Wreckage from Japan's tsunami – fishing gear and furniture, footballs and ships – has swept across the Pacific far faster than expected, with thousands of tonnes projected to land on North American shores this year.

Scientists believe lighter objects such as buoys and oil drums began reaching land last November or December. The rest is spread over thousands of miles of ocean between the Midway atoll and the northern islands of Hawaii.

About 95% will probably never come ashore and is destined for that massive swirl of floating plastic known as the north Pacific garbage patch. The remaining fraction is due to reach the west coast of the US and Canada in October.

No one expects to wake up one morning to a tsunami of rubbish. "It is not like you are going to be standing on the beach looking at the horizon and see a wall of debris come in," said Nicholas Mallos, a marine debris expert at the Ocean Conservancy.

But there have already been some bizarre finds. This week a beachcomber in British Columbia found a moving crate containing a rusting Harley-Davidson motorcycle registered to Japan's Miyagi prefecture, which absorbed the brunt of the tsunami. The crate also contained a set of golf clubs.

Last month a a football washed up on an uninhabited island off Alaska and was traced to its owner, a Japanese schoolboy from the town of Rikuzentakata which was almost flattened by the tsunami. A 160ft fishing boat, the Ryou-Un Maru, drifting to within 300 miles of the British Columbia coast before it was deemed a hazard to shipping and sunk by the US coastguard, was also found.

Washington state officials last week put up posters advising residents what may arrive on their beaches, from common litter to aluminium canisters possibly containing insecticide, and derelict boats.

Personal belongings should be treated with respect, the posters said. "It is extremely unlikely any human remains from the tsunami will reach the US," they added, but if people did find a body they should call the authorities.

The wreckage stems from a vast stretch of Japan's northern coast and was swept away several days before the meltdown at the Fukushima reactor, so radiation is not seen as a potential hazard, according to the National Oceanic and Atmospheric Administration (Noaa).

The Japanese government estimates 4.8m tonnes of debris – parts of factorybuildings, houses, cars and trees – were swept into the ocean during the March 2011 earthquake and tsunami.

Heavy items sank to the ocean floor close to shore. But at least 1.5m tonnes of debris, including small ships, lumber, and even steel beams was carried off by currents and began making the 4,500-mile journey across the Pacific. Within a month of the tsunami, the debris had dispersed and now stretches across 4,000 nautical miles of the north Pacific. Scientists have only a sketchy notion of what is still out there, how fast it is travelling, and where and when it might land.

"Most people probably think there is a huge pile of debris moving across the ocean like a carpet," said Jan Hafner, of the International Pacific Research Centre, in Honolulu, Hawaii. "But it is very sparse, very patchy."

Projections made by Nikolai Maximenko and Hafner suggested most of the wreckage would reach North America between March 2013 and March 2014.

But that did not account for buoyant materials, such as oyster floats, foot- and volleyballs and lightbulbs, which bobbed on the water, and were propelled by winds and the ocean current.

A football or a big float sitting on the water exposed to the wind is carried by the wind more than the currents," Hafner said. "Wind blows faster than the typical surface current so those types of debris are moving faster."

Officials logged hundreds of reports of those outliers from Oregon, Washington state, and British Columbia before conceding it was tsunami wreckage.

They argued many of the findings could not be definitively identified. People were overly excited, said Kinji Shinoda, the deputy consul general in Vancouver. "Several newspapers were reporting that they found pop bottles or cans," he said. "But in some of the photos the bottles actually had Chinese characters, not Japanese, so nobody knows where it came from."

A more definitive picture of the debris is unlikely to emerge before June or July when two privately-funded expeditions are due to travel into the north Pacific. But the latest computer models from the Japanese government and Noaa suggest most of the wreckage that will make landfall will begin washing up this October and continue into late 2013.

Washington, Oregon, British Columbia and Alaska will get much of the debris, while most of California will be protected by currents pushing objects back out to sea. Hawaii, however, is in line for several deposits of tsunami trash.

"It's going to bounce off the western shore of North America, swing back south and come back towards Hawaii and enter that big circular area called the North Pacific Garbage Patch," said Bill Francis, board president of the Algalita Marine Research Foundation in Long Beach, California, which will be leading one expedition. "I heard someone say it's like a big toilet that never flushes. Anything that floats is going to stay out there and stay out there."

The US navy and coastguard will be tracking accumulations of debris which could pose a danger to shipping.

The tsunami swept as much debris into the ocean in one day as is usually dumped in a year, threatening wildlife and the Pacific's ecology, conservationists said. Coral is smothered by plastic, fish get trapped in drifting nets. Birds die from eating plastic.

"It is clearly already an ocean problem. We know that all of these hundreds of tonnes of debris are in the ocean. We know that actually all of the plastic debris contains a lot of toxins, and we know there are other types of toxins that would have got into the ocean from the tsunami and so all of this debris represents a hazard to navigation and a terrible distress to the ocean ecosystem," said Mary Crowley, founder of the Ocean Voyages Institute, which will also be leading an expedition.

© Guardian News and Media 2012

[Japanese earthquake aftermath via yankane / Shutterstock]