Fault Scarp or Ranch Road? Review of USGS paleoseismic trenching near Wallula, WA

Trenching at Wallula, WA

In 2020, the U.S. Geological Survey excavated a trench across a suspected fault scarp near Wallula, WA in an effort to better understand earthquake hazards in south-central Washington (Brocher and Sherrod, 2018; Angster et al., 2020). In this post, I review the land use history of the SUK Trench site recorded in several decades of aerial photographs.


The site is located along the trace of the Olympic-Wallowa Linement and locally the Wallula Fault Zone. The site itself is located in open rangeland - geomorphically stable, sparsely vegetated, undeveloped, and never glaciated. Some windblown dust is accumulating today, but no other sediment. No oral history or historical accounts of a fault rupturing the ground here are known. The combination of the Horse Heaven Hills ridge (s-wave amplification?) and the Columbia River trough (s-wave deamplification?) provide neaby topographic relief.


Location of SUK Trench. The trench is located along the Wallula Fault Zone east of the Columbia River off Hwy 12 west of Walla Walla, WA.


Namesake. Grafitti on an old shed visible from Hwy 12 just east of Wallula Jct provided the name for the SUK Trench.


Trench site. The SUK Trench site on a bench above the mouth of the Walla Walla River near Wallula, WA. Hwy 12 is seen at top of this June 2019 photo.


Mapped scarp. Yellow line highlights a feature USGS Geologist Stephen Angster interpreted to be a young fault scarp. The dissected bench traversed by the feature is composed of late Pleistocene Touchet Beds (Missoula flood slackwater deposits, 18-14 ka) capped by <1m of Holocene loess. A weak soil is developed into the loess. Local gullies are dry, but carry intermittent flow during high rainfall events. The water table is located several meters below the surface. Vegetation cover is sagebrush, grasses, and some forbs. Base image is a bare earth lidar hillshade.


Aerial photo interpretation. I used nine sets of aerial photo sets (1996-2021) to compile a map of features created by humans and livestock in the vicinity of the SUK Trench. Mapped features include ranch roads, cattle trails, fencelines, watering structures, a borrow pit, and a single small building. The same features appear in the lidar image. A network of two-track ranch roads (black lines) were established along fencelines and other intuitive routes. Some appear to have been built decades ago by a dozer, but are still in use today. The trace of Angster's "scarp" follows one such track (yellow line, black line).


Overlap. A road and the "scarp" appear to be the same feature.



Review and Commentary

No fault was found in the SUK Trench.


The spatial correspondence between a two-track ranch road and the "fault scarp" mapped and trenched by USGS is striking. The scarp appears to be a road created by a dozer sometime after WWII, not a break in the Earth's crust. Remnants of the road surface are seen in the trench log. A cultural feature, not a fault.


"Widespread liquefaction" was identified in dry Holocene loess well above the water table. No evidence of wetland sediment or soil is present. The underlying silty-sandy Touchet Beds (18,000-14,000 years old) are undeformed. In south-central Washington, modern upland soils are commonly developed into a half meter of post-glacial loess (windblown silt). While new discoveries are certainly possible in the Channeled Scablands, widespread liquefaction in Holocene loess has never been reported. There are hundreds of excellent roadcuts, railcuts, and cutbanks within 100 km of the SUK Trench site that expose identical sediments that have been examined by thousands of geologists, researchers, soil mapping crews, road engineers, dam engineers, Hanford scientists, guide book authors, field trip participants, and students over the past century. Someone prior to Angster would long ago have noticed liquefaction features in Holocene loess if present. None has. Alternative explanations should have beeen more thoroughly considered.


Angster reports finding MSH Set J tephra (~13,000 years old; 12-10.5 ka according to Mullineaux, 1996) in the trench. Ash beds provide precise age control on sediments. Finding the Set J ash suggests faulting at Wallula is a bit younger than at other sites in the region. However, it is unlikely the ash bed is actually the Set J. The distinctive Set J tephra is all but absent from south-central Washington due to its limited eastward drift during eruption and extensive post-depositonal soil mixing. Geologists who live and work in the region would be quite surprized to find Set J in any upland exposure this far east. More likely, Angster found the bright white MSH Set S tephra (16,000 years old), which is ubiquitous throughout the Walla Walla Valley, Yakima Valley, Umatilla Basin, Lewiston Basin, Tucannon Valley, and Pasco Basin. Set S ash is readily observable in nearby roadcuts near Wallula, even at 65 mph. The tephra in the SUK Trench is amost certainly Set S, not Set J.


Out Over Your Skis

Mistaking a road for a fault, confusing Set J for Set S, and taking soil features (piping? rodent burrows?) for liquefaction features are errors of irrational exuberance. Rookie mistakes. No one benefits from overinflated seismic hazard rating.


Press

Angster's team presented their SUK Trench findings, including the claim of "widespread liquefaction" and the discovery of "Mount St. Helens Set J tephra", to the 2020 American Geophysical Union meeting (poster session) and via Zoom to the Wenatchee Valley Erratics in 2021. Northwest Public Broadcasting ran a story on their trench work at Wallula in December 2019. The word has gotten out. See references below for links to reports and media.


Who's In Charge?

I am mystified how managers at the USGS Seattle Field Office-Earthquake Science Center (on the UW campus) let this through review. A casual ten minutes spent surfing around in Google Earth would surely have raised questions about the nature of Angster's "scarp". Internal reviewers should have terminated his proposal to trench the site early on. Instead of, "Find a better prospect, young man," Angster was told, "Load up the backhoe." Local experts were invited to the site after the trench had already been opened, rather than during the planning stages. Many thousands of tax payer dollars were wasted.


Publish or Abandon

At this point it is unclear whether Angster and Sherrod intend to publish an article on their work as it stands, or submit a heavily-revised version for peer review, or exclude the SUK Trench from regional seismic hazard syntheses altogether. I'm guessing findings at the SUK trench will make their way into a region compilation by USGS.


What am I missing?


Your comments are always welcome: skyecooley@gmail.com


..................................


References


Angster, S.J.; Sherrod, B.L.; Lasher, J.P., 2020, Constraining the paleoseismic record on the Wallula fault from widespread liquefaction features in southeastern Washington: Nice scarp, where's the fault...?, Link to AGU poster below:

Angster_AGU2020_Wallula Fault and Liquifaction_D3_FINAL
.pdf
Download PDF • 1.02MB

Brocher, T.M.; Sherrod, B.L., 2018, Intensities, aftershocks, and location of the 1936 Milton‐Freewater Earthquake near the Oregon–Washington Border, USA Intensities, Bulletin of the Seismological Society of America, v. 108, p. 2594-2613


Courtney Flatt, "In southeastern Washington, geologists dig into the past to find fault signs of future shaking", Northwest Public Broadcasting (December 6, 2019)

https://www.nwpb.org/2019/12/06/in-southeastern-washington-geologists-dig-into-the-past-to-find-fault-signs-of-future-shaking/


Mullineaux, D.R., 1996, Pre-1980 tephra-fall deposits erupted from Mount St. Helens, WA, USGS Professional Paper 1563


YouTube: Stephen Angster talks to Wenatchee Valley Erratics - December Meeting 2021

https://www.youtube.com/watch?v=51ofR0raCCo






Last 50 Posts
All Posts by Month