Volcanic Disaster
Tsunamis are usually caused by earthquakes, underwater landslides, or volcanoes. They can also be triggered by meteor strikes, but there was no evidence that a meteor landed near Tonga, Frohlich said.
Initially, earthquakes were the most obvious candidate. But the wave struck Tongatapu's western shore, and an earthquake-triggered tsunami would have come from the east, where the fault line lies.
(Related: Explore a map of Earth's tectonic plates.)
"It just didn't smell like an earthquake," Frohlich said.
The scientists used computer models to determine the power that nearby underwater landslides could have had, but "we couldn't get waves high enough," he said.
The researchers believe the wave was most likely triggered by one in a string of underwater volcanoes about 21 miles (35 kilometers) away from the island.
A 2007 mapping survey of the seafloor near Tonga revealed flank collapses and calderas, geological features generated by volcanic instability or extraordinarily large eruptions.
Eyes Opened to Tsunamis
Team member Allan Morton, a retired geology professor from Central Arizona College, first identified the massive boulders in a 2003 paper on evidence of tsunamis in Tonga.
"There's a general feeling in Tonga they don't have tsunamis," Morton said, noting that the most recent recorded event was in 1919.
The study could help raise awareness of the dangerous waves in Tonga, helping its people to be better prepared, he said.
(See aerial views of tsunamis)
Dale Dominey-Howes, director of the Australian Tsunami Research Center, was not involved in the research.
"What they have found is interesting and, if proved, is profound in terms of improving our understanding of potential volcano-triggered tsunami in the Southwest Pacific," he said.
The finding could also have implications for Australia's eastern seaboard. An extension of the Tonga volcano line runs north-south about 1,600 miles (2,500 kilometers) east of Australia.
"Work is underway to try and evaluate which marine-based volcanoes in the Southwest Pacific might have the capacity to generate eruption tsunami," Dominey-Howes said in an e-mail. "We'll have a better idea in a year or two."
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