1Characterization of Fault Zones

There are currently three major competing views on the nature of earthquake fault zones. The first is that faults are (possibly segmented and heterogeneous) Euclidean zones in a continuum solid. The second focuses on granular aspects of faults and deformation fields. The third is that faults are fundamentally rough fractal objects at all relevant scales. The existing data can not distinguish une-quivocally between the three different views or determine their scale of rele-vance. However, in each observational category, the highest resolution results associated with mature faults are compatible with the continuum-Euclidean framework. A positive feedback mechanism associated with strain weakening attracts the long-term evolution of faults toward progressive regularization and Euclidean geometry. A negative feedback mechanism associated with strain hardening during initial deformation phases and around persisting geometrical irregularities generates renewed complexity. We conclude that long term slip ac-commodation may be accounted for to first order within the continuum-Euclidean framework