The "Big One" Might Not Happen Where You Expect It To

Seattle could be in trouble. dibrova/Shutterstock

Robin Andrews 21 Nov 2017, 20:02

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 sediment compaction is so high in the highlighted section that the plates there are more likely to get stuck and accumulate stress. UTIG

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.

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