Nucleation of large earthquakes determined by the seismic–aseismic boundary: agreement between models and observations

Using a slip-dependent constitutive law, we investigate how the seismic–aseismic boundary determines the nucleation process of large earthquakes. We model the depth variation of the breakdown strength drop and the critical weakening displacement on the basis of several studies by Ohnaka and co-workers. In this model, the breakdown strength drop increases linearly with depth in an unstable seismogenic zone and decreases sharply in a seismic–aseismic transition at the bottom of the seismogenic zone. On the other hand, the critical weakening displacement is almost constant in the seismogenic zone, and increases sharply with depth in the seismic–aseismic transition at the bottom of the seismogenic region as the fracture energy remains constant. It has been reported that large intra-plate earthquakes appear to initiate at the place where the cut-off depth of seismicity changes sharply. We examine a case in which there is a steep depth change in the seismic–aseismic transition at the bottom of the seismogenic region. The results of the simulation show two stages of the accelerating slip: slip velocity enhancement widely along the seismic–aseismic transition at the bottom of the seismogenic region, and subsequent localized accelerated slip in the region of a steep depth change in the seismic–aseismic transition at the bottom of the seismogenic region. The first stage of the accelerating slip will be important for the intermediate-term earthquake prediction. We also consider a broad aseismic weak zone in the seismogenic zone. In this case, first, slip velocity is enhanced in a broad weak zone, then nucleation starts at lower corner of the surrounding strong region. Our numerical results can explain the location of rupture initiation of some recent large earthquakes.