The researchers conducting a seismic survey off the coasts of these two states to find out what this sedimentary wedge was actually like. As these waves travel differently through various materials, geophysicists can use them to build up a picture of massive sections of the planet.
Combining their surveys with sediment core samples, the team discovered that the wedge was incredibly compact, far more than expected. This means that water can’t get through it very easily to the subducting plate juncture, where it usually promotes the smooth slip of one plate beneath another.
Drier subduction zones are more likely to experience frictional build-ups, which will eventually culminate in huge, devastating releases of energy. So not only does the northern CSZ wedge increase the likelihood of a more powerful quake occurring in the region, but its positioning also makes an accompanying tsunami more probable too.
Although the shores of Central Oregon appear to feature less compact wedges, northern Oregon and Washington State in general appear to feature thrust fault sections that are nothing less than strongly locked. That places them at an extremely high risk of damage should the subducting plate slip.
“The Cascadia subduction zone is unusually quiet,” the authors ominously note at the start of their study. They add that the last megathrust quake 300 years ago caused a tsunami so powerful that it managed to make its way across to Japan.
The CSZ produces a major earthquake once every 200 to 530 years. The last subduction zone quake happened all the way back in 1700, and although this makes it sound like the region is potentially nearing its due date for another big show of force, it's worth pointing out that quakes don't follow any sort of clealry defined schedule.
Something powerful will take place in this region at some point, though, which is why careful research like this, and preparation, is paramount.