A small landslide occurred in Gig Harbor today (Nov. 16), damaging a garage and two vehicles. The landslide occurred at 29th Street Court Northwest near Shawnee Drive around 1:30am.

Landslides in urban areas, especially during these first storm events, could be caused by misdirected water or water inadvertently being directed into an unstable area. Something like a drain blocked by leaves causing water to flow onto the hillside. Unfortunately, I have been dealing with my computer dieing this morning, so I haven’t been able to investigate this landslide on the scene. Hopefully I will make it out there tomorrow.

This ground is starting to get to a good saturation level. The chance is probably low to moderate for at least one or two more landslides around the Puget Sound Area from this storm.

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It has been awhile since I completed a Landslide of the Week. I think the Sanford Pasture Landslide is a good candidate since it has gotten so much press lately and what we know about it is fairly limited (at least, in publications).

The formation of the Sanford Pasture Landslide started back in the late Miocene and early Pliocene epochs, where the eruptions covered much of Eastern Washington with basalt, known as the Columbia River Basalts. Between the eruptive cycles, sandstones, generally fluvial in origin, deposited on top of the flows, only to be covered by the next pulse of magma. These are known as interbeds and are suspected to be Ellensburg Formation. At the Sanford Pasture Landslide, the dominant flows of the Columbia River Basalts are the N2 and R2 flows of the Grand Ronde Basalts, some of the last recorded flows of the eruptive cycle. Much of the deposits were lain horizontally, but as we know them today, the geologic units are folded and faulted. This is accomplished by stress from the subductive oceanic plate pushing its way underneath the continental crust that we live on here in Washington State. The force of the collision compresses Washington State, forming wrinkles and faults as the stress is dissipated through the plate. In the Naches area, this folding resulted in the formation of Cleman Mountain as a steeply dipping anticline. The area was not able to just fold to reduce the stress on it, it faulted as well, forming the Nile Thrust Fault. The failure mechanism is something that we probably do understand. The oversteepened anticline combined with the weak interbed layers of sandstone created a perfect weak plane for the above rock to slide on. An earthquake, probably on the Nile Thrust, or perhaps something larger like a Cascadia Subduction Earthquake, probably reduced the restraining forces enough to start the material moving downhill, depositing where we see it today (more on that below). These events occurred after the Columbia River Basalts and interbeds were lain in place, giving us a limiting age on the landslide. Given the flow age, coupled with the folding and faulting of the area, the general estimation of the landslide is 2 million years old.

Determining the age of a landslide is often difficult. Dates can be acquired through a couple of different methods, most often coring into sag ponds, or lake bed deposits (on older landslides that have dammed rivers), or by coring old tree snags that have been drowned. The goal is to find datable material or stratigraphic reasoning to determine a specific of general age. For the Sanford Pasture, there are no found lake bed deposits up valley of the landslide initiation and the landslide is too old to support sag ponds that formed during its initial movement. The general thought is that the landslide occurred prior to glacial times.

The Sanford Pasture landslide moved across what is today the Naches Valley and deposited material almost a mile inward from the valley’s edge. During this time, the Naches Valley was less incised and contained much less water (remember, no lake beds deposits), so whatever damming of the paleo-fluvial system here, it was minor. During the age of glaciation in the Quaternary Period (predominantly alpine glaciation influences at the Sanford Pasture). Advances and retreating of the glaciers, combined with their constant run-off carved much of the valleys and fluvial systems we see today in the area. I should point out, I don’t think any glaciers have reached the Sanford Pasture Landslide area. The melt water flowing through what is now the Naches Valley would have eroded out the landslide and continued to incise into the valley, exposing in-place Columbia River Basalt Flows on the western side and eastern side of the valley. Unfortunately, all of this erosion created yet another unstable element into the system. The eroding river removed much of the lateral strength that the landslide had when its mass continued for another mile. It literally shortened the landslide by half. In response, the Sanford Pasture landslide didn’t fail as one large piece, but as smaller failures within the older landslide material.

This image of the Sanford Pasture Landslide is a quick drawing of the possible major landslide events. There are dozens of smaller events throughout the landslide. The most difficult part to figure out is the northwest section of the landslide, that appears to have gone through a series of deformations, probably more than I have drawn here. That is something we are going to try and unravel down the road. It is difficult to determine if the last major movement was on the eastern or western section of the landslide. The only sag pond that exists on the landslide is on the eastern side, known as Dog or Mud Lake. This makes me suspect that the last major movement has been on the eastern side. Other evidence also suggests that the morphology is younger, less stream development and incision on the eastern side. Regardless, the western side is the side where the Nile Landslide initiated off of and probably has a much more active, smaller landslide activity.

