Tracking the millions of tons of debris washed into the ocean from the tsunami that struck Japan last year is virtually impossible. But scientists can get an idea of where the debris likely is with the help of a computer model that uses measurements of currents and other factors.
Based on estimates from the Japanese government and the U.S. space agency, NASA, the monstrous tsunami that hit the coast of Japan after the 9.0 earthquake of March 11, 2011, swept up more than 5 million tons of debris. About 70 percent of that debris sank to the seafloor, leaving some 1.5 million tons floating across the ocean's surface.
More than a year later, that floating debris is still drifting with the Pacific Ocean currents, spreading out across the sea, though no one knows exactly where and how far it has spread.
With the help of a computer model, though, scientists can make their best guess at where the debris has gone.
Following the tracers
The scientists' SCUD (for Surface Currents from Diagnostic) model starts with the release of 678,000 "tracers" from various points along the northeastern coast of Japan on March 11, 2011. The initial distribution is based on the density of population and development along the coast.
The model then runs over the course of time, with swirls of orange- and red-shaded areas that represent parcels of water likely to hold pieces of debris moving along with the currents. The deeper the red, the higher the likely concentration of debris.
The above still image shows the expected extent of the debris field on April 3, 2012. The debris field stretches roughly 3,000 miles by 1,200 miles (5,000 kilometers by 2,000 kilometers) across the North Pacific, according to a NASA statement.
The model, developed by Nikolai Maximenko and Jan Hafner of the International Pacific Research Center at the University of Hawaii, uses real satellite data on sea surface heights and ocean surface winds, as well as information from a set of drifting scientific buoys.
Hafner and Maximenko also have collected information on debris sightings and used them to test the accuracy of the model; so far, they back it up.
Where the debris could go
Debris was initially carried by the Kuroshio Current, which whips past eastern Japan much like the Gulf Stream in the North Atlantic. The surface waters carried by this current eventually move east in the Kuroshio Extension and then the North Pacific Current.
Based on the model, some debris could reach the west coast of North America within a year or two. Most of it, however, is likely to end up in the Pacific Garbage Patch, a floating debris field in the middle of the North Pacific Gyre.
As of April 3, 2012, there had been very few reports of debris at Midway Island and Kure Atoll. North winds have been minimal in recent months, and ocean currents have favored keeping the debris from the island. But those currents may be shifting, Hafner said, and debris should eventually wash up with greater frequency.