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Preferred orientation of montmorillonite reduces friction by holding water
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  • Hiroshi Sakuma,
  • Ikuo Katayama,
  • Kenji Kawai,
  • Kenji Tamura
Hiroshi Sakuma
National Institute for Materials Science

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Ikuo Katayama
Hiroshima University
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Kenji Kawai
University of Tokyo
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Kenji Tamura
National Institute for Materials Science
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The presence of smectite is critical for weakening the frictional strength of natural faults. The friction coefficient of smectite changes depending on water presence, chemical composition, and preferred orientation. These various factors determine the frictional properties of smectite in a complex manner, and it is difficult to understand the synergistic effects on friction. Here, we examine the synergistic effect of preferred orientation, high affinity of clays to water, and water lubrication. Highly preferentially oriented montmorillonite was prepared as self-supporting sheets, which were dried at temperatures of 70–200°C for 12 h before conducting shear experiments. The humidity-controlled double direct-shear tests of these sheets and powdered montmorillonite were conducted at room temperature under an applied normal stress from 5 to 40 MPa. No effect of drying temperature was observed for the friction coefficients of the powdered gouges, while those of the oriented sheets increased with increasing drying temperature. The slow dehydration of water in the oriented samples was confirmed by thermogravimetry-differential thermal analysis. These results indicate that the preferred orientation of smectite reduces the frictional strength by water lubrication without increasing the pore pressure. Water supplied from the ground and sea during sedimentation enhances the lubrication of oriented smectite at relatively shallow depths and approximately 200°C. The presence of oriented smectite on the subducting plate can hold more water than estimated using smectite powder, and this water in smectite may deepen the updip seismogenic zone boundary by water lubrication.