CSZ History

As of January 26th, 2020, it has been 320 years since our last major CSZ earthquake. How do 320 years compare to other intervals in our past? Note: the data used to find these intervals is the same data used to calculate the ‘37% chance in 50 years’ risk we hear about on the news. I argue that using that Log-normal model to discuss our risk is not the best narrative.

Take a look at this 10,000-year historical chart. Note how few lines stretch above 320 years… We have highlighted the years that do in red.

Only 8 of the past 46 intervals were longer than 320 years. Humm… 46-8=38. 38/46=82.6%. Let’s play a game.

If you knew that roughly 80% of the time, your child was asleep by 8:00pm, would you expect that child to be up until 9:00pm tonight? Probably not.

Knowing that roughly 80% of the time, our fault breaks within a 320-year time frame… should we expect the fault to truly hold for another 50 years? 20 years? History cautions us against doing so.

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One Step Farther

Placing a yellow border around the longest intervals helps illustrate how they are grouped toward the oldest chunk of the 10,000-year history (intervals 34-46).

Intervals 1 (our current 320 years) through 33 make up the most-recent 6,029 years. Notice that only 2 lines stretch beyond 320.

… and they don’t reach far above!!

So, 33-2 = 31. 31/33 = 93.9%  Let’s play the game again.

If you knew that 94% of the time, your child was asleep by 8:00pm, would you expect that child to be up until 9:00pm tonight? Nope.

We know that during the most recent 6,000 years of history, 94% of the time, our fault has broken within a 320-year time frame. 320 years have passed without a Cascadia megathrust earthquake. Should we truly expect our fault to hold much longer?

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Right about now, you are probably asking why the difference between the two time frames is so obvious. In image A below, the earthquake behavior more closely matches our history from our oldest 4,000 years in the CSZ.

Image B is an excellent illustration of what we have been experiencing during the most recent 6,000 years (more magnitude 8 earthquakes coming between the magnitude 9s).

https://eos.org/meeting-reports/addressing-cascadia-subduction-zone-great-earthquake-recurrence

Our last major earthquake was a magnitude 9, so perhaps we should expect the M8.5 next, based on image B above. Check out segments B and C on the segments page of this website. They are the only two capable of M8.5s (other than A, which does the M9s).

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

Professor John Vidale

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Professor Chris Goldfinger

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Something to keep in mind: the ‘37% chance in 50 years’ is a Log-Normal probability model based on the entire 10,000 years, rather than just the most recent 6,000.

If our current fault behavior is more likely to reflect that of our most recent history (6,000 years), then we should not be looking 50 years out when we think about risk.

Remember, 93.9% of the time, earthquakes have struck faster than 320 years during that portion of history. 93.9%… as of today. Forget the 50-year outlook. That’s as of today. We need a different narrative.

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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.