Earthquakes in the PNW
Downtown Seattle, seen from Alki Point in West Seattle (King County). Seattle is at high risk for a large earthquake in the future.
Introduction
One marquee phenomenon associated with subduction zones are earthquakes. In Washington, the risk for a devastating earthquake is some of the highest in the country, and this is due entirely to the geologic setting in which Washington and the PNW is situated. As aforementioned, the PNW sits on a Subduction Zone, which we now know is a place where two tectonic plates collide. Before diving into the types of earthquakes that rock the PNW and the risk of a deadly earthquake in Washington, let’s define what exactly an earthquake is, how it works, and the scale at which earthquake strength is measured.
What is an Earthquake and How Do They Happen?
An earthquake is generally defined as the shaking of the Earth’s surface in response to a sudden release of energy in the Earth’s lithosphere that creates seismic waves. The lithosphere is simply the outermost, rocky layer of the Earth. Most earthquakes occur on faults- planar fractures or discontinuities between two blocks of rock where there has been displacement due to rock mass movement. Earthquakes and faults vary in size, and the size of an earthquake and size of the fault at which the earthquake occurred are directly proportional. Earthquakes range in size from so small that they can’t be felt to events that can destroy entire cities. Similarly, faults range in size from tiny breaks in a rock to plate boundaries thousands of miles long. Large earthquakes can only occur on large faults, as one of the determining factors in the strength of an earthquake is how much of a fault has ruptured. The longer the fault, the more potential rupture.
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To further complicate things, the type of fault is also a determining factor in the potential strength of an earthquake. There are three main types of faults- normal, reverse (thrust), and strike-slip (transform) faults. Normal faults are dip-slip faults in which the block above the fault has moved downward relative to the block below the fault. Reverse, or thrust faults are dip-slip faults in which the block above the fault has moved upwards relative to the block below the fault. Lastly, strike-slip, or transform faults are faults where neither blocks move up or down, but slide past each other laterally. Dip-Slip Faults are faults that move up and down. Different types of plate boundaries, as discussed earlier, are exemplified by different types of faults. Divergent Plate Boundaries, where the crust is spreading apart, are characterized by normal faults. Transform Plate Boundaries, where the crust is sliding past one another, are characterized by strike-slip or transform faults. Lastly, convergent plate boundaries and subduction zones are exemplified by reverse/thrust faults as both plates smash into each other. Due to the immense mass of each tectonic plate and the attitude at which they interact (converging into each other), earthquakes on reverse/thrust faults associated with subduction zones are the most powerful types of earthquakes on the planet. All of the world’s strongest earthquakes have occurred on subduction zones, including the strongest earthquake ever recorded (Magnitude 9.5, 1960, Chile), and one of the deadliest earthquakes ever (Magnitude 9.2, 2004, Sumatra, 230,000 casualties).
Though tectonic plates are always moving, motion is not always seamless and tectonic plates and fault planes often get stuck on eachother, as the Earth’s crust is solid and brittle. When these plates/planes get stuck on each other, they build tension, stress, and strain as the molten/ductile part of these plates continue to move. When the tension, stress, and strain has reached a critical point on the fault and the fault suddenly slips is when an earthquake occurs.
Map of the Cascadia Subduction Zone, drawn by Solomon Feinstein
Diagram of different types of faults, credit to National Park Service (NPS).
