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Page 1: MoEFRwx - 東京大学

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High-resolution seismic reflection profiling across the

Shiraiwa fault, eastern margin of the Yokote basin

fault zone, northeast Japan : data acquisition and

processing

Kyoko Kagohara+)*, Toshifumi Imaizumi,�, Tomoo Echigo-�, Takahiro Miyauchi.�, Shin

Koshiya/�, Masaru Noda/�, Hajime Kato0�, Shigeru Toda1�, Tatsuya Ishiyama2�, Hiroshi

Sato3�, Shinsuke Okada+*�, Satoshi Kanda++�, Naoto Kamiya++�, Nobuto Morishita,�, Shuichi

Takahashi,�, Kosuke Hashimori,�, Satoko Shimizu+,�, Kota Yamazaki+-�, Taro Koike+.� and

Takeshi Ikawa+.�

+�Graduate School of Science, Tohoku University, ,�Graduate School of Science, Tohoku University,-�Graduate School of Science, the University of Tokyo (Now at Geo-Research Institute), .�Graduate

School of Science and Technology, Chiba University, /�Faculty of Engineering, Iwate University,0�Faculty of Education and Human Sciences, Yamanashi University, 1�Faculty of Education, Aichi

University of Education, 2�Active Fault Research Center, National Institute of Advanced Industrial

Science and Technology, 3�Earthquake Research Institute, the University of Tokyo, +*�Graduate

School of Science, the University of Tokyo, ++�Graduate School of Engineering, Iwate University,+,�Faculty of Science, Tohoku University, +-�Faculty of Education, Aichi University of Education

(Now at TSUDA INDUSTRIES CO., LTD.), +.�Geosys Inc.

Abstract

The eastern margin of the Yokote basin fault zone extends about /0 km at the western foot of

the Ou Backbone Range, northeast Japan. The Rikuu earthquake (Mv1.,) occurred in the Ou

Backbone Range (Mahiru Range) on -+st August, +230. Associated with this earthquake, four thrust

faults-Obonai, Shiraiwa, Ota, and Senya fault- appeared on the surface of the western foot of the

Mahiru Range. These faults were highly sinuous with numerous gaps and en echelon steps.

We conducted a high-resolution seismic reflection profiling survey across the Shiraiwa fault.

The obtained seismic reflection data were processed by conventional common mid-point methods,

post-stack migration, and depth conversion. The subsurface structure across the Shraiwa fault is

characterized by branched low-angle reverse faults and conjugate back-thrust. The emergent

thrust associated with the +230 earthquake is regarded to be a subsidiary reverse fault.

M o E F R w xBull. Earthq. Res. Inst.

Univ. TokyoVol. 2+ I,**0� pp. +,3�+-2

* e-mail : d*-*[email protected] Iy32*�2/12 z{|}T~���}T 0�-�

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Key words : Riku-u earthquake, Shiraiwa fault, thrust fault, subsurface structure, high-resolution

seismic reflection profiling

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Page 3: MoEFRwx - 東京大学

���������� ���������������� �� +*�2* Hz, +*�+** Hz, +*�+,* Hz

����� ���� / �!"#���� +2* ch$%&'�� ()� *�+�,-"�%&�� /** msec�AGC�.'��/� �����,-"�%&�01'�23$�� 456� *.-7*.28�9:+�/;<=>�?@ABCDE��/ +*�+** Hz��� ��F'� ��G� ,*8� HI!"#�� +* $J�'� KLM� �NOP��: 1 !"#�QRS� T+�/U��V�WX!Y�Z[�\�,�] GDAPS-

. KK^SU_`abcdefgS �h�� +2* ch$i&

'�� 23$� RP+**+-++,*� RP+-*2-+-02�jklmF'� RP++,+-+-*1�no 0* ch, po +,* ch�qrsVlm�t�Yutvwxy$i&'�� zh'�{|}� +* Hz K3~�S ${|�N�� +* mF'��-�, �����23W�!�� ;<�W�!@�h��� Super X-C

KK^SU_`abcdefgS�h���L��(��;<��c�$@�'� KFig. .S� W�!@�����[�!� Table ,��'� �����(��������/�

Fig. +. (A) Shaded relief map based on ,/* m DEM showing the locations of active thrusts in the Tohoku district.

