Many earthquakes are preceded by foreshocks. However, the mechanisms that generate foreshocks and the reason why they occur before some earthquakes and not others are still poorly understood. We investigate here the evolution of seismic activity before 65 large earthquakes which have occurred in well instrumented areas. We show that most of the earthquakes occurring along the plate interfaces are preceded by foreshocks. Intraplate earthquakes which result from the internal deformation of the plates are less prone to foreshocks. This difference supports that slow slip of the plate interface precedes many large interplate earthquakes. Within this group a large difference exists in the spatial distribution of foreshocks: While foreshocks to transform fault earthquakes tend to localize close to the future hypocenter, those preceding subduction earthquakes are often spread over a broad area of the plate interface, suggesting that these earthquakes are preceded by the slow slip of a large patch of the subduction interface.
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