When 200 million metric tons of rock tumbled down a remote Southeast Alaska mountain in October, nobody was around to see it. But thanks to a beefed-up seismic network and a new system that can distinguish landslides from earthquakes, scientists knew it had happened.

The slide happened near Icy Bay and sent debris onto Tyndall Glacier, reported scientists from Columbia University's Lamont-Doherty Earth Observatory. Some of the debris was dumped into Taan Fiord, setting off a tsunami that was big enough to be measured at the nearest tidal gauge nearly 100 miles away.

It was one of the world's biggest landslides in recent years and the largest in North America since the collapse of Mount St. Helens in 1980, according to the observatory.

It was also part of a series of slides that have been detected in the mountains of Southeast Alaska and neighboring parts of Canada that would have likely gone unnoticed if not for a new system -- developed by the Lamont-Doherty Earth Observatory scientists -- that distinguishes the signals of such slides from those produced by earthquakes.

It is no coincidence that so many big slides have happened in that region, said Colin Stark, a professor of geology and geophysics and one of the Lamont-Doherty scientists who developed the new landslide detection system.

"Southeast Alaska has probably the most rock avalanches of huge magnitude of any place in the world," said Stark, who presented information Friday on the slide at the American Geophysical Union's annual meeting in San Francisco. "It's the world's hotspots for these huge rockslides and rock avalanches that drop onto glaciers."

One possible explanation for the frequent debris slides in that region: Rapid retreat of the glaciers in the region is removing the ice that used to hold slopes together, Stark said. It is also possible that softening of permafrost is allowing the big collapses, he said.

The Oct. 17 Icy Bay slide, which was documented by satellites, happened over the course of a minute and caused a local tsunami that could have been extremely dangerous to people if any had been nearby, Stark said. It could have been 100 feet high; the closest tidal gauge, at Yakutat, measured a 1-foot wave, he said.

Some colleagues have expressed concerns about a similar event happening at the massive Columbia Glacier, which has retreated dramatically and is closer to towns, Stark said.

Are these landslides happening more frequently than in the past? It's difficult to tell, Stark said.

Technology allowing scientists to learn about these slides is vastly improved, he said. There is a much better seismic network, featuring a sophisticated system called the Transportable Array, that has been deployed around the nation, including in Alaska, he said.

And the expansion of satellite imagery is a big factor, especially in mostly cloudy Southeast Alaska.