Off the United States’ northwest coast the Juan de Fuca Plate is diving beneath North America along the Cascadia subduction zone. Ensuing crustal shortening has created the Yakima Fold Province, a seismically active region in central Washington where deformation is focused along a series of arch-shaped anticlinal folds. The relative timing and rate of deformation along individual structures in this region are still poorly constrained, however, despite its potential to unleash earthquakes at least as powerful as the M6.8 Entiat event that occurred there in 1872.
To better understand the province’s tectonic history, Staisch et al. used several independent but complementary methods to analyze the signatures of deformation focused along three anticlines in the Yakima Canyon region. By employing stream profile inversions, balanced cross sections, and geophysical mapping techniques, the team was able to constrain local slip rates and fault geometries and use the results to calculate the amount of time required for each fault to accumulate enough strain to rupture.
The results indicate that stream incision rates accelerated during the Pleistocene, a change the team attributes to tectonically driven uplift rather than climate. The researchers also estimate that modern slip rates range between 0.4 and 0.5 millimeter per year, with motion accommodated along reverse faults coring each anticline, and that the region has been compressed by a total of 3.5 kilometers (11.5%) since the mid-Miocene.
The team’s calculations indicate that large earthquake (M ≥ 7) seismic events could recur as often as every 200 to 6,000 years on faults within the fold province, a region that was previously considered aseismic. This study demonstrates how independent analyses of diverse data sets can complement one other and collectively improve our understanding of deformation history, as well as help estimate potential hazards, in seismically active regions. (Tectonics, https://doi.org/10.1029/2017TC004916, 2018)
—Terri Cook, Freelance Writer