Fault strength drop due to phase transitions in the pore fluid

We conducted tri-axial friction experiments for quartz gouge at hydrothermal conditions to examine the effect of phase transition of water. We observed a decrease of similar to 0.05 in its coefficient of friction during the transition between liquid and vapor. This can be interpreted as a local pore pressure increase in the gouge-filled layer during the phase transition, which is caused by the surface tension of bubbles created between the two different phases. The local pore pressure increase deduces effective normal stress in the gauge layer, which makes a decrease of the frictional strength. The transient friction drop on a fault can play an important role in triggering an earthquake not only in hydrothermal areas but also in typical seismically active areas in the crust where water often contains CO2 of various concentrations because the CO2 density in the binary system of H2O-CO2 controls the pressure-temperature condition of liquid/vapor phase equilibrium.