37% in 50 Years

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

What’s commonly known

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.

That 37% chance is derived from the knowledge that there have been 46 major CSZ earthquakes in the last 10,270 years, which averages out to one earthquake every 223 years on the southern end of the fault (the northern end of the fault does not rupture as often so the probability is lower for that region).

322 years have passed since the Pacific Northwest saw its last major earthquake at around 9:00 pm on January 26th, 1700. Check out this OPB article on how scientists know the date and time so precisely.

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 322 years (the current amount of time we have gone without one, for an easy visual reference).

Intervals longer than 322 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 322 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 322nd year… should we expect the fault to hold for another 50 years? 20 years?

50 Years Out?

Let’s consider the quoted risk probability from above—a 37% chance it will happen in the next 50 years. If the earthquake doesn’t happen in the next 50 years, how different will the interval chart look? Take a look. The two bars in purple were red in the previous graph, reaching above the 322-year line. If we go 50 years without it occurring, these purple lines will join the green…. leaving all the red intervals more than 6,000 years in the past.

Now, One Step Further

Placing a yellow border around intervals 34 through 46—as of 2022, not 2072— helps illustrate how the largest intervals are grouped toward the oldest chunk of the 10,272-year history (intervals 34-46).

Intervals one through thirty-three make up the most recent 6,030 years. Looking at that timeframe in the chart, we see that the longest interval was 344 years—only 22 years from our current 322-year break. 22. Not 50. So saying there is a 37% chance in the next 50 years (which could also be read as a 63% chance it won’t happen) seems problematic.

Let’s Play That Game Again

To be clear, regarding the most recent 6,000 years of history on the CSZ, this means 94% of the time (31 intervals out of 33) the Cascadia Subduction Zone did not have to wait 322 years between ruptures.

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 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 10,272-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. Note: this is just one theory.

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

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When Did the M8s vs M9s Occur?

Looking at the chart below, it is again 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 megathrust earthquakes, for example. Still, those two earthquakes came fairly close together with only 232 years between.

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

So Are We doomed?

Referencing the 2011 Tōhoku earthquake and tsunami in the OPB documentary Unprepared, Oregon State University Professor 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.

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

If you need extra incentive, here is a list of very reputable organizations asking you to take it seriously and 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.