No abstract

In natural fault zones, shear strain is mainly accommodated in fault rocks (or fault materials) that make up the fault core (i.e., a central high-strain zone of a brittle fault; Fossen, 2016). The shear strain may be localized in short-term and long-term scales. The long-term localization has been reported from both seismogenic and aseismic fault zones (e.g., Sibson, 2003). Seismic rupture events may repeatedly occur in a slip zone over a long period, expressed by progressive displacement of geomorphic features (e.g., Lensen, 1968). Also, aseismic fault slip may be accommodated in narrow zones of meters to decimeters in thickness (e.g., the Hayward fault; Bilham & Whitehead, 1997). Aseismic shear localization observed from statically weak faults appears to be promoted by the operation of time-dependent, fluid-assisted deformation mechanisms (e.g., dissolution-precipitation creep) in fault rocks (e.g.,

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 (Tm); 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 Tm and the melting mechanisms should be considered for understanding the mechanics of earthquakes, landslides, and caldera collapses.