The Cascadia Subduction Zone is one of the secret threats lurking beneath Earth’s surface, where the small Juan de Fuca plate is being slowly pulled beneath the Pacific Northwest. The last time the fault dramatically spasmed, in 1700, it caused a magnitude 9.0 earthquake and spawned massive tsunamis. We know the fault will quake again, but we don’t know when. Understandably, that means geologists are pretty interested in what’s happening in the area.
A key part of the challenge is that we don’t really know what’s going on under the surface in this region, but a new paper published in the journal Science Advances sheds some light. In it, Pascal Audet, a geophysicist at the University of Ottawa in Canada, and a colleague made use of temporary sensors on the seafloor off the coast of Washington state, as well as land-based seismic devices.
Taken together, those sensors allowed the pair of scientists to study what’s called the low-viscosity layer, which is made up of rock that’s full of high-pressure water. The low-viscosity layer is important because the waves of an earthquake can travel through it only very slowly. The new paper’s findings suggest the low-viscosity layer around Cascadia begins closer to the shoreline then some scientists had thought. That means it could buffer nearby cities like Seattle and Portland, Oregon, from the worst earthquake risk.
The new data also suggest that it’s the low-velocity layer that explains a weird local quirk that has been stumping scientists, an apparent gap between the edges of the fault zone. “This sort of makes it look less likely that an earthquake could occur in that gap,” Chris Goldfinger, who studies the Cascadia subduction zone at Oregon State University but wasn't involved in this study, told Newsweek.
That said, the low-viscosity layer isn’t without its unique features. “The fluid pressure is supporting the weight of North America, essentially,” Goldfinger said, comparing the situation to a bus hydroplaning on a layer of water on a road. “Then all sorts of mayhem might happen.”
In the case of Cascadia, the new paper suggests that mayhem might manifest as a strange phenomenon called episodic tremors, which are small shakes deep below the surface of the Earth that are much too small to feel from the surface. They aren't dangerous, but they're the sort of thing that bugs scientists who want to understand what's going on.
"This helps explain a phenomenon that we've been scratching our head over," Goldfinger said. "It doesn't nail it to the wall perfectly, but I think it limits the possibilities."