Cascadia earthquake turns 324. What an ‘ancient’ quake says about the next one
PORTLAND, Ore. (KOIN) – January 26, 2024, marks the 324th anniversary of the last Cascadia Subduction Zone earthquake — a massive magnitude 9 quake spanning Northern California to British Columbia.
With Cascadia earthquakes expected every 300-500 years, researchers are using geographic clues from the ancient quake to prepare for the next tremor.
“We know a surprising amount for an earthquake that’s 324 years old. It’s not very often that we know so much about ancient quakes. In this case we do. We know it was about a magnitude nine, which is really big. We know that it probably ruptured from Northern California, from the Mendocino region, through Oregon, Washington and into British Columbia. So, we’re talking about 600-700 miles of fault that ruptured in a single event,” said Diego Melgar, director of University of Oregon’s Cascadia Region Earthquake Science Center.
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The devastating earthquake left evidence that a tsunami — triggered by the quake — traveled to Japan and created a “ghost forest” on the Oregon Coast.
“That’s actually one of the more beautiful geologic stories that we have in science. Folks that are from the coasts know that there’s these things called ‘ghost forests,’ tree trunks that are left standing that are no longer alive, sometimes they’re petrified. And they are a direct product of the earthquake in 1700. During the big events, the coast drops, sometimes by as much as 4, 5, 6, 7, 8 feet and then the trees end up being submerged into the salty water at these coastal marshes,” Melgar explained.
“Trees don’t like salty water, they die. And so, all of a sudden, we see these massive die-offs of forest all across our region and people can date them with tree rings, and carbon 14, and things like that. Then that’s how we know that they more or less all died at the same time. And so, people started piecing together the story that there’s really only one thing that can explain swaths of forest dying all at the same time, from California to B.C., and that’s a really big earthquake,” Melgar said. “It really is difficult for the human mind to understand, you know, 700 miles of a fault line just unzipping in a single event.”
Melgar says modern earthquakes like the 9.1 magnitude earthquake in Indonesia in 2004 and the magnitude 9 earthquake in Japan in 2011 tell scientists what to expect when “the big one” strikes in the Cascadia Subduction Zone.
“We’re looking at big tsunamis, lots of strong shaking, and the shaking goes on for a really long time. That’s something that’s really different about these events. Yes, it’s strong shaking, it can last minutes. So, imagine not being able to stand for three, four, five minutes and imagine the forces that our buildings, and bridges, and dams have to put up with in order to weather that event,” Melgar said.
Small 0.5 and 1 magnitude earthquakes rattle the Pacific Northwest “all the time,” Melgar says.
“They’re there to remind us that we live in an earthquake country, so to speak,” Melgar explained. “Thankfully, big earthquakes, like the 1700 events, are very few and far between. We think they happen every 300-500 years.”
While seismologists cannot predict when the next Cascadia earthquake will happen, Melgar says now is the time to prepare for the quakes.
“If they happen every 300-500 years, and we’re 324 years into what we call the earthquake cycle since the last one, well, that’s why we tend to think that we really need to treat preparing for events with some urgency because while we can’t say that it’s going to happen tomorrow, we can say that we live in an active region and that we’re late into that earthquake cycle. So, it’s time to get ready,” Melgar said.
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“Right now, there is no way that is scientifically proven to say, ‘oh, it’s imminent, it’s about to happen.’ Seismology is a very young science; we still have a lot of way to go and ground to cover. So right now, we can’t say. There have been many precursors and signals that have been proposed by people over the years, and none of them have panned out. So, what we do now is we try to use science to understand if it happens, and when it happens, how strong is the shaking going to be? How big is the tsunami going to be? How much is the coastline going to drop or subside in a big event? Because then we can give engineers useful information, ‘Hey, when you build a bridge, you need to build a bridge that is able to withstand this kind of shaking,” Melgar explained.
It’s possible that the next big earthquake will be a smaller magnitude than the 1700 tremor, but Melgar warns that a seven or eight-magnitude earthquake could still be damaging.
“While we can’t tell you what day, what time the earthquake is going to happen, we can still give a lot of useful information so that when it does happen, our society is ready, our buildings remain standing, our bridges remain upright, our power plants remain operating. That’s really what we want,” Melgar said.
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As pressure builds between the Juan de Fuca plate and the North American Plate, Oregon’s Office of Emergency Management says there’s about a 37% chance of a megathrust earthquake in the Cascadia Subduction Zone in the next 50 years.
The office warns “With the current preparedness levels of Oregon, we can anticipate being without services and assistance for at least two weeks, if not longer, when the Cascadia Subduction Zone earthquake occurs.
Melgar reminds Oregonians that even though major earthquakes can be scary, we can prepare.
He explained, “I like to tell people that it can be scary to think about what this big event is going to look like, and I want people to focus less on that because it is scary. I don’t dispute that. But there’s so much that we can do to prepare. And so, if we get our buildings up to code, if we get our bridges stiffened up if we get our schools moved out of tsunami inundation zones, even if it does happen, while that moment when it happens is going to be scary, if we’ve prepared adequately, we should do okay. I’ll remind people that in Japan, in 2011, they had a magnitude nine earthquake, and there were very, very few building collapses. We know how to build buildings that can withstand earthquakes. It’s just up to us to get it done.”
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