Predicting liquefaction extent in the Yakima Fold Belt
Geologists have long debated how large of an earthquake faults in the Yakima Fold Belt are capable of producing. Some claim these shallow (<10km), intraplate structures are capable of generating quakes up to 7.0 magnitude. Others estimate their capability no higher than about 6.0 magnitude. The difference between 6.0 and 7.0 is huge. I've personally experienced two quakes larger than 7.0 M. Believe me, crazy sh*t happens above magnitude 7. I've also been in several 6.0 M quakes. Those, by contrast, were little more than brief amusements. Draw a bright line at magnitude 6.8 (VII). And ask that paleoseismologist how many M 7 quakes has he/she personally experienced.
Here's an excerpt from the the Walla Walla Union on July 17, 1938 describing the 1872 Chelan/North Cascades Earthquake,
And the same article's description of the 1936 Umapine/Stateline Earthquake,
New seismic hazard database and probability maps. Oregon just completed their latest earthquake hazard assessment, the Oregon Seismic Hazard Database 1.0 (Madin et al., 2021). The map above is an excerpt from Plate 3 showing the "probability over the next 50 years of experiencing shaking strong enough to damage weak buildings". Medium blue to light blue colors, which correspond to Low (6-10%) to Very Low Probability (<5%), appear in the southern Walla Walla Valley, Umatilla Basin, and Columbia Gorge between Wallula Gap and Hood River.
I throw my lot in with the lower-estimate camp, based on my experience working in the unconsolidated sediments of the greater Columbia Basin for more than 20 years.
The evidence of liquefaction in Neogene, Pleistocene, and Holocene sediments from Hunters, WA to The Dalles, OR and from Lewiston, ID to White Swan, WA is almost non-existent. In fact, the region's most obvious liquefaction features (t-shaped sand blows) are located way up in Priest River, ID in clay-rich proglacial lake sediments.
Local deformation along local faults, yes. Widespread liquefaction, no.
The U.S. Geological Survey is currently assessing liquefaction evidence in the Yakima Fold Belt by examining and dating deformed strata exposed in the walls of shallow trenches they have excavated across scarps found in lidar images. Its not yet clear what all they have found, but early press reports suggest liquefaction features are present in southern Pasco Basin and perhaps elsewhere. No liquefaction was documented in trenches in the western Boylston Mountains (Barnett et al., 2013) or at Wenas Creek (Sherrod et al., 2013). Whether the evidence attests to "strong shaking" (above 7.0 M) is not clear at this point.
The Pliocene Ringold Formation at White Bluffs, a continuous exposure stretching 65 km, is not tectonically deformed (Brown and McConiga, 1960; Hays and Schuster, 1983). Ringold sediments contain the usual suite of "normal sedimentary and compaction features of these type of lacustrine deposits". (Newcomb, 1972) and, from what I've seen, little evidence of strong seismic shaking (>VII). Elsewhere, the Ringold is undoubtedly involved with young folding and faulting (Warren, 1941; Jones, 1945; Strand and Hough, 1952; Newcomb, 1958), but full rip events? Um, no.
Tight corridors. Light blue ovals are my prediction of where liquefaction evidence in the Yakima Fold Belt in south-central Washington will be reported by USGS in the future. Liquefaction will be limited to tight corridors at low elevations in unconsolidated sediments along mapped faults, consistent with modest shaking (MCE <M 6.8, <VIII).
Trenching and age dating of sediments is the modern way Geomorphologists and Paleoseismologists assess fault activity and the threat faulting poses to local infrastructure and human safety. One limitation of fault trenching is that trenches are shallow, typically just a few meters deep. Shallow trenches provide clear views of the subsurface, which may be better than natural outcrops. But trenches are a compromise. Cost, safety issues, environmental impacts, and landowner restrictions all factor into the decision of where and how deep to dig. Faults and the strata they cut reach much deeper than the view a trench provides. The good stuff might be 5m, 50m, or 500m down and unreachable by the excavator's bucket. Also, scarps with clear surface expression (lidar imagery, aerial photos) may not actually correspond to the main fault or express well in the subsurface; trenches can sometimes miss the real action. Nevertheless, trenches are the closest thing we've got to drilling and high wall mining - and they're a whole lot cheaper than even the simplest drill rig or open pit operation.
Bedrock in the Yakima Fold Belt is Miocene Columbia River Basalt. I contend that layered flood basalts, when folded, simply crumble. They don't store elastic energy in the same way many other rocks do. Sedimentary interbeds between some of the flows serve as slide surfaces and accommodate slip without much fuss. A modest-magnitude estimate is therefor appropriate for the region - an maximum credible estimate (MCE) somewhere below 6.8 M. Earthquakes, yes. Large quakes, no.
New avenues for exploration. Faults in the Yakima Fold Belt have been growing over the past 10+ million years - presumably generating large earthquakes all the while. So why have geologists not found widespread, unambiguous evidence of soft sediment deformation ("seismites") in the a.) Ringold, Ellensburg, Thorp Formations, b.) borehole logs and physical cores stored at Hanford, c.) Ocean Drilling Program cores collected offshore of the Columbia River mouth, d.) fine grained non-flood deposits in the Channeled scabland, e.) fine grained Pliocene and Pleistocene deposits just beyond the floodway margins (i.e., Kittitas and Naches-Tieton Valleys), f.) cores from Cascade Range alpine lakes and other lakes in Eastern Washington, g.) Okanogan/Okanagan Valley glaciolacustrine sediments, h.) Glacial Lake Missoula beds, and i.) thick bodies of Holocene alluvium in Eastern Washington. Lindsey's detailed stratigraphic sections make almost no mention of disturbed beds, nor do Newcomb's (Newcomb, 1972). Are a few trench logs through surface faults enough (i.e., West and Shaffer/GEI, 1988 work)?
Attention graduate students: Start looking here. The Kittitas Valley and Naches-Tieton Valley are two basins that do not contain flood deposits, but do contain significant volumes of young, semi-consolidated, fine grained sediment deposited during growth of the Yakima Folds. Both lie on the east slope of the Cascade Mountains in close proximity to active faults. A field-based survey for features attributable to strong shaking in exposures of post-basalt sediments along the Yakima River corridor (Cle Elum to Richland) would be cheap and relatively quick to do - a perfect project for a couple of field-savvy graduate students. Simply plot the types of features found along a longitudinal profile of the river.