Measuring Earthquake Strength & Location
When an earthquake occurs, terms such as magnitude, epicenter, and hypocenter are often thrown around. For starters, an earthquake’s magnitude is simply the size of the earthquake. Since 1979, earthquakes have been measured on the Moment Magnitude Scale. Most people think of the Richter Scale when they think of an earthquake’s magnitude, but the Richter Scale is outdated and relatively inaccurate in most conditions. The Moment Magnitude Scale, however, is the gold standard for accurate seismic energy measurement. It is a logarithmic scale and it measures an earthquake’s seismic moment- taking shear modulus (ratio of shear stress to shear strain) of rocks involved in the earthquake [measured in newtons/square meter (pascals)], area of rupture along the involved fault (in square meters), and average slip of the involved fault (in meters). This scale records earthquakes from 1-10 to 1 decimal place (tenths), and magnitudes of specific earthquakes are denoted as “Mw” on this scale. For example, the 2019 Ridgecrest Earthquake, which I personally experienced while I was living in Las Vegas, was a Mw 7.1 on the Moment Magnitude Scale. As the scale is logarithmic, the strength of earthquakes greatly increases as you go up in the scale. A magnitude 6.0 earthquake is 31.622 times more powerful than a 5.0. A 7.0 is 1000x more powerful than a 5.0. An 8.0 is 31,620x more powerful than a 5.0. Lastly, a Magnitude 9.0 earthquake is 999,902x more powerful than a magnitude 5.0. This is difficult to grasp for many people, and the illusion that a magnitude 9.0 and a 5.0 aren’t that different due to them only being “4 apart” is a dangerous one. To further illuminate this, a magnitude 9.0 earthquake releases as much energy as 31 billion tons of TNT, or 2 million Hiroshima atomic bombs.
Though the Moment Magnitude Scale measures the amount of energy released in an earthquake, it’s not the only scale of measurement for earthquake strength. The Modified Mercalli Intensity Scale (MMI) also measures the strength of an earthquake, but it does so with regard to shaking intensity rather than raw energy released. The MMI is utilized by estimating shaking intensity at specific locations during an earthquake, taking its effects on people, objects, and buildings into account. The MMI goes to 10, and anything above a 6 is considered “high intensity”, causing damage to buildings.
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The terms hypocenter and epicenter are also important in earthquake science. The hypocenter, also known as the focus, is the point inside the earth where the earthquake occurs. The epicenter is the geographic location directly above the hypocenter. For example, the 2001 Nisqually Earthquake was a magnitude 6.8 earthquake with an epicenter near Olympia, WA, almost 200 miles from the Cascadia Subduction Zone, though its hypocenter was actually on the boundary of the Juan De Fuca and North American Plates at a depth of about 35 miles in the earth.
Diagram showing the difference between the epicenter and hypocenter (focus) of an earthquake, Difference Between . com
Types of Earthquakes in Washington & the PNW
Subduction Zones are complex geological areas, and the Cascadia Subduction Zone is no exception. Three main types of earthquakes occur in Washington and the greater PNW, each type distinct in strength, geophysics, and fault rupture and type. These three types of major earthquakes in the CSZ include Megathrust Earthquakes, Deep Intraplate Earthquakes, and Crustal Faulting Earthquakes.
Diagram of different types of earthquakes in the Cascadia Subduction Zone, drawn by Solomon Feinstein
Megathrust Earthquakes
Megathrust Earthquakes are the largest types of earthquakes on the planet. They occur when the reverse/thrust fault that forms the contact between the two plates in a subduction zone ruptures, and can often be larger than magnitude 9.0. All magnitude 8.5+ earthquakes that have ever occurred have been megathrust earthquakes. Some infamous megathrust earthquakes include the 2011 Tohoku Earthquake (Japan, Mw 9.1), the 2004 Sumatra Earthquake (Indonesia, Mw 9.2), the 1964 Alaska Earthquake (USA, Mw 9.2), and the 1960 Valdivia Earthquake (Chile, Mw 9.5). Most subduction zones are located under the sea, and as such, these megathrust earthquakes produce tsunamis (a series of waves in a large body of water caused by displacement of a large volume of water), as the tectonic plates suddenly slip in an earthquake.
