Predicting liquefaction extent in the Yakima Fold Belt, south-central WA

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.

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 to Lewiston, ID to White Swan, WA is almost non-existent. In fact, the region's most obvious liquefaction features (sand blows) are located way up in Priest River, ID in clay-rich proglacial lake sediments.

The U.S. Geological Survey is currently assessing liquefaction evidence in the Yakima Fold Belt (south-central Washington) by examining and dating deformed strata exposed in the walls of shallow trenches they have 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" (above 7.0 M) is not clear at this point.

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, landowner conditions, and the size of equipment factor into the decision of just 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, a scarp with clear surface expression (lidar imagery, aerial photos) may not actually be the main fault or express well in the subsurface, thus 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 most basic 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 or otherwise accommodate slip. A modest-magnitude estimate is therefor predicted for the region, with a maximum credible estimate no greater than 6.8 M.

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). If the faults in the Yakima Fold Belt, which have been growing over the past 5-10 million years (presumably 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.

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