Recent modeling of tsunami waveforms caused by tsunami earthquakes indicates that fault slip occurred along the subduction boundary close to the trench axis. On the other hand, it is well known that seismicity is generally very low within ca. 50 km from the trench axis, and this has been attributed to the stable frictional sliding properties of the subducted unconsolidated sediments. This makes tsunamigenic slip near the trench difficult to explain. In order to solve this dilemma, I propose that transition from stable sliding to nearly zero friction, with a velocity-weakening property, occurs as a result of areal increase of zones of elevated fluid pressure, which may make fairly rapid seismic slip possible following the breakage of asperities at normal seismogenic depths. This transition can be identified as seismic reflections with negative polarities, which may help to rate hazards for a coming large tsunami earthquake