Cascadia Subduction Zone Earthquake
“Every place and in every day of living there are hazards, and people are very bad at sorting out what are the most dangerous.”
That’s what Bill Steele, seismologist and spokesman for the Pacific Northwest Seismic Network, said addressing the response of the public after the New Yorker article, “The Really Big One,” came out in July 2015.
The article painted ugly scenes of “unrecognizable” regions along the West Coast, explaining that nearly 13,000 people would die and 1 million would be displaced after the historic earthquake. One of the more dramatic quotes saying that “everything west of Interstate 5 would be toast.”
According to Steele, who lives just eight blocks east of Interstate 5, he’s not any safer than those to the west, but certainly no one will be toast.
But the “really big one,” of which the article is referring to, is a very real thing, and the pressure of one tectonic plate sliding underneath another is happening right at the border of North America. This particular fault is called the Cascadia Subduction Zone, and right now it’s at a standstill waiting to slip.
“We live on a plate boundary between the ocean plate called the Juan de Fuca plate, which is converging with North America,” Steele said. “We are coming together and because the two plates are locked together what’s happened is the crust is forming a ripple.”
The coastline of Washington and Oregon is rising steadily. We are being pushed east a little bit, and the toe of North America under the ocean is being pushed down. It’s been doing that for 315 years since the last earthquake in 1700.
“We know that there’s enough strain accumulated to produce a great earthquake,” Steele said. “But the average recurrence is 500 years. So it could be a century or two before it happens.”
If such an earthquake happened in the Cascadia Subduction Zone, which is 700 miles from Vancouver Island, Canada and goes all the way to Cape Mendocino, California, the region could experience anywhere between an 8.7 and 9.3 earthquake.
The actual probability of a great earthquake occurring in Cascadia in the next 50 years is about 15 percent, according to Steele.
“A 15 percent chance in 50 years is a high enough probability for a great earthquake that we need to take it seriously,” Steele said
This fault is off shore about 100 miles from the Puget Sound region and a 700-mile long fault ripping is going to take several more minutes compared to that of the average earthquake, which is 20 to 40 seconds long.
“The first feeling that you’ll have are short pulse like motions, and that won’t last very long. But then the ground will start shaking,” said Sandi Doughton, Seattle Times reporter and author of the book “Full-Rip 9.0: The Next Big Earthquake in the Pacific Northwest.” “It’ll be low frequency (and) slow shaking. It won’t be hammer, hammer, hammer. It will be wave-like for four to five minutes.
After the quake, a cascade effect of events will take place – some are more certain than others.
According to Doughton the thing that will do the biggest damage outside of the tsunami area (coastal towns) is that the shaking will be destructive on our buildings and infrastructure. Doughton said that almost half of our high-rises were built before 1990, and it wasn’t until the early 1990s that building codes considered CSZ quakes in building codes.
According to Steele, a University of Washington research group is looking further into the effect a massive quake will have on high-rise buildings.
“In most cases you’ll see strong shaking in pockets where there are poor buildings, where you have a lot of damage, and then you’ll have neighborhoods (where) you’ll be able to walk and see very little signs of damage,” Steele said.
The longer the ground shakes, the more water comes out of the particles in the soil, causing liquefaction, which causes structural problems for buildings. Buildings on hills and river valleys where soils are loose and not hard-packed can create structural problems for buildings on these areas as well.
“A lot of our infrastructures run through valleys. This will create electrical line and pipeline damage and flooding,” Doughton said.
For bridges built to more modern standards, Steele said they will probably be just fine. But delays and inspections after the quake will cause closures.
You will want to move away from the water or waterfront if you can because, even though the tsunami generated will take hours to get down to the Puget Sound or more, the amplitude is going to be limited to a meter or two and depending on where the high tide is – that could mean some localized flooding.
“We expect a small tsunami in the Puget Sound,” Steele said. “The currents are going to be very tricky and very strong. The marinas are really going to be bad place to be. Landslides can generate tsunamis that can cause problems and flood waterfront areas.”
Steele explains that most of time when you have a great earthquake, you won’t see a response from volcanoes, but occasionally it does happen
“If a volcano is getting ready to erupt already, I think being pulsed by these long period waves that are compressing and dilated in the ground can crack things up a bit more,” Steele said. “This could cause debris flows and mountain landslides. Most likely that would be contained in the park.”
“Life as we know will cease to exist for months.” Eric Holdeman, director for the Center of Regional Disaster Resilience said.
Pacific Northwesters should expect to be on their own for up to two weeks, and the best place is to be in your home.
“Recognize that no one is coming to help you,” Holdeman said. “With this size of disaster all our resources are engaged – they are going to the nursing homes, schools and higher occupancy types of facilities.”
Create a plan with your family and consider that cell phone towers may not be working. Use your neighbors and know their strengths and skills.
“Know the capabilities of schools and nursing homes,” Holdeman said. “Ask to see a copy of the plan. Ask when the last time they simulated an earthquake was. How long are they prepared to take care of your children?”
Besides food and water, Holdeman said that enough medication for two weeks and a battery – powered radio are necessities.
“It could happen tomorrow or 100 years from now, that’s the geologic time scale. That doesn’t mesh with the human time scale,” Doughton said. “We have all these other things competing for our time and attention and money. It’s an easy thing to push to the back of the list.”
“We will pull through and have a really nice memorable experience out of it, if we do it right,” Steele said.
For Doughton, preparedness also includes wine because two weeks or longer without wine is a very long time.
