{"id":8516,"date":"2014-03-25T08:20:11","date_gmt":"2014-03-25T08:20:11","guid":{"rendered":"http:\/\/blogs.agu.org\/landslideblog\/?p=8516"},"modified":"2014-03-25T08:20:11","modified_gmt":"2014-03-25T08:20:11","slug":"the-steelhead-landslide-1","status":"publish","type":"post","link":"https:\/\/blogs.agu.org\/landslideblog\/2014\/03\/25\/the-steelhead-landslide-1\/","title":{"rendered":"The Steelhead landslide in Oso, Washington State"},"content":{"rendered":"
The Steelhead landslide<\/h5>\n

The death toll from the Steelhead landslide near to Oso in Washington State<\/a> is continuing to rise.\u00a0 Latest reports suggest<\/a> that there are now 14 known fatalities, but 176 people are reported to be missing.\u00a0 It is quite normal in this sort of event for the number of reported missing people to exceed substantially the actual number of victims, so this maximum toll may reduce in the next few days.\u00a0 However, it is still likely to be the costliest landslide in terms of lives lost for many years in the USA.<\/p>\n

Details are slowly emerging of the landslide history of this site.\u00a0 It is clear that major landslides have occurred here on many previous occasions; indeed so much so that the landslide is known as either the Hazel landslide or the Steelhead landslide; at this stage I am opting for the matter given that the inundated area is known as Steelhead Drive.<\/p>\n

Better images of the Steelhead landslide<\/h5>\n

I very much appreciate the help that numerous people have given me over the last few days to put together this post – too many to name, but thanks to you all.\u00a0 The best graphic that gives an overview of the slide is in the Seattle Times<\/a>:<\/p>\n

\"Steelhead<\/a>

Seattle Times<\/a><\/p><\/div>\n

..<\/p>\n

The original version has a very impressive slider function that allows the user to flip from one image to the other.\u00a0 I can’t replicate that, but putting the two images side-by-side shows the extent of the devastation.\u00a0 The number of inundated houses is large, suggesting that the loss of life will be high, especially bearing in mind that the slide occurred on a Saturday.<\/p>\n

The best set of aerial images of the slide are on the Flicker page of Governor Jay Inslee<\/a> – there are some wonderful images there.\u00a0 This image shows the source of the landslide:<\/p>\n

\"Steelhead<\/a>

Image from Jay Inslee<\/a><\/p><\/div>\n

..<\/p>\n

Whilst this image from the same source shows the entire landslide mass.<\/p>\n

\"Steelhead<\/a>

Image from Jay Inslee<\/a><\/p><\/div>\n

..<\/p>\n

The landslide has been widely reported as a mudslide.\u00a0 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.\u00a0 However, the upper portion is a rotational landslide – the rotated block with the fallen trees is very clear.\u00a0 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.<\/p>\n

The history of the Steelhead landslide<\/h5>\n

The Yakima Herald has a very nice article<\/a> that details the chronology of events on the Steelhead landslide.\u00a0 This includes:<\/p>\n