"On land, thicknesses of the tsunami's deposits were usually only about 30 centimeters [12 inches], but on the shallow sea bottom, it was meter-scale," Goto said.
It remains difficult to say how many dunes the tsunami might have created.
"The tsunami wave current was very strong and I would not be surprised if dunes were formed across the entire bay, plus slightly deeper areas, but some of them may have been erased since then by normal post-tsunami wave activity," Goto said.
The fact that the tsunami drastically altered conditions on the seafloor should affect Japan's marine ecosystem. "Future monitoring of the marine ecosystem is highly required," Goto said.
Past tsunami evidence
These findings also suggest that geological evidence of past tsunamis may be well-preserved on the seafloor, helping shed light on how often an area might experience tsunamis in the future, and how powerful those killer waves might be.
Usually scientists look for such evidence on land, but in urban areas, such traces are often destroyed as people reshape the earth there, making it difficult to see what tsunami risks those cities might face, Goto explained.