Predicting liquefaction extent for Yakima Fold Belt faults, south-central WA
March 9, 2020
Geologists have long debated how much shaking the faults in the Yakima Fold Belt can produce. Some claim these shallow, intraplate structures are capable of generating earthquakes 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 been in two >7.0 M quakes; crazy sh*t happens above magnitude 7. I've also been in several 6 M quakes. All kind of fun.
I throw my lot in with the lower-estimate camp, based on my experience looking at unconsolidated sediments throughout 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 to Lewiston, ID to White Swan, WA is almost non-existent. In fact, the region's best features are way up in Priest River in clay-rich proglacial lake sediments.
The U.S. Geological Survey is currently assessing liquefaction evidence in south-central Washington by examining and dating deformed strata exposed in the walls of trenches excavated across faults that emerge at the surface. Its not yet clear what they have found, but early press reports suggest liquefaction evidence is present in southern Pasco Basin and perhaps elsewhere. Whether the evidence attests to "strong shaking" or not isn't clear at this point.
Trenching and age dating of sediments is the modern way Geomorphologists and Paleoseismologists assess fault activity, thus the threat poses to local infrastructure and human safety. One limitation of fault trenching is that trenches are shallow, typically just a few meters deep, but faults and the strata they cut go much deeper. The good stuff might be 5m, 50m, or 500m down and unreachable by the excavator's bucket. Also, a scarp with clear surface expression may not actually be the main fault, thus trenches can sometimes miss the real action. Nevertheless, trenches are the closest thing we've got to drilling - and they're a whole lot cheaper and simpler than even the most basic drill rig.
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 beneath hundreds of landslides in Eastern Washington. Interbeds likely accommodate tectonic deformation in a similar way. A modest-magnitude quake regime should be expected on small folds in the YFB.
Light blue ovals are my prediction of where liquefaction evidence in south-central Washington will be found by USGS in the future. Liquefaction limited to narrow zones at low elevations in unconsolidated sediments along mapped faults is consistent with modest estimates of shaking (MCE < M 6.5). If the faults in the Yakima Fold Belt, which have been growing over the past 5-10 million years (and generating earthquakes all the while), then geologists should have by now found widespread evidence of soft sediment deformation in the Neogene Ringold Fm, Pleistocene outburst flood deposits, and Holocene alluvium across the region (gray areas). We have not.