Characterization of fault zone structures

Yehuda Ben-Zion*
USC

*based on a joint paper with Charlie Sammis

We review conceptual frameworks and data on the character and properties of 
earthquake fault zones. There are currently three major competing views on 
the nature of faults. 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. Each framework 
has very different implications for the mechanics of earthquakes and faults. 
The existing data can not distinguish unequivocally between the three different 
views or determine their scale of relevance. However, in each observational 
category, the highest resolution results associated with mature faults are 
compatible with the continuum-Euclidean framework. This can be explained by a 
positive feedback mechanism associated with strain weakening rheology and 
localization, which 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 and conjugate sets of faults 
generates new fractures and granularity at different scales. We conclude that 
long-term deformation in the crust, including many aspects of the observed 
spatio-temporal complexity of earthquakes and faults, may be explained to 
first order within the continuum-Euclidean framework.