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Earthquakes: A Deep Dive

This is a supplemental article to “Earthquakes: What to know and how to prepare.”  Many of these sections are a continuation of sections from that article.  Reading that article first is highly recommended.

Thank you to Corina Forson, Brian Terbush and Jerry Thorson for taking the time to answer my questions and helping us share some great and fascinating information about earthquakes!

Why Do Earthquakes Happen?

The earth’s surface is made up of a series of ‘plates.’  These plates move at roughly the same pace as fingernails grow.  “It doesn’t seem very fast, but moving an inch a year, over 100 years, adds up to more than 8 feet,” said Terbush.  “Since there is a limited amount of space on Earth’s surface for that movement to happen, these plates are constantly running into one another or sliding past one another.  Where two surfaces of rock move past each other are called a fault.”

These plates are made of rocks, which are not always smooth.  Sometimes the plates stick together, instead of sliding past one another smoothly. “These faults are then “stuck,” by the friction between them.  Since the plates continue moving, all that distance is built up as the energy behind it as the years go on. Eventually, the force built up is enough to overcome the force of friction, and the plates become unstuck.  In that moment of unsticking, the rocks slide past one another, making up for the distance they were unable to travel all at once.  So, if they were stuck for a hundred years, while the plates continued to move at 1 inch per year, the plates would suddenly slide 100 inches past each other,” explained Terbush.

“Small Earthquakes might be the kind where two plates slide past each other an inch or two.  Many of these happen every day, and most are not even felt. [However], think about how much different it is when the ground suddenly shifts 5-6 feet at once, along several tens of miles of fault?  This is more typical of magnitude 6 to 7 earthquakes,” added Terbush.

What are Aftershocks

Several factors can cause aftershocks. The plates causing an earthquake may not slip all at once. This is not as likely, as sliding objects don’t tend to stick together as easily.  “But this might be where a Fault has built up 8 feet of movement, and the “main shock” only moves it forward 6 feet. Aftershocks may be it moving that additional 2 feet,” said Terbush.

“Aftershocks may occur on other, nearby areas of a fault,” Terbush continued. “Sometimes part of that surface will slip in an earthquake, and another part nearby, or possibly even within the middle of that fault surface will have to slip to catch up.  Earthquakes also have foreshocks sometimes.  [Foreshocks] are aftershocks that happen before the Main shock.  It is sometimes difficult to determine which was the main shock until after studying a series of earthquakes.”

Terbush explained that “Some “faults” are also not just one plane, but a system of stacked and connected faults.  An Earthquake may occur on one of these, and aftershocks may occur in others in this system as it readjusts to the change in crustal pressure.  Sometimes, they all rupture at once, in one earthquake, but this is (at least historically, since we’ve begun measuring earthquakes) rare.  In 2016, the Kaikoura Earthquake in New Zealand was a rupture of 21 different faults.”

There are wildfire, tornado, and hurricane “seasons,” but is there an “earthquake season”?

“Steve Malone, Professor Emeritus at the University of Washington likes to remind everyone that all Earthquakes that have ever occurred in Earth’s history have been within three months of an Equinox.  Other than that, there is not any evidence that earthquakes occur during a particular time of year,” said Terbush.

How Frequent Do Earthquakes Occur?

Terbush explained that “There is an established relationship between the magnitude of earthquakes and how frequently they occur.  While hundreds of Magnitude 1 and 2 Earthquakes occur around the world every day, earthquakes of magnitude 9 occur roughly once every ten years. M8’s roughly once a year, there are about 10 M7s a year, and so on by factors of 10 down the line.”

“This relationship has been roughly shown to be true based on observed and historically-cataloged earthquakes,” continued Terbush. “Again, it is important to consider that even a lower-magnitude earthquake may be catastrophic, depending where it strikes.  The 2010 earthquake in Haiti was a magnitude 7, but resulted in 250,000 deaths – more than the magnitude 9.2 Sumatra Earthquake and Tsunami in 2004.  This earthquake was so catastrophic because it was shallow, and located very close to a city with a population of 2 million, which doesn’t enforce any building codes.  Our Nisqually Earthquake caused $4 Billion in damage, and at least 400 injuries; we were incredibly lucky that no one was killed.  Other magnitude 6.8 earthquakes around the world have been catastrophic.”

What is a Subduction Zone?

Forson explains that “subduction zones are a type of fault. All faults are related to the movement of Earth’s tectonic plates. The biggest faults mark the boundary between two plates (subduction zone faults, such as Cascadia are this type. However, some subduction zones are smaller.). There are also crustal faults, where the rocks that move past each other are on the same tectonic plate (such as the Seattle fault). During an earthquake, the rock on one side of the fault suddenly slips with respect to the other side.  The fault surface can be horizontal or vertical or some arbitrary angle in between.”

