Media playback is unsupported on your device Media caption Animation of how the Japanese tsunami debris field has spread since March 2011. Courtesy of the International Pacific Research Center.

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.

Shipping forecast

Video pictures at the time showed all manner of materials caught in the flow, from upturned boats and cars to whole buildings picked up off their foundations.

The dominant movement of water off Japan is the Kuroshio Current, the North Pacific equivalent of the Gulf Stream in the Atlantic.

It hugs the Asian continental slope until about 35 degrees North, where it is then deflected due east into the deep ocean as the Kuroshio Extension. A lot of the floating material rode this extension.

Media playback is unsupported on your device Media caption Jan Hafner: 'It's good to know where the debris is'

The IPRC, which is based at the University of Hawaii, was already studying debris paths in the Pacific at the time of the tsunami, and was perfectly placed to produce exquisite forecasts of where the Japanese material was likely to spread. These forecasts continue to be updated daily.

The team's model incorporates sea surface height and wind data acquired by satellites. They developed an animation that shows the likely evolution of the field up to this week.

It is important to note that although the field appears as a block of colour in the simulation, the block does not represent a physical island of debris.

Rather, it is the proximity of the large number of tracer points used in the model which, when bunched together at the reduced resolution of the animation, appear as a tightly-fitted mass of colour.

What the simulation shows is the area of ocean where debris might be found, but look over the side of any ship and you would very probably see no debris at all because the individual items have now become widely separated.

However, the information is of immense interest to shipping authorities because objects in the water, depending of their size, can be a serious collision hazard.

Patchwork

Another keenly interested party is the Papahanaumokuakea Marine National Monument, the marine park encompassing the northwestern Hawaiian islands and atolls, such as Midway. It is an area of outstanding natural beauty and is home to many interesting and endangered species.

So far, the modelling has suggested the bulk of the field is passing to the north of the monument.

"However, the currents have changed and so we expect reports [of debris washing up] from Midway and the Kure Atoll soon," said Mr Hafner.

The debris may touch the west coast of the US in another year or two, but what does make landfall will be a small percentage of the overall floating mass.

Ultimately, the IPRC work suggests, at least 95% of the debris that has not sunk will move into the North Pacific "Garbage Patch", a long-lived circulation of floating rubbish trapped by the North Pacific Gyre.

Over time it will decay and sink. The concern for conservationists is that smaller, particularly plastic fragments can be ingested by marine organisms.

The modelling work at IPRC is led by Nikolai Maximenko.

Image caption Houses and cars were swept out to sea in Kesennuma City

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter