Tsunamis

Welcome to Surviving Cascadia’s tsunami page. Scroll through to learn a bit about what tsunamis are, how they are expected to impact the PNW, and what you should know in order to survive one.

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What are tsunamis?

Tsunami is Japanese for “harbor wave”. Asteroids, landslides, volcanic eruptions, and subduction zone megathrust earthquakes can all displace massive amounts of water, creating a series of incredibly long waves known as tsunamis. Unlike surface waves, tsunamis are columns of energy; pushing water from the ocean floor to its surface.

Here’s a simulation of a tsunami caused by a subduction zone earthquake.

“A single magnitude 9.0 CSZ event may generate multiple tsunamis that last for approximately 10 to 12 hours after the initial earthquake. Aftershocks of magnitude 7.0 or greater may follow the initial earthquake and tsunami, generating additional tsunamis.”

United States Government Accountability Office (GAO)

In contrast to the above quote, the Cascadia Rising Exercise Scenario Document states that tsunami waves may surge for up to 24 hours after the earthquake.

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Expected Impacts from a CSZ Tsunami

2001 Data

A 2001 document, Designing for tsunamis: seven principles for planning and designing for tsunami hazards, prepared by the National Tsunami Hazard Mitigation Program, states:

“900,000 people in Alaska, California, Hawaii, Oregon, and Washington
live in areas in danger of being inundated by a 50-foot tsunami”.

Details for the figure can be found in Jackson State University’s paper, Assessing the Vulnerability of the US Pacific North West Region to Tsunami Hazards, from which the following table comes.

It’s important to note that these figures were based on population counts, which as of March 2023, are 22 years old. The population at risk in each state has risen. In fact, the US Census Bureau shows the US experiencing a net gain of one person every 22 seconds.

2015 Data

Oregon’s 2015 Cascadia Rising Exercise Scenario Document estimated the following. Strangely, the total number of residents, non-resident workers, and tourists for the data in Washington aline with the 2001 data. However, Oregon’s 2015 numbers are much larger than those from the 2001 estimate.

How many people will actually be located within the inundation zone is hard to predict. As with most things, the answer is “it depends”. Unfortunately, looking at just the first and last rows above, it’s clear that a large portion of those in the inundation zone when the earthquake happens, however many there may be, may not survive if they don’t move quickly enough.

Current Data

Resiliency 2025: Improving Our Readiness for the Cascadia Earthquake and Tsunami states, “Oregon’s at-risk population is approximately 40,000 on the outer coast, excluding tourists and visitors that seasonally swell the population manyfold”.

Future Projections

It’s worth noting, too, that the population is expected to continue to grow in the Pacific Northwest. The following quote relates to expected population changes between 2011 and 2061.

Across the region, 2061 population exposure in tsunami-hazard zones was projected to increase by 3880 households and 6940 residents. 

Projecting community changes in hazard exposure to support long-term risk reduction: A case study of tsunami hazards in the U.S. Pacific Northwest – USGS

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How to Survive

How fast will you need to move? Based on the table in the section above… definitely faster than a slow walk. But what’s a “slow walk”? The Oregon Department of Geology and Mineral Industries (DOGAMI) has the following information on walking speeds, speeds by age, and speed adjustments based on the type of path. Note that when moving on loose beach sand, your normal walking speed is cut by more than half.

A reminder: tsunamis travel at about 30mph once they hit the shoreline. As you can see below, it is unlikely you can sprint faster than 10mph, even if you are in incredible shape. Don’t wait until you can see the waves. You can’t outrun them once they get to shore.

Figuring out how fast you’ll need to move based on where you are when this happens is tricky! Thankfully, DOGAMI has developed Beat the Wave maps. They show the speed at which you will need to move based on specific locations you may be in when the tsunami hits. To see the map for your area (only covers Oregon) or a coastal area you plan to visit, click here.

Visit the American Red Cross for more tsunami preparedness information.

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Some Final Data to Consider

Oregon Health Authority’s 2021 Cascadia Tsunami Casualty Estimates Report states:

“Casualty estimates are assumed to occur within hours of the earthquake (as opposed to days after the earthquake). Estimates, which include permanent residents and visitor populations, are for a summer “night” (i.e., 2 AM) when visitor populations are high. It is important to note the modeling assumes that all persons quickly evacuate by foot using the most optimal tsunami evacuation route; these estimates are not a worst-case scenario.”

The report lists the following expected injuries and fatalities per county in Oregon (reminder: figures include residents and tourists).

The above estimates raise some questions about why the north is predicted to experience so many of the casualties. Based on 2020 census data, there isn’t a huge difference in population between the north and south.