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Internal Structure and Permeability of the Yanagase Fault

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Keiji Sato* and Akito Tsutsumi*

Abstract : We investigated the internal structure and permeability of the Yanagase

Fault, an active fault in SW Japan, at two localities, Kamishimizu in Fukui Prefecture

and Nakakawachi in Shiga Prefecture, through field survey and laboratory measure-

ments. The Yanagase Fault of the studied area is developed in the Jurassic ac-

cretionary complex of the Mino-Tamba Belt. In both outcrops, the Yanagase Fault

consists of - zones : fault gouge zone, fault breccia zone and host rock. Permeability of

the core samples from the fault outcrops was measured with a gas-medium tri-axial

deformation apparatus using a pore-pressure oscillation method. Test results per-

formed at 3*MPa e#ective pressure indicate that the highest permeabilities are found

in the fault breccia zone and the lowest permeabilities in the host rock. In the

Kamishimizu outcrop, permeability is +*�+1-+*�+0m, for the fault breccia, +*�+3-+*�+1m,

for the fault gouge and +*�+3m, for the sandstone host rock. In the Nakakawachi

outcrops, permeability is +*�+2-+*�+1m, for the fault breccia and +*�+2m, for the fault

gouge. The permeable nature of the fault breccia zone of the Yanagase Fault is similar

to the previously reported permeability structure of the fault developed within a

granitic basement rock.

Key words : Yanagase Fault, permeability, fault rocks, active fault

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Smith, +321E' F�"�<� 34���-�GH9I*9JKL�+$M�6�NAOPQR����� ��ST���)���������)UV��6� �WXY)Z[�������ST�����\�$&�' ������� ��]^�$+,NC����<&�6��_?@��@ � F��� !"�"`N�1' ��ST������� �������_`Jab�%���M�2c$&�' de

���S������c1A� fghH�ij<�?@A1�DEvans et al., +331 ; Seront et al., +332 ;

Lockner et al., +333 ; �kl9� ,***E' N9N�<:� mn���ST������<I�W*�o$&p� ;@<�o��q�I�W*��rL��9�1*s ���t� l�uIvw��xy:@A1�1' ;NA� de���S����z{|�s}1C� 34�5�6�����v\��~���K<�?@A1�1�<��$&�' ��S������ ��ST�����r� ��S��� ������ �@�� -�������I��n��MJ��NAh�$&�' Wp�������z��s}1C34�5"�JKNA1HCb�� ��� mn���S�T���������J��N� ?:��F@:��<34�P����n��6������T���o��q�I�W*��rL��9J� ����T���W�A�:9�NA19��@ �:�1'�n� ��� �������2c$&�������c1A� ,� ����¡$T��J¢£�¤¥N� ¦§NC¨��c1A©ª«�¨� D�¬¨

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,**,g 3­ +*�®�' ,**-g -­ ,.�® '¯°±²±²³ ²ij´3µ!¶´²·¸Division of Earth and Planetary Sciences, Graduate

School of Science, Kyoto University, Kyoto 0*0�2/*,,

Japan

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Fig. + (a) The distribution of active faults around

the study area, SW Japan. (b) Localities of the

Kamishimizu and Nakakawachi outcrops (·) along the

northern part of the Yanagase Fault. Based on

Sugiyama et al., (+33.).

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Fig. , A photograph (a) and a sketch (b) of the Kamishimizu outcrop. Open circles represent the

sampling points for the permeability measurements.

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sampling points for the permeability measurements. The stereogram shows equal-area, lower hemisphere

projections of the fault plane (great-circles) and striations on the fault plane (arrows).

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e#ective pressure.

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Fig. 0 Variation of permeability at the Kamishimizu (a) and the Nakakawachi outcrops (b) across the Yanagase Fault.

Permeability of each sample at 3*MPa e#ective pressures are plotted against the distance of the sample point across the fault.

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� �

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quake faulting and a mechanism for partial or

total stress relief. Nature, ,.-, 00�02.

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