The Oregon Surveyor Sept/Oct 2018

24 The Oregon Surveyor | Vol. 41, No. 5 The Big One Parts of the Pacific Northwest’s Cascadia Fault are More Seismically Active than Others T he Pacific Northwest is known for many things—its beer, its music, its mythical large-footed creatures. Most people don’t associate it with earthquakes, but they should. It’s home to the Cascadia megathrust fault that runs 600 miles from Northern California up to Vancouver Island in Canada, span- ning several major metropolitan areas including Seattle and Portland, Oregon. This geologic fault has been relatively quiet in recent memory. There haven’t been many widely felt quakes along the Cascadia megathrust, certainly nothing that would rival a catastrophic event like the 1989 Loma Prieta earthquake along the active San Andreas in California. That doesn’t mean it will stay quiet, though. Scientists know it has the potential for large earthquakes—as big asmagnitude 9. Geophysicists have known for over a de- cade that not all portions of the Cascadia megathrust fault behave the same. The northern and southern sections aremuch more seismically active than the central section—with frequent small earthquakes and ground deformations that residents don’t often notice. But why do these vari- ations exist and what gives rise to them? Our research tries to answer these ques- tions by constructing images of what’s happening deep within the Earth, more than 100 kilometers below the fault. We’ve identified regions that are rising up beneath these active sections which we think are leading to the observable differences along the Cascadia fault. Cascadia and the ‘Really Big One’ The Cascadia subduction zone is a region where two tectonic plates are colliding. The Juan de Fuca, a small oceanic plate, is being driven under the North American plate, atop which the continental U.S. sits. Subduction systems—where one tecton- ic plate slides over another—are capable of producing the world’s largest known earthquakes. A prime example is the 2011 Tohoku earthquake that rocked Japan. Cascadia is seismically very quiet com- pared to other subduction zones—but it’s not completely inactive. Research in- dicates the fault ruptured in a magnitude 9.0 event in 1700. That’s roughly 30 times more powerful than the largest predict- ed San Andreas earthquake. Researchers suggest that we are within the roughly 300- to 500-year window during which another large Cascadia event may occur. Many smaller undamaging and unfelt events take place in northern and south- ern Cascadia every year. However, in central Cascadia, underlying most of Or- egon, there is very little seismicity. Why would the same fault behave differently in different regions? Over the last decade, scientists have made several additional observations that highlight variations along the fault. One has to do with plate locking, which tells us where stress is accumulating along the fault. If the tectonic plates are locked—that is, really stuck together and unable to move past each other—stress New Imaging Data Suggests Why The Juan de Fuca plate meets the North American plate beneath the Cascadia fault. A GPS geosensor in Washington. Reprinted with permission from www.theconversation.com Miles Bodmer, PhD Student in Earth Sciences, University of Oregon Doug Toomey, Professor of Earth Sciences, University of Oregon