

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� ������������ ������� ��� ��� �� ����� �� ���������������� �� ������ �!"#$� Hi-net, K-NET %&' �����(�����()*+�������� �����(,�-./���� �0#$�(��1#$2� 3�4�(56789(:;��<;�=>�������� �?���(@�ABCD� 3�4�(E��<;�FG �������� �����(�!HI#$2� �J�"KL�� MN;�G#�����������($�IOP�%&(Q"R(�S %&''T()(U*��(+�VWP(XYZ[ �J�\]^_`aL�,- b���� �./(cd GMT

efg9 [Wessel and Smith, 1995]��������hi(�S�j��kl;���^�

� �

Crosson, R. S., 1976, Crustal structure modeling ofearthquake data. 1. Simultaneous least squares es-timation of hypocenters and velocity parameters,J. Geophys. Res. 81, 23036�23046.

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WNEW/PRESS-RELEASE/2004/1215.html� ��� 2006�8�18��

Korenaga, M., S. Matsumoto, Y. Iio, T. Matsushima, K.Uehira, and T. Shibutani, 2005, A Three dimen-sional velocity structure around aftershock areaof the 2004 mid Niigata prefecture earthquake(M 6.8) by the Double-Di#erence tomography,Earth Planets Space, 57, 429�433.

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Okada, T., T, Matsuzawa, J. Nakajima, N. Uchida, T.Nakayama, S. Hirahara, T. Sato, S. Hori, T. Kono, Y.Yabe, K. Ariyoshi, S. Gamage, J. Shimizu, J. Suga-nomata, S. Kita, S. Yui, M. Arao, S. Hondo, T. Mizu-kami, H. Tsushima, T. Yaginuma, A. Hasegawa, H.Zang, and C. Thurber, 2005, Aftershock distribu-tion and 3D seismic velocity structure in andaround the focal area of the 2004 mid Niigataprefecture earthquake obtained by applying dou-ble-di#erence tomography to dense temporary seis-mic network data, Earth Planets Space, 57, 435�440.°�±I 2002 �9²Vh³(>T�N �O´Pn QRn��SW �'��=��2µ(�:;���g£[¤£f 2|��}<~ 111�121.

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310 pp.¶T·¸��U 2005 ¹ 16G �2004G� 12� 14=����@A(��VQI� 29 pp.

Takahashi, H., M. Kasahara, F. Kimata, S. Miura, K.Heki, T. Seno, T. Kato, N. Vasilenko, A. Ivash-chenko, V. Bahtiarov, V. Levin, E. Gordeev, F. Kor-chagin, and M. Gerasimenko, 1999, Velocity fieldof around the Sea of Okhotsk and Sea of Japanregions determined from a new continuous GPSnetwork data, Geophys. Res. Lett., 26, 2533�2536.����n�E F 2005 ����º»A(���C�����A(g£[¤£f �������H��#$I� 68, 199�218.

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Tamaki, K. and E. Honza, 1985, Incipient subductionand deduction along the eastern margin of JapanSea, Tectonophysics, 119, 381�406.�� ���� ����� 2003� ���������������������� �!"#� ��2, 55, 337�350.�� ��$%&'(�)*+,�-./0�- 12� 2005,34 165 126 147�89:;<�=>?@A���B�CDEF��GHI�����JKL�MN�� ���O�GPQREF� 76, 113�128.

Thurber, C. H., 1983, A fast algorithm for two-pointseismic ray tracing, J. Geophys. Res., 88, 8226�

8236.Um, J. and C. H. Thurber, 1987, A fast algorithm for

two-point seismic ray tracing, Bull. Seism. Soc.Am., 77, 972�986.S� T�U�VW� 1991� XYZ[\Y]^_���������`abc�:d][\e� 7f��ghijklm� No. 1, 70.nopq� 2001� ��g� r 3s� tOus� 376 pp.Wessel, P. and W. H. F. Smith, 1995, New version of

the generic mapping tools released, EOS Trans.Am. Geophys. Union, 76, 329.

221