23 July 2014

Oso landslide: the last set of remains been recovered and a new report has been released

Posted by Dave Petley


Image of the upper block of the Oso landslide, from the GEER report (image collected by Eric Jensen, King County Sheriff Department)

Oso landslide – the last set of remains

Two significant events occurred yesterday with respect to the Oso landslide in Washington State,  First, it was announced that the last set of human remains were recovered from the site.  In the tragic circumstances of the landslide it is an extraordinary achievement to have recovered all of the remains, given the size and mobility of the slide.  The search and rescue teams at Oso deserve great credit for what they have achieved.  In case you haven’t seen it, the leader of the rescue effort posted a comment on one of my earlier posts, providing insight into how this was achieved:

I was in charge of the planning the search operations in Oso Washington. I just read your blog where you detailed what would have to happen to find the victims. We did exactly what you wrote in your blog would need to be done to find the victims and we have recovered all but two of the victims. We mapped where the survivors and initial victims were found. We searched the debris fields of building remnants and vehicles and found more victims. We searched specific locations based on K9 indications and found more. We plotted each find on the map then geo located where we believed each victim was at the time of the slide. Based on trajectories of each victim we were able to focus our search efforts in two distinct areas finding all but two of the missing victims.  Just thought I would let you know that we did this based on no prior experience in mudslide search operations but good solid Search Management theory. You are spot on when it comes to finding people in this type of event and we validated it with greater success than we could have ever imagined. We believe we will eventually find the remaining two people once we can dewater the areas we think they are in.

I can only salute the team for this – recovery of bodies is a technically very difficult and deeply distressing process, and I have great admiration for what has been achieved.

Oso landslide – mechanisms of movement

On the same day a detailed report was released on the Oso landslide, compiled by the NSF funded GEER scientific response team.  The report is available online (NB it is a large pdf) and makes very interesting reading.  It is too long and detailed for me to have been able to go through in detail as yet, but on first inspection it is a very impressive analysis.  I thought it would be interesting to reflect on two aspects – the mechanism of failure and the long runout.

The former is interesting because the seismic signals indicated two movement events at Oso a few minutes apart.  The question of course is what these two events represent.  The report looks at this in detail and concludes that:

The first major stage of movement (Stage 1) is interpreted to be a remobilization of the 2006 slide mass and a headward extension that included part of the forested slope of the pre-historic slide. As such, Stage 1 was comprised largely or entirely of previous landslide deposits, some as recent as 2006, and others ancient. It is believed that Stage 1 initiated partly on a shear surface that is essentially the lower portion of the ancient slide surface…and mobilized as a debris flow, traveling across the valley and causing most or all of the damage and destruction south of the river.


The second stage (Stage 2) occurred subsequently in response to the unloading (i.e., loss of “buttressing”) and the redirection of principal stresses and possibly, to groundwater seepage forces. Stage 2 was a retrogression into the Whitman Bench of up to nearly 90 m horizontally from the ancient slide scarp. The Stage 2 slip surface probably joined the slip surface of Stage 1 (and that of the 2006 and ancient slides) at depth, but also included up to 300 m or more of previously deeper in-place outwash, till and lacustrine deposits…The Stage 2 landslide moved rapidly on the existing Stage 1 slip surface until it essentially collided with the more intact blocks at the trailing edge of the Stage 1 slide mass, and came to rest.

To me this is an entirely plausible analysis that captures all of the known information, and is in keeping with our understanding of other events of this type.  Indeed this is broadly in keeping with the mechanism for Oso about which I speculated in a post at the time, which I illustrated with this figure:


Oso landslide – the mobility and runout

The second aspect that is really interesting is the mobility and runout of the Oso landslide.  In many ways it is this component of the landslide that generated such high human losses.  If the slide had moved less fast then most of the victims could have escaped.  It has been speculated by some that the Oso landslide showed exceptional mobility, and indeed I was heavily criticised by someone whom I respect greatly for the post in which I stated that in my view that the landslide should have been foreseen.  My understanding is that it was felt that I did not appreciate the apparent exceptional mobility of the Oso landslide.

The report examines the runout of the landslide in detail using a range of empirical techniques.  This an example, in which the travel distance of the landslide is compared with the landslide volume for this landslide alongside a dataset of other slides from the literature:



The mobility the Oso landslide compared with a dataset from the literature, taken from the GEER report


As can be seen above, the mobility of the Oso landslide as described by the runout distance does not appear to be exceptional from the perspective of other large landslides.  Indeed the report notes that:

“We found that the runout of this debris flow was indeed long (greater than 1 km); however, it was not exceptional for a landslide of its size.”

I’ll conclude by noting that poorly consolidated glacial materials do show comparatively high levels of mobility in landslides – that is well established.  Coincidentally, yesterday Jon Parsons sent this youtube video to me of the aftermath of a landslide in Lower Churchill, in Labrador, Canada:

The scarp height appears to be somewhat lower than at Oso, but the runout of the slide blocks is estimated to be 500 metres.

I do agree that we may have been underestimating the risk associated with landslides in these materials, but their behaviour should not be a surprise.