Landslides in eastern Washington

Picture of the crack on Rattlesnake Ridge near Yakima. Telephoto flattens depth of image. (KOMO News.)

I have been captivated by the crack in the Earth that has opened up on a ridge near Yakima, in south central Washington state. It makes me think of other landslides that I have seen and read about over the years.

If you are following this landslide story in the news, you may want to skip to the bottom of this post to see more recent updates.

The new crack is reported to be 250 feet deep. Such a deep crack in the Earth is also known as a fissure. On the slope below the crack, the ground has recently been moving over an inch per day, according to the Yakima Herald, and the cracks have been described as part of a "slow-moving landslide."

It is possible that the landslide could turn abruptly from slow and creeping to abrupt and catastrophic. Technicians and government agencies have been working to monitor the site and make things as safe as possible for people in the area.

Steven Mack's eye-catching drone video of the crack and its surroundings has been widely shared in the news and social media.

Interstate 82 passes through Union Gap. City of Yakima behind and to the left. The image, taken from Dave Petley's blog, is a still from the drone video by Steven Mack (below).

Interstate 82 passes through Union Gap near the base of the slow-moving landslide. US Route 97 passes through Union Gap on the other side of the river.

Each time I have driven through there, I have been struck by the narrow gorge that the Yakima River has cut through the mountain, and how it is an example of an antecedent stream. The river was there first. As the ridge developed from tectonic forces, it rose slowly enough that the river, with its erosive power, was able to maintain its course. The result is the gap in the ridge where the river passes through.

From Google Maps. Labels added. Satellite image taken before cracks occurred.

An evacuation warning was issued to people who live in the potential landslide runout zone, but it is not easy to just pack up and move out of your house. The Seattle Times had an article yesterday that included stories from families faced with the challenge of living in the newly defined landslide hazard zone.

The Torres family packs up to evacuate their home below Rattlesnake Ridge. (From the Seattle Times, Jan. 4, 2018.)

Landslides happen. Where? When? Why? As a general geologist, but not a landslide expert or engineer, I can do my best to provide some basic answers.

All slopes - all parts of the Earth that stick up, rise, slope - have gravity pulling down on them. If they are made of material that cannot hold up to the gravity, the slope will move - fail, slide, creep, break. There are lots of ways for it to go, depending on how fast the material moves, how much it stays together as one or a few coherent pieces, how small its pieces are if it moves in many pieces, and how wet and liquid or dry and solid its behavior.

There is a quarry overlapping the newly developing crack system. It looks in the pictures like bulldozing has cleared the vegetation and some of the soil from the ridge up above the quarry. Landslide experts are very much aware of the several ways that clearing and excavating a slope can destabilize it, including by:

1. Undermining the foot (toe, base) of the slope. This removes material which was providing resistance to slope failure.
2. Steepening slopes. The steeper a slope is, the more directly the force of gravity is acting to pull it downhill. Therefore, if a quarry makes its backwall into the slope too steep, the quarry wall itself may fail. Quarry operators are aware of this and work to avoid over-steepening.
3. Changing the hydrology of the place - the flow of surface water and groundwater, the pressure of the groundwater. The most common effect is that the excavation of the Earth allows allows more groundwater to infiltrate into the ground more quickly. (Much less commonly, a mine or quarry may inadvertently tap a confined aquifer and release pressurized groundwater, causing a small flood.)
4. Removing vegetation that stabilized the soil and the slope. The vegetation can stabilize a slope by a combination of the living web of roots and other organic material making the soil more cohesive (more held together). The plants and other living things that grow in the soil also slow the runoff of surface water and the infiltration of water into the ground.

Types of landslides. From idahogeology.org  Landslides page.

I do not know whether the quarry at the end of Rattlesnake Ridge has contributed to the cracking and sliding of the Earth there. But the possibility occurs to me, and it seems a reasonable question.

Rattlesnake Ridge has a fault zone that has been active in the Holocene epoch, as the map below suggests.

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Image from USGS map "Faults and Earthquakes in Washington State" by Jessica L. Czajkowski and Jeffrey D. Bowman. Label and arrow added. Red faults are active (or very recently active), orange faults have been active at least in the last 26,000 years, green faults may no longer be active. 

Although I don't know if any of the mapped faults overlap the slow-moving landslide zone, geologic history makes a connection. The faults along Ahtunum Ridge and Rattlesnake Hills have uplifted the ridge and tilted the layers of rock. This has raised the risk of slope failure by making the slopes steep and the elevation high.

The Google Map satellite image, below, was taken before the fissure opened up. You can zoom the map in or out, or move around in it.



The land around Yakima, Washington is known as "landslide country" because of the active tectonics and tilted and faulted structures of the Yakima Fold and Thrust Belt. The rapid uplift of sections of the landscape, which drives nearly as rapid erosion, has created the steep, high slopes. Between the layers of Columbia River basalt that make up most of the upper crust in the area there are weak, slippery layers of fine-grained stream and lake sediment, volcanic ash and debris, and broken up basaltic material mixed with fine-grained sediment where the basalt eruptions entered wetlands and lakes. These layers tend to collect a lot of groundwater. They also tend to weaken and form the major slide planes of landslides in the area.

Another landslide has some possible parallels to the new one on Rattlesnake Ridge. On October 11, 2009, a big landslide in the Nile Valley about 25 miles west of Yakima buried 2,000 feet of a state highway that leads west from Yakima to a pass over the Cascade Mountains, damaged 4 houses, and diverted the Naches River, causing a flood that damaged several more houses. Fortunately, no casualties occurred.

From  Nile Valley Landslide Geotechnical Report-WSDOT

The geology of the Nile Valley landslide has some similarities to the (thus far) slow-moving landslide at Union Gap. The Nile Valley is also in the Yakima Fold and Thrust Belt. The Nile Valley slide came from the side of Cleman Mountain, which is a ridge of tilted, up-folded, thrust-faulted Columbia River Basalt (and other, interbedded layers of rock) like Rattlesnake Ridge. The side of Cleman Mountain is on the south side of a thrust-fault-cored anticline in the Yakima Fold Belt.

A noticeable difference in the setting of the two Earth movements is that the one at Nile Valley occurred in a landslide complex, a zone where previous landslides had occurred, going back into prehistoric times. But from what I have seen in Google Maps and other imagery, it does not appear that the landslide by Union Gap is in a pre-existing landslide complex.

Besides the new crack that has opened up, the Rattlesnake Ridge landslide does not have a large, arc-shaped or scallop-shaped scarp, the headwall that defines the upper boundary of a landslide source. In addition, the Rattlesnake Ridge landslide zone does not appear to have the hummocky terrain that is typical of a landslide after it comes to rest.

The Nile Valley landslide of 2009 (red) is dwarfed by the prehistoric Sanford Pasture landslide. This is a Lidar image from "Sliding Thought Blog - Washington's Landslide Blog," with labels added in yellow.

There was a quarry along part of the base of the Nile Valley landslide, which led to the question of whether the quarry contributed to triggering the landslide. Tension cracks started forming on Cleman Mountain above the quarry in 1998, nine years before the big landslide. The quarry was a gravel pit, smaller than the quarry on Rattlesnake Ridge. Investigations into the causes and mechanisms of the Nile Valley landslide did not decide conclusively whether the quarry played a role.

No two landslides are exactly alike. They each have their own history and process. If the Rattlesnake Ridge landslide fails completely, whether by a continued slow process that continues to let it down gradually, or if it picks up pace into a more rapid Earth failure, I expect that the question of the role of the quarrying and soil-clearing will be closely studied and debated.

Cargo containers filled with concrete being placed along Thorpe Road at bottom of Rattlesnake Ridge. (From the Seattle Times.)

In the meantime, according to the Seattle Times, technicians have installed seismometers (to detect Earth vibrations) and lasers (to detect how the surface of the ridge is moving) and are monitoring the Rattlesnake Ridge landslide. Thorp Road is closed, and cargo containers partly filled with concrete are being put into place along the road, in the hope of holding back landsliding material from going further out and blocking Interstate 82.

Image from the Seattle Times, January 4, 2018.

We cannot know in advance how that landslide will play out. It reminds us how unstable the Earth is in many places. In the long run, the ground beneath our feet doesn't just sit there, and on the larger scales, we have virtually no control over where it goes. Our job is to predict what it will do, and prepare accordingly.

Updates follow.

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Update the following day (January 6, 2018).

The Seattle Times ran an article this morning, by Hal Bernton and Sandi Doughton: "Geologist warns Yakima-area landslide could be worse than officials expect." The geologist is Bruce Bjornstad. Bruce is a colleague of mine with whom I have interacted on a few field excursions, and heard during some talks he has given to the public and to groups of geologists. Bruce has written two in-depth, well-illustrated guidebooks to the geology of the Channeled Scablands in eastern Washington. 

In the comments after the article online, and in Facebook where I follow Bruce Bjornstad, there are some differing opinions offered about the Rattlesnake Hills landslide and how it is being reported in the news. 

Bjornstad is emphasizing the worst possible scenario of landsliding, in his view. Based on his experience studying the geology of the area, and his knowledge of a prehistoric landslide a few miles down the Yakima Valley from the currently active site, he thinks it likely, or at least quite possible, that the slide will travel west, onto I-82 and possibly into the Yakima River. Here is a diagram he drew on a Google Earth image:

Bruce Bjornstad’s depiction of the potential landslide in what may be a worst-case, and according to other geological analyses unlikely, scenario. (Copied from nwnewsnetwork.org)

The quarry operators hired a geotechnical consulting company several months ago, Cornforth Consultants from Portland, and their preliminary report is available online. The report points out that (1) the layers of rock in the hillside are tilted south, towards the quarry, at 10-15 degrees of dip, and (2) their measurements of motion of the hillside found that, above the quarry, it is moving south toward the quarry. They report that the landslide is likely to continue sliding toward the quarry, even if it moves bigger and faster, and in the end it may not do more than fill the quarry.

The preliminary report discusses and shows (page 3 and Figure 8) previously mapped landslides along the north edge of Rattlesnake Ridge, on the other side of the hill from the quarry.

In addition, the report suggests that there may be some older landslides in the area that were not previously recognized. The largest of these newly proposed landslides overlaps and contains the quarry and the currently moving zone of concern above the quarry. There are, it seems to me, some questionable aspects to this proposed ancient landslide, including how small a distance it would have moved, given that the top layer of basalt within it still extends to the top of the ridge.

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One more update (Jan. 7, 2018).

For a balancing of viewpoints, see the last part of this Seattle Times article about the landslide and how residents from the evacuation zone are being given temporary lodging in motel units with kitchens.

The relevant part of the newspaper article is subtitled "Experts disagree." It features a response from Stephen Reidel, a geologist who is a research professor at Washington State University - Tri-Cities.

Steve has worked with the Columbia River Basalt and its sedimentary interbeds, and their geological contexts, structures, and origins, as much as anybody I know. As with Bruce Bjornstad, I have seen Steve give talks to the public and to professional geology groups, and have been on field trips he helped lead. I will finish with a diagram provided by Dr. Reidel to the Seattle Times, showing a schematic, simplified cross-section view of the landslide situation on Rattlesnake Ridge.

From the Seattle Times.

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Here is a four-page fact sheet about the landslide from the Washington Geological Survey, which is in the Washington State Department of Natural Resources-Division of Geology and Earth Resources.



Co-agencies on the fact sheet are the Washington State Department of Transportation and the Yakima County Office of Emergency Management.

Below is an image that the the Yakima County Office of Emergency Management tweeted on January 7. What is interesting about this image is that it maps, as red lines, cracks west of the ridge crest, making an arc when joined with the more obvious cracks everybody can easily see above the quarry on the south-facing slope of the ridge. If those cracks west of the ridgeline are there and are interpreted as marking the limit of the landslide, that may be a big reason why the geotechnical company hired by the quarry considers it most likely that the landslide is a slab which will continue moving south toward the quarry.

From Yakima County Office of Emergency Management (Yakima OEM). The original caption is, "Map of DNR's geological monitoring equipment network deployed at #RattlesnakeRidge to track changes and provide instant data to incident managers." The map is excerpted from the four-page fact sheet posted by the Washington State Department of Natural Resources.

On this still from a drone video, you can see what may be the continuation of the crack on on the hillside west of (to the left of) the ridge crest.

From a video on YouTube made by KING5 News, Seattle. (Arrows added.)