This page examines why online resources quote different probability numbers regarding the risk of the next CSZ earthquake. (For information about why some online sources cite the actual number of Cascadia Subduction Zone megathrust earthquakes occurring in the last 10,000 years to be 40, 41, 43, or 46, visit CSZ Frequency).
Have you heard that the chance of the next CSZ hitting within 50 years is less than 37%? Chances are yes. But why? On our 37% Chance in 50 Years page, you will see that 37% is the most commonly reported figure. But there are others. This page is designed to help you understand why other figures are out there and also help highlight some of the potential confusion around them.
The lowest probability figures are based on the chances that the Pacific Northwest will experience a magnitude 9.0 earthquake in the next 50 years.
19 CSZ megathrust earthquakes in the past 10,000 years have been full ruptures. These are shown in Segment A (T19, not shown here takes this number to 20 M9 events in 10,271 years). The white lettering on the left side lists the names of the earthquakes. The red shows the rupture length extending the full length of the fault (full-rips).
Not all CSZ megathrust earthquakes rupture the entire fault. Sometimes, a segment breaks. As shown here, image A from above is accompanied by other segments, and there are probabilities for each of these.
(OSU paper from which this image comes)
Page 129 of the 2012 USGS publication, Turbidite Event History—Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone, lists the following:
In 2016, updated research published through Oregon State University increased the probabilities for the 2 northern-most sections of the fault. Here’s how the numbers changed.
Hopefully seeing these figures helps clarify why different probabilities are provided at times. Looking again at segment A (magnitude 9.0s, also known as Full-Rips), we see that the probability is less than 37% (That figure considers the possibility that any of these segments might happen and includes all ruptures during the past 10,000 years).
Because these full-rip earthquakes account for less than half of the recorded Cascadia Subduction Zone megathrust earthquakes, the intervals from one M9 to the next are, of course, larger than intervals that take into account M8s (like very destructive pitstops between destinations).
It’s important to distinguish between the two interval timelines. For example, one State of Oregon page states, “There have been 41 earthquakes in the last 10,000 years within this fault that have occurred as few as 190 years or as much as 1200 years apart.“
This statement can, unfortunately, create confusion. As stated above, 41 was the number of known CSZ earthquakes back in 2012. It includes magnitude 7 and 8 ranges. However, the 1200-year interval discussed in the very same sentence relates only to the number of years between M9s.
The same State of Oregon page discussed above states, “Currently, scientists are predicting that there is about a 37 percent chance that a megathrust earthquake of 7.1+ magnitude in this fault zone will occur in the next 50 years“. While it is clear in this sentence that all earthquakes are included, the 1,200-year statement isn’t clarified.
As shown below, had even a single 1,200-year period of inactivity occurred between major CSZ earthquakes, then no other interval could have exceeded 194 years. With 190 years being listed as the smallest interval, all remaining intervals would have all been close to equal.
Statements like this, which don’t clarify whether data relates to all CSZ megathrust earthquakes or just M9s, could lead communities to believe that, at times, the Pacific Northwest went 1,200 years without a major CSZ earthquake occurring.
Curious how the probabilities change when 41 earthquakes are used in the calculation, rather than 46? Here are the calculation pages and probability models for both. (See bottom of this page for the Oregon State University research supporting this data.)
That’s not the case. If we consider M8s, the longest interval during the past 10,000-years is only 577 years. Only 8 of the past 45 intervals have stretched longer than 322 years. Visit the 37% in 50 Years page for more details.
A recent article in The Spokesman Review states, “scientists predict the chances that a mega-CSZ earthquake will occur within the next 50 years are about one in ten.” Again, that quote relates to M9s only, but the article doesn’t explain that. This kind of confusion pops up often… and impacts our understanding of urgency.
We prepare—or don’t—based on our perceived risk.
Check out this USGS graph below. It illustrates how M9s appear to cluster on the CSZ. The most recent grouping shows average intervals of only 330 years… between M9s! (We have currently gone 322 years)
This is the same data that supports the long 1,200-year interval, but again, there are no M8s included here. The PNW has not gone 1,200 without a major CSZ hitting… at least not during the last 10,000 years.
In 2010, Professor Goldfinger was quoted as saying, “We’re just starting to understand the whole idea of clusters and there isn’t consensus on whether we are in one or not, but that possibility does exist, which further suggests that we may experience a major earthquake sooner than later.”
More information about clustering can be found in the research article, Statistical Analyses of Great Earthquake Recurrence along the Cascadia Subduction Zone, as well as on my When Will It Happen page. Here is a look at the M8s mixed with the M9s.
Chris Goldfinger, Steve Galer, Jeffrey Beeson, Tark Hamilton, Bran Black, Chris Romsos, Jason Patton, C. Hans Nelson, Rachel Hausmann, Ann Morey, The importance of site selection, sediment supply, and hydrodynamics: A case study of submarine paleoseismology on the northern Cascadia margin, Washington USA, Marine Geology, Volume 384, 2017, Pages 4-46, ISSN 0025-3227, https://doi.org/10.1016/j.margeo.2016.06.008.