A US state governor thanked Japan on Thursday for help in dealing with debris washed up on America's West Coast from the 2011 tsunami, including $5 million in aid to fund the clean-up.

Washington state governor Jay Inslee voiced gratitude after the latest major piece of debris, a dock that washed ashore on a marine sanctuary stretch of coastline, was removed some two years after the catastrophic tidal wave.

"Our state deeply appreciates Japan's friendship and the generous gift of $5 million in goodwill to the United States to support response efforts related to tsunami debris," he said in a statement.

Japan's diplomatic mission in Seattle has coordinated with local authorities in dealing with debris, including the latest dock removed from the Olympic National Park and Olympic Coast National Marine Sanctuary.

"We thank the Consulate-General of Japan in Seattle for their assistance in identifying the dock as marine debris from the tsunami and for obtaining needed information to prepare for its disposal," he said.

Japan recently marked the second anniversary of the March 11, 2011 9.0-magnitude earthquake that sent a huge wall of water into its northeastern coast, killing some 19,000 people and triggering a nuclear calamity.

"We continue to offer our condolences for the... tragedy that claimed so many lives in Japan, and admire the strength and resilience of the Japanese people as well as the nation's ongoing efforts to recover and rebuild.

"Japan is under absolutely no obligation to provide this type of assistance, and the nation's generous gift is yet another demonstration of Japan's continued contributions to the international community," said Inslee.

A recent study in California, further down the West Coast, warned that more than a quarter of a million people live in coastal areas which could be hit by a devastating tsunami in the quake-prone US state.

The US Geological Survey (USGS) study, published to mark Tsunami Awareness Week, says tidal waves of eight meters or more could hit northern California following a quake of magnitude 8 or more from a well-known tectonic fault line.