Active faults are after Ikeda et al. (,**,) (B) Simplified geological map in and around the Yokote basin. Surface

geology is compiled from Usuda et al.(+310, +311, +32*), Osawa and Suda (+32*) and Osawa et al. (+322).

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� ������� �CMP Sorting������� ���� ������ �������� ������� �� CMP!"�#�$%�&' !"�#( Fig. ,)*+' � �#),-.���� /0�1-&' CMP 23( / m)4%�� CMP!"�#)5-&!"6(� 78 329 -&:Fig. /;'

� �� �Gain Recovery<=> ��� ?@ ABCDE�FG.� tnH�IJK9LM�IJK :AGC; �1G� �I NO�PQRS&'

tnH�IJK :n; : +.,, AGC Gate length : 0** msec

� ������ �First Break MuteCMP sortT)UVWM XYGZ��[\�&' F

G&ABCDE� Table -)*+'� ���������� �Deconvolution��]^_`�] ab6cde� bMfghi)^jklm ab6nop� qr�sDE��t%�\u�� �vw xGyz�{k&�� s|}~��D��}���1-&' ���(?@ ABCDE�FG&'

Operator length : +** msec, Gate length : +*** msec

White noise factor : /� Prediction length : , msec

� ����� �Static Correction�. ���>yz��UVWM�������� � �������)^jk Time-term�� ^_`����������69+k�}�D��} :��E��ED��; �1G� ��� ���&' v¡)(iRAS ::¢;£¤¥¦§"¨©ª«; �¬F�&' �}

Fig. ,. Location map of the Kotaki ,**/ seismic line and CMP stacking line.

Table +. Data acquisition parameters for the Kotaki

,**/ seismic line.

­®¯° ±

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Page 5: MoEFRwx - 東京大学

Fig. -. Examples of shot gathers at di#erent sweep

frequency.

A. Sweep frequency +*�2* Hz.

B. Sweep frequency +*�+** Hz.

C. Sweep frequency +*�+,* Hz.

Fig. .. Flow chart of data processing.

Table ,. Processing parameters for the CMP of the Kotaki ,**/ seismic data.

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Page 6: MoEFRwx - 東京大学

���������� �������� Time-

term�� ������������� Time-term�������� !��"�#$% Fig. 0&'() *+� ,�-./0�1234�� ��5 +�������67 0** m�sec%89�):����&�;<� ��5 +��=!�>?+@ABC>?&@D7EFGHI>?�JK L��MJKN %OD�)

� ���� �Velocity Analysis� �NMO� �Nor-

mal Move Out Correction�P��QRS LConstant Velocity StackN &@G��TU%OD�) TU�V ,* CMPW&XY� Z[ 3 CMP

�\�]�P��QR^_`%ab67OD�) ��QR��#$% Fig. 1&'() :���&�;c7def-.gh%ij/0%(k7 Normal Time&JK6�) JK&lmno�p2%qr(G�s&� no�tuvAw% ,.+x&ry6� :�%z9G{|VCMP gather%�&� }~�.%X�6��6�) �c�^���]% Table .&'()� ��� �Residual Static Correction�

NMOJK[�\�]&�67� ��MJK%Oc�MJK�VJK�<*�D���n�HI��j<%JK6�) JK������V 0 msec���6�)

Fig. 0. Surface structure determined by time-term analysis.

(A) Time-terms.

(B) Velocities of second layer.

(C) Topography and geometry of surface low velocity layer.

Fig. /. Distribution of number of folds along the CMP stacking line.

Table -. First break mute parameters for the CMP of

the Kotaki ,**/ seismic data.

