Aftershocks to large earthquakes pose a significant hazard, a fact that was highlighted by damaging aftershocks to the 1999 Turkey and Taiwan mainshocks. A new approach is introduced to quantify this hazard in terms of physical parameter maps that forecast the probabilistic aftershock hazard based on the observed spatially varying aftershock activity. For two case studies, the Landers and Northridge mainshocks, probabilistic aftershock hazard maps are shown to improve conventional aftershock hazard assessment. ¿ 2000 American Geophysical Union