Megathrust Earthquakes

The Awesome Power of Megathrust Earthquakes

A magnitude 9.0 earthquake releases the equivalent energy of 480 million tons of TNT—or 30,000 nuclear bombs (Figure 1). It’s massive.

According to the Woods Hole Oceanographic Institute (OCEANUS) shaking from the Japan 2011 earthquake lasted 6 minutes. Penn State has the 2004 Sumatra–Andaman Earthquake (Boxing Day earthquake) lasting between 8 and 10 minutes.

Japan’s topography is a near mirror image of the Pacific Northwest, giving us a glimpse of what’s to come.

What kind of impacts could the earthquake have on the Pacific Northwest? One major impact will be to 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’ 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 Power of Sister Tsunamis

Subduction zone megathrust earthquakes displace massive amounts of water, creating series of incredibly long waves known as tsunamis. Rather than just the surface water moving in waves, a tsunami is made up of a column that extends all the way from the ocean floor to the surface. According to the National Oceanic and Atmospheric Administration (NOAA), a tsunami wavelength from crest to crest can be hundreds of miles wide. In the deep ocean, they can travel over 500 miles per hour, slowing to around 30 mph near the shoreline.

The sister tsunami that followed on the heels of Japan’s 2011 9.1 earthquake, is estimated to have released 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.

Only 5 earthquakes have reached magnitude 9 since 1950—about 1 every 15 years. Like those shown below, the Cascadia Subduction Zone is capable of great earthquakes.

(Source: Graphic by Dan Coe, DOGAMI)

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

Forty-six of these megathrust earthquakes have occurred in the Pacific Northwest during the last 10,270-years, averaging one every 223 years. The year 2022 marks 322 years since the last major event.

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.

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.

Figure 1

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