I believe they thought a tsunami of the height it was - 14 meters or 46 feet - was not a credible scenario. Alas for all of us, it was all too credible.

According to Wikipedia:
Quote
The Fukushima Nuclear Accident Independent Investigation Commission found the nuclear disaster was "manmade" and that its direct causes were all foreseeable. The report also found that the plant was incapable of withstanding the earthquake and tsunami. TEPCO, regulators Nuclear and Industrial Safety Agency (NISA) and NSC and the government body promoting the nuclear power industry (METI), all failed to meet the most basic safety requirements, such as assessing the probability of damage, preparing for containing collateral damage from such a disaster, and developing evacuation plans.[21][22] A separate study by Stanford researchers found that Japanese plants operated by the largest utility companies were particularly unprotected against potential tsunamis.

Last edited by IolantheAlias; 08/31/14 11:05 PM. Reason: Add citation