Water vaporization promotes coseismic fluid pressurization and buffers temperature rise

We investigated the frictional properties of carbonate-rich gouge layers at a slip rate of 1.3 m/s, under dry and water-saturated conditions, while monitoring temperature at different locations on one of the gouge-host rock interfaces. All experiments showed a peak frictional strength of 0.4–0.7, followed by strong slip weakening to steady state values of 0.1–0.3. Experiments which used a pore fluid with a constant drainage path to the atmosphere showed the development of a temperature plateau beyond 100°C, contemporaneous with the dynamic slip weakening and consistent with thermodynamic considerations of ongoing vaporization of pore water. Upon pore fluid vaporization, the pore pressure increases, while the temperature is buffered endothermically, such that the pore water moves along the liquid-vapor transition curve in a pressure-temperature phase diagram. Pore fluid phase transitions of this kind are expected to occur in natural earthquakes at relatively shallow crustal levels, enhancing fluid pressurization while impeding the achievement of high temperatures. Therefore, the operation of vaporization may help explain the low downhole temperature anomalies obtained shortly after large earthquakes.