Ted S. Warren/Associated Press
At 10:37 a.m. local time on March 22, as much as 5 million cubic meters of sediment slid down a hillside outside Oso, Wash., killing dozens and damming a stretch of the North Fork of the Stillaguamish River. The debris scraping and bouncing downhill generated seismic waves like those unleashed during an earthquake.
The pattern of these ground vibrations, recorded at regional seismic stations, allowed geologists to decipher the landslide’s sequence of events. The stations first detected a roughly two-minute rumble, probably from a part of the hill that had experienced a landslide in 2006, says Kate Allstadt of the Pacific Northwest Seismic Network. The Stillaguamish River, she says, has been cutting into the toe of that landslide’s debris pile and destabilizing it. Recent heavy rains may have further weakened the hillside, contributing to its collapse.