Orthophoto and Fissures

November 9, 2009

WSDOT released an orthophoto of the landslide last Friday (at least, this is when I got it). The image is spectacular and helps give us some much needed data. This weekend I worked on mapping out all of the fissures in the “Woodshed Restraint”, as well as other places (that was much quicker, since the cracking was predominantly localized there).

A note of caution on this map, these fissures haven’t been field verified, so they could change, move, or disappear. Especially some of the cracks outside of the Woodshed Restraint area. We have some data on these as well as for the types of movement (uplift, down dropped or translational movement) and that will help us map the stresses and block movement within this mass of earth.

Last week DNR issued an order to suspend mining at the Simmons Quarry. There is a long story within that, but also one that may lead to some legal issues. The continued potential for danger and instability in the area gave us concern for public safety in the area, especially since we have residences living in houses right on the landslide. More on that later.

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Precipitation is an important component into landslide movement. During the investigation into the Alderwood-Banyon and the Carlyon Beach-Hunters Point Landslides, long-term precipitation (over five years) had been above the mean average. This is thought to be the main driver of these landslides. In the same thought, maybe the Nile Landslide has experienced above average rainfall over a period of time, similar to the other two landslides. We asked Cliff Mass (click here for his Blog) at the University of Washington Atmospheric Sciences to help us figure out the the precipitation history of this area. The data, emailed from Mark, an colleague of Cliff Mass, isn’t a smoking gun. The email below:

I have looked at water year annual precipitation for 2 snotel sites situated on the east slope of the Cascades but somewhat north of the Niles Landslide. They are Blewett Pass and Grouse Camp snotel sites.
There are no snotel sites in the immediate vicinity of the Niles Landslide.

Over the past water year (Oct 2008-Sept2009) precipitation totalled 10% above the long term average (1983-2008) at a composite of the two snotel sites.
Over the past 2 years ==> 2% above the long term average (1983-2008).
Over the past 3 years ==> 6% above the long term average (1983-2008).
Over the past 4 years ==> 7% above the long term average (1983-2008).
Over the past 5 years ==> 0% above the long term average (1983-2008).
The 2005 water year was unusually dry bringing the 2005-2009 5-year average back to nearly the same as the long term average.

Hmm, well, looks like we are back to the drawing board.

This landslide has brought together an surprising amount of scientists from various agencies around Washington State. One that certainly deserves mention is John Vidale, Director of the Pacific Northwest Seismic Network, and his crew, who has been of invaluable help to us in helping to unravel the timeline of this landslide. Here is an excerpt and some data that John had given us:

These are spectrograms, which plot frequency content of the seismogram the vertical axis against time on the horizontal axis. The number on the horizontal axis is hours after the start of Saturday, for example, 34 is 10am Sunday. I think you can see more detail on them by looking at them in a graphics program. This 1st plot runs from 25 to 35 hrs, the bright red spot is the landslide noise at 10am Sunday. The vertical axis is frequency – 0 at the top grading to 10Hz at the bottom. The 5Hz sound of the landslide grows from imperceptible on the left until the racket at 34, then fades slowly.The industrial source at 9Hz is visible as the pulsation on the bottom, and the pops are too short to see in this plot. The 5-10Hz smears in the lower right are probably unrelated cultural noise that starts at daybreak after a quiet night.


Close-up of the noisy part, spanning about 1.3 hrs or 80 minutes.

This is the noisiest part of Saturday, hours 5-21, on the same color scale. More cultural noise 5-10Hz starts about 7am, as appeared above for Sunday. There is not a signal similar to the 5Hz band above, which is apparently how the landslide appears on this station. Also, the patches of signals present do not match the timing of energy on the other nearby station ELL. So maybe some Saturday landslide noise could be invisible on this plot, but it would be less than the noise on Sunday.

Here is an example of the pops at their most frequent, 2 hours before the big noise. The plot spans about 15 minutes, and the pops appear on the upper half of the plot, 1-5Hz, and agree in timing with pops seen on station ELL.


This is the burst at 7:38 Sunday in a 15 minute window. Note the strong 1-2Hz energy, more so than during the rest of the landslide-related signals, and most of the action takes place within 1 minute.

This is the 4:55am Sunday burst in a 15-minute window, weak but with the same frequency range and gradual onset as the other slide related shaking.

