37% in 50 Years

There’s no harm in hoping for the best as long as you’re prepared for the worst.”
– Stephen King

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

They have found a “37% chance the Pacific Northwest will have a major Cascadia Subduction Zone (CSZ) earthquake in the next 50 years“.

The probability is based on 10,000 years of data. But there’s a catch! The fault experienced very different interval behavior during the oldest 4,000 years compared to the most recent 6,000.

The Oregon Seismic Safety and Policy Advisory Council (OSSPAC) has some good resources on its page. In accordance with Oregon Public Law, all of OSSPAC’s meetings are open to the public, and materials are posted on the website. Under OSSPAC’s Presentations and Testimony section is a link, Lifeline Resilience Teams Overview – Steve Robinson Submission (2019-09-10). It states:

Wait… 15-20 years? It’s worth noting here that those facts were posted in 2019. 15-20 years out from 2019 would have been 2034-2039. On January 26, 2023, those years will only be 11-16 years out.

A Deeper Dive

Take a look at this ten-thousand-year historical chart1. Each vertical bar represents the years between major Pacific Northwest earthquakes along the Cascadia Subduction Zone. The horizontal blue line runs at 323 years (the current amount of time we have gone without one, for an easy visual reference).

Intervals longer than 323 years are shown in red. There aren’t many. In fact, only eight out of forty-three lines are red… meaning 81% of the time, the fault has not had to wait 323 years for the strain to break it.

Let’s Play a Game

If you knew that 81% 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 81% of the time, our fault has broken prior to reaching its 323rd 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 323-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 31 through 43—as of the year 2022, not 2072— helps illustrate how the largest intervals are grouped toward the oldest chunk of the 10,272-year history.

Intervals one through thirty make up the most recent 6,031 years. Looking at that timeframe in the chart, we see that the longest interval was 344 years—only 21 years from our current 323-year break. 21. 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 93% of the time (28 intervals out of 30) the Cascadia Subduction Zone did not have to wait 323 years between ruptures.

If you knew 93% of the time your toddler was asleep by 8:00 pm, would you expect her to be up until 8:10? Maybe. 9:00 pm? 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).

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.

Click the links below to jump to a new page!

Megathrust EarthquakesTsunamisHow Big Will it Be?
How Big is The Fault?CSZ Frequency10,000 Years
When Will it Happen?37% in 50 Years17% in 50 Years
Summer EarthquakesThe True RiskWorst Case Scenario
Expected Regional ImpactsWhy PrepareDrop, Cover, Hold?
Water PreparednessFood PreparednessSanitation… & Whales
Life Without ElectricitySlow-Slip Events (ETS)Offshore Earthquakes
ResourcesCERT SalemWest Salem CERT
Research PapersBook RecommendationsContact

References

The importance of site selection, sediment supply, and hydrodynamics: A case study of submarine paleoseismology on the northern Cascadia margin, Washington USA
Goldfinger, C., Galer, S., Beeson, J.W., Hamilton, T.S, Black, B., Romsos, C., Patton, J., Nelson, C.H., Hausmann, R., Morey,A., 2017, Marine Geology, v. 384, p. 4-46.
https://www.sciencedirect.com/science/article/abs/pii/S0025322716301220

Turbidite Event History—Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone
Goldfinger, C., Nelson, C.H., Morey, A.E., Johnson, J.E., Patton, J.R., Karabanov, E., Gutiérrez-Pastor, J., Eriksson, A.T., Gràcia, E., Dunhill, G., Enkin, R.J., Dallimore, A., and Vallier, T., 2012, U.S. Geological Survey Professional Paper 1661–F, 170 p. https://pubs.er.usgs.gov/publication/pp1661F