When Will It Happen?

“We learn geology the morning after the earthquake.”
-Ralph Waldo Emerson

First things first: Science is not yet capable of predicting earthquakes. That doesn’t stop us from wishing we had some handle on when they might happen. Dr. Lucy Jones’ book The Big Ones: How Natural Disasters Have Shaped Us (and What We Can Do About Them) does a fabulous job explaining why we are so drawn to the idea of predicting these types of major events. For now, one way science helps us to answer the “When will it happen?” question is by providing probabilities during a particular timeframe. And those timeframes often impact our actions.

In this case, the probabilities are incredibly problematic. In fact, I highly recommend reading this particular page in its entirety. It may change your sense of urgency.

We prepare—or don’t—based on our perceived risk.

Let’s face it. When there is an 80% chance of snow sticking in the Willamette Valley, grocery stores turn into zoos! Humans are really good at preparing for emergencies when the risk is high and the timeframe is short. (Image from FEMA: https://www.ready.gov/winter-weather)

In some areas, earthquakes happen often enough that being prepared for one is common among residents. In locations like the Pacific Northwest, we experience few. The low frequency makes it much harder to take the risk seriously. The way we discuss the risk can also impact how motivated we are to prepare. Looking at the percentages below—all based on a 50-year outlook—it can feel like the level of risk depends on who you ask. Click the buttons to read the articles.


So, which percentage from above is correct? Well, it depends. Each percentage is correct because each percentage uses different criteria. Frustrating, right? Check out this Northwestern University paper for more information on why there are so many different probabilities out there. The answers are complex because earthquakes are complex. In this COGS Interview, Professor Goldfinger explains why we hear so many different probabilities. I highly recommend watching the entire interview, as it’s packed with information, but for a brief intro, start the video at 1:02:00.

If you’re curious about how the CSZ earthquake probabilities are calculated, here is some additional information. If not, don’t leave this page without reading the Bottom Line.

Explore the Probabilities

Step 1
Choose the mean and standard deviations you are curious about from one of the rows below. Gaussian and Log-Normal results are listed in this image for 50-year outlooks, but you can use the mean and SD in STEP 2 to see how the probabilities change in different timeframes (What are the chances this earthquake hits by the time I retire? By the time my kids go to college? etc.) Details for the calculations are available at the bottom of this page. Feel free to reach out if you have questions or comments!

T1 to T19 refer to specific earthquake ‘names’. See the segments in the How Big Will It Be page for more info on this. 
References for this data are listed at the bottom of the page below calculations.

Step 2
Edit the input fields in the Excel spreadsheet below, provided by Oregon State University Professor Chris Goldfinger and his colleagues. The probability numbers used in the “37% chance in 50 years” stat are based on all 46 major earthquakes.

If you would prefer to download this spreadsheet, Northwestern University Professor Seth Stein has it available. Simply click on the Download Excel spreadsheet for calculations link on Seth Stein’s website*Note: the Download Excel spreadsheet of Cascadia earthquake history option on the webpage lists only the full-rip (M9) occurrences.



Step 3
Edit the “Start Year” or “End Year” to see how the risk probabilities (in blue) change. According to Professor Goldfinger, the Log-Normal model fits the data for our region better, but the Gaussian model (think bell curve) is better at predicting over time.

Take a look at this chart and the comparison images below. 50-year timeframes that are further into the future show a higher probability of the next earthquake hitting… according to the Gaussian model. In the Log-Normal model, however, the risk is currently “maxed out”. The probability begins to decrease in the 2032-2082 50-year timeframe.

That would be great, right! If we could just make it 10 more years without an earthquake, the risk would start going down… Unfortunately, that’s definitely not the case. The probability of this occurring doesn’t decrease over time. Pressure continues to build where the Juan de Fuca plate is stuck beneath the North American plate. It will continue to build until eventually, the built-up stress breaks.


Bottom line
The Log-Normal probability model shows a “37% chance a CSZ megathrust earthquake will hit the PNW in the next 50 years” (36.92% rounded up). The model would have given the same risk in 1982 (36.80 rounded up) and will still give the same risk in 2072 (36.70% rounded up). It’s good to know where the numbers come from, but it’s also good to know how much stock to put into the numbers.

Again, we choose to prepare—or not—based on our perceived risk. If it looks like the risk never changes, the region may be less likely to act. But the risk does rise. The stress continues to build. This is why I recommend looking at intervals rather than probability figures. Did you know that 81% of the time, the fault has not had to wait 322 years for the strain to break it? Check out Surviving Cascadia’s 37% in 50 Years page for more info!


One Last Note
Neither the Gaussian nor the Log-Normal calculation accounts for the clustering behavior of the fault, so Professor Goldfinger cautions against getting too consumed by the numbers.

Professor Seth Stein and his team are currently working on a new probability model. For more information, visit their research publication, A More Realistic Earthquake Probability Model Using Long-Term Fault Memory, explanatory YouTube video, and PDF with graphics like the one.

Further discussion on the topic from the experts can be found on Facebook’s Pacific Northwest Earthquake Discussion Group’s post. This comment and the images below are of particular interest.

We live in a region where these large earthquakes occur. While we don’t know exactly when the earthquake will happen, there is no doubt that the fault is going to rupture at some point. Being 2-Weeks-Ready is the right choice.

Details for the Calculations


References

Goldfinger, C., Galer, S., Beeson, J.W., Hamilton, T.S, Black, B., Romsos, C., Patton, J., Nelson, C.H., Hausmann, R., Morey,A., 2017, The Importance of Site Selection, Sediment Supply, and Hydrodynamics: A Case Study of Submarine Paleoseismology on the Northern Cascadia margin, Washington USA, Marine Geology, v. 384, p. 4-46.

Statistical Analyses of Great Earthquake Recurrence along the Cascadia Subduction Zone
Ram Kulkarni; Ivan Wong; Judith Zachariasen; Chris Goldfinger; Martin Lawrence Bulletin of the Seismological Society of America (2013) 103 (6): 3205–3221. https://pubs.geoscienceworld.org/ssa/bssa/articleabstract/103/6/3205/331820/Statistical-Analyses-of-Great-Earthquake