Debris from Japan’s 2011 tsunami will continue to litter the North American coastline over the next three years, with everything from refrigerators to lumber and sports balls still floating offshore in the Pacific, an expert said on Tuesday.

About 1m tons of debris was still lingering in the Pacific Ocean four years after a magnitude 9.0 earthquake – the most powerful ever recorded in Japan – set off a series of massive tsunami waves that devastated a wide swath of Honshu’s Pacific coastline and killed nearly 20,000 people.

It also damaged the Fukushima Daiichi nuclear plant, leading to a series of explosions and meltdowns in the world’s worst nuclear disaster since Chernobyl 25 years earlier.

The Oregon Sea Grant, a program at Oregon State University funded by the National Oceanic and Atmospheric Administration (NOAA), has been working with researchers at Japan’s Tottori University to monitor tsunami debris.

An estimated 5m tons of wreckage – everything from cars to building materials, boats and docks – washed into the Pacific, and about 70% sank quickly to the ocean floor, experts in the US and Japan said.

But items with buoyancy eventually caught the Pacific Ocean’s currents and have since lodged as close as 25 miles (40 km) off the North American coast, covering an area from California to Alaska, said Sam Chan, an aquatic invasive species expert at Oregon Sea Grant.

“When it comes into our Pacific shores in North America it [the debris] tends to stay offshore for months and sometimes a year,” he said. “It’s not until we actually end up with local storm events or changes in the season that debris comes ashore.”

Over the past year, warmer temperatures and a lack of major storms have kept most of the refuse from washing up on land, but wave patterns and other oceanic conditions were likely to send more items heading to west coast beaches this summer, he said.

Last summer, 26 Japanese boats floated ashore in Oregon, Washington state and Canada’s British Columbia, Chan said.

In 2012, a 66ft (20-meter) long dock which had ripped from its moorings in Japan floated up on a beach north of Newport, Oregon, south-west of Portland.

The Japanese team has sent out about 70 battery-operated tracking devices that float at different heights to simulate different kinds of debris in order to understand where in the Pacific the refuse has clustered.