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� ������� �CMP StackNMO��� CMP gather����� �� ���

CMP������ +������������ � ��� !"#$% *�2** m&'����()���� TV�� � �Time-Varying Filter*�+�,-.�/0�1&2� 3456789� :9;<�=>?@ABCDEF GHIJ� CMP� KL�M��� NO��PQR�89�ST3456�*�+�UV�3W'X��

Operator length : ,.* msec

Time Pass-band frequency

*� .** (msec) ,*/,2�+**/++* (Hz)

.**� 2/* (msec) +2/,.� 3*/+** (Hz)

2/*�-*** (msec) +*/+/� 2*/ 3* (Hz)

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Filter*�+�:9Y6�Z[\]^_`a GHIJ�NO�b)�\� cCdefgh�iP�� jkl� S�N�<mno�pq�rM��� )[�EcsJI�UV��t�

Operator length : - CMPs, Gate length : -* CMPs,

Time window length : /** msec

�� �������� �Migration� KL�uv��wx���yz{lA|����}~�� SW��A�V���������l��\��H��" 89�g��J��C�b)��� ��A�������-./0 �� ���+� ���;<������ 1*��F�J�C��M�����)[��

Fig. 1. Optimum stacking velocities determined by velocity analysis.

Table .. Post NMO mute parameters for the CMP of the Kotaki ,**/ seismic data.

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Page 8: MoEFRwx - 東京大学

�� ���� �Depth Conversion����������� ������� �������������� ������� ��! � �"���#$%��&' +.* m()*!

.� ��� �������������+,���-�� Fig. 2-A, ���������-�� Fig. 2-B,��� -�� Fig. 3�./0/12!./0/�-�('� 345�� *.26� ������*.1 km7(�89:;<=>?/�! @�ABC�'D�%� ��=9EF��G� H��/�IJ(�,K�LMN� OPQRSTSU� +31.�WS-+.VPQRSTSU� +31/�WS-,* ; WX' Fig. +YZ[ �\]��^��_`abc�de�� ��� -�c�fg�� OFig. 3[! 9h� ij('� klmE O+310[ �_`�����no2*! p_q('�� ,** m7(=rst� �� ,**�/** m=�t� �� /** muvjW=wxytVz{|/*!

}~p_j OCMP+�,/*[ ('� �tE?� *.06 O�� .** m[ �E��� m���9:;�=����G?/*=� CMP+/*�,**('@/?�:;����� O,**

mi�[ =�! @/����� CMP-/*i� Owx�_q[ ('� :;�=-��()v� p_���2*��tV���=��()*! 7�� �_����'wxytE?9v� y�-ti�(' CMP-**��7(�t=����hv� ��� �j���e2*@V=�EF�! @��t' CMP,/*�-/*� ¡(�¢£¤���*�¥=��()v� CMP--*�./*('� @��t�¦§m���9:;��=�G?/*! @�:;��'� p_¨���2*©ª��|�:;���«¬��­��hv� rst��^|/*V®¯?/*!@�rst' CMP,**�--*�� Om� +** mi�[ ('�°o��t�¦F�±V���¢£¤2*!rst��²�³�?2-t'_��-t´ Oµ¶-t ; CMP

,**[ E?� �¢ ,/��£¤�³F� CMP.**���.** m��7(·¸|/*! |?�� :;-�(�t

Fig. 2. Filtered stacked section (A) and post stacked, migrated time section (B).

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����������� ���� �CMP-.*��� �� ������������� �Fig. ,�! "#� $%� CMP+/*�,**�&' +**�,** m���(���� �)��� �*+��,�� CMP+/*�&',** m��-� CMP-/*�&' /** m+./0�12���34 5�.67��! ���� CMP/**�0**�&' ,**�.** m��� 89:��;<= ��>?@A��� B>?@��� @����� C�C.-�A�����@DE��� F���1G�HI������J !

/� ���K�����<=�LMN�+�OPQR�-� C.QST.UV� WX�Y���Z.[�\]^�

�Q_� -.2 km�`�89a�YbcQdeU#!fg89hija�klmnop�kqV� &'�:QrsU#! ]^����� tuQvw/0���@�F�x#<=�\.DE��! yz�{|U#�:�} ~�����T���� ���������Q_��: ������� +332 ; ���-� ,**1 ; ���-� ,**1a, ,**1b� .����[�V�������E�\!

� �y��QMw�\#�� ��� ������ � ¡¢£Q���� }¤¥¦}��§¨©£��ª«¬�N­��� ®¯°Q�#±�#! 89a�Ybc²³�� �� ´µ��¶·.UV®¯°Q�#±�#! bc�\

Fig. 3. Depth converted seismic section (above) and its geologic interpretation (bottom). Black, heavy lines and thin

lines indicate main faults and subsidiary faults, respectively. The lines are dashed where inferred.

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