Causes and Consequences of the great strength variability among soft Nankai Accretionary Prism Sediments from offshore SW-Japan

Geophysical Research AbstractsVol. 16, EGU2014-10666, 2014EGU General Assembly 2014© Author(s) 2014. CC Attribution 3.0 License.Causes and consequences of the great strength variability among softNankai accretionary prism sediments from offshore SW-JapanMichael Stipp (1), Kai Schumann (1), Bernd Leiss (2), and Klaus Ullemeyer (3)(1) GEOMAR, Marine Geodynamics, Kiel, Germany (mstipp@geomar.de, kaschumann@geomar.de), (2)Geowissenschaftliches Zentrum, Universität Göttingen, Germany (bleiss1@gwdg.de), (3) Institut für Geowissenschaften,Universität Kiel, Germany (kullemeyer@geomar.de)The Nankai Trough Seismogenic Zone Experiment of the International Ocean Discovery Program (IODP) isthe very first attempt to drill into the seismogenic part of a subduction zone. Offshore SW-Japan the oceanicPhilippine sea plate is subducted beneath the continental Eurasian plate causing earthquakes of magnitude 8.0 to8.5 and related tsunamis with a recurrence rate of 80-100 years. For the tsunamigenic potential of the forearc slopeand accreted sediments their mechanical strength, composition and fabrics have been investigated. 19 drill coresamples of IODP Expeditions 315, 316 and 333 were experimentally deformed in a triaxial cell under consolidatedand undrained conditions at confining pressures of 400-1000 kPa, room temperature, axial shortening rates of0.01-9.0 mm/min, and up to an axial strain of64% (Stipp et al., 2013). With respect to the mechanical behavior,two distinct sample groups could be distinguished. Weak samples from the upper and middle forearc slope ofthe accretionary prism show a deviatoric peak stress after only a few percent strain (< 10%) and a continuousstress decrease after a maximum combined with a continuous increase in pore pressure. Strong samples fromthe accretionary prism toe display a constant residual stress at maximum level or even a continuous stressincrease together with a decrease in pore pressure towards high strain (Stipp et al., 2013). Synchrotron textureand composition analysis of the experimentally deformed and undeformed samples using the Rietveld refinementprogram MAUD indicates an increasing strength of the illite and kaolinite textures with increasing depth downto 523 m below sea floor corresponding to a preferred mineral alignment due to compaction. Experimentallydeformed samples have generally stronger textures than related undeformed core samples and they show alsoincreasing strength of the illite and kaolinite textures with increasing axial strain. Mechanically weak samples havea bulk clay plus calcite content of 31-65 vol.-% and most of their illite, kaolinite, smectite and calcite [001]-polefigures 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 figuremaxima are mostly