Sumatra, hit by deadly earthquakes and tsunamis in recent years, is at risk from another natural phenomenon, researchers say -- major volcanic eruptions.

Researchers from Oregon State University, working with colleagues in Indonesia in a study funded by the National Science Foundation, have documented six major volcanic eruptions in Sumatra in the past 35,000 years, and say most of them equaled or surpassed the explosive intensity of the eruption of Washington's Mount St. Helens in 1980.

"Sumatra has a number of active and potentially explosive volcanoes and many show evidence of recent activity," Morgan Salisbury of the university's College of Earth, Ocean, and Atmospheric Sciences said in an OSU release Wednesday. "Most of the eruptions are small, so little attention has been paid to the potential for a catastrophic eruption.

"But our study found some of the first evidence that the region has a much more explosive history than perhaps has been appreciated," he said.

The Indian Ocean region has a violent volcanic history. The 1883 eruption of Krakatoa between Sumatra and Java was likely the most violent volcanic explosion in recorded history, researchers said.