Media playback is unsupported on your device Media caption The BBC's Tim Allman says the dock was mistaken for a barge at first

A huge dock torn from a Japanese port by the 2011 tsunami has washed up 8,050km (5,000 miles) away on the US West Coast after crossing the Pacific.

The 165-tonne structure made of concrete, metal and tyres, and studded with starfish and barnacles, arrived on a beach south-west of Portland, Oregon.

It has tested negative for radiation, but scientists say a host of invasive marine species may have hitched a ride.

Police are guarding the dock while officials decide what to do with it.

A plaque on the 20m-long (66ft) structure, which was first mistaken for a barge, shows it came from the port of Misawa in northern Japan.

It has taken 15 months to drift across the Pacific to Agate beach since the earthquake and resulting tsunami shook it loose. Two other docks from the same port are still missing.

The structure is one of a number of items that have washed up on north American shores. Experts expect a surge of debris in the coming months, with the bulk of it due in the winter.

Danger to shipping?

The senior senator for Oregon, Ron Wyden, has asked the body tracking the debris to redouble its efforts saying something as big as a dock could pose a danger to ships at sea.

This April, the US Coast Guard used cannon to sink a crewless Japanese ship that drifted to Alaska after the tsunami.

A month later, a Japanese owner of a Harley-Davidson motorbike swept away by the tsunami was amazed to find out that it had been washed up inside a container on a beach in Canada - about 6,400km away.

Japanese scientists estimate that some 20 million tonnes of debris were generated by the earthquake and the incoming rush of water.

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 of debris is still afloat.

A starfish native to Japan was among the marine life still clinging to the structure.

John Chapman, a research scientist at Oregon State University said hundreds of other organisms, such as tiny crabs and algae, posed a "very clear threat".

"It's exactly like saying you threw a bowling ball into a china shop. It's going to break something. But will it be valuable or cheap glass? It's incredibly difficult to predict what will happen next."

Police have been deployed to stop people climbing on it.

Traffic jams

A spokesman for the Oregon Parks and Recreation Department, Chris Havel, said two options were being considered for its removal.

The dock could be towed off the beach and floated somewhere for disposal, or cut up on the beach. The plaque has been put in storage.

Image caption A derelict Japanese fishing boat was sunk by the coastguard after drifting to the coast of Alaska

"We're working with some salvage experts to get advice on whether it's even possible to move it, whether we have to wait for some certain conditions, like another high tide, or whether there's some other way to do it," he said.

The owners of the port in Misawa have said they do not want it back.

Meanwhile, local residents have been causing traffic jams in the beach car park as they come to see it for themselves.

Kirk Tite, who was visiting the beach with his son, said: "It's kind of scary seeing this wash up here because we all surf.

"If this crossed the Pacific Ocean and it's this big, that means that just about anything of our worst nightmares could cross the Pacific Ocean. So we're kind of frightened of what's to come".

Nearly 16,000 people were killed by the quake and tsunami in Japan.