preted as being due to slip weakening on a single major fault versus a network of dynamically stressed secondary faults?

  • When does the rupture path follow a fault that branches off from the major failure surface? What is the role of pre-stress magnitudes and orientations and of the dynamically altered stress distribution near the rupture front? How do ruptures surmount stepovers? Are elastic descriptions adequate for the stepped-over material, or is there an essential role for damaged rock and smaller fault structures within the stepover region?

5.4 RUPTURE DYNAMICS

Earthquake rupture entails nonlinear and geometrically complex processes near the fault surface, generating stress waves that evolve into linear (anelastic) waves at some distance from the fault. Better knowledge of the physics of rupture propagation and frictional sliding on faults is therefore critical to understanding and predicting earthquake ground motion. Research on rupture processes may also contribute to improvements in earthquake forecasting because of the dynamical connection between the evolution of the stress field on interseismic time scales and the stress heterogeneities created and destroyed during earthquakes.

Rupture Initiation

The process leading to the localized initiation of unstable stick-slip in laboratory (82) and theoretical (83) models of the earthquake process is referred to as earthquake nucleation. In frictional fault models, stick-slip instabilities can begin only in regions where the progression of slip causes the fault friction to decrease. For the rate-state model, this situation corresponds to velocity weakening—when the steady-state friction µss decreases with velocity V:

(5.1)

The dimensionless rate dependence a – b can vary with rock composition, temperature, and pressure. Equation 5.1 defines the condition at which earthquake nucleation can occur (Figure 4.30). However, a correspondence between the depth range at which earthquakes occur and the region where a – b is negative has not been confirmed by independent observations of velocity weakening, and there is no micromechanical theory that can be used to extrapolate laboratory data to crustal conditions. Nevertheless, the available lab information on the effect of temperature on the constitutive parameters, combined with inferred geotherms, suggests



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