DISTRIBUTED DAMAGE, FAULTING, AND FRICTION

We present a formulation for mechanical modeling of geological process es in the seismogenic crust using damage theology. The seismogenic lay er is treated as an elastic medium where distributed damage, modifying the elastic stiffness, evolves as a function of the deformation histo ry. The model damage rheology is based on thermodynamic principles and fundamental observations of rock deformation. The theoretical analysi s leads to a kinetic equation for damage evolution having two principa l coefficients. The first is a criterion for the transition between st rength degradation and recovering (healing), and is related to frictio n. The second is a rate coefficient of damage evolution which can have different values or functional forms for positive (degradation) and n egative (healing) evolution. We constrain these coefficients by fittin g model predictions to laboratory data, including coefficient of frict ion in sawcut setting, intact strength in fracture experiments, first yielding in faulting experiments under three-dimensional strain, onset and evolution of acoustic emission, and dynamic instability. The mode l damage rheology accounts for many realistic features of three-dimens ional deformation fields associated with an earthquake cycle. These in clude aseismic deformation, gradual strength degradation, development of process zones and branching faults around high-damage areas, strain localization, brittle failure, and state dependent friction. Some pro perties of the model damage rheology (e.g., cyclic stick-slip behavior with possible accompanying creep) are illustrated with simplified ana lytical results. The developments of the paper provide an internally c onsistent framework for simulating long histories of crustal deformati on, and studying the coupled evolution of regional earthquakes and fau lts. This is done in a follow up work.