A M4.7 earthquake hit off the Oregon coast on February 8, 2020. It should remind us that an indescribable disaster in the offing is the Cascadia Subduction Zone earthquake, a scenario that involves a M9.0+ earthquake in the Pacific Northwest that would be felt all the way down to Northern California and would shake the earth for three to six minutes.
The Pacific Coast would have perhaps an hour of warning before the tsunami hit. And the devastation would be great.
If it happened tomorrow, there would be perhaps more than 10,000 deaths and 30,000 injuries. Whole cultures of people could be wiped out and the recovery would take decades. The good news? There’s probably ample time to prepare, and that preparation has begun. A similar quake is thought to have occurred in or before January 1700.
Scientists estimate that a recurrence would be due in about 500 to 600 years. But they don’t know for sure. They also don’t know how much seismic slip occurred during that quake in 1700.
Did the subduction zone save anything?
“Faults tend to be pretty good at this. They don’t always spend everything they have,” said Brian Atwater of the U.S. Geological Survey Earthquake Hazards Program. There may have been a lot of “breakage” during the last quake or there could be parts of the fault that didn’t break. That would be a way to get “better than 500 years’ worth of slip in 300 years,” Atwater said. Or maybe the earlier quake spent the “whole bank account” and the future event won’t amount to much.
“That’s part of the challenge of trying to present to a public audience scenarios for future earthquakes and their unknowns,” said Atwater. “They don’t happen like clockwork.”
Cascadia Rising: Alarming detail about the impending Cascadia earthquake and tsunami
The document Cascadia Rising takes you through the scenario of The Quake. The daytime quake will sneak up on the region’s population, feeling somewhat like a semi-truck passing by.
As the shaking continues, some will forget thei initial training. Some may run, but will make it only a few steps before falling. After about a minute, the shaking will begin to toss people about. Things not anchored will fall. Some people will have gotten under chairs, tables or something they think will protect them. Many will not.
The document says that coastal areas will likely feel it the worst. But down in Oregon and even Northern California, residents will know something is afoot.
Cascadia Earthquake Damage
Besides the dead and injured, the damage to infrastructure would be enormous.
One of the big threats during such a quake is liquefaction. Many critical structures stand on silt and sand that become unstable. The grainy soil will begin to act like liquid, and structures — such as bridges, ports, airports and industrial facilities — may shift position or sink.
Local residents will have 20 to 30 minutes to get to high ground and away from the effects of the inevitable tsunami, which will consist of multiple waves over several hours. Some areas will remain flooded even as the tsunami retreats.
There are complete cultures in Washington that could be completely wiped out. “It’s a catastrophic event that will affect a wide swath of our state and Oregon, British Columbia and Northern California,” said Paci-Green. “It is absolutely catastrophic for outer-coast communities, and beyond catastrophic for some of the tribes where a large percentage of their reservation is in the inundation zone.”
The area will be prone to landslides and falling rock. The effect on transportation would be immense as the shaking and tsunami could damage 16,000 miles of highway and 7,000 highway bridges. The resulting economic losses could exceed $80 billion.
But those are estimates. Paci-Green said things could be worse. “The concern among the engineering community is that we don’t have a lot of data on how infrastructure, especially buildings and bridges, responds over that very long period of shaking,” she said. “We don’t know what we don’t know. The other fear is that these events are so far apart, that it’s much harder to get people’s attention and we’re only recently understanding the significance and the potential for a major Cascadia event.”
The immediate impacts will be devastating, but the bigger issue is long-term recovery. That means a consistent and sustained effort to build a more resilient community. “In terms of how to plan for this, it’s long term,” Paci-Green said. “Our best strategies for earthquakes and tsunamis are not response and preparedness, but mitigation, mitigation, mitigation. The six minutes of shaking are going to be really, really terrible. But it’s the decades of recovery that we actually need to be worried about.”
As Paci-Green said, it’s difficult to develop a plan for something that may not happen for 100 years, but recent attention has sparked interest. “The risk is so overwhelming and so large, but also so infrequent that just the slow movement of mitigation is more likely going to put us in good stead,” she said.
Washington is slowly seismically upgrading its bridges. Oregon is working on seismically upgrading its schools, and outer-coast communities are creating bonds and funding structures to do vertical evacuations or move critical assets out of inundation zones.
Vertical evacuation is used when there’s no way to walk or drive out of the inundation zone. It consists of creating berms or multistory structures, strategically placed in communities where people can climb up and out of danger.
Native Americans Preppers
The Ocosta School District in Washington state is building a gym for vertical evacuation that will hold not only students and staff but also the local community.
The Quileute Tribe, which is completely in the inundation zone, has spent the last 10 years fighting with the federal government about treaty boundaries and reservation boundaries, and was recently granted a land swap to be able to exit the inundation zone, according to Paci-Green.
The Native American tribe is swapping with the federal government some of its reservation, which happened to be an access point to one of the state’s most scenic beaches, for about 750 acres of National Forest where its members can get to high ground. As a whole, tribes struggle to compete with other jurisdictions in the state when it comes to hazard mitigation grants funding and developing mitigation plans. “They’re very small jurisdictions and these kinds of tasks are overwhelming,” said Paci-Green. “At the same time, this is beyond catastrophic for the tribes because they may literally lose everything in their community.”
Paci-Green said the region needs to look at “innovative resilience solutions,” a lot of redundancy and also smaller-scale solutions such as developing local food production. “We may struggle to get food in across the mountains and up Interstate 5, and so more localized meeting of basic needs like food and water [is important].”
Lots of Unknown
The trouble is that scenario is not similar to the San Andreas Fault in California, where there’s a long history and considerable knowledge. “This is literally a new concept,” she said, “and our codes and planning and all that have not considered it for the vast majority of development in our communities, so we have a whole lot of small towns in the inundation zones.”
So When Will It Happen?
Paci-Green said things are moving slowly and that’s OK. It’s best not to engineer a response based on fear. Plus, there’s time — but how much?
“The fault doesn’t have a good memory in terms of when the last event happened,” Atwater said. “So when the next one happens is independent of how much time has elapsed since the previous one.”
Most of the knowledge about the Cascadia Zone has been garnered relatively recently. “When I first showed up here in 1985, it was really a hot question among scientists as to whether this fault produces any earthquakes at all,” said Atwater. “And it’s only been during the past 30 years that a series of discoveries have been made that convinced earth scientists that it does pose a threat.”
The fact that the last one happened some 300 years ago makes it difficult for people to understand the potential. “The earthquakes that have happened in the last decades haven’t approached the kind of thing that the big faults are capable of doing,” said Atwater. “People don’t get the full sense of what nature can do. But that’s part of the detective story.”
This new moderate quake near the dangerous Cascadia should remind you of the earthquake risks you face if you live along the Pacific Northwest Coast. Because when the subduction zone explodes, it will be terrifying. So be ready and start to prepare for the next Big One… Just because it could hit tomorrow!