We consider three questions related to the 2011 Tohoku mega‐earthquake: (1) Why was the event size so grossly underestimated by Japan’s national hazard map? (2) How should we evaluate the chances of giant earthquakes in subduction zones? (3) What is the repeat time for magnitude 9 earthquakes off the Tohoku coast? The maximum earthquake size is often guessed from the available history of earthquakes, a method known for its significant downward bias. We show that historical magnitudes systematically underestimate this maximum size of future events, but the discrepancy shrinks with time. There are two quantitative methods for estimating the corner magnitude in any region: a statistical analysis of the available earthquake record and the moment conservation principle. However, for individual zones the statistical method is usually ineffective in estimating the maximum magnitude; only the lower limit can be evaluated. The moment conservation technique, which we prefer, matches the tectonic deformation rate to that predicted by earthquakes with a truncated or tapered magnitude–frequency distribution. For subduction zones, the seismic or historical record is insufficient to constrain either the maximum or corner magnitude. However, the moment conservation principle yields consistent estimates: for all the subduction zones the corner magnitude is of the order 9.0–9.7. Moreover, moment conservation indicates that variations in estimated corner magnitude among subduction zones are not statistically significant. Another moment conservation method, applied at a point on a major fault or plate boundary, also suggests that magnitude 9 events are required to explain observed displacement rates at least for the Tohoku area. The global rate of magnitude 9 earthquakes in subduction zones, predicted from statistical analysis of seismicity as well as from moment conservation, is about five per century; five actually happened.

You do not currently have access to this article.