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The Cascadia Subduction Zone is prone to megathrust earthquakes. The fault between the North American and Juan De Fuca Plates is a thrust fault that generally moves at a rate of 40mm/yr, though the brittle part of the fault has been stuck since 1700. The subduction of the Juan de Fuca Plate under the North American Plate is not seamless, and as the brittle parts of the plate grind past one another, tension builds as the plates lock. Stress is eventually relieved in the form of an earthquake. The region has not had a major megathrust earthquake since 1700, when a Magnitude 9.0 struck, rocking the Pacific Northwest and sending a tsunami across the Pacific. In Japan, this was known as the infamous “minashigo tsunami”- “orphan tsunami”, since it hit Japan without an apparent earthquake. Local native tribes recall Thunderbird and Whale battling during the night of the earthquake, violently shaking the earth and causing tsunamis. Tsunami waves as high as 130’ pounded the Washington and Oregon coasts. During periods of inactivity (when there are no major earthquakes), the North American continent on the coast of the PNW bulges, since the North American Plate moves westward but gets stuck at the plate boundary at the CSZ. As such, the continent has actually been deformed and raised higher than it “should be” at the coast, and when the stress is too much to handle for the fault at the CSZ and a megathrust earthquake occurs, the coast experiences rapid subsidence (rapid sinking of the ground) as the continent slips back into place during the earthquake. This occurred during the 1700 earthquake as North America sank by 36’.
Map of the shaking intensity of a hypothetical Mw 9.3 earthquake on the Cascadia Subduction Zone, using the Modified Mercalli Intensity Scale (MMI), USGS. This would be a full rupture of the fault and would be felt strongly across the entirety of the west coast of the contiguous US.
Large megathrust earthquakes occur every 200-500 years in Cascadia, and the subduction zone has been eerily silent since it unleashed hell in 1700. Geophysicists calculate that there is a 37% chance that the Cascadia Subduction Zone unleashes a Magnitude 8.0+ earthquake in the next 50 years. The probability for a 9.0+ earthquake is about 10-14% in the same time frame. An earthquake in the 8’s would likely be a rupture of one segment of the subduction zone, while a 9.0+ would be a complete rupture of the fault.
Deep Intraplate Earthquakes
The most common type of earthquake that occurs in Washington is known as a Deep Intraplate Earthquake. These earthquakes occur deep in the subduction zone where the subducting plate experiences significant changes in temperature and pressure, on normal faults branching from the main subduction zone. These tremors occur every 30 years or so, and notable past deep intraplate earthquakes that have occurred in the Puget Sound Area include ones in 1949, 1965, and the famous 2001 Nisqually Earthquake. The 2001 Nisqually earthquake was a Mw 6.8 earthquake with its epicenter near Olympia. Though it was relatively large and its epicenter was located within the greater Seattle-Tacoma Megapolis, it was not immensely damaging due to the fact that its hypocenter was very deep (57 km / 35 miles). With that being said, maximum shaking was recorded as an 8 on the Modified Mercalli Scale (MMI), which is categorized as “severe”. The earthquake lasted between 20-30 seconds and 400 people were injured, 1 died (due to a heart attack), and total damage was estimated between $1-4 billion. It was a good wake up call that Seattle is indeed susceptible and prone to earthquakes, and it spurred the decommissioning and demolition of the Alaskan Way Viaduct in downtown Seattle, which was badly damaged due to this earthquake. Thousands of unreinforced masonry buildings were damaged in downtown Seattle and beyond, spurring widespread seismic retrofitting in the greater Seattle-Tacoma area. Geologists calculate an 84% chance that another deep intraplate earthquake of magnitude 6.5 or greater will strike Seattle within the next 50 years.
