The very large slip up to the subduction front of the 2011 Tohoku-Oki (Japan) earthquake has challenged our classic view of the megathrust undergoing only aseismic slip at shallow depth. Furthermore, the enhancement of tsunamis during frontal rupturing has increased concern about tsunami risks. Recent seismic reflection images from the Sumatra subduction zone show frontal landward-vergent thrusts in the accretionary prism in the area of supposed shallow ruptures and enhanced tsunamis. Using mechanical analysis, we here show that sudden and successive decreases of the effective friction along the megathrust are required to form landward-vergent frontal thrusts. These decreases are most likely caused by dynamic weakening mechanisms, such as thermal pressurization of the pore fluids related to the propagation of earthquakes to the seafloor. Therefore, landward vergence in accretionary prisms is indicative of past seafloor frontal ruptures and consequent tsunamis. The presence of landward-verging structures in the Cascadia and Sumatra accretionary prisms might indicate future frontal rupture of the shallowest portion of the megathrust, resulting in large tsunamis.