ANCHORAGE, Alaska - Scientists say they have discovered a massive landslide in an uninhabited area of eastern Alaska that's the largest detected in North America since the 1980 eruption of Mount St. Helens.

The Oct. 17 landslide unleashed 200 million tons of rock down the Taan Fiord valley onto Tyndall Glacier in Icy Bay, according to Columbia University's Lamont-Doherty Earth Observatory. No one witnessed it, but scientists picked up its "seismic signature" with a method of reading patterns they have been honing for six years.

"We're reading the data, processing the data, in a way that lets us detect the landslides and figure out where they are," Lamont geomorphologist Colin Stark said Tuesday.

The landslide had a magnitude of 4.9 and was confirmed by satellite images taken a week later, according to Stark. Scientists say the images appear to show tsunami damage along the sides of the fiord.

As large as it was, however, the slide was a distant second from the Mount St. Helens eruption in Washington state that accounted for a 2.5 billion-ton landslide from a flank collapse, he said Tuesday.

Because of the difference in size, the two debris dumps are not really comparable, but the Washington event was included for reference, Stark said.

"This landslide in Alaska, in Tyndall, was a pretty normal landslide," he said. "But the Mount St. Helens was pretty freakish."