A huge floating dock cast adrift by Japan's killer tsunami has washed up on an Oregon beach 15 months later, among the biggest pieces of flotsam to make landfall on the US West Coast so far.

The 66-foot (20-meter) long rectangular structure, made of concrete and metal, was spotted floating off the coast on Monday, and then washed in with the high tide on Agate beach, 100 miles (160 kilometers) southwest of Portland.

Local authorities contacted the Japanese consulate in Portland, who confirmed that it was from the March 11, 2011 tsunami, thrown up by a massive earthquake off Japan's coast 5,500 miles across the Pacific.

"Four floating docks were washed away by the tsunami. This is one of them," deputy consul general Hirofumi Murabayashi told AFP.

"The other three we don't know where they are, if they're floating somewhere or they sank in the ocean or not," he added.

Confirmation that the dock came from the tsunami came after Japanese writing and markings were found on various parts of the seaweed-covered dock, including "Shibata, Japan" on tires, apparently designed to make it buoyant.

Photos of the structure, and at least one Japanese-language placard attached to it, have been sent to the local Japanese consulate, Oregon State Parks Department spokesman Chris Havel told KATU 2 television news.

Japanese officials confirmed that the dock -- 66 feet long, 19 feet wide and 7 feet tall -- came from the port of Misawa, in Aomori Prefecture in the northern part of Japan. The metal plaque was dated June 2008.

He said the dock was able to float due to being made of concrete and styrofoam.

"The surface is concrete, there's some steel cables, inside is styrofoam," Murabayashi said. "That's why it floats on the sea. The dock itself doesn't contain any hazardous materials."

The owner of the dock doesn't want it back. "They said they don't wish to have it returned," Murabayashi said. "I believe it will be disposed (of) in Oregon."

Various debris from the Japanese tsunami have begun washing up on the US and Canadian west coast, and experts predict a surge of flotsam in the coming months.

Kirk Tite, visiting the Oregon beach with his young son Trevor, 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," he told KATU 2.

The broadcaster said it had traced a phone number on a Japanese placard to a business located in Tokyo, called Zeniya Marine Services Company, Ltd., which builds docks and other floating marine structures.

The dock has been checked for radioactivity -- the killer earthquake and tsunami triggered a disaster at the Fukushima nuclear plant on Japan's east coast -- but had proved negative, it reported.

Trevor Tite, examining shellfish attached to the side of the dock, was bemused. "I just think it's kind of weird that it could have come over from Japan... the tsunami, that happened quite a while ago."

In early April, the US Coast Guard sunk a deserted Japanese trawler that had appeared off the coast of Alaska more than a year after being set adrift by the tsunami.

In the same month, a Japanese schoolboy was told he was getting his football back, after it was spotted by an observant beachcomber on Middleton Island in the Gulf of Alaska.