The creation of new fractures controls fault slip, produces rock damage, and contributes to the dissipation of energy during earthquakes. Pulverized rocks around faults contain grains with a wide range of sizes, but the mechanisms that produces nanoscale grains remain elusive. Using molecular dynamics simulations, we model tensile rupture propagation in alpha-quartz under conditions of stress that occur during earthquake propagation. Our results show that for rupture speeds below 15 % of the Rayleigh wave speed, the fracture propagates straight. At higher speeds, fracture propagation undergoes path instabilities with crack oscillations and microbranching. We show that microbranching can lead to nanoscale fragments. Energy dissipation occurs by the creation of fracture surfaces and material damage; the dissipated energy increases with rupture speed. This nanoscale mechanism of irreversible deformation during earthquake propagation conditions contributes to the energy budget of earthquakes, and damage production in fault zones.