When it comes to American earthquakes, everyone’s understandably worried about the “big one” – a catastrophically energetic slip on the San Andreas Fault, several sections of which haven’t moved for centuries. However, as a new Nature Geoscience study reminds us, many in the general public may be focused on the wrong place.

A team led by the University of Texas Institute for Geophysics (UTIG) have been poking around the Cascadia Subduction Zone (CSZ), which runs from northern California all the way up to Vancouver. It’s 1,000 kilometers (621 miles) long, and features the Juan de Fuca tectonic plate slipping under the North American plate at a strange angle.

A careful peek at the CSZ’s geological record has suggested that it’s not only primed to rupture again sooner rather than later, but it’s also more likely to generate another “big one” than many previously suspected. There's no sign than anything is imminent, but this nevertheless highlights that it’s not just California that’s at risk of a devastating earthquake – it’s the entire western seaboard.

The CSZ, as aforementioned, is a subduction zone, and the world’s most powerful earthquakes are always generated along them. Any major movement after a period of friction along the CSZ would primarily affect Oregon and Washington State, home to a combined 11.4 million people.

The danger of the next “big one” occurring here has always been on the minds of the United States Geological Survey (USGS), who are acutely aware that a 9.0M megathrust quake could devastate cities like Seattle and Vancouver.

The team note that a thick sediment band overlies the oceanic Juan de Fuca plate. As with many subducting plates, this material accretes as a wedge above the convergence point between the two plates, which, among other things, weighs it down.

This influences how stress is released during an earthquake. Unfortunately, the team’s discovery reveals that the wedge above part of the CSZ is inadvertently doing all it can to prevent stress from being unleashed.

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.