Many seismological studies interpret earthquake sources as shear cracks and use concepts of fracture mechanics to shed light on earthquake source processes. This approach is partially justified by extreme localization of shear deformation on mature faults in the earth’s crust, with kilometers of relative plate motion sometimes accommodated by millimeter-wide shear layers filled with micrometer-sized rock particles. The presentation will discuss recent progress in using laboratory-derived shear resistance laws in the presence of pore fluids for numerical elastodynamic simulations of earthquake source processes and the associated insights, including similarities and differences with the traditional fracture mechanics interpretations