Seismic Fault Rheology and Earthquake Dynamics

As preparation for a workshop on Dynamics of Fault Zones (95 th Dahlem Workshop, Berlin, January 2005), specifically on the sub-topic Rheology of Fault Rocks and Their Surroundings, we addressed critical research issues for understanding the seismic response of fault zones in terms of the constitutive response of fault materials. That requires new concepts and a host of new observations and experiments to document material response, to understand the shear localization process and the inception of earthquake instability, and especially to understand the mechanisms of fault weakening and dynamics of rupture tip propagation and arrest during rapid, possibly large, slip in natural events. We examine in turn the geological structure of fault zones and its relation to earthquake dynamics, the description of rate and state friction at slow rates appropriate to the interseismic period and earthquake nucleation, and the dynamics of fault weakening during rapid slip. The last topic gets special attention here in view of the important recent advances in theoretical concepts and experiments to probe the range of slip rates prevailing during earthquakes. We then address the assembly of the constitutive framework into viable but necessarily simplified conceptual and computational models for description of the dynamics of crustal earthquake rupture. This is done principally in the slipweakening framework, and we examine some of the uncertainties in doing so, and issues of how new understanding of the rapid large slip range will be integrated to model the traction evolution and the weakening process during large slip episodes.