Tohoku earthquake: a surprise?

Yan Y. Kagan, & David D. Jackson

Published May 2013, SCEC Contribution #1670

We consider three questions related to the 2011 Tohoku mega-earthquake:
(1) Why was the event size so grossly under-estimated by Japan's national hazard map?
(2) How should we evaluate the chances of giant earthquakes in subduction zones? and
(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 under-estimate 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.

Kagan, Y. Y., & Jackson, D. D. (2013). Tohoku earthquake: a surprise?. Bulletin of the Seismological Society of America, 103(2B), 1181-1194. doi: 10.1785/0120120110.