Tsunamis

This page provides information on 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

The United States Government Accountability Office (GAO) estimates the initial tsunami wave that will follow a Cascadia Subduction Zone (CSZ) earthquake will measure between 3 and 80 feet high, depending on the topography of each location along the coastline. It will reach the coastline in as little as 10 to 30 minutes after the shaking.

The initial wave isn’t always the largest.

The report states, “In general, if a person is caught where the tsunami is over six feet deep, then death is likely.”

How many people will be located within the inundation zone is hard to predict. As with most things, the answer is “it depends”. The table below contains data derived from Cascadia Rising. Looking at just the first and last rows, it’s clear that a large portion of residents may not survive if they don’t move quickly enough.

Residents on vacation or traveling outside the inundation area for any other reason will obviously lower the number of people at risk on that day. The three rows in the middle provide the largest gray areas, however, with thousands of people who may or may not be in the zone on any given day.

The Cascadia Rising Exercise Scenario Document was written in 2015. Increases in the coastal populations since then should be considered. For example, 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”.

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It’s worth noting, too, that population is growing 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.