propagation processes. Second, it defines the dimensions of the region of precursory strains related to the earthquake nucleation process. Small scaling lengths impose severe restrictions on numerical calculations and could also mean that precursory phenomena related to earthquake nucleation may be difficult or impossible to detect.

Stress Interactions and Short-Term Clustering

Although major earthquakes generally tend to be associated with large faults easily recognized at the surface, instrumentally recorded seismicity indicates that smaller earthquakes become more diffusely distributed as their size decreases. The smallest earthquakes often arise on faults with no known surface expression. Stress-mediated interactions among these fractal fault systems can be explored by using the scaling behavior of the seismicity to monitor system organization as a function of time. This type of regional seismicity analysis offers the most promising approach to intermediate-term prediction.

A widely studied type of fault interaction arises from the permanent change of the stress field following an earthquake. According to the Coulomb stress condition for frictional failure (Equation 2.1), an increase in the magnitude of the shear stress acting across a fault should push it closer to failure, while an increase in normal stress should increase the effective frictional strength, thus retarding failure. An important recent discovery is that regional seismicity appears to be correlated with the relatively small Coulomb stress increments calculated from static dislocation models of large earthquakes (31). This interpretation of seismicity has been largely successful in explaining the patterns of aftershocks as well as regions of reduced seismicity (“stress shadows”) following large events along the San Andreas fault system (32), the 1999 Izmit earthquake in Turkey (33), and various earthquakes in Japan, Italy, and elsewhere (34).

The Coulomb stress calculations usually assume purely elastic interactions at the time of the mainshock. This is a reasonable approximation in the outer layers of the brittle crust, but it does not describe known postseismic processes, which include ductile flow below the seismogenic zone, fault creep (earthquake afterslip), and poroelastic effects (due to fluid flow) that all result in extended intervals of stressing in the region of a large earthquake (Section 4.2). The role these postseismic effects have in controlling, or altering, aftershocks sequences is presently not well understood, but the stress changes due to these processes are usually rather small compared to the immediate stress change caused by the mainshock.

Aftershocks are thought to be primarily a response of the surrounding fault system to stress changes caused by the mainshock fault slip. That is, the Coulomb stress changes drive the aftershock fault planes to failure.

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