The most powerful earthquake to hit Japan since records began has hit the country's north-east and triggered a devastating tsunami. Japanese TV showed cars, ships and buildings swept away by a vast wall of water after the 8.9-magnitude quake. A state of emergency has been declared at a nuclear power plant but officials said there were no radiation leaks.At least 40 people have been killed by the quake, which struck about 400km (250 miles) north-east of Tokyo.