A Decade of Lessons Learned from the 2011 Tohoku‐Oki Earthquake

The 2011 Mw 9.0 Tohoku‐oki earthquake is one of the world's best‐recorded ruptures. In the aftermath of this devastating event, it is important to learn from the complete record. We describe the state of knowledge of the megathrust earthquake generation process before the earthquake, and what has been learned in the decade since the historic event. Prior to 2011, there were a number of studies suggesting the potential of a great megathrust earthquake in NE Japan from geodesy, geology, seismology, geomorphology, and paleoseismology, but results from each field were not enough to enable a consensus assessment of the hazard. A transient unfastening of interplate coupling and increased seismicity were recognized before the earthquake, but did not lead to alerts. Since the mainshock, follow‐up studies have (1) documented that the rupture occurred in an area with a large interplate slip deficit, (2) established large near‐trench coseismic slip, (3) examined structural anomalies and fault‐zone materials correlated with the coseismic slip, (4) clarified the historical and paleoseismic recurrence of M∼9 earthquakes, and (5) identified various kinds of possible precursors. The studies have also illuminated the heterogeneous distribution of coseismic rupture, aftershocks, slow earthquakes and aseismic afterslip, and the enduring viscoelastic response, which together make up the complex megathrust earthquake cycle. Given these scientific advances, the enhanced seismic hazard of an impending great earthquake can now be more accurately established, although we do not believe such an event could be predicted with confidence. Plain Language Summary The Mw 9 Tohoku‐oki earthquake was one of the most disastrous earthquakes in recent history. In this review, we first clarify the knowledge of the earthquake and tsunami potential before the earthquake. Pre‐Tohoku‐oki studies partly recognized the potential of Mw 8 or larger earthquakes. However, the knowledge based on different types of observations was incomplete and the occurrence of such a great event was not considered in the official earthquake probabilities. The improved understanding of earthquake‐cycle and rupture processes since the Tohoku‐oki earthquake advanced the leading edge of efforts to characterize megathrust earthquake hazards. We can summarize the lessons as follows. (1) The incorporation of interdisciplinary research is essential to advance our understanding of the processes underlying the occurrence of ear