A small amount of water reduces the friction of the preferentially oriented montmorillonite gouge

The presence of smectite may control the updip seismogenic boundary of a subducting plate. The smectite near the boundary can be dehydrated and highly preferentially oriented in the smectite-to-illite transition zone. Herein, the frictional properties of preferentially oriented (PO) montmorillonite gouge were examined under dry conditions. Highly PO montmorillonites were prepared as self-supporting sheets, which were dried at 70–200 °C for 12 h before conducting shear experiments. The double direct-shear tests on these PO sheets and nominally randomly oriented (NRO) montmorillonite were conducted at room temperature and dry conditions under applied normal stress from 5 to 40 MPa. The friction coefficients of the NRO gouges (0.42–0.45) were not affected by drying temperature, while those of the PO sheets increased with increasing drying temperature from 0.32 to 0.52 at normal stress of 5 MPa. The estimated water contents for both samples depended on the drying temperature, but it was significantly lower (<1 mass%) than the upper limit of the water content of 4.5 mass% for dry friction as estimated from previous studies. A small amount of water (<1 mass%) might become vapor by frictional heating in the low porosity, low permeability PO smectite gouge and increased pore pressure resulting in the decrease of the effective normal stress and shear stress. Although there is much to consider under the natural conditions of smectite-to-illite transition, it was found that preferentially oriented smectite showed a low friction coefficient, even in a dehydrated state.