The devastating 2011 Tohoku earthquake was observed by dense networks of geophysical instruments. A unified source model was constructed through joint inversion of teleseismic, strong motion, and geodetic datasets. The result indicates that the earthquake consists of three main ruptures. After small rupture in the initial 50 s, the first rupture expanded at a slow speed of 1.8 km/s to the northeast and east. The second rupture began 20 s later at the slowest speed of 1.5 km/s and became dominant with the largest slip of 36 m. The third rupture then played the leading role, propagating southward at a speed of 2.5 km/s. Only the tsunami inversion requires an extension of the source fault to a shallow part between the first rupture area and the Japan Trench, recovering tsunamigenic slips larger than 40 m. The slow rupture speed and tsunamigenic slips can explain the features of the disaster by the earthquake.