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When it comes to Cascadia, not all parts are equal – Times-Standard
At some point in the next 15 million years, the Cascadia Subduction Zone (CSZ) will no longer pose a prolonged seismic threat to northern California and the Pacific Northwest. This isn’t a long time geologically, but from a human perspective, it means that for the foreseeable future, the CSZ threat is real.
The CSZ represents the convergent boundary between the Juan de Fuca plate system and the North American plate. It extends from Cape Mendocino to Vancouver Island and west to Gorda, Juan de Fuca and Explorer. The edges are offset by transformation errors, giving the western boundary of this plate system a jagged appearance. These transformations also divide Juan de Fuca into three distinct geographic regions: Gorda (in the south), Juan de Fuca (central and largest), and Explorer (the smallest part off the coast of Canada).
I was recently asked if researchers are still working on CSZ and the Juan de Fuca panel system. Yeah. I recently participated in a workshop focused on the Gorda region of Northern California and Southern Oregon. About 30 scientists attended with a wide range of backgrounds including seismology, paleobiology, geology, engineering and geodesy. The purpose was to summarize what we know, where we agree/disagree, and the highest priorities for future research, as there is nothing wrong with controversies in science. Without controversy, we will never question or test ideas and science will advance very slowly. Awareness of the Cascadia subduction zone and the potential for major earthquakes is less than four decades old. The last Cascadia earthquake was more than three centuries ago, and the information we have about this event is incomplete. We have tantalizing bits of data from tsunami deposits, marine and terrestrial geology, oral accounts from indigenous peoples, and modeling studies. But just like the person trying to describe an elephant when confined to a small space, it is difficult to make a complete picture of the beast.
There were many areas of agreement. The Central District or Juan de Fuca District off the coasts of Oregon and Washington is the only part that behaves like a classic plate with seismic focus on the border and an earthquake-free interior. Gorda and Explorer areas are full of internal earthquakes. Since 2000, more than 200 earthquakes of magnitude 4 and greater have occurred within the Explorer panel, including nine in the M6 range. The Gorda Plate is also prone to earthquakes. More than 130 M4 and larger earthquakes have occurred with the inner portion of the plate including 7.2 and 3 in the M6 range since 2000. 5.9s were recorded just three weeks ago far from the California-Oregon border. In contrast, only nine earthquakes were recorded within the central Juan de Fuca plate in the same time period.
The rate of internal seismic activity in Gorda’s and Explorer’s interiors has led some scholars to argue that “plates” should not be called and that “deformation zone” is a more appropriate designation. But here lie some big questions and some disagreements. How do earthquakes within the plates affect what happens in the next Great Cascadia earthquake?
Like most of you reading this column, CSZ Southern is what interests me most. I live directly above the CSZ interface. It’s only eight miles below my house. This is the facade that will be broken in the next Cascadia earthquake. I would like to know how strong the shaking is, how big a tsunami we can expect, and what will happen to our infrastructure.
The answers to the two thirst-related questions lie in stress accumulation and the properties of basic rocks and rock structures. This is what determines the characteristics of the rupture and the amount of slip that occurs under the sea floor. We cannot directly measure these variables, but we can measure surface deformation and estimate the subsurface structure from geophysical studies.
I am in the camp that thinks the Gorda region differs from Juan de Fuca in Oregon and Washington in several important ways. The CSZ fault plane is a gently submerged surface that extends from the continental shelf offshore for 60 to 80 miles. In Humboldt County, perhaps a third of this fault plane is underground. By Central Oregon, it’s quite offshore. Second, the rate of plate convergence along the southern Cascadia margin is lower than that of the northernmost. There is also evidence that this area has more partial ruptures (~M8-8.5) than areas further north. Partial rips and slower convergence in my mind suggest less slippage in the next earthquake.
Everyone in the workshop agreed that the Cascadia tsunami waves reach our coast more quickly than they do in central Oregon and Washington. A tsunami is caused by slipping under the sea floor, and we are closer to the source area. The first tsunami waves can reach the coasts of Humboldt, Del Norte and Cary County, Oregon in less than ten minutes after the earthquake. We also agreed that this region would experience stronger ground shaking because some of the rupture zone is below populated areas.
Determining the likely height of a tsunami in the future is more difficult. Paleotsunami studies that determine the extent of past tsunamis in the CSZ are the only direct measurement of what happened in the past. But such studies require the preservation of sediments that can easily erode in dynamic landscapes. The relative scarcity of tsunami sand in Humboldt County convinces me that tsunami heights here are likely to be lower than those along the Oregon and Washington coasts.
We all agreed that learning more about our marine environment, especially how the sea floor is deformed, can be one of the best ways to get a better treatment of stress buildup. So this is a path I will be pushing out loud for. We also agreed that all jurisdictions along the Cascadia fringe are working hard to update risk information based on the best science currently available and that the best way to survive the shock and tsunami is to stay informed and prepared now.
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