Contrasting hydrological and mechanical properties of clayey and silty muds cored from the shallow Nankai Trough accretionary prism

• Published in: Tectonophysics 2013-07, Vol.600, p.63-74
• Main Authors: Takahashi, Miki, Azuma, Shuhei, Uehara, Shin-ichi, Kanagawa, Kyuichi, Inoue, Atsuyuki
• Format: Article
• Language: English
• Subjects: Clayey mud; Failure; Failure strength; Friction; Hydrology; Mechanical properties; Mud; Nankai Trough accretionary prism; Permeability; Prisms; Seismic phenomena; Silty mud; Strength
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2013.01.008

Two mud samples cored from the shallow (≈1000mbsf) Nankai Trough accretionary prism at Site C0002 of IODP Expedition 315 are found to be distinctly different in hydrological and mechanical properties. At confining pressures, pore water pressures and temperatures close to their in situ conditions, a clayey mud sample has lower permeability of 2.92×10−19m2, while a silty mud sample has higher permeability of 2.29×10−18m2. Triaxial compression experiments at these conditions and an axial displacement rate of 10μm/s reveal that the clayey mud sample exhibits lower failure strength of 14.2MPa followed by a slow failure lasting for ≈40 s, while the silty mud sample exhibits higher failure strength of 20.1MPa followed by a rapid failure within ≈5 s. Friction experiments at these conditions and axial displacement rates changed stepwise among 0.1, 1 and 10μm/s reveal that the clayey mud sample has a much lower steady-state friction (μss≈0.25) than the silty mud sample (μss≈0.53). Although both samples exhibit velocity strengthening, the former has more than three times larger velocity-dependence of steady-state friction than the latter. Such contrasting hydrological and mechanical properties of the clayey and silty mud samples as revealed in this study suggest the following implications for deformation and faulting in the shallow mud-dominant Nankai Trough accretionary prism. Deformation results in a possible increase in pore pressure, and hence, in strength reduction in clayey mud, but not in silty mud. Faulting would preferentially occur in the weaker clayey mud, and its slow failure may result in a slow slip. Faults formed in clayey mud are weak and easily reactivated, but stable and not seismogenic. In contrast, once the stronger silty mud is faulted, its rapid failure may become a seismic slip. Faults formed in silty mud are strong and not easily reactivated, but possibly unstable and seismogenic. ► Two mud samples from the Nankai Trough accretionary prism are different in lithology. ► One is a clayey mud, and the other is a silty mud. ► Experiments revealed that they differ in hydrological and mechanical properties. ► We discuss the reasons and implications of such different properties.