This sort of data allows us for form a timeline to the landslide movement. Combined with eye witness reports, we can reconstruct the various parts of the landslides and when they moved. With that data, we can look at the places of initial movement and evaluate the pre-failure conditions to see if there is any likely event that might have triggered this landslide. Vary preliminary data, however, has been suggesting that landslide movement might have been prior to 2002 (we are still working on this), but this movement was quite slow, just about creeping. I am currently working on tying together a series of aerial photos to determine the amount of movement and hopefully to constrain the first start of movement.

slide3

Sampling the Ice

October 22, 2009

I just got back from GSA, so I apologize for the delay in the posts. We have had a lot of talks about this landslide at the conference and have a lot of heads thinking and working on it. Particularly, the ice. It actually might play a big part in the landslide failure. If the landslide formed deep cracks through previous slower movement, it could have allowed warm air into the area where the ice was. This could have resulting in melting of the ice, which would have added in water to the system or just made the perfect slide for a block of ice to flow on. The low angle of the landslide would suggest something like this could have occurred. Once we gain a sample of the ice, we will run some tests on it, try and determine its age and so forth. Hopefully it will shed some further clues on this landslide.

UPDATE:
As I thought might have occurred, the places where we first found the ice had collapsed and it was impossible to safely gather a sample of ice (ever try digging on an unstable talus slope?). So, that clue looks like it might be lost unless we can recover something from drilling.

Ice in the Nile Landslide?

October 14, 2009

One of the odd things that we discovered during our investigation on Sunday was chunks of ice in the Nile Landslide. Ice. I guess I should qualify this a bit better. The ice is between the coarse rocks and it doesn’t look much like ice when you are next to it. Here is what I mean:

Ice in the Nile Landslide

Ice in the Nile Landslide

To get it out of the way, the ice probably didn’t play much, if any role in the landslide failure. It is probably a product of the talus, which in Eastern Washington can sometimes form ice cores. These are well known to many in Eastern Washington and even were mined early on as natural cold storage. This isn’t something I know a whole lot about, but my geologist friend Jack from Eastern Washington explained it to me. Cold air from the surface is able to permeate into the talus, forming a barrier from the hot air. As water infiltrates down into the talus, either through precipitation, snow melt, or shallow groundwater (or other ways), the water hits that cold air and starts to freeze (as long as the air is colder than freezing). The cycle continues as cold air can continue to permeate down, keeping the ground cool and protecting it from the hot air above.

I will be heading into the field on Thursday, either with WSDOT on a helicopter flight or by vehicle. Either way I will be hiking on the west side of the Sanford Pasture Landslide, checking for stability. I’ll keep you posted.

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It is a strange thing to see how TV works from the production side. The interview I had was maybe 15 minutes or so, but it all boiled down to about 10 seconds of film on air. I am glad to see that this landslide is getting the attention it deserves. This is a major event in Washington’s history, at least, landslide history.

Features of the Nile Landslide

Features of the Nile Landslide

This is a generalized map of the different features present in the Nile Landslide. The main landslide mass is located in red and deformation at its toe is yellow. The salmon color block is an area where cracks were noticed along the hillside. The orange is a block of material that was pushed and cracked by the Nile Valley Landslide main landslide body. The diverted Naches River is blue in color. These offer a representation and a general boundary of the features. More detailed delineation will be conducted once the landslide is safe to work on.

The Nile Landslide is part of a much bigger landslide system. The Sanford Pasture Landslide is perhaps the second largest landslide in Washington State, it is about 6 miles long an over 2 miles wide. The Malaga (aka Stemilt) landslide is the largest landslide in Washington State.

Sanford Pasture Landslide

Sanford Pasture Landslide

That little red blob at the bottom left of the map, that is the Nile Landslide that just occurred. The Nile Landslide is actually on a secondary failure (probably resulting some point after the initial movement of the bigger landslide), probably on the toe of that landslide. The worry is that the removal of mass on the toe might weaken the upper material by the removal of lateral material. There is a lot of rock up there and if the landslide propagated up or actually triggered a larger block up slope, the result could be the entire valley filling up with sediment. Bare in mind that there is a very low chance of this occurring, but it is something we need to checking out.

The age of the Sanford Pasture Landslide is unknown, but probably Holocene. The landslide, when it moved, crossed the valley and flowed up onto the other valley wall. This might have been done is a rapid, catastrophic failure. The landslide probably formed lakebed deposits upstream and sometime in the near future, we might be able to date this landslide.

I will heading out into the field again soon. I will try and keep you all updated. Most of our photos will be posted on our DNR flickr site located here:

Western edge of the Nile Landslide

I am going to try and post some new photos either tonight or early tomorrow morning.

Nile Landslide Map

October 14, 2009

Nile Landslide Map

Nile Landslide Map

Woodshed Restraint
I am not sure if Jack (a geologist here at DNR who works in Forest Practices, who created this map) intentially put the Woodshed Restraint or not. Either way, it is a good term. As the landslide came down, it hit a block of material, probably an old landslide block. The weight and movement of the landslide behind it slowly pushed the old block (which on the surface is a small hillside). The block had many fissures forming and the surface ruptures trended north-south, lining up with the stress from the main landslide mass.

Cracks on the Woodshed Restraint

Cracks on the Woodshed Restraint