Along the Cascadia Subduction Zone, preparedness and understanding are key to being prepared for the potential threats of earthquakes and tsunamis. New research from Cascadia CoPes Hub member Audrey Dunham and colleagues have uncovered that the standard models might be underestimating one hazard while overestimating another. 

Historically, many studies calculate how much the land moves during a massive megathrust earthquake using an “elastic homogeneous half-space model” (EHSS). However, the Cascadia Subduction Zone is far from homogeneous. It features low-rigidity materials, such as soft accretionary wedge sediments right near the coast, and much higher-rigidity materials, like the deep subducting slab and continental mantle, at depth.

By running 3D ground motion simulations, researchers found that these geological complexities change our estimates of the earthquakes impact.

The new models, which include 3D structures, predict significantly less coastal sinking compared to simpler, homogeneous models. This decrease is due to the presence of higher rigidity materials, like the deeper continental mantle, which resist downward movement.

This finding has major implications for how scientists interpret the geological record. If the land-level change was less than previously modeled, it suggests that future earthquake scenarios might need to incorporate even deeper slip on the fault to match the sinking observed during the 1700 event.

While the coast sinks less in the new model, the seafloor offshore is predicted to rise more. When a heterogeneous 3D structure is included, offshore uplift increases. This greater seafloor uplift displaces more water, leading directly to higher tsunami waves. The simulations showed that maximum tsunami wave heights offshore increased on average by about 25%, and sometimes up to 75%, compared to the homogeneous models.

The takeaway from this research is that accurate hazard assessments in complex subduction zones like Cascadia require moving beyond simplified models. The study highlights the importance of using realistic 3D structural models when estimating coseismic vertical displacements and comparing them to global paleoseismic estimates. 

When asked how this study has improved the understanding of the Cascadia Subduction Zone, Dunham had the following to say. 

“One of the best ways that we can quantify future hazard in a place like the Cascadia Subduction Zone is to characterize how past earthquakes ruptured, which takes a combination of geologic evidence of those past earthquakes and models that fit that evidence. This study really highlights that the modeling approach matters and that using realistic 3D Earth structure can impact the choice of scenario earthquakes and tsunami models, ultimately shaping our estimates of future shaking and tsunami hazard.” 

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