Fault Scarp or Ranch Road? Review of USGS paleoseismic trenching near 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. I have worked on the surficial geology of this area for more than 20 years and am familiar with the site.


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.


Trench site. The SUK Trench site 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 fault scarp. The dissected bench the feature traverses is composed of late Pleistocene Touchet Beds (Missoula flood slackwater deposits) capped by <1m of Holocene loess. Local gullies are mostly dry, but carry intermittent flows now and then. The water table is several meters below the surface. Vegetation cover is sagebrush, grasses, and some forbs. Base image is a lidar DTM 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 lidar images. A network of two-track ranch roads (black lines) follow intuitive routes and facilitate ranch work. Some were built decades ago by a dozer. The trace of Angster's "scarp" follows one such track (yellow line).



Commentary

The spatial correspondence between a two-track ranch road and the "fault scarp" identified by USGS is striking. The scarp appears to have been created by a dozer, not recent tectonism. USGS located no fault in their SUK Trench. The Touchet Beds and the overlying loess are undeformed.


Nevertheless, Angster, Sherrod, and Lasher presented their findings, including claims of "widespread liquefaction" and the discovery of "Mount St. Helens Set J tephra", via poster to the 2020 American Geophysical Union meeting 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. Links below.


I am mystified how managers at the USGS Seattle Field Office-Earthquake Science Center (on the UW campus) let this work 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 review should have terminated his proposal to trench immediately. Instead of, "Find a better prospect, young man," Angster was told, "Load up the backhoe."


A second element in their preliminary report struck me as strange. Angster reports finding MSH Set J tephra in the trench wall. Ash beds can be precisely dated and provide age control on sediments and deformation. Finding Set J ash emphasizes how young the deformation is. However, the Set J is all but absent in south-central Washington. This is common knowledge among geologists who live and work in the region. On the other hand, the bright white MSH Set S tephra, is ubiquitous in correlative sediments throughout the Walla Walla Valley, Yakima Valley, and Pasco Basin. The tephra in the SUK Trench is the Set S, not the Set J.


A third red flag is the identification of "widespread liquefaction" in the loess that caps the trenched section. In south-central Washington, modern soils are commonly developed into a half meter of post-glacial (Holocene) loess. While new discoveries are certainly possible in the Channeled Scablands, widespread liquefaction in Holocene loess is simply not a thing. There are hundreds of excellent roadcuts, railcuts, and cutbanks within 100 km of the SUK Trench site that expose identical sediments. These outcrops have been visited 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 have noticed liquefaction features in Holocene loess. None has.


At this point it is unclear whether Angster and Sherrod intend to publish their work as it stands, abandon it altogether, or submit a heavily-revised version for peer review. Let's hope its the latter.


What am I missing?


Your comments are always welcome: skyecooley@gmail.com


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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/


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

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






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