Figure 1. The Oso, Washington area before the March 22, 2014 landslide as seen onGoogle Earth (top) and after the landslide, as photographed by the Washington Department of Transportation (bottom.) The landslide blocked the North Fork of the Stillaguamish River and Highway 530.
The death toll has grown to fourteen from Saturday's massive landslide near Oso, Washington, located about 50 miles north-northeast of Seattle. At least seven were injured, and 176 are listed as missing, though this total is likely to decrease dramatically as missing people check in. The landslide was triggered by unusually heavy rains over the past 30 days in the region.
A personal weather station located about ten miles west of the slide recorded 13.81" of precipitation in the 30 days prior to the slide, including 5.17" in the ten days just before. Precipitation imagery from NOAA's Advanced Hydrological Precipitation Service (Figure 2) shows that the 30-day precipitation amounts in the region were more than 8" above average--about double the usual amount
of rain for this time of year.
According to Dave Petley, Professor of Hazard and Risk in the Department of Geography at Durham University in the United Kingdom, it is clear that major landslides have occurred here on many previous occasions, so much so that the landslide is known as either the Hazel landslide or the Steelhead landslide. In his excellent Landslide Blog, my go-to source of information for any landslide, he writes:"The landslide has been widely reported as a mudslide. In terms of the lower portion, which did the damage, this is correct, although in places it might have been more of a mudflow than a mudslide. However, the upper portion is a rotational landslide–the rotated block with the fallen trees is very clear. A working hypothesis would be that this block failed catastrophically, transferring load onto the block below, which in turn generated very high pore water pressures,
causing fluidisation and a very rapid mudflow that struck the settlements across the river."
He writes that the last event on a similar scale he knows of was the 25th December 2003 debris flow in San
Bernadino County, California, which killed sixteen people. Weather historian Christopher C. Burthas a post about the worst landslides in U.S. history, which puts this week's landslide in context.
http://www.wunderground.com/blog/JeffMasters/archive.html?year=2014&month=03
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