New study shows Cascadia Subduction Zone earthquake risk to be much higher than previously thought

New study shows Cascadia Subduction Zone earthquake risk to be much higher than previously thought

America's Pacific Northwest has a 37% chance of being hit by a magnitude 8 or larger earthquake in the next 50 years, a new study shows.

That's more than double previous estimates of a 10-15% risk. Great Subduction Zone earthquakes are the largest earthquakes in the world, and can exceed magnitude 9.0. Earthquake size is proportional to fault area, and the Cascadia Subduction Zone is a very long sloping fault that stretches from mid-Vancouver Island to Northern California.

It separates the Juan de Fuca and North American plates. Because of the very large fault area, the Cascadia Subduction Zone could produce a very large earthquake. Thermal and deformation studies indicate that the locked zone is fully locked for 60 kilometers (about 40 miles) downdip from the deformation front. Further downdip, there is a transition from fully locked to aseismic sliding.


Ocean floor is sinking below the continental plate offshore of Washington and Oregon. The North American Plate moves in a general southwest direction, overriding the oceanic plate. The Cascadia Subduction Zone is where the two plates meet. The zone separates the Juan de Fuca Plate, Explorer Plate, Gorda Plate, and North American Plate. Here, the oceanic crust of the Pacific Ocean has been sinking beneath the continent for about 200 million years, and currently does so at a rate of approximately 40 mm/yr. Major cities affected by a disturbance in this subduction zone would include Vancouver and Victoria, British Columbia; Seattle, Washington; Portland, Oregon; and Sacramento, California.

Cascadia subduction zone

Area of the Cascadia subduction zone (USGS)

Written by researchers at Oregon State University, and published online