The paper by James R. Rice explores the thermal weakening mechanisms of faults during earthquake slip, focusing on mature crustal faults with highly granulated cores. Field observations suggest that slip occurs primarily within a thin shear zone (1-5 mm) within a finely granulated fault core. The thermal weakening processes involve two main mechanisms: (1) thermal pressurization of pore fluid, which reduces the effective normal stress and shear strength, and (2) flash heating at frictional micro contacts, which reduces the friction coefficient. The author uses theoretical modeling constrained by laboratory data on fault core materials to predict that strength drop should be nearly complete at large slip and that macroscopic melting should be precluded over most of the seismogenic zone. The paper also discusses other weakening mechanisms, such as macroscopic melting and gel formation, and provides a model for the friction coefficient at high slip rates. The findings are supported by seismic data on fracture energy and low heat outflow from major faults, suggesting that thermal weakening mechanisms play a significant role in earthquake dynamics.The paper by James R. Rice explores the thermal weakening mechanisms of faults during earthquake slip, focusing on mature crustal faults with highly granulated cores. Field observations suggest that slip occurs primarily within a thin shear zone (1-5 mm) within a finely granulated fault core. The thermal weakening processes involve two main mechanisms: (1) thermal pressurization of pore fluid, which reduces the effective normal stress and shear strength, and (2) flash heating at frictional micro contacts, which reduces the friction coefficient. The author uses theoretical modeling constrained by laboratory data on fault core materials to predict that strength drop should be nearly complete at large slip and that macroscopic melting should be precluded over most of the seismogenic zone. The paper also discusses other weakening mechanisms, such as macroscopic melting and gel formation, and provides a model for the friction coefficient at high slip rates. The findings are supported by seismic data on fracture energy and low heat outflow from major faults, suggesting that thermal weakening mechanisms play a significant role in earthquake dynamics.