Worst-Case Scenario

01/26/2023 marks the 323rd anniversary of the last major CSZ earthquake. For last year’s anniversary, I wrote an op-ed, “The next Cascadia earthquake could be devastating. Start your preparations”, published in the Statesman Journal. It is also available on Yahoo! News if you’d like to check it out!

USGS KEY: blue: San Jacinto fault, black: San Andreas fault, green: Cascadia Subduction Zone, red: tectonic plate boundaries

When natural disasters strike, the size of the impacted area can affect how prepared you need to be for the aftermath. Let’s consider a worst-case scenario for the Western US.

Part 1

We know that a Cascadia Subduction Zone megathrust earthquake will be paired with a devastating tsunami.

Part 2

According to an interview with OSU Professor Chris Goldfinger (see references at the bottom of this page), eight of the past twelve Cascadia Subduction Zone megathrust earthquakes appear to have triggered a San Andreas earthquake1 via either static stress or seismic waves2. It is believed that the faults may be partially synchronized. In other words, during the past 3,000 years, the two faults have ruptured ‘together’ 2/3 of the time. This information is also discussed in OPB’s article, The Big One, Times 2: Research Shows Cascadia Quakes Sometimes Trigger San Andreas Fault.

Contrary to the popular movie, a San Andras earthquake will not result in a large tsunami. Local tsunamis from offshore landslides could create localized tsunamis. For more info, visit Earthquake Country Alliance.

Part 3

Further research shows that volcanoes along the Cascade Mountain range have occasionally erupted within a reasonably short timeframe (days to months) after CSZ earthquakes. Based on current data, Goldfinger estimates there is a close correspondence between the two roughly 20% of the time. This phenomenon has been observed in other locations around the world, such as Chile, Sumatra, and Japan3. Check out these two USGS images showing volcano locations and eruption history.

Part 4

New research by the University of California, Davis, and San Diego State University finds that 20% to 23% of earthquakes on the San Andreas fault are shared with earthquakes on the San Jacinto Fault. The article cited at the bottom of this page states, “each of these faults on their own can generate a large-magnitude (7.5 or above) earthquake” 4. Here is a closeup from the first image on this page showing where the two faults meet. The black line shows the southern end of the San Andreas; the blue line, the San Jacinto. This image was generated using USGS ARCGIS MAPS.

While we know a tsunami will follow a CSZ earthquake, the probability of the CSZ, the San Andreas and the San Jacinto rupturing together (especially to their full length) followed by a volcanic eruption (or two) is highly unlikely. However, any combination of these disasters will impact available resources and response times and may possibly create homeland security issues.

If response times are slowed, and supplies are limited due to simultaneous disasters, having additional supplies on hand for you and your family may be crucial. Food and water are essential. What else might you need? The City of Salem has an awesome 2-Weeks-Ready Challenge. It’s a great starting point.

Part 5

Now focus on only the CSZ earthquake. Imagining different scenarios can help diversify the supplies you store. As an exercise, close your eyes and imagine the earthquake occurring in the following scenarios (if you dare):

  1. A calm spring morning (daylight) where current temperatures and those in the coming weeks are mild. Some rain is forecasted (rainwater collection).
  2. At midnight in winter, with temperatures in the low to mid-thirties. Heavy winds and rain are forecasted for the coming week.
  3. A hot summer day where rain isn’t expected in the ten-day forecast, and wildfires are getting closer to town. More fires sparked in town during the shaking. Smoke fills the air.
  4.  You and your partner are at work. Your kids are at three different schools. Reunifications become the first order of business.
  5. You and your loved ones are safe, but your home is uninhabitable.

These scenarios are scary to work through, but each scenario changes your needs. What do you need for each of these? Remember, knowing what you’ll need is only half the equation. Your next task is to start preparing!

One last thought

You can see the direction a tornado is moving, can watch it shift direction. You can see floodwaters rise, and landslides and avalanches flow. You can see the volcano that’s spewing, the thunderclouds approaching.

You won’t see the earthquake. It will be nowhere, then everywhere at once. There isn’t an earthquake season, and aside from the precious few seconds that ShakeAlert can now bring, there will be little warning. You have to be ready in advance. Knowledge is power, and hopefully, in this case, it’s motivation, too. Just be ready.


*Goldfinger mentions the record we have for San Andreas is not as complete as that of the CSZ earthquakes. Geologic records for the volcanoes are also not as extensive as the earthquake history. It’s possible some of the correlations will prove to be higher when additional records are uncovered.

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


Central Oregon Geoscience Society, The Next Great Cascadia Earthquake – How did we get here? January 26th, 2021 Webinar
1. Video at 58 minutes
2. Video at 1 hour 16 minutes)
3. Video at 1 hour 20 minutes

UC Davis December 2021 report:
4. Evidence for Shared Earthquakes Between San Andreas and San Jacinto Faults