A unified seismic source model is proposed which can explain diverse earthquake rupture processes, an earthquake faulting consisting only of aseismic slip, an aseismic fault slip preceded and/or followed by seismic faulting, mainshock-aftershocks sequence, multiple-rupture event, an earthquake swarm and the like.
We analyse, as a mathematical model, quasistatic extension of longitudinal shear cracks in a viscoelastic medium. A Dugdale model is adopted as a fracture model. An inhomogeneously distributed stress drop is assumed on the crack surface. It is shown that the viscoelastic properties of the medium, the fracture strength at the crack tip and the inhomogeneity of stress drop distribution play important roles in the occurrence of the diversity of earthquake rupture processes.