Environmental Hazards of an Earthquake:

There are a number of potentials hazards caused by earthquakes.


“In general, the sandy, silty, wet soils (think river valleys) are more susceptible to liquefaction than the hard bedrock. Below the ground-water table, the pore space among sand grains is filled with water. The weight of the overlying soil mass is ordinarily supported by grain-to-grain contact. Strong shaking during a large earthquake can disrupt the grain-to-grain contact, causing a decrease in the grain support,” explained Forson.

“If strong shaking lasts long enough, the grain structure of a loose sandy soil may completely collapse,” Continued Forson. “In the extreme case where the grain support is completely lost, the pore water must bear the entire weight of the overlying soil mass. At this point, the sandy soil is liquefied and will temporarily behave as a viscous fluid.”


“Engineers can do incredible things to compact the soil, drive pilings deep enough, and construct buildings to be able to withstand the shaking,” shared Forson. “There are things you can do to reduce risk, such as limiting vulnerability and exposure near geologically hazardous areas; [for] example not building critical facilities in tsunami hazard zones.”

Forson admitted that “[adjusting] land to prevent hazards is not something I know enough about to elaborate on. It does remind me of one of my favorite quotes, [from Will Durant] “Civilization exists by geological consent, subject to change without notice.”


“A tsunami from the Cascadia Subduction Zone would impact ports and Harbors within the Salish Sea. Even a 1-3 foot tsunami projected this far inland, can cause significant damage to boats,” said Terbush. “A 1-m tsunami wave in March 2011 from the Japanese Tohoku earthquake, caused over $1 million in damage to the harbor in Crescent City, California.  It’s important to realize that a tsunami is not just an ocean wave: it’s a displacement of water, much more like a slow flash-flood, it carries a lot of force within a small amount of water.”

Will Mt. Rainier erupt after an earthquake?

There is no historical data, in the PNW or around the world, that would set a precedent for this. ”We know that the Cascadia Subduction Zone last ruptured on January 26th, 1700. However, geologic records and stories from the Native Americans living here at that time do not talk about any of the volcanoes erupting then, or shortly after,” shared Terbush.  “This is the same case with smaller earthquakes happening near volcanoes.”

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Mt. Rainier from Auburn

Terbush went on to explain that “[occasionally], an earthquake occurs close to a volcanic eruption, but for the most part, a volcano already needs to have a magma source near the surface, ready to erupt before shaking might cause it to erupt.  Magma movement within a volcano causes small earthquakes, as it forces itself through cracks, or breaks new pathways open, so looking for these earthquakes, and watching whether they are rising towards the surface is an important way to monitor a volcano’s potential activity.  So like most things in geology: it’s not impossible, but it’s certainly infrequent, and isn’t something you should assume will happen.”

“Just be aware of all your hazards, how to get information on them, and how to respond to them,” continued Terbush, “You would respond to a lahar the same way after an earthquake, that you would before one, there may just be other hazards to consider, such as damaged roadways, landslides, etc.  Know multiple evacuation routes, and have a way to receive hazard warning information, such as a NOAA Weather Radio.”

Can Earthquakes be Predicted:

No, earthquakes cannot be predicted.  However the USGS is currently seeking pilot users to assist in the development of the MyShakeAlert Early Earthquake Warning System.  Terbush shared that “Pilot Users are systems that will develop a way to cause something to happen when an EEW is received.  For example, several sewer and water districts in Washington are working with engineering companies to have electricity to their pump stations shut off in an earthquake and prevent water from flowing out of one tank. These actions will ensure at least one source of water and prevent the possibility of a pump station from catching on fire due to any electrical issues, saving that infrastructure.”

For more information on Earthquake Early Warning and/or how to become a pilot user of the ShakeAlert Earthquake Early Warning System, please contact:

Chuck Wallace – WA ShakeAlert EEW Coordinator, (360) 280-8278

Maximilian Dixon – WA State EMD Earthquake Program Manager, (253) 512-7017

Bill Steele – PNSN, Director of Outreach & Information Services, (206) 685-5880

The Big One:

Scientists know the last megathrust earthquake in the northwest (from the Cascadia Subduction Zone) occurred in January 1700.  This is known “from tsunami sand deposits from onshore and offshore on the PNW,” explained Forson.  “It is also known because the tsunami waves that resulted from the CSZ earthquake made it to Japan and they have written a record of an “orphan tsunami” (a tsunami that came without an earthquake). Tsunami modelers were able to trace the waves back to the CSZ and find the exact date of the earthquake based on that written record. They are also able to date the tsunami deposits (or the soils that surround the sand deposits) and confirm this date.”

