Despite evidence that events of similar magnitude the this month's tsunami have happened in Japan before, the Fukushima nuclear power complex was not designed to withstand such a catastrophe.
Designers of Japan's stricken nuclear power plant ignored risk evidence
TOKYO // In planning their defence against a killer tsunami, the people running Japan's now-hobbled nuclear power plant dismissed important scientific evidence and all but disregarded 3,000 years of geological history, documents show.
The misplaced confidence displayed by Tokyo Electric Power Company (Tepco) was prompted by a series of overly optimistic assumptions that concluded the Earth couldn't possibly release the level of fury it did two weeks ago, pushing the six-reactor Fukushima complex to the brink of multiple meltdowns.
Instead of the reactors staying dry, as contemplated under the power company's worst-case scenario, the plant was overrun by a torrent of water much higher and stronger than the utility argued could occur, according to an analysis of records, documents and statements from researchers, the utility and Japan's national nuclear safety agency.
Tepco and government officials have said no one could have anticipated such a massive tsunami. However, there is ample evidence that such waves have struck the north-east coast of Japan before, and that it could happen again along the culprit fault line, which runs roughly north to south, offshore, about 350 kilometres east of the plant.
Tepco officials say they had a good system for projecting tsunamis. They declined to provide more detailed explanations, saying they were focused on the ongoing nuclear crisis.
What is clear is that Tepco officials discounted important readings from a network of GPS units that showed that the two tectonic plates that create the fault were strongly "coupled", or stuck together, thus storing up extra stress along a line hundreds of kilometres long. The greater the distance and stickiness of such coupling, experts say, the higher the stress buildup, pressure that can be violently released in an earthquake.
That evidence, published in scientific journals starting a decade ago, represented the kind of telltale characteristics of a fault being able to produce the truly overwhelming quake - and therefore tsunami - that it did.
On top of that, Tepco modelled the worst-case tsunami using its own computer program instead of an internationally accepted prediction method.
It matters how Japanese calculate risk. In short, they rely heavily on what has happened to figure out what might happen, even if the probability is extremely low. If the view of what has happened isn't accurate, the risk assessment can be faulty.
That approach led to Tepco's disregard of much of Japan's tsunami history.
In postulating the maximum-sized earthquake and tsunami that the Fukushima complex might face, Tepco's engineers decided not to factor in quakes earlier than 1896. That meant the experts excluded a major quake that occurred more than 1,000 years ago - a tremor followed by a powerful tsunami that hit many of the same locations as the recent disaster.
A Tepco reassessment presented only four months ago concluded that tsunami-driven water would push no higher than 5.7 metres once it hit the shore at the complex.
However, the wall of water that thundered ashore two weeks ago reached about 8.2 metres above Tepco's prediction.
The fault involved in the tsunami is part of what is known as a subduction zone. In subduction zones, one tectonic plate dives under another. When the fault ruptures, the sea floor snaps upward, pushing up the water above it and potentially creating a tsunami. Subduction zones are common around Japan and throughout the Pacific region.
Tepco's latest calculations were started after a magnitude-8.8 subduction zone earthquake off the coast of Chile in February 2010.
In such zones over the past 50 years, earthquakes of magnitude 9.0 or greater have occurred in Alaska, Chile and Indonesia. All produced large tsunamis.
When two plates are locked across a large area of a subduction zone, the potential for a giant earthquake increases. And those are the exact characteristics of where the most recent quake occurred.
Tepco "absolutely should have known better", said Costas Synolakis, a leading US expert on tsunami modelling and an engineering professor at the University of Southern California. "Common sense," he said, should have produced a larger predicted maximum water level at the plant.