Crustal Faulting Earthquakes
The third main type of earthquake that western Washington experiences in relation to the Cascadia Subduction Zone are Crustal Faulting Earthquakes, also referred to as Shallow Crust Earthquakes. Similar to megathrust earthquakes, these earthquakes can be quite damaging, albeit on a more local scale than the aforementioned megathrust events. Crustal faulting earthquakes are shallow earthquakes that occur on thrust faults within the fore-arc basins of the CSZ, usually of magnitude 6.5-7.5 at their largest. Knowing what the hypocenter or focus of an earthquake is is very important in understanding why these types of earthquakes are so damaging, despite their smaller size than megathrust events. The reason why these earthquakes are so damaging is because they do not occur deep in the Earth’s crust- they occur on very shallow faults, close to the surface of the Earth. As such, there is less substrate for the seismic waves to travel through when these types of earthquakes occur, so ground motion is much more intense and strong. Moreover, the locations of the major faults that generate these types of earthquakes are located right underneath densely populated areas. The Seattle Fault is perhaps the most infamous example of this, stretching from the Kitsap Peninsula to Fall City, parallelling I-90 and running right underneath the heavily populated areas of downtown Seattle, Bellevue, and Issaquah, as well as Bainbridge Island and northern Bremerton. The South Whidbey Island Fault is another such fault that is culpable of damaging shallow crustal earthquakes, running from just south of Victoria, British Columbia to the Cascade Range near North Bend.
Map of the shaking intensity of the 2001 Nisqually Earthquake (Mw 6.8) using the Modified Mercalli Intensity Scale (MMI), USGS
Map of the shaking intensity of a hypothetical Mw 7.2 earthquake on the Seattle Fault, using the Modified Mercalli Intensity Scale (MMI), USGS
An earthquake on either of these shallow thrust faults would be absolutely devastating to the greater Seattle-Tacoma Megapolis, though would not be as widespread as the infamous “Big One” on the Cascadia Subduction Zone. As aforementioned, the shallow depth of the hypothetical hypocenter of one of these quakes would make for very intense shaking and widespread destruction throughout Seattle, Bellevue, and surrounding municipalities. The last time the Seattle Fault ruptured, a Mw 7.5 earthquake struck near Alki Point in West Seattle in the year 900 AD, generating a tsunami in the Puget Sound and absolutely annihilating Native American villages in the area. The local tribes have oral histories of these seismic events. Recurrence rates of earthquakes on these shallow crustal faults is not known with great confidence, though geologists estimate a 5% chance of a magnitude 6.5 or greater earthquake to strike the Seattle Fault within the next 50 years.
Is the Pacific Northwest Prepared for "The Big One"?
Given the sheer strength of earthquakes in subduction zones, the Pacific Northwest’s widespread lack of seismically conscious engineering, the geography of the PNW, and human experiences with past earthquakes, I postulate that major PNW cities, including Seattle and Portland, are far from prepared for the “Big One” (Cascadia Megathrust Quake). FEMA estimates that the rupture of the Cascadia Subduction Zone will be the deadliest natural disaster in the history of the United States, with at least tens of thousands of casualties.
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A complete rupture on the Cascadia Subduction Zone would produce one of the largest earthquakes ever recorded. Geologists estimate a complete rupture would produce an earthquake between 8.7-9.3 on the Moment Magnitude Scale, unleashing an unimaginable amount of energy and destruction. To attempt to illuminate the sheer amount of energy involved in megathrust earthquakes, a Magnitude 9.0 earthquake releases as much energy as 2 million Hiroshima atomic bombs. Intense shaking will commence for at least five minutes, and possibly as long as 10 minutes in the entirety of the region, from California up to Canada and everywhere in between. Due to the fact that much of the ground that Seattle and Portland is built on is highly moist glacial till and alluvium, rather than bedrock, soil liquefaction is a huge concern, effectively rendering much of the ground as quicksand. Additionally, several landslides will also be triggered from this earthquake, leveling the areas immediately affected by them.
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The lack of seismically conscious engineering in the Pacific Northwest is alarming. The earthquake threat for cities such as Seattle and Portland was not widely known and accepted until the 1990’s, and as such, buildings built before then were not built to code, though newer buildings have been constructed to withstand a megathrust earthquake. Many buildings built before earthquake code have not been seismically retrofitted, spelling disaster for their structural integrity when the earthquake occurs. Thousands of buildings and structures in Seattle, Portland, and elsewhere in the PNW will likely collapse when this earthquake occurs, and utilities such as water, electricity, and gas will likely be severed for months. Fires will occur afterwards, and a tsunami as high as 120’ will completely annihilate coastal communities. Buildings will collapse, bridges will fail, and engineering marvels will be reduced to rubble.
