The authors argue that there were very serious and repeated shortcomings in the methodology used to estimate the highest possible flooding caused by a tsunami.

The authors argue that there were very serious and repeated shortcomings in the methodology used to estimate the highest possible flooding caused by a tsunami. The plant was designed for 6-meter tsunami from an M=7.5 earthquake, but what actually hit them was a 13-meter tsunami from an M=9.0 quake. I understood it's international standard practice to consider worst-case events from historical record spanning thousands of years in nuclear plant risk analysis, and it is known that almost as strong earthquakes hit Japan in 1933 (M=8.5) and 869 (M=8.6), for example. A plant by a different company just 100 km north of Dai-ichi was built for a 13-meter tsunami, so there was little consistency in what preparations the tsunami analysis actually induced, and the regulator seems to have been indifferent to these disparities. Further, there were for some reason no mandatory "safety margins" in tsunami level precautions to offset modelling inaccuracies, unlike in precautions for the earthquake ground shock – eventhough computational models for tsunami levels were and remain much less accurate than the ground shock models. Although by 2000s the shortcomings of tsunami analysis like the one done for Dai-ichi were already well-known, a methodologically-flawed further analysis by TEPCO in 2010 failed to introduce any new recommendations – against the published opinions of their own scientists. The cost of relocating the diesel generators higher up in the building would have been just in the hundreds of thousands of dollars, rather than millions.