Frontal view of the interplate megathrust and possible scenario for slip propagation during gigantic earthquakes. A to C, rupture domains defined by Lay et al. (2012). (a) Plate boundary structure during an interseismic period. The asperities associated with M9- and M7-class earthquakes are indicated by broken lines. A part of the M9 asperity is tightly locked by an unruptured seamount. The color gradation from red to blue represents the velocity dependence of shear strength. (b) Triggering of a gigantic earthquake (red star) by breakdown of the locked part of the seamount. The M9 asperity is broken by dynamic rupture. The slow slip region propagates downward. (c) Rupture propagation to the down-dip side of the megathrusts. See text for details.