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Though the earthquake itself is going to be deadly, the aftermath of the quake is perhaps the greatest area of concern, as utilities will be severed for months. Many people who survived the quake itself will die in the aftermath of the quake as they are stranded without food or water, and due to the geography of the Pacific Northwest, it will be exceedingly difficult to bring humanitarian aid and supplies to everyone in need of them that survives the quake. Washington and Oregon alone have over 11 million residents, most of whom live west of the Cascade Range, where the damage of the earthquake will be greatest. Only three major highways connect Seattle to the rest of the US (I-90, I-5, US-2), and only three connect Portland to the rest of the US as well (I-84, I-5, US-26). If any bridges on any of these highways collapse, which they most likely will, Seattle and Portland will be marooned from the rest of the United States from a ground transportation standpoint. Moreover, the Portland International Airport (PDX) is built in a zone of liquefaction near the Columbia River, rendering it useless if it succumbs to liquefaction during the earthquake. It will be exceedingly difficult to get supplies for all 11 million people during the aftermath of the earthquake.
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From a scientific standpoint, there have been several earthquakes in recent memory that are comparable to what the “Big One” will be, at least in terms of seismic strength and probable societal effect. There are two main comparable earthquakes that I would like to discuss in this section, and these include the 2004 Sumatra Earthquake and the 2011 Tohoku (Japan) Earthquake. Both events occurred in the same tectonic setting as Cascadia (submarine thrust fault in a subduction zone), and both were of comparable size to what geologists estimate the “Big One” to be in the PNW. Sumatra’s was a Magnitude 9.1-9.3 (which I will simply state as a 9.2), and Japan’s was a 9.0-9.2 (which I will simply state as a 9.1). In Sumatra, the earthquake lasted ten minutes. Ten minutes. Imagine ten minutes of intense shaking. It’s mind-boggling. Over 230,000 people died and 90,000 buildings were destroyed in the 2004 Sumatra earthquake, and a tsunami as high as 167’ completely plowed everything in its path, rushing as far as three miles inland in some localities in the Indian Ocean. In 2011, a Magnitude 9.1 earthquake struck off the coast of Sendai, Japan, and intensely shook the islands for over five minutes. A tsunami as high as 128’ plowed Japan, killing 20,000 and causing the single worst nuclear disaster in recent history, the infamous Fukushima-Daiichi disaster. Now, imagine all of this happening in the Pacific Northwest. Imagine intense shaking in Seattle, Portland, and elsewhere lasting over five minutes. Japan was lucky in that not many buildings collapsed due to Japan’s widespread implementation of seismically conscious engineering. The PNW is not so lucky. Imagine buildings throughout Seattle and Portland crumbling to their foundations. People trapped inside of those buildings won’t be so lucky. Imagine bridges collapsing and the ground turning to quicksand, swallowing up everything on it. Imagine coastal cities such as Aberdeen, Ocean Shores, Astoria, Seaside, Newport, and Eureka reduced to rubble due to a 100+’ tsunami plowing everything in its path on the coast. That will be the reality.
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Though this all sounds apocalyptic, it is honestly just the reality in my estimation. In Turkey, over 60,000 have died in the most recent earthquake that hit in February 2023. That earthquake was a Mw 7.8. As aforementioned, many buildings in the PNW are just as susceptible to collapse as the ones in Turkey, because they were built before an earthquake code was enacted in the region. Given that many people in the PNW don’t take the risk of this disaster seriously, that few people have plans and kits ready for this disaster, and that many buildings and structures either aren’t built to code or haven’t been seismically retrofitted, I don’t have a very positive outlook on the survivability of western Washington and Oregon when the Cascadia Subduction Zone inevitably ruptures. I strongly believe that people have been downplaying the risk of the “Big One” hitting the PNW, and I wouldn’t want to be anywhere near here during that earthquake. Most people don’t understand just how huge a magnitude 9 earthquake is, and unless you’ve personally experienced one, it’s understandable to not be able to wrap your head around it. One spokesperson for FEMA was famously quoted as saying “Our operating assumption is that everything west of Interstate 5 will be toast”.
