A huge dock torn from a Japanese port by March 2011 tsunami has washed up in the US state of Oregon – 8,050 km (5,000 miles) across the Pacific.

The 20m-long (66ft) concrete dock weighing 165 tons was spotted on Agate beach, south-west of Portland.

A Japanese consulate official said a commemorative plaque showed it had come from the fishing port of Misawa.

Radiation checks proved negative, but scientists say invasive species foreign to the area may have hitched a ride.

A huge dock torn from a Japanese port by March 2011 tsunami has washed up in the US state of Oregon

A starfish native to Japan was among the marine life still clinging to the structure, Oregon Department of Parks and Recreation spokesman Chris Havel was quoted as saying by the Associated Press news agency.

“This is tsunami debris, not just from Japan, but from the tsunami itself,” Chris Havel said.

Oregon police have now been deployed to keep people from climbing on the dock, which was first mistaken by local residents for a barge.

Misawa lost four docks during an earthquake and resulting tsunami on 11 March 2011. Two docks are still missing.

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 6,400 km away.

Japanese scientists estimate that some 20 million tons 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 tons of debris is still afloat.

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

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