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| Home > Publications database > Synchrotron texture analysis reveals the cause for mechanical strong behavior of soft Nankai accretionary prism sediments from offshore SW-Japan |
| Home > Publications database > Synchrotron texture analysis reveals the cause for mechanical strong behavior of soft Nankai accretionary prism sediments from offshore SW-Japan |
| Conference Presentation | PUBDB-2015-00781 |
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2014
Abstract: The International Ocean Discovery Program (IODP) is currently exploring subduction and seismogenesis offshore SW-Japan where earthquakes of magnitude 8.0 to 8.5 and related tsunamis recur every 80-100 years. For the tsunamigenic potential of the forearc slope and accreted sediments their mechanical strength, composition and fabric have been investigated. 19 drill core samples were experimentally deformed in a triaxial cell under consolidated and undrained conditions at confining pressures of 400-1000 kPa, room temperature, axial shortening rates of 0.01-9.0 mm/min, and up to 64% axial strain. Mechanically weak samples show a deviatoric peak stress after only a few percent strain (< 10%) and a continuous stress decrease after a maximum combined with a continuous increase in pore pressure. Strong samples display a constant residual stress at maximum level or even a continuous stress increase together with a decrease in pore pressure towards high strain. Synchrotron texture and composition analysis of the experimentally deformed and undeformed samples using the Rietveld refinement program MAUD indicates an increasing strength of the illite and kaolinite textures with increasing depth down to 523 m below sea floor. Experimentally deformed samples have generally stronger textures than related undeformed core samples and they show also increasing strength of the illite and kaolinite textures with increasing axial strain. When comparing mechanically weak and strong samples it can be seen that the weak samples have a bulk clay plus calcite content of 31-65 vol.-% and most of their illite, kaolinite, smectite and calcite [001]-pole figures have maxima >1.5 mrd. Strong samples which were deformed to approximately the same amount of strain (up to ∼40%) have no calcite and a bulk clay content of 24-36 vol.-%. Illite, kaolinite and smectite [001]-pole figure maxima are mostly <1.5 mrd, except for one sample which was deformed to a considerably higher strain (64%). The higher clay and calcite content and the stronger textures of the mechanically weak samples can be related to a collapsing pore space of the originally flocculated clay aggregates. This process is not effective in the strong samples, but it could be crucial for earthquake rupture, surface breakage and tsunami generation.
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