The Mainichi answers common questions readers may have about debris from the Great East Japan Earthquake and ensuing tsunami that has arrived or is expected to arrive on the shores of the North American continent.

Question: How much debris from the earthquake and tsunami drifted out to sea?

Answer: The Ministry of the Environment estimates that about 5 million tons of debris was swept into sea following the tsunami, of which around 70 percent, or 3.5 million tons, sunk to the sea floor. The remaining debris -- about 1.5 million tons -- floated out into the Pacific Ocean.

Q: What kind of things drifted out to sea?

A: Many things like soccer balls and volleyballs used by schools have been found, but around 1.33 million tons of what floated out is wooden debris from houses. Boats, fishing equipment like buoys used in aquaculture, and plastic foam are also arriving on North American coastlines.

Q: How will the arrival rate change from now?

A: The ministry and other government organizations of Japan and the United States are predicting the arrival of debris based on ocean currents, satellite imagery and other data. Around 1.2 tons is expected to arrive in October, and 41,300 tons by February 2013. Fishing boats, buoys and other floating debris arrived quickly due to the influence of the westerlies. Wooden debris from houses receives little influence from the wind as only a small amount of it lies above the surface. However, such debris is expected to begin arriving in large amounts in the future.

Q: Objects with shellfish or seaweed attached to them have arrived. How could they affect ecosystems?

A: Even before the earthquake and tsunami, ocean garbage has been a serious problem, and many birds and ocean organisms die from getting caught in or consuming the garbage. Due to ocean currents and winds, garbage arrives on the west side of Japan from China, South Korea and other countries, while garbage from Japan arrives in Hawaii, the North American continent or elsewhere. It has long been pointed out that foreign species can attach themselves to ships or drifting objects that end up overseas. In the Great East Japan Earthquake, though, a large amount of debris was created at once -- a different scenario that needs more study.

It is thought that the amount of radioactive material on the debris is extremely small, as much tsunami debris had already drifted out to sea when the disaster at the Fukushima No. 1 Nuclear Power Plant broke out.

Q: Other countries are disposing of debris that arrives in accordance with customary practice, but what is Japan doing?

A: The Japanese government is compiling information at meetings of the Ministry of Foreign Affairs, the Ministry of Land, Infrastructure, Transport and Tourism and other relevant organizations, and sharing the information with other countries. In August, the nongovernmental organization JEAN, which is involved in activities to protect the marine environment, will be sent by the Environment Ministry to meet with U.S. NGOs. (Answers by Hiromi Nagano, Foreign News Department)