There was a recent New Yorker Article (The Really Big One, by Kathryn Shulz) that discussed the potential Cascadia Subduction Zone earthquake and how it will affect the Seattle area. As a structural engineer living and working in Seattle, I am familiar with the Cascadia Subduction Zone. I’ve written reports referencing it and talked to clients about its possible effects.
The first thing to realize is that subduction zones are interesting places. You have one tectonic plate being shoved under another plate. As one of these plates moves in fits and starts, you have the potential for big earthquakes if a large movement occurs and smaller earthquakes if not so much movement occurs. These are the subduction earthquakes.
Then, as the plate going under heats up and breaks apart in the earth’s mantle, you can have more earthquakes. This was the type of earthquake the Puget Sound region experienced last, the Nisqually Earthquake in 2001. But because of their nature, these are very deep earthquakes that have to go through a lot of earth and dissipate a lot of energy before they can do damage at the surface.
Finally, you have earthquakes caused by surface faults, which are caused as the plate is wrinkled up and moves to accommodate this. The movement can be side to side or up and down, or some combination. These are the most common type of earthquake in California. There are many surface faults running through the Pacific Northwest as well, they just don’t move as frequently as in California.
So, what makes the Cascadia Subduction Zone so special? Mostly, its size. The subduction zone stretches from Vancouver Island in Canada to Northern California. Now, that doesn’t mean the whole thing will move at the same time. Maybe only the northern section only will move or just the middle. But worst case scenarios assume the whole zone breaks at the same time. That is a lot of energy to release. That’s where you get a Richter Magnitude 9+ earthquake.
So let’s compare that to an event occurring on the Seattle Fault. It’s a surface fault running east-west in the approximate location of I-90, through Mercer Island and Seattle. It’s about 43 miles long and capable of generating about a magnitude 6.7 earthquake.
Which one will be worse for Seattle? The Cascadia event is about 100 miles away. The Seattle Fault is about 0 miles away. Strong shaking during the Cascadia event will last 1 – 2 minutes. Strong shaking during the Seattle Fault will last around 30 seconds.
Both events are large and both will cause extensive damage to existing infrastructure. Either event would severely affect Seattle, though the Cascadia event would affect more people in the entire region. Which earthquake may cause more damage is a question for scientists and academics. It’s valuable knowledge, to be sure, but the important thing for Northwest residents to get from this information is to be prepared. I’m not just talking about having supplies in your house and walking shoes in your car. I’m talking about making sure the buildings you live and work in are as safe as possible.
In your house: Are your heavy bookcases and dressers anchored to the wall or floor? Are delicate valuables (TVs, computers, etc.) restrained against falling? Is your brick chimney in good shape? Is your house anchored to the foundation?
In your business: Are servers or other important equipment adequately restrained? Are storage racks and inventory properly restrained? Do you have a recovery plan for you and your employees?
Property Owners: Is your building seismically vulnerable? Have you had it retrofitted? Are critical service items like water heaters, furnaces, ceilings, and ductwork restrained or anchored? Do you have a seismic shutoff valve for your gas intake?
The Puget Sound area will have an earthquake again, and though it’s important to be prepared to survive without city services like water and power for a while, it’s also important to try to prevent as much damage to building stock and businesses as we can before it happens
Could your business survive being out of your building or without your equipment or servers for months? If not, think about what can be done ahead of time to prevent it.