Thermal environment of the Southern Washington region of the Cascadia subduction zone

What is it about?

The recent occurrence of large subduction earthquakes and their associated tsunamis have demonstrated their capability for destruction, including the 2011 Magnitude 9.0 Tohoku Japan earthquake-induced tsunami that killed an estimated 16,000 people. This demonstration of the destructive capability of great subduction earthquakes has heightened the awareness of Pacific Northwest communities to the inevitability of a similar event happening locally within the Cascadia Subduction Zone. The last large megathrust earthquake on the Washington, Oregon and British Columbian coasts occurred over 300 years ago and the statistical odds of another ‘big one’ increase with each passing year. Understanding the mechanisms that underlie this active subduction zone is essential for evaluating the potential shaking hazards and local tsunami inundation maps for the entire west coast of the U.S. Any earthquake movement on a subduction zone fault is temporarily blocked by only a very small portion of this plate interface - where friction temporarily prevents relative plate motion and where tectonic stress from the massive collision is stored. This ‘locked zone’ on a fault is similar to the brakes on a bicycle wheel, where the very small caliper pads arrest the motion of the entire wheel. Identification of the specific off-shore location of the ‘locked zone’ of the contact fault between the North America and Juan de Fuca plates determines if an individual megathrust earthquake will produce largely terrestrial damage to infrastructure from shaking but only a small tsunami. In contrast, if this critical ‘locked zone’ where plate tectonic energy storage is located lies instead beneath the submerged portion of the outer continental margin, only a small amount of terrestrial damage from earthquake shaking will result, but can produce very large tsunamis that can inundate the coast communities. Because of this dramatic difference in possible types of impact resulting from a megathrust earthquake, it is critical to locate the exact position off-shore of the portion of the subduction zone fault that is locked where these massive amounts of energy will be released when an earthquake occurs.

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Why is it important?

Mapping of the specific off-shore distance where the locked zone is located can accomplished by modeling the thermal environment of the entire subduction zone, including the fault-damaged rocks on both of the colliding tectonic plates. This is possible since both the upper and lower boundaries of the locked portion of the plate interface are determined by the temperature of the rocks that surround the subduction fault zone. The current research program used heat flux calculations obtained from data that were obtained from a 2012 multi-channel seismic research cruise aboard the R/V Langseth. This research cruise used a large array of airguns and hydrophone streamers to map the sub-seafloor structure of the Cascadia Subduction Zone along a 60 mile North-South distance lying offshore Southern Washington State. The recently published results from this new thermal model clearly predicts the location of fault breakage and movement during a large subduction earthquake is likely to occur fully offshore the Washington coast. This new location places the earthquake motion further from large population centers such Seattle and Portland, but shows there is a higher chance of the fault movement reaching all the way to the seafloor at the western edge of the margin. This type of fault movement dramatically increases the risk for the production of a very large tsunami on the Washington coast, similar to what occurred during the 2011 Tohoku Japan earthquake and the new publication will have an impact on the evaluation of geohazards in the Pacific Northwest.