The area where the Nile Landslide has occurred has experienced several large landslide events. Looking at the history, the Nile Landslide is probably the 4th in a series of movements in the area (Sanford Pasture, Largest block in purple, smaller block in green, then Nile Landslide). That is the larger movements. Further evidence looks like smaller landslides have been recent in the same area as the Nile, maybe being able to form and move every couple of hundred years (not sure how far back this might go, but maybe a thousand or two years, depending on when the major movement of the largest block in purple and smaller green block occurred). Granted, that is a bit of speculation. In the 1940’s photo, there is clearly areas without vegetation that look hummocky that might indicate recent movement, like within the last 50 years. Comparing that 1940’s photo to today, areas that were once void of vegetation now are supporting sparse tall trees, indicating a possible regrowth period. Maybe we are looking at something that is geologically common here.

The last work, Sanford Pasture Reactivation. This has been pushed around in the media about State Geologists concerned about future movement of the Sanford Pasture Landslide. They are right, we are concerned, I being on of them. The removal of lateral support by the Nile Landslide could reactivate something larger uphill. Remember, this is really torn up landslide material, it has its strength reduced and it looks like it is sliding on something that is fine grained. Reactivation of the Sanford Pasture Landslide, worst case scenario, would completely block the Nile Valley, forming a massive lake (Lake Naches?) behind the debris. The threat would then continue into the competency of the material to hold the water, a race to safely dewater the lake and the possible major dam-burst flood into the Yakima Valley. The destruction of that last one would be unlikely, but something we have not seen the likes of in modern society.

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.

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.

Quick Report

October 16, 2009

Okay, lack of sleep and long hours have been catching up to me. Here is the latest:

On October 15, 2009, Jack Powell (FP), and Isabelle Sarikhan (DGER) investigated the Nile landslide on Highway 410 and the older Sanford Pasture Landslide approximately 10 miles west of Naches. There is concern that the Sanford Pasture Landslide could reactivate as a result of the recent movement of the Nile Landslide. We also investigated potentially new activity within the Nile Landslide, continued risk to public safety and the safety of crews working on the ground, and collected additional data.

No immediate danger was found for continued progression of failures. No cracks or fissures were found above slope of the Nile Landslide and a traverse of the crest of the hill above the Nile Landslide also found no cracks or fissures. Further, evidence was found that the Nile Landslide occurred below the failure plane of the Sanford Pasture Landslide, greatly reducing the risk of movement by the release of pressure from the Nile Landslide. However, because of the size and wide scale impact that the movement of Sanford Pasture can cause, further surveys for recent cracks and fissures further uphill on the western edge of the Sanford Pasture Landslide should be conducted.

The Nile Landslide had moved several feet since Sunday. During our visit on Oct. 15, no rapid signs of movement were occurring, but the landslide is probably still moving at a speed of inches per day. The speed will gradually slow, but movement might not completely stop for months. Sudden catastrophic movement is unlikely and the risk of public safety and workers on the ground should be low if they stay a safe distance from the landslide. The most unstable parts of the landslide are the toe, which is oversteepened and can result in shallow failures and the scarps, both internal and the main scarp, which can ravel or result in shallow failures without warning.

Nile Landslide Field Visit

October 15, 2009

Tomorrow looks rainy, really rainy out by the landslide. In the dryer climate of Central Washington, dirt roads are not always built to drive on when wet. So, we might not be able to drive very far up on the Sanford Pasture Landslide tomorrow. We are still going to investigate on and around the Nile Landslide. We have heard that there is uplift of up to 8 feet on the old Naches River bed. We will be examining for cracks around the landslide to see if the landslide is propagating (expanding) and potentially going to continue to fail. If we can, we will also try and hike or drive up above the Nile Landslide to look for stress or tension cracks from the release of material below. I will be posting probably on the trip on Friday Morning.

Oct. 13 SR 20 Landslide

October 15, 2009

In all of the focus on the Nile Landslide, I just about missed this landslide up on SR 20.
WSDOT Alert

DATE/TIME: October 13, 2009, 2:19 p.m.
DESCRIPTION: Both lanes of SR 20/North Cascades Highway are blocked by a rockslide at milepost 145.5, the Skagit/Whatcom County line.

LOCATION: SR 20 at the Skagit/Whatcom county line
START: October 13, 2009, 2:05 p.m.
Est. END: unknown
OTHER: WSDOT maintenance crews are en route to the slide and will provide an estimated reopening time once they assess the area.
CONTACT: Bronlea Mishler, 206-440-4455