CSZ History

Oregon Resilience Plan page 4

Scientists know it isn’t a matter of if but when the Pacific Northwest will experience a major
Cascadia Subduction Zone earthquake. But when?

What’s commonly known
1) There is a ‘37% chance the Pacific Northwest will have a major Cascadia Subduction Zone (CSZ) earthquake in the next 50 years’.
The 37% figure comes from research headed by OSU Professor Chris Goldfinger, who helped me understand what data was used for the calculations and why the Log-Normal probability model was used.

2) That 37% chance is derived from knowledge that there have been 46 major CSZ earthquakes in the last 10,270 years, which averages out to one earthquake every 223 years.

3) 321 years have passed since the Pacific Northwest saw its last major earthquake.

What’s less well known
Take a look at this ten thousand year historical chart1. Each vertical bar represents the number of years between major Pacific Northwest earthquakes along the Cascadia Subduction Zone. The horizontal blue line runs at 321 years (the current amount of time we have gone without one, for an easy visual reference).

Intervals longer than 321 years are shown in red. There aren’t many. In fact, only eight out of forty-six lines are red… meaning 83% of the time, the fault has not had to wait 321 years for the strain to break it.

Let’s play a game
If you knew that 83% of the time, your toddler was asleep by 8:00 pm, would you expect her to be up until 9:00 pm tonight? Probably not.

Knowing that 83% of the time, our fault has broken prior to reaching its 321st year… should we expect the fault to hold for another 50 years? 20 years?
Now, One Step Farther
Placing a yellow border around the longest intervals illustrates how they are grouped toward the oldest chunk of the 10,270-year history (intervals 34-46).

Intervals one through thirty-three make up the most-recent 6,030 years. During that time only 2 lines have stretched beyond the 321-year horizontal blue line. They don’t reach far above. To be clear, this means 94% of the time the Cascadia Subduction Zone did not have to wait 321 years between ruptures.

Let’s play that game again
If you knew 94% of the time your toddler was asleep by 8:00 pm, would you expect her to be up until 9:00 pm tonight? Nope. Just nope.

A New Perspective
So why do the oldest 4,000 years and the most recent 6,000 years look so different on the chart? This EOS’s image is one possible explanation.

Graphic A, the steady earthquake cycle, is similar to the CSZ’s behavior in the oldest 4,240 years (with regards to the 10,270-year timeframe). This pattern flows from a magnitude 9.0 to a single, smaller 8.0 range, then back to a 9.0.

Graphic B, earthquake clustering, resembles what the CSZ has experienced over the last 6,030 years (more magnitude 8 earthquakes coming between the 9s). Below is chronological chart in which each bar represents an earthquake.

Again, it is clear that the most recent history on the right side looks ‘busier’ than the left, with more 8.0s (short lines) between the 9.0s (long lines). Occasionally, there aren’t any 8s between 9s. Look at the two most recent quakes, for example. Still, those two earthquakes came fairly close together with only 232 years between. (Curious how big the next one be? Click here for some related research/thoughts.)

The short bars above represent magnitude 8.0 – 8.9 earthquakes (smaller events), while the tall bars represent 9.0s & above.

Here is what a couple of professionals in the field have to say about the matter.

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Note: The longest interval in the recent 6,030 years (earthquake clustering) was 344 years, only 23 years from our current 321-year break. 23. Not 50. So saying there is a 37% chance in the next 50 years (read 63% chance it won’t happen) seems problematic.

Check out the following pages in Surviving Cascadia to delve a little deeper.

So are we doomed?
Referencing the 2011 Tōhoku earthquake and tsunami in the OPB documentary Unprepared, Dr. Chris Goldfinger says, “Japan is a great example of how an earthquake like this, as bad as it is, is survivable. And to just throw up your hands and say ‘we’re all gonna die’, is wrong.”

We’re not doomed. Nor are we totally safe. Denial isn’t an effective preparedness strategy. A Cascadia Subduction Zone megaquake is a very real threat to our region.

It’s critically important to understand the risk of that threat. After all, we prepare (or don’t prepare) based on our perceived risk. For that reason alone, understanding the risk is key to Surviving Cascadia.

It is my hope that you will consider this information, as I did, when you take steps toward getting prepared for the earthquake. If you need extra incentive, here is a list of very reputable organizations asking you to take it seriously & get 2-weeks-ready. (There are many others!)


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*The age of the 5th earthquake in this series is currently unknown. Due to the ambiguity, intervals 5 & 6 are unknown, as well. What we do know is that the interval between the 4th & 6th earthquakes equals only 115 years. Therefore, intervals 5 & 6 are each far less than 318 years. The same is true for the 25th & 31st undated earthquakes, with 235 and 37-year intervals surrounding them, respectively.