We propose a theoretical model which can explain the occurrence of foreshocks and pre-events associated with the nucleation of large earthquakes. We consider a broad weak zone with a locally strong part (asperity) on a fault plane in a two-dimensional framework and examine the transition process from quasistatic nucleation to high-speed rupture propagation through numerical simulations. The dynamic rupture of the asperity occurs in three different manners, as aseismic slip, foreshock or pre-event, depending on the peak stress of the asperity. In all cases the rupture of the asperity accelerates the nucleation of main rupture. ¿ American Geophysical Union 1995