For the 2011 Tohoku earthquake, we propose a mechanical model to explain rare giant (M9-class) and frequent large (M7-class) earthquakes on a thrust fault in the subduction zone. Observations implied, in the M9 Tohoku earthquake, that extremely large slip on the order of tens of meters occurs in a shallower part to release a slip deficit, as well as substantial slip about ten meters or so in a deeper part including the source area of the M7-class earthquakes. Here, we present a model in which the extremely large slip is caused by hydrothermal weakening (dynamic thermal pressurization of pore fluid) on the fault plane, not by contrast of frictional properties in terms of rate- and state-dependent friction. The model explains that the Tohoku earthquake followed a M7-class earthquake in two days, but M7-class earthquakes are not always followed by a giant earthquake. In a giant event, large coseismic slip can occur over an area where quasistatic slip, namely, afterslip of M7-class earthquakes or spontaneous slow slip events, takes place. Slight differences of stress state in the shallow part can result in drastically different coseismic slips. We further perform numerical experiments varying hydraulic parameters and the length of effective hydrothermal weakening area. The experiments imply that observations for monitoring the effective hydrothermal weakening area need spatial resolution on the order of 10 km or finer. (C) 2012 Elsevier B.V. All rights reserved.