• To what extent do fault-zone complexities, such as bends, stepovers, changes in strength, and other “quenched heterogeneities,” control seismicity? How applicable are the characteristic earthquake and slip-patch models in describing the frequency of large events? How important are dynamic cascades in determining this frequency? Do these cascades depend on the state of stress, as well as the configuration of fault segments?

  • How does the fault system respond to the abrupt stress changes caused by earthquakes? To what extent do the stress changes from a large earthquake change nearby seismicity rates and advance or retard large earthquakes on adjacent faults? How does stress transfer vary with time (49)?

  • What controls the amplitude and time constants of the postseismic response, including aftershock sequences and transient aseismic deformations? In particular, how important are the induction of self-driven accelerating creep, fault-healing effects, poroelastic effects (which involve the hydrostatic response of porous rocks to stress changes), and coupling of the seismogenic layer to viscoelastic flow at depth?

  • What special processes occur at borders or transition regions between creeping zones, whether localized on faults or distributed, and fault zones that are locked between seismic events? Do lineations of microseismicity provide evidence for processes along such borders?

  • What part of aseismic deformation on and near faults occurs as episodes of slip or strain versus steady creep?

5.3 FAULT-ZONE PROCESSES

The move toward physics-based modeling of earthquakes dictates that research be focused on relating small-scale processes within fault zones to the large-scale dynamics of earthquakes and fault systems. Earthquakes have many scale-invariant and self-similar features, yet numerical simulations must assume some smallest length scale in a grid or mesh, as well as a shortest time step, in order to discretize the computational problem. The issue then becomes how to refine the discretization adequately so that principal phenomena are represented qualitatively, if not at the quantitatively correct small size scale. There is also the question of whether it is possible to capture the wealth of processes that occur on sub-grid scales through judicious parameterizations. For example, rate- and state-dependent friction laws suggest that processes at a scale smaller than the coherent slip patch size can be swept into the macroscopic constitutive description. This characteristic dimension appears to be a very small, however—on the order of 0.1 to 10 meters (see Section 5.4). Numerical resolution of processes at that size scale is well



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