How has climate change impacted earthquakes?

Both Forson and Terbush were unaware of any data indicating a direct impact climate change has had on earthquakes.  “That doesn’t mean there aren’t any that science hasn’t had an opportunity to discover yet,” Terbush clarified.  “100 years is pretty short in Geological time, but Earth takes a long time to respond to factors.  For example, the crust under the northern U.S. is still slowly rising from the weight of the glaciers leaving it 10-15,000 years ago.”

The Pacific Northwest Isn’t As Prepared As Japan:

Forson warns that the PNW is not nearly as prepared for a major earthquake as Japan was in 2011, leaving us at [higher] risk. “I absolutely agree [with Forson],” said Terbush.  “Infrastructure can always be hardened and think about uses of earthquake early warning!  Could a PA system in schools or other public facilities in Auburn be used to help warn students, faculty and staff to Drop, cover, and hold on, seconds in advance of an earthquake to protect them from falling objects?  Unreinforced masonry buildings can be retrofit to prevent bricks, facades, parapets and chimneys from falling onto people below them when the shaking begins: these falling objects are what cause the most injuries and deaths in an earthquake: NOT building collapse.”

What to do if you’re in an earthquake:

Forson shared the following advice for what to do in the event of an earthquake.

Survive DURING

  • Drop, Cover, then Hold On like you practiced. Drop to your hands and knees. Cover your head and neck with your arms. Hold on to any sturdy furniture until the shaking stops. Crawl only if you can reach better cover without going through an area with more debris.
  • If in bed, stay there and cover your head and neck with a pillow.
  • If inside, stay there until the shaking stops. DO NOT run outside.
  • If in a vehicle, stop in a clear area that is away from buildings, trees, overpasses, underpasses, or utility wires.
  • If you are in a high-rise building, expect fire alarms and sprinklers to go off. Do not use elevators.
  • If near slopes, cliffs, or mountains, be alert for falling rocks and landslides.


  • Expect aftershocks to follow the largest shock of an earthquake.
  • Check yourself for injury and provide assistance to others if you have training.
  • If in a damaged building, go outside and quickly move away from the building.
  • Do not enter damaged buildings.
  • If you are trapped, cover your mouth. Send a text, bang on a pipe or wall, or use a whistle instead of shouting so that rescuers can locate you.
  • If you are in an area that may experience tsunamis, go inland or to higher ground immediately after the shaking stops.
  • Save phone calls for emergencies.
  • Once safe, monitor local news reports via battery-operated radio, TV, social media, and cell phone text alerts for emergency information and instructions.
  • Use extreme caution during post-disaster clean-up of buildings and around debris. Do not attempt to remove heavy debris by yourself. Wear protective clothing, including a long-sleeved shirt, long pants, work gloves, and sturdy, thick-soled shoes during clean-up.

If you are trapped under debris

  • Ensure you do the following if you are trapped under debris during or after an earthquake:
  • Do not light a match.
  • Do not move around or kick up dust.
  • Cover your mouth with a handkerchief or clothing.
  • Tap on a pipe or wall so rescuers can locate you. Use a whistle if one is available. Shout only as a last resort. Shouting can cause you to inhale dangerous amounts of dust.
Above All, Be Prepared

Terbush warns that “a rupture on the 700-mile long Cascadia subduction zone, and the associated Tsunami, and hazards like landslides and liquefaction, will impact all critical infrastructure (energy, water/wastewater, transportation, communications), and it’s going to take a long time, and a lot of work to get those back to normal. Two weeks of preparedness is the bare minimum that we recommend.  Again, responders will be coming to repair things; but with the number of people a large earthquake will impact, and the difficulty of bringing supplies in on damaged roads, and to damaged ports, it is going to take time.  It will take even more time after that, to get things back to “normal.”  Understanding this and being prepared for it are important.”

“Overall, like with any hazard, it’s all about looking at the information that’s been learned about it, understanding what that means for you and your daily life, and understanding that this knowledge and time gives you power to do something about it,” Terbush continued.  “Think about all the things that might possibly be impacted, and take steps now to lessen any hardship those might cause.  And it might be a little bit inconvenient initially, but one you start, you’ll realize how easy it is, and when something happens, be it a short term power-outage, or two-plus weeks of lasting impacts from a catastrophic earthquake, it will suddenly seem very convenient.”


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