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 ar, the temperature increases with cr.r and the earthquake magnitude, Mw. If the thickness of the heated zone, w, is of the order of a few mm, then, even for a modest a1, the temperature rise, ll.T, would exceed 1000° for earthquakes with Mw=5 to 6, and melting is likely to occur, and reduce friction during faulting. If fluid exists in a fault zone, a modest ll.T of 1 00 to 200° would likely increase the pore pressure enough to significantly reduce friction for earthquakes with Mw=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, ER, 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 e =ERIM0 and Mw. The observed e for large earthquakes is 1 0 to 1 00 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.