Rapid seismological analyses, carried out within minutes of the March 11, 2011, Tohoku earthquake, were crucial in providing an earthquake ground shaking and tsunami early warning and in hastening the evacuation of the population along Japan’s northeastern coast. By 20 to 30 minutes after fault rupture began, these analyses had established that the event had a moment magnitude of Mw = 9 and involved shallow thrust faulting on the plate boundary megathrust. Preparation for future large earthquakes on megathrusts in Japan and elsewhere should include onshore and offshore geodetic monitoring of strain accumulation, implementation of rapid earthquake and tsunami warning systems, and public training and education for shaking and tsunami response.