Nearly forgotten research from decades ago complicates the task of quantifying earthquake hazards in the Pacific Northwest, according to a new report from scientists at the U.S. Geological Survey, the University of Washington and other universities.
The report focuses on the Cascadia subduction zone — a giant active fault that slants eastward beneath the Pacific coast of southern British Columbia, Washington, Oregon, and northern California.
Geologic studies in the past three decades have provided increasingly specific estimates of Cascadia earthquake sizes and repeat times. The estimates affect public safety through seismic provisions in building design and tsunami limits on evacuation maps.
The new report does not question whether the Cascadia subduction zone repeatedly produces enormous earthquakes. What the report asks instead is how much geologists can say, with confidence, about the history of those earthquake going back thousands of years. How big was each of the earthquakes? Did they occur twice as often along one part of the subduction zone as another?
The report concludes that extracting such details from deep-sea sediments is more complicated than was previously thought.
The report reappraises sediment cores that were collected near the foot of the continental slope offshore Washington. A subset of cores from this area underpins influential estimates of Cascadia earthquake size and recurrence that were published in 2012.
The new report points to confounding evidence from a much larger suite of cores that were collected and first analyzed by University of Washington and Oregon State University scientists in the late 1960s and early 1970s.
Those Nixon-era cores were the work of researchers unconcerned with earthquakes. Plate tectonics was then such a new idea that scientists were just beginning to recognize the Cascadia subduction zone as a tectonic plate boundary.
The sediment cores were collected to learn about turbidites — beds of sand and mud laid down by bottom-hugging, sediment-driven currents that infrequently emerged from submarine canyons onto the deep ocean floor. Not until a 1990 report would turbidites be reinterpreted as clues to Cascadia earthquake history.
"Rethinking turbidite paleoseismology along the Cascadia subduction zone" is freely available online in Geology, a leading Earth-science journal. The authors are Brian Atwater (U.S. Geological Survey), Bobb Carson (Lehigh University), Gary Griggs (University of California Santa Cruz), and Paul Johnson and Marie Salmi (University of Washington).