Synopsis of GSOC Friday Night Lecture, April 13, 2018
by Carol Hasenberg
Tom Pierson has been a research scientist at the USGS Cascades Volcano Observatory (USGS CVO) since 1981. His field-based research focuses primarily on the hydrological response to volcanic eruptions—lahars, debris avalanches, and floods. Pierson spoke to GSOC in February 2014 about the debris flows following the eruption of the Chaiten Volcano in Chile. Pierson’s April 2018 topic hit a lot closer to home, although the Pacific Northwest also has plenty of volcano hazards.
Pierson’s involvement in Skamania County, Washington, landslide risk assessment began in February 2007 when he and other CVO colleagues got a call from the Skamania County emergency personnel about a large landslide in Rock Creek just west of the town of Stevenson in the Columbia River Gorge.
Pierson showed some slides of this landslide in motion. In a couple of days after the call, the landslide had become much larger, moving downslope at about a foot a minute. It ended up engulfing a lot of trees and threatening some houses. The landslide was aggravated by a lot of rain earlier in the season. This is a general rule of landslides – they respond to an accumulation of rainfall. In response to this disastrous event, a team from the CVO set about the task of examining the landslide hazards in Skamania County.
At this point Pierson laid out the geological framework for the Columbia Gorge landscape and landslides. A visitor to the gorge cannot help but notice that the land abutting the Oregon side of the river is very steep and that on the Washington side has more gently sloped, hummocky terrain. In fact, in some places on the Washington side one can see layers of rock dipping towards the river at a moderate angle.
What really distinguishes the geology of the gorge is that there is a large, heavy, rigid sequence of Columbia River Gorge Basalt rocks sitting atop layers of weaker, variably weathered volcaniclastic rocks of the ancestral Cascade mountains. The area has been experiencing uplift for millions of years, causing layers to dip from 2 to 10 degrees toward the southeast, and also has been downcut by the Columbia River, which predates the Cascade Range. Add to that the abundant rainfall of the area, and you have terrain that is highly prone to sliding.
The dip of the stratigraphic layers also accounts for the difference in the landscape between the north and south margins of the Columbia River. This talk was predominantly about the north margin in the state of Washington, where the gentle slopes leading down to the river are in fact the remnants of ancient landslides, many of which are still active. The southern Oregon side is mostly steep cliffs of Columbia River Basalt and is prone to debris flows and rockfalls.
But any discussion about landslides must include a discussion of modern tools and techniques to find and analyze them. LiDAR (Light Detection and Ranging) is a method of surveying terrain using a laser mounted on an airplane with the sophisticated computer algorithms to analyze the data. Bursts of laser light are emitted and then the return times are collected and analyzed. The technique can be used to map the canopy and buildings, or these can be filtered out to reveal the ground surface beneath to high accuracy. What this means is that geologists have a clearer picture of the ground surface than they did in the past century using photogrammetry (airphoto analysis), and the landslides really pop out due to their shape and can be easily identified. In effect, for landslide analysis LiDAR removes the trees from the image with resolution of objects in feet or smaller. In the context of this study, LiDAR was used in mapping the landslides and the resolution was 6 feet.
Another, related technique is called InSAR (Interferometric Synthetic Aperture Radar) is satellite-based and uses radar emissions rather than laser light. InSAR data is much cheaper to acquire than that of LiDAR and the measurements are repeated every time the satellite passes over the terrain to be analyzed, every few weeks. Measurements between passes can be compared and changes in position over time can be monitored. This technique is highly effective in monitoring landslides, volcano inflation or deflation, ground subsidence and other such conditions. In this study, InSAR was used to determine movement on the landslides in multiple satellite flybys.
There were other techniques used in the studies of these landslides for dating, documenting movement and other information. Dating of landslides was done by dendrochronology (tree ring dating), lichenometry (measuring lichen growth on rocks), radio-carbon dating, and historic records. Besides InSAR, GPS and historic records helped to document the movement of active landslides.
Due to the research initiated by Skamania County, the area of found landslides in the study area increased dramatically. At least 215 landslides were identified, covering 64% of the land area -- an increase of 56% over previous studies. At least 12 slides are active. Parts of old slides have been shown to have reactivated. About two-thirds of the slides have translational motion, in other words they move downslope in a horizontal direction, rather than rotating along a curved failure plane.
About one quarter of the slides are less than 1000 years old and categorized as recent in the study. They all show rather sharp and clear mapping features in the LiDAR images, whereas older slides have smoother, more amorphous features. The well-known Bonneville slide near Stevenson, Washington, occurred sometime between years 1421-1447 AD, dated using dendrochronology and radio-carbon dating. The Bonneville slide is part of a major set of landslides, the Cascade Landslide Complex, originating from a set of scarps strung between Table Mountain to the west and Greenleaf Peak to the east. Another major slide in this complex, the Red Bluffs slide, has had a preliminary dating window that brackets the date of the last great Cascade Subduction Zone earthquake, 1700 AD, and it is possible that this slide was triggered by the earthquake.
An active landslide of particular interest is the Crescent Lake Landslide in the Cascade Landslide Complex, located a little to the east and overlapping the Bonneville slide. An ominous finding using InSAR was a new slump block developing above this slide on the 500-foot-high head scarp. There is a back tilt to the top of the slump block at the head scarp, so it may indicate back rotation motion in the block. But it is not known for sure if that is this type of motion or a block translational glide. The slide has moved about 30 cm in 4 years. They are currently monitoring this slide with GPS. Some movement studies on this slide have correlated the movement of the slide with periods of prolonged rainfall, a not infrequent occurrence during winters in the Columbia River Gorge. The same researcher was able to calculate the relative thickness of the landslide itself, which showed that this landslide is overlapping the old Bonneville slide.
In the calculation of risk to society from these landslides, one considers population, infrastructure, transportation and other ways the slides can impact human activities. Although the population is not great, there is lot of infrastructure in this area. The small town of Stevenson is built upon the toe of slides from the Cascade Complex. There is also the Bonneville dam, a railway, highways, electrical transmission, and a 26-inch gas pipeline running across the toes of these slides.
One important question asked by the researchers is how will all these slides react to a magnitude 9 Cascadia Subduction Zone quake? Predictions are so hard to make it’s difficult to assess risk, but the knowledge that there is a risk can be used to guide future development.
"Imagery Reveals More Landslides in Western Columbia Gorge," USGS web page, November 4, 2016
Thomas C. Pierson, Russell C. Evarts, and Joseph A. Bard, "Landslides in the Western Columbia Gorge, Skamania County, Washington," USGS Pamphlet to accompany Scientific Investigations Map 3358, 2016. This pamphlet gives background info on and summarizes the results of the study and also gives an excellent primer to landslide types and failure mechanisms.
Thomas C. Pierson, Russell C. Evarts, and Joseph A. Bard, "Landslides in the Western Columbia Gorge, Skamania County, Washington," USGS Scientific Investigations Map 3358, 2016.