Megathrust Earthquakes

The Pacific Northwest is home to the Cascadia Subduction Zone (CSZ); a thrust fault, like the one shown in this video by the Incorporated Research Institutions for Seismology (IRIS). It’s capable of producing M9+ megathrust earthquakes.

(*Scroll down the page to find a tsunami simulation and a video on magnitude classes)

The National Earthquake Information Center records about 20,000 annual global earthquakes. That’s roughly 1,440,000 earthquakes since 1950. Of those 1.4 million, only 5 reached magnitude 9 or larger.

YearLocationEvent NameMagnitude
1952Kuril–Kamchatka TrenchSevero-Kurilsk earthquake9.0
1960Peru-Chile Subduction ZoneValdivia Megathrust Earthquake9.5
1964Alaska/Aleutian Subduction ZonePrince William Sound Earthquake9.2
2004Sumatra Subduction ZoneSumatra–Andaman Earthquake9.1
2011Japan TrenchGreat Tohoku Earthquake9.1

The Awesome Power of Megathrust Earthquakes

The 1960 earthquake listed above was a whopping magnitude 9.5. A quote from a National Public Radio (NPR) piece titled, “When The Biggest Earthquake Ever Recorded Hit Chile, It Rocked The World” reads:

What became known as the Great Chilean Earthquake revealed something new about the planet — that the world itself can vibrate like a guitar stringThe seismic waves went through every part of the globe, even its core. And because they were so strong, scientific instruments from around the world picked up the signal. When it was over, seismologists realized the earthquake had given them a window into Earth’s structure. Nature had given the planet something like an ultrasound scan.

Again, the 9.5 was the largest ever recorded earthquake. By comparison, Oregon’s Department of Geology and Mineral Industries’ (DOGAMI) included a magnitude 9.4 CSZ earthquake as the largest hypothetical earthquake in its research, Variable Rupture Scenarios for Tsunami Simulations Inferred From a 10,000-Year History of Cascadia Megathrust Earthquakes.

To get a better idea of how large these megathrust earthquakes are, check this 2-minute video comparing the magnitude classes.

A magnitude 9.0 earthquake releases the equivalent energy of 480 million tons of TNT—or 30,000 strategic nuclear bombs. It’s massive.

Freund, Friedemann. (2003). Rocks that crackle and sparkle and glow: Strange pre-earthquake phenomena. Journal of Scientific Exploration. 17.

What kind of impacts could the earthquake have on the Pacific Northwest? One major impact will be on infrastructure. The Oregon Seismic Safety Policy Advisory Commission (OSSPAC)’s Oregon Resilience Plan lists the following expected times for restoration of services. *Oregon assessment only*

Washington State Department of Natural Resources paper, Modeling a Magnitude 9.0 Earthquake on the Cascadia Subduction Zone off the Pacific Coast shows some of their expected outcomes.

Table 1. Summary of significant losses in the M9.0 Cascadia subduction zone earthquake scenario. *Washington assessment only*

The Awesome 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.

An Oregon Health Authority 2021 report estimates 18,667 fatalities in Oregon alone (not including those caused directly by the earthquake) should a magnitude 9.0 strike on a Summer night.

Visit the Oregon Department of Geology and Mineral Industries (DOGAMI) Beat the Wave Technical Reports & Maps to prepare.

How Big Is The CSZ?

According to the Pacific Northwest Seismic Network (PNSN) and the United States Geological Survey (USGS), the length of the offshore Cascadia Megathrust Fault where the Juan de Fuca plate meets the North American Plate, is 1,000 km (621.37 miles).

However, the section of land that lies east of that junction bends. According to the Cascadia Region Earthquake Workgroup (CREW), the CSZ’s coastal/inland region stretches 800 miles (1287.47 km) from Vancouver Island’s Brooks Peninsula down to Cape Mendocino, California.

Page 113 of Cascadia’s Fault: The Coming Earthquake And Tsunami That Could Devastate North America by Jerry Thompson states, “[In a magnitude 9 scenario] Cascadia’s fault is going to cause damage to all the cities and towns along a swath more than 800 miles (1,300 km) from north to south and as much as 125 miles (200 km) inland.” That’s 100,000 square miles (64,000,000 acres).

In an Oregon Office of Emergency Management presentation, The Impacts of the Cascadia Subduction Zone Earthquake on Oregon, Dr. Althea Rizzo shows that 15 million people live in the impact zone. This figure doesn’t take into account tourists who may be in the region when the earthquake hits.

Image credit: USGS Topo-bathymetric map of the Cascadia subduction zone. Cascadia megathrust fault (white line); approximate shelf break along 200-m isobath (yellow line); MTJ, Mendocino triple junction.

Will the Whole Fault Rupture?

Not every CSZ megathrust quake is a magnitude 9+. Magnitude is basically a function of rupture length. The longer the ‘unzipping’, the larger the magnitude. Based on core samples, scientists have discovered 4 primary CSZ rupture patterns shown in the DOGAMI image below. *Mw = moment magnitude.

Oregon Department of Geology and Mineral Industries (DOGAMI)

It’s worth noting that the above image was published before the number of known CSZ earthquakes jumped from 41 to 43. Those two extra earthquakes changed the recurrence intervals for segments A & B above. You can see the details about the change on Surviving Cascadia’s 17% in 50 Years page. More information about the different fault segments is available on Surviving Cascadia’s How Big Will It Be? page.

So What Now?

When the earthquake hits the Pacific Northwest, you will need to be self-sufficient for at least two weeks until help from outside the disaster area can assist. Visit the food, water, and electricity pages to learn about aftermath expectations and ways to get prepared.

The purpose of this website is to (hopefully) answer questions about the Cascadia Subduction Zone that others like me have wondered about. Every time questions about Cascadia have led me to search out information in books or online, had me reaching out to the scientific community or running analyses, I added my findings to the pages of Surviving Cascadia.

The information comes from reputable scientific research. I do my best to ensure the data is accurate, reproducible, and verifiable, and encourage you to travel through the pages of this website. Reach out if you have any questions or suggestions. Surviving Cascadia is a constant work-in-process, so I absolutely welcome conversation and feedback.