Frictional melting mechanisms of rocks during earthquake fault slip.

Rapid slip, at rates in the order of 1 m/s or more, may induce frictional melting in rocks during earthquakes. The short-lived melting has been thought to be a disequilibrium process, for decades. We conducted frictional melting experiments on acidic, basic, and ultrabasic silicate rocks at a slip rate of 1.3 m/s. The experiments and microstructural observations reveal that all minerals in the rocks are melted at temperatures below their known melting temperatures (T); e.g., quartz is melted at ~ 1000-1200 °C, not ~ 1720 °C, while olivine at ~ 1300 °C, rather than ~ 1700 °C. The low-temperature melting is incompatible with the conventional disequilibrium melting, and may be caused predominantly by grain size reduction and phase boundary reactions during the early and later stages of slip, respectively. The newly estimated T and the melting mechanisms should be considered for understanding the mechanics of earthquakes, landslides, and caldera collapses.