Thanks a Lot, Erwin
In 1941, the talented cartographer Erwin Raisz of Harvard University's Institute of Geographical Exploration published a map showing the Olympic-Wallowa Lineament, a "straight four-hundred-mile-long structural line extending from Cape Flattery to the Wallowa Mountains".
The Olympic-Wallowa Lineament crosscuts the whole of Washington, extending from Cape Flattery to the Wallowa Mountains in Oregon. Map by Reidel and Martin (2003).
The prominent line diagonally bisects Washington State and was immediately suspected to be deep crustal structure expressed at the surface as a series of aligned basins and faults. That is, the sum of all fears: a huge, long-lived fault zone running through major population centers, near several hydroelectric dams, and the Hanford Nuclear Reservation.
Countless geologists have pondered the OWL's importance since publication of the Raisz map, but little has been made of its "structural" relevance to Washington's tectonic history. It may follow the edge of Siletzia. It seems to align with lithospheric flow from the Juan de Fuca Ridge. So began the fascination with fault zone hazards in Eastern Washington.
A portion of the 1941 Raisz map showing south-central Washington, where the OWL is best expressed as a series of aligned ridges in the Yakima Fold Thrust Belt.
Consensus concerning the OWL has not been achieved, but many believe it is probably "an accidental alignment of different features" (Raisz, 1945), rather than a long-lived, through-going, and seismogenic crustal structure that remains active today.
Undeterred, engineering-minded geologists shifted their focus to the most conspicuous portion of the OWL, the Cle Elum-Wallowa deformed zone (CLEW). New maps of faults were produced and their potential for damaging earthquakes was assessed. Much progress was made, but faults in the CLEW never garnered the attention from safety officials that the Straight Creek Fault, Seattle Fault, or Cascadia Subduction Zone have. Maximum magnitude estimates consistently came in <6.5 M.
Faults and the earthquake hazard they pose to local residents and infrastructure of Washington. Map by Washington Geological Survey.
The Hite Fault Zone in the Blue Mountains received attention for a time, but a major investigation of the structure remains in the planning stages. Likewise, a bit of work has been done on gorge-crossing faults in the Umatilla Basin, but there's never been much enthusiasm for the Arlington-Shutler Butte area. No brew pubs.
Nevertheless, paleoseismological efforts continued in south-central Washington as attention shifted to the Yakima Fold Thrust Belt. Exploratory drilling for hydrocarbons in the fold belt provided new insights to fold belt age, architecture, basalt stratigraphy, and the nature of sub-basalt sediments. Some wells penetrated thrust faults, resolving questions surrounding the big picture geometry of the thrust belt (its a foreland thrust system). Long-term geodetic GPS monitoring of plate motions shows the YFTB ridges grew over the past 15 Ma by north-south compression related to plate rotation about a pole located near the ID-WA-OR border. New aeromagnetic data and lidar images have clarified long-suspected structural connections beneath the Cascade Mountains, specifically links between active faults in Puget Sound and those in the YFTB.
But there are nine major ridges in the fold belt, each dozens of kilometers long, some in rugged terrain, and each with a fault at its core. A robust assessment of the entire belt would not be feasible. Best to target just a couple ridges instead.
Aeromagnetic map interpreted by Blakely et al. (2011) shows structural connections between the Puget Sound and Yakima Fold Thrust Belt run beneath the Cascades.
The U.S. Geological Survey, building on the past work of field geologists, consultants, contractors, state geologists, and other federal scientists, is currently trenching faults, flying lidar, conducting detailed stratigraphic studies, and dating key marker beds in the Saddle Mountains and Horse Heaven Hills. Constraining Holocene fault recurrence intervals and the geographic extent of earthquake-generated liquefaction is their focus. Excitement builds over "the possibility that Puget Lowland and YFTB structures are kinematically linked" (Blakely et al., 2011). Results have not been published as of 2020.
In the end, we have the humble cartographer, Erwin Raisz, to thank for the nearly century-long quest to find Bigfoot...er....The Big Earthquake in Eastern Washington. Thanks a lot, Erwin. Thanks a lot.