Two Of The World's Biggest Earthquakes Seem To Be Synched Together

Residents of the US’s West Coast have long feared “The Big One” – the apparently inevitable massive earthquake that will one day hit where the Pacific tectonic plate meets its North American neighbor. 

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Well, we’ve got good news and bad news. 

The good: according to a new paper, that expectation may be unrealistic. The bad: the alternative? A Big Two.

“I’m from the Bay Area originally,” said Chris Goldfinger, a paleoseismologist at Oregon State University and lead author of the new study, in a statement this month. “If I were in my hometown of Palo Alto, and Cascadia went off, I think I would drive east. There looks to me like a very high risk the San Andreas would go off next.”

That’s right: the Cascadia subduction zone – the part of the seam north of Cape Mendocino, California, where the tiny Juan de Fuca plate is being subducted underneath the North American plate – is not only capable of devastating the continent with magnitude 9+ earthquakes and tsunamis, but it’ll likely set off a chain reaction and trigger Southern California’s San Andreas fault in the process.

“It's kind of hard to exaggerate what a M9 earthquake would be like in the Pacific Northwest,” Goldfinger said. “And so the possibility that a San Andreas earthquake would follow, it's movie territory.”

A Cascadia cascade

That one earthquake could trigger the other is perhaps not surprising. After all, the Cascadia subduction zone is huge – it stretches roughly from Vancouver to northern California, covering around 1,100 kilometers (684 miles) of latitude along the way – and, partly because of that size, it can produce incredibly large earthquakes.

More important than its size alone, however, is the way in which the two plates meet. The Juan de Fuca plate is now a “microplate” – it’s only about 250,000 km² (96,526 square miles), which is tiny compared to the 75.9 million km² (29.3 million square miles) of the North American plate or the 103.3 million km² (39.9 million square miles) of the Pacific plate, which press on it from either side. But it wasn’t always so small: it’s in fact one of the very last remnants of the Farallon plate, one-third of the base of the vast Panthalassa Ocean that once surrounded Pangea.

The rest of this ancient tectonic plate is gone now – specifically, forced underneath the North American plate. The Juan de Fuca plate is still following – and while it does so, the boundary between the two plates will continue to be home to so-called megathrust earthquakes. 

“A megathrust earthquake is a very large earthquake that occurs in a subduction zone, a region where one of the earth's tectonic plates is thrust under another,” explains Earthquakes Canada. “The two plates are continually moving towards one another, yet become ‘stuck’ where they are in contact. Eventually the build-up of strain exceeds the friction between the two plates and a huge megathrust earthquake occurs.”

The upshot of this process – as you may have guessed from the name – is an earthquake larger than any other kind on Earth. “The last Cascadia earthquake is estimated at magnitude 9,” points out Earthquakes Canada. “A megathrust earthquake in Chile in 1960 was magnitude 9.5, and one in Alaska in 1964 was magnitude 9.2.”

All of which might make it somewhat unexpected that it’s actually not the size of the future Cascadia earthquake that will set off the San Andreas activity. At least, not directly.

So what is to blame?

Finding fault

Goldfinger and his team weren’t originally looking for this discovery. Their initial plan was just to conduct seismic surveys along the Pacific Northwest, drilling and excavating cores from the ocean to investigate the long-term sediment record in the region.

But when a grad student plugged in the wrong latitude for their next destination, sending the whole team off course by some 90 kilometers (56 miles) to the south, things got interesting.

“We wound up off northern California,” Goldfinger said. “When I woke up, I was pretty hot. But, once we were there, I thought, ‘well, let's take a core here.’”

Now, what they were expecting was – well, you can think of it kind of like a tectonic sundae: layers upon layers of sediment, built up through 3,000 years of various geological activity. Scattered throughout, particularly in an area like the Pacific Northwest, are “turbidites” – sediment layers created by events strong and violent enough to change the density of the material being deposited, and therefore often a sign of an ancient landslide or pyroclastic flow. 

To find all or any of that should not have been unusual. But when they investigated the samples, the team were puzzled: the turbidites they found “seem[ed] to be upside down with all the sand at the top,” Goldfinger told Scientific American last week. “And as far as we know, gravity hasn’t changed.”

Well, they say the simplest explanation is usually the truth – and indeed, rather than gravity flipping over every so often throughout history, a closer look revealed two turbidites, not one, stacked on top of each other. Even curiouser: there were quite a few of these “doublet events” throughout the cores, with radiocarbon dating showing some might have occurred as close as minutes apart from each other. What could be the culprit?

“A lightbulb went on,” Goldfinger said in the statement, “and we realized […] if Cascadia went off and triggered a weak turbidity current near the San Andreas, and then the San Andreas went off some time later and triggered a very coarse, sandy deposit to come down. It would create this upside-down doublet stratigraphy.”

Synchronized seismology

It was, the team reasoned, unlikely that these doublet events would occur so many times thanks only to luck. Could the Cascadian events actually be causing the San Andrean events? 

Worryingly, the answer appears to be yes.

It’s all a case of synchronization, Goldfinger explained. Consider tuning an analog radio: it works because “you’re essentially causing one oscillator to vibrate at the same frequency as the other one,” he told Scientific American. 

Similarly, “when these faults synchronize, one fault could tune up the other and cause earthquakes in pairs,” he explained.

In other words: if Cascadia hits, there’s a pretty good chance San Andreas will hit pretty soon afterwards. And as you might imagine, that would… not be a good day for North America.

“We could expect that an earthquake on one of the faults alone would draw down the resources of the whole country to respond to it,” Goldfinger told The Guardian. “If they both went off together, then you’ve got potentially San Francisco, Portland, Seattle and Vancouver all in an emergency situation in a compressed timeframe.”

The only good news – or, let’s be more accurate, less bad news – is this: firstly, that those of us living around the San Andreas fault might have an early warning in the form of the Cascadia event; secondly, that the two quakes might have a reasonable length of time separating them. “Even if these two faults are synchronized, the time interval between earthquakes can still be decades,” Goldfinger told Scientific American.

Still, it’s bleak news for anybody hoping for reassurance about “The Big One”. And while Goldfinger and his team are careful not to doom-monger in their paper, one thing is clear: the US and Canada ought to be prepared for the worst.

“Our preparedness level is poor,” Goldfinger told The Guardian. “We have a long way to go, and all these areas were built on top of ticking time bombs.”

The study is published in the journal Geosphere.