vinces99 writes: The disastrous March 22 landslide that killed 43 people in the rural Washington state community of Oso involved the "remobilization" of a 2006 landslide on the same hillside, a new federally sponsored geological study concludes. The research indicates the landslide, the deadliest in U.S. history, happened in two major stages. The first stage remobilized the 2006 slide, including part of an adjacent forested slope from an ancient slide, and was made up largely or entirely of deposits from previous landslides. The first stage ultimately moved more than six-tenths of a mile across the north fork of the Stillaguamish River and caused nearly all the destruction in the Steelhead Haven neighborhood. The second stage started several minutes later and consisted of ancient landslide and glacial deposits. That material moved into the space vacated by the first stage and moved rapidly until it reached the trailing edge of the first stage, the study found. "Perhaps the most striking finding is that, while the Oso landslide was a rare geologic occurrence, it was not extraordinary," said Joseph Wartman, a University of Washington associate professor of civil and environmental engineering and a team leader for the study.
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