Megathrust Earthquakes

“Civilization exists by geological consent, subject to change without notice. ”
― Will Durant

The Pacific Northwest is home to the Cascadia Subduction Zone (CSZ), a thrust fault capable of producing some of the earth’s most powerful earthquakes and tsunamis. Oregon’s Cybersecurity and Infrastructure Security Agency’s (CISA) 2021 Resiliency Assessment states:

“Along this fault, three regional tectonic plates—the Explorer, Juan de Fuca, and Gorda plates—are pulling away from the larger Pacific plate and moving toward the North American plate. At the North American plate boundary, these three regional plates are descending—or subducting—underneath the North American plate.”1

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 has some extra twists and turns. 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.

The expected impact area for a magnitude 9.0 earthquake covers 140,000 square miles — an area just slightly larger than all of Germany3. It is home to roughly 15 million people2.

(Image Credit4)

by the Incorporated Research Institutions for Seismology (IRIS)

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. All occurred along subduction zones similar to the Cascadia Subduction Zone.

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

Check out this quote regarding the 1960 subduction zone earthquake in Chile, shown in the table above, which measured a whopping magnitude 9.5.5

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.

By comparison, Oregon’s Department of Geology and Mineral Industries (DOGAMI) included a magnitude 9.4 as the largest hypothetical CSZ earthquake in its research, Variable Rupture Scenarios for Tsunami Simulations Inferred From a 10,000-Year History of Cascadia Megathrust Earthquakes.6

According to a presentation by Oregon State University professor Chris Goldfinger, “The well-known AD 1700 earthquake is thought to be Mw=9.0, yet it is only “average” in the turbidite record. There are a number of others like it in the 43-event record over 10,000 years. The largest events are T11 and T16, which is about three times the mass of the ~ M9.0 1700 turbidite.

The USGS paper, Simulated tsunami inundation for a range of Cascadia megathrust earthquake scenarios at Bandon, Oregon, USA, states the following:

The extra-large (XL) size class reflects a single event (T11) in the Holocene record of Cascadia turbidites (Table 1). T11 is the second largest turbidite by mass and has the longest post-event time interval (∼1150 yr). We rounded the interval for the extra-large scenario to 1200 yr for the purpose of modeling a hypothetical maximum earthquake with peak slip (>40 m) similar to the Mw 9.5 1960 southern Chile earthquake (Barrientos and Ward, 1990; Moreno et al., 2009).

To help visualize how large subduction zone megathrust earthquakes are, check out these quick videos comparing magnitude classes.

A magnitude 9.0 earthquake releases the equivalent energy of 480 million tons of TNT—or 30,000 strategic nuclear bombs.7 It’s massive.

It’s worth noting, however, that not all CSZ megathrust earthquakes have measured magnitude 9.0 or above. In fact, nearly half have been smaller.

Size of The Next Big One

Media outlets often report about the inevitable next “Really Big One”, but just how big will the next one be?

Scientists don’t know for sure, but GPS offers some clues. Check out this article by the American Association for the Advancement of Science which states:

“At the Cascadia subduction zone off the Pacific Northwest, for example, GPS stations on land suggest that enough strain has accumulated to drive a magnitude-9 earthquake when the fault finally ruptures. But the land-based measurements also hint that strain along the fault’s midsection, off the coast of Oregon, is being relieved by a type of harmless slip called creep. That suggests the fault could rupture in pieces, in a series of independent, smaller quakes. But without offshore measurements, scientists only see half the picture, says Harold Tobin, a geophysicist at the University of Washington in Seattle.”

Image Credit: National Park Service

Magnitude is basically a function of rupture length. The longer the ‘unzipping,’ the larger the magnitude. According to the USGS:

* M4 ~ 1 km long
* M7 ~ 40-60 km long
* M9.1 Sumatra fault ~ 100’s of km long

Based on core samples, scientists have discovered 4 primary CSZ rupture patterns shown in the DOGAMI image below.

To learn more about how magnitude is a function of rupture length, check out this KVAL interview with Chris Goldfinger.

Here is a second image with three additional segments. Earthquake “names” are listed in white lettering on the left of each segment (example, T1). As you can see, the southern section of the fault breaks in nearly every event.

Events where the entire fault ruptures (Segment A in the images above), also known as full-rips, create ∼M9 earthquakes. When a partial rupture (shown in the other segments) occurs, the result is an earthquake in the ∼M8 or high M7 range. *Of note, there is some debate about whether the three events shown in Sement B above should be considered full or partial ruptures. More information is available on Surviving Cascadia’s Likelihood of a 9.0 page.

“You can, you should, and if you’re brave enough to start, you will.”
― Stephen King

When (not if) the earthquake and tsunami happen, water, wastewater, food distribution, electricity, and natural gas will all be heavily impacted. Bridges, air transportation, rails, roadways, and seaports will be compromised. Some fire and police departments, hospitals, and schools will be heavily damaged. So will some homes. Most communication systems will be down. (See Surviving Cascadia’s Expected Impacts page for more details).

Daily life will look all but unrecognizable.

Getting ready for that eventuality takes planning! You are smart & resourceful. But being resourceful requires having access to resources. Those resources are plentiful now. Toilet paper anyone? (Hint: The resources won’t be plentiful after a major event).

Learning about the earthquake and tsunami that will hit the Pacific Northwest is flat-out scary. Striking without warning, earthquakes are some of the greatest ghost stories ever told. They are nowhere, then suddenly everywhere around us at once, unseen but for the destruction they cause.

Be Ready.

Be ready so that you don’t become a victim in need of saving. There are people (and pets!) who can’t prepare in advance the way you can. Be ready so you can help others when the time comes.

Just be ready.

Because you see, for most people, the true risk won’t be the earthquake itself. The true risk is not being ready for the earthquake’s aftermath.

As you travel along this journey seeking knowledge, as you take steps to prepare yourself and your loved ones, the fear will dissipate. It will transform into determination. Let this website serve as a guide on this journey. You’re here. That, in and of itself, is courageous. You’re taking the first step. Not all those around you will be so brave. Keep learning. Start planning. Prepare. Your loved ones will need you to be ready when the time comes.

There is a ghost on our horizon.

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