The Power of Sister Tsunamis

Approximately 86,000 PNW residents are located in the CSZ tsunami inundation zone (*That’s not just Oregon). This figure doesn’t account for tourists.

Subduction zone megathrust earthquakes displace massive amounts of water, creating a series of incredibly long waves known as tsunamis. Rather than just surface waves, tsunami waves are columns of water that extend all the way from the ocean floor to the surface. Check out this two-minute simulation.

According to the National Oceanic and Atmospheric Administration (NOAA), a tsunami wavelength from crest to crest can be hundreds of miles wide. They can travel over 500 miles per hour in the deep ocean, slowing to around 30 mph near the shoreline.

The sister tsunami that followed on the heels of Japan’s 2011 9.1 Tōhoku earthquake released roughly 3 Petajoules (PJ) of energy, equivalent to 717,017 tons of TNT, 3.0*1022 ergs. The University of Hawai‘i notes that the Japan tsunami generated waves that reached as high as 131 feet (40 meters) and traveled as much as 6 miles (10 km) inland. Our topography is very similar.

This image has an empty alt attribute; its file name is fatalities-pic.png

The 2021 Oregon Health Authority’s Cascadia Tsunami Casualty Estimates report lists the following expected injuries and fatalities per county:

The 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 image raised some questions about why the north is predicted to experience so many of the casualties. Were the figures based on population? I did some digging. Based on 2020 census data, there isn’t a huge difference in population between the north and south. Turns out, topography is the leading factor.

The coastal cities listed to the left are color-coded by county with northern counties at the top and southern at the bottom.

As with most things, the casualty estimates are multi-faceted. Topography appears to play a major role, as does access to evacuation routes and tourist flows. If you are traveling to the PNW coast, please make sure you know your evacuation routes! Practice them. Time how long it takes to make it from your favorite places to the safe zones. Does it take longer on stormy days? Are there roads and bridges that may fail in an earthquake, making the route more difficult?

Have you checked out Oregon State University’s O-Help page to see if the area will be prone to landslides or liquefaction, possibly making routes more difficult? Do you have a backup route in mind? Will you be with children, elderly, or disabled persons? It can take as little as 30 minutes to plan and practice. Those 30 could save your life. Check out the Oregon Department of Geology and Mineral Industries (DOGAMI) tsunami evacuation maps to get started.