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TitleLimitation of fluid flow at the Nankai Trough megasplay fault zone
AuthorHammerschmidt, S B; Davis, E EORCID logo; Hüpers, A; Kopf, A
SourceGeo-Marine Letters vol. 33, no. 5, 2013 p. 405-418,
Alt SeriesNatural Resources Canada, Contribution Series 20182827
PublisherSpringer Nature
Mediapaper; on-line; digital
File formatpdf (Adobe® Reader®); html
AreaShikoku; Kii Peninsula; Pacific Ocean; Japan
Lat/Long WENS 133.0000 139.0000 35.0000 31.0000
Subjectstectonics; bedrock geology; structural features; fault zones; splays; faults, thrust; fractures; fluid migration; fluid flow; tectonic interpretations; subduction zones; boreholes; hydraulic analyses; formation pressures; permeability; porosity; models; ocean tide loading; Nankai trough; Kumano Basin; Izu-Bonin Trench; Kyushu-Palau Ridge; Pacific Plate; Eurasian Plate; Philippine Sea Plate
Illustrationsgeoscientific sketch maps; cross-sections; schematic diagrams; models; tables; profiles; lithologic sections; seismic profiles
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2013 08 03
AbstractAlong the Nankai Trough megasplay fault off SE Japan, the effect of fluid migration on subduction-related seismogenesis and tsunamigenesis remains unresolved. To investigate the existence and role of fluid flow, a SmartPlug borehole observatory was installed at Site C0010 of the Integrated Ocean Drilling Program NanTroSEIZE Kumano transect, where a shallow branch of the fault was intersected and in situ fluid pressure monitored from August 2009 to November 2010. The tidal signal in the formation showed no phase shift relative to seafloor loading. The attenuation of 0.73 reflects the loading efficiency accurately, and enabled calculation of a formation compressibility of 1.0×10-9 Pa-1 and a hydraulic diffusivity (HD) of 1.5×10-5 m2 s-1. A similar HD is predicted by a tidal response model based on SmartPlug pressure data. By contrast, permeability measurements on intact samples from Site C0004 SE along-strike the splay fault and from Site C0006 in the frontal thrust zone were found to be similar and one magnitude smaller respectively, despite having a higher porosity. This is explained by the presence of fractures, which are covered by the larger scale of in situ measurements at Site C0010. Consequently, HD can be set to be at least 10-5 m2 s-1 for the splay fault and 10-6 m2 s-1 for the frontal thrust fault zone. Considering recent publications makes fluid flow at the splay fault unlikely, despite the presence of fractures. If the influence of fractures is limited, then processes leading to fault weakening may be enhanced.
Summary(Plain Language Summary, not published)
Data from the first 15 months of recording with the 'SmartPlug' temporary observatory instrument, deployed in the borehole that intersects the Nankai subduction zone mega-splay fault, were analysed for the response of the formation to ocean tidal loading at the seafloor. This provides in-situ estimates for the compressibility of the fault zone rock matrix, 1 x 10-9 Pa-1, and for the hydraulic diffusivity, 1.5 x 10-5 m2 s-1. The former suggests that the fault zone is highly compliant (allows large elastic deformation under small loads); the latter value, higher than values determined on small core samples, belies the influence of fractures and helps to explain the relatively low levels of overpressure observed with the same instrument.

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