13 December 2012

Secret tsunamis of the South Pacific

Posted by kramsayer

Cocos Island Atoll. Underwater landslides that shape atolls may create thousands of undocumented tsunamis. (Credit: PalawanOz / Wikimedia Commons)

The crumbling volcanic islands of the southern Pacific Ocean could be a major source of undocumented – and potentially dangerous – tsunamis.

When geologist James Goff from the University of New South Wales, Sydney, looked at the thousands of atolls that dot Pacific, he spotted something odd. Most atolls seem to have chunks missing. Charles Darwin had described atolls as “great rings of coral-rock,” yet only 3 percent of atolls are complete circles.

These gaps are the scars of large underwater landslides of the atoll’s volcanic rock, each of which can create a tsunami. With nearly 30,000 atolls in the Pacific, these landslides could be the source of hundreds of thousands of previously unknown tsunamis, Goff said as he presented a poster on his findings Wednesday at the American Geophysical Society’s Fall Meeting.

“When I look at the Pacific, I just see 30,000 disasters waiting to happen,” said Goff. “We don’t understand just how bad this problem is—and that’s scary.”

An atoll is what remains of a volcanic island that once jutted out from the sea. As the island formed, a coral reef grew around it. Over time the volcanic rock of the island eroded into the ocean, leaving behind a circular coral reef surrounding an empty lagoon.

One atoll Goff looked at, Mangaia in the Cook Islands, appeared to have several bites taken out of it. As he talked with the local population, Goff learned that in the past 30 years, this one island has had six underwater landslides, causing six tsunamis. Since Mangaia is isolated and the tsunamis were relatively small, these major events went unnoticed by the scientific community.

“Many of these tsunamis go undetected, maybe 80 percent of them or more,” Goff said. “One tsunami only three people knew about—two fishermen and the chief of police, that’s it.”

Many Pacific atolls are unpopulated and isolated, but that doesn’t mean these tsunamis can’t be devastating. Twelve years ago, an underwater earthquake created a tsunami similar in size to the ones on Mangaia, killing close to 100 people in the Republic of Vanuatu, a small island nation.

“These things might not kill a relatively large amount of people,” said Goff. “But if an island has only got 400 people on it and you kill 100 of them, that’s 25 percent of the population; that is significant.”

Most research in the field is focused on tsunamis from earthquakes. When asked how many people are studying these atoll tsunamis, Goff replied, “I can count how many on three fingers.” Extensive underwater surveys of atolls are needed to understand and possibly predict the landslides that created these tsunamis. With further research and millions of dollars needed, the question is who will pay.

“It’ll probably be close to a million dollars per island—these island countries don’t have that kind of money,” said Goff.

Thomas Sumner is a science communication graduate student at UC Santa Cruz