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Velocity‐weakening behavior of Westerly granite at temperature up to 600°C

The deep limit to seismicity in continental crust is believed to be controlled by a transition from velocity‐weakening to velocity‐strengthening friction based on experimental measurements of the rate dependence of friction at different temperatures. Available experimental data on granite suggest a transition to stable creep at about 350°C (∼15 km depth). Here we present results from unconfined experiments on Westerly granite at both dry and hydrated conditions that show increasingly unstable slip (velocity‐weakening behavior) at temperature up to 600°C. A comparison of previously published experimental results with those presented in this study suggests that the rate and state friction parameters strongly depend on normal stress and pore pressure at high (>400°C) temperature, which may help explain regional variations in the depth distribution of earthquakes in continental crust. Temperature dependence of the rate and state friction parameters may also contribute to strong dynamic weakening observed in high‐speed friction experiments on crystalline rocks such as granite and gabbro.