Microscopic and macroscopic physics of earthquakes

Frictional melting and fluid pressurization can play a key role in rupture dynamics of large earthquakes. For faulting under frictional stress σ_ƒ, the temperature increases with σ_ƒ and the earthquake magnitude, M_w. If the thickness of the heated zone, w, is of the order of a few mm, then, even for a modest σ_ƒ, the temperature rise, ΔT, would exceed 1000° for earthquakes with M_w = 5 to 6, and melting is likely to occur, and reduce friction during faulting. If fluid exists in a fault zone, a modest ΔT of 100 to 200° would likely increase the pore pressure enough to significantly reduce friction for earthquakes with M_w = 3 to 4. The microscopic state of stress can be tied to macroscopic seismic parameters such as the seismic moment, M_0, and the radiated energy, E_R, by averaging the stresses in the microscopic states. Since the thermal process is important only for large earthquakes, the dynamics of small and large earthquakes can be very different. This difference is reflected in the observed relation between the scaled energy ẽ = E_R/M_0 and M_W. The observed ẽ for large earthquakes is 10 to 100 times larger than for small earthquakes. Mature fault zones such as the San Andreas are at relatively moderate stress levels, but the stress in the plate interior can be high. Once slip exceeds a threshold, runaway rupture could occur, and could explain the anomalous magnitude-frequency relationship observed for some mature faults. The thermally controlled slip mechanism would produce a non-linear behavior, and under certain circumstances, the slip behavior at the same location may vary from event to event. Also, slip velocity during a large earthquake could be faster than what one would extrapolate from smaller earthquakes