Almost a year after the Japanese Tohoku earthquake and mega-tsunami, the Pacific Ocean is still dealing with the consequences of the catastrophe.

A mass of debris was washed out to sea as floodwaters receded from the land, and some of that wreckage continues to float around the ocean.

Most of it headed eastwards, according to modelling work by the Hawaii-based International Pacific Research Center.

Its staff have given an update to this week’s biennial Ocean Sciences meeting.

"We can only use our model to make projections," explained International Pacific Research Center (IPRC) scientific computer programmer Jan Hafner.

"So far, the debris field has spread in length more than 2,000 nautical miles, and is more than 1,000 nautical miles wide," he told BBC News.

That is approaching 4,000km by 2,000km.

Japanese estimates suggested perhaps 20 million tonnes of debris were generated by the earthquake and the incoming rush of water on 11 March last year.

Most would have stayed on land, and a fair proportion pulled out to sea would have sunk rapidly. But it is possible a million tonnes is still floating on the ocean.