Seattle & Mt. Rainier from Kerry Park
Portland from Burnside Bridge
Seattle & Mt. Rainier from Columbia Center
I-84 is a major artery connecting Portland to the rest of the US.
Bellevue from Bellevue Downtown Park
One of 90,000 collapsed buildings in the 2004 Great Sumatran Earthquake (9.2), image taken from NPR.
A ship ended up on top of a building due to the tsunami in the 2011 Magnitude 9.1 Tohoku Earthquake of Japan (New Statesman).
Tsunami waves as high as 128' inundated coastal Japan following the Great Tohoku Earthquake of 2011 (9.1).
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This image is from Minato Bay. The red object is a lighthouse. This is right before the tsunami hit. I screenshotted this from a youtube video, which can be seen here:
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The same sea wall and lighthouse from the previous picture, after the tsunami hit.
A tsunami wave 100' (30 m) hitting Kesennuma immediately following the earthquake. Screenshot was from this video:
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These three images (2 above, 1 to the left) are the 2011 Japan tsunami inundating Minami-Sanriku. Screenshotted from this video:
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Surviving "The Big One"
Now that we’ve gotten all the doom and gloom out of the way, what can you do to survive a full rupture on the Cascadia Subduction Zone and be prepared for it? Well, truthfully, there are a few ways to do this. Step one would be to download the MyShake App. The MyShake App is an app that can alarm you when an earthquake occurs, and it has been implemented in California, Oregon, and Washington. It could give you up to 90 seconds of warning when an earthquake hits, and those 90 seconds could be critical in saving your life. It can warn you because earthquakes emit two main types of waves, P Waves and S Waves. P Waves travel much faster than S Waves and are not felt by people, though they are detected on seismographs. When the P Waves from a megathrust earthquake are initially read, MyShake will alert you in similar fashion to an Amber Alert on your phone, giving you valuable seconds to do what you need to do in about a minute to get ready for the earthquake. If you’re in a building, find the most sturdy table or desk or bed and get under it. Step Two would be to have a bug-out kit, complete with food, water, and supplies. Many websites will tell you to have enough supplies for a few weeks, but my personal opinion is that it would be wiser to have enough supplies for a few months, though in full disclosure, that could just be the neurotic pessimist in me. Basically, whatever you would need to go camping, have in your kit. Food, water, water purification, tools, warm clothes, bedding, fuel, camp stove, walkie talkies, possibly weapons, etc, etc. I would keep it in an accessible place that hopefully won’t collapse when the earthquake hits. I personally keep my supplies in my car. Moreover, I would have a bug-out plan if you have a family of where to meet, what to do, etc, etc. The possibility that you’re both at work when the quake hits is very real. Thirdly, and this is the one that you have the least control over, I would just pray you get lucky. If you live in a building that will definitely collapse, hopefully you aren’t home. Hopefully you aren’t stuck in traffic under an overpass. Hopefully you aren’t on a bridge that will collapse. Hopefully you’re in the middle of a large field on sturdy ground with few tall objects that could fall on you. And if you live on the coast, as soon as that shaking is over and you can stand up again, run for the hills like your life depends on it. Because it does.
MyShake Shake Alert when an earthquake in your vicinity greater than magnitude 4.5 occurs.
"Home Base" function on MyShake.
MyShake map of recent earthquakes in the area. Only earthquakes stronger than magnitude 3.5 are shown on the map.
As you can see, the Cascadia Subduction Zone is relatively quiet in comparison to the rest of the Pacific Ring of Fire.