Microscopic and macroscopic physics of earthquakes

Abstract

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 a 1 , 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 =ERIM 0 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.

Source parameter scaling and radiation efficiency of microearthquakes along the Irpinia fault zone in southern Apennines, Italy

“…An approximate estimation of the dynamic stress drop, Δσ d , e.g., the difference between the yield stress and the dynamic friction during the rupture, can be derived through Δσ d = τ a + Δσ/2 [Kanamori and Heaton, 2000], assuming that fracture energy is negligible. In our case, considering the median estimates of static stress drop (Δσ = 1.4 MPa) and apparent stress (τ a = 0.1 MPa), we obtain Δσ d = 0.8 MPa, that is, the dynamic stress drop is about 50% smaller than the static stress drop, suggesting a significant positive dynamic overshoot during the microearthquake rupture process.…”

Fault Characteristics, Energy Estimates, and Earthquake Recurrence: What One Seismologist Wants from Fault Drilling

“…What is the fracture/healing process on the fault? Kanamori and Heaton (2002) emphasize the importance for understanding and integrating small and large scale results for fault zone studies. The physical processes that occur on microscopic scales of the fault zone are reflected in the macroscopic observations made by seismologists.…”

“…The actual physical process that controls the stress accumulation and large slip of the asperities is currently an active topic of discussion in seismology. Yamanaka and Kikuchi (2004) Kanamori and Heaton (2002) emphasize the importance for understanding and integrating small and large scale results for fault zone studies. The physical processes that occur on microscopic scales of the fault zone are reflected in the macroscopic observations made by seismologists.…”

ESR Techniques for the Detection of Seismic Frictional Heat

“…However, the frictional heat has been hardly estimated from fault rocks except melting-originated pseudotachylytes but calculated from seismological or frictional test data. The frictional heat calculated often gives a large component of the total energy budget although the San Andreas fault heat flow paradox suggests that the contribution of frictional heat may be rather small (Kanamori et al, 1998;Kanamori & Heaton, 2000;Lachenbruch & Sass, 1980. Thus, we had required any new technique to directly estimate frictional heat from fault rocks.…”

“…According to one-dimensional equations on frictional heating (McKenzie & Brune, 1972;Cardwell et al, 1978), the frictional heat strongly depends on the width of heat generation, which is equivalent to the thickness of the slip zone but not to the thickness of a pseudotachylyte vein. The thickness of the slip zone is considered to be commonly an order of millimeters or less (Kanamori and Heaton, 2000;Sibson, 2003). To actually estimate the frictional heat from a fault rock, we must sequentially detect FMR signals at a resolution of 1 mm or less.…”