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CORVALLIS, Ore. - The large tsunami two years ago in American Samoa has given scientists a chance to examine an issue that often seems of little significance in the immediate aftermath of these massive disasters - the little-seen, rarely studied but often frightening damage done to offshore coral reefs.

A new study by scientists from Oregon and Michigan, done with a remotely operated undersea vehicle, or ROV, surveyed large areas of that area's coral reefs, and revealed significant damage from sediment, debris, and the enormous forces of both the incoming and outgoing waves.

Corals are delicate living organisms that can only survive in shallow, nearshore areas where they get adequate sunlight. That's also where the tsunami wave action is most violent, and they are especially vulnerable to its impacts - but often ignored in the understandable concern about terrestrial damage and loss of life.

"Very little until now has been known about the impact of tsunamis on coral reefs," said Solomon Yim, a professor of structural and ocean engineering at Oregon State University and co-author of the study, which was supported by the National Science Foundation.

"These are huge forces and often these events have happened in remote locations of the world where we had little opportunity to study them," Yim said. "American Samoa gave us the chance to use some very sophisticated equipment to gain a much better understanding of what damage is being done to coral reefs, and what might be done in the future to help reduce it."

On Sept. 29, 2009, a magnitude 8.3 subduction zone earthquake near American Samoa sent waves crashing into many islands, destroying buildings and eroding coastlines with waves up to 20 feet high that came almost a mile inland and killed more than 180 people. It was the world's largest earthquake that year.

The onshore devastation was heavy. Although not seen at the time, so was the underwater damage to coral reefs.

"We found tires, clothing, sheet metal roofs, and window frames littered on the reefs," Yim said. "Much of the coral was broken or covered with sediments, and some of it died as a result. Both the run-up and run-down of the tsunami waves were very destructive. It will probably take years to decades for the reef to recover."

The sediments and debris carried by the rapid drawdown back into the sea can be harmful to the delicate marine ecosystem, the researchers noted in their report. They introduce bacteria and toxic chemicals, erode the seafloor and destroy the reef.

Work with the ROV examined the reefs five weeks after the tsunami, when they were still deeply scarred. Some corals were ripped up and tossed onshore, others broken and sucked back into deep water. In either case they would not survive. Hours of video footage were made of the damage, and the research indicated the drawdown of the water was even more destructive than the incoming waves.

Most of the damage and debris was found in comparatively shallow ocean waters, about 30 to 70 feet deep.

Since so little is known about the damage to coral reefs by tsunamis, more studies are needed to examine the influence of water depth, three-dimensional effects, wave-wave interactions and coral strengths, the researchers said.

"In the aftermath of a destructive tsunami, there may be some things we could do to aid reef recovery after the more immediate needs onshore are tended to," Yim said. "There's probably not much we can do about the fine sediments that bury the coral, but we could perhaps clean up some of the larger debris and building materials like sheet metal roofing that cover up the coral. It's a significant challenge."

Collaborating on this research, which was published in Marine Geology, a professional journal, were Y.L. Young and D.L. Witt, scientists from the University of Michigan. The research was funded by the National Science Foundation and the video was produced by Paul Hillman of the National Oceanic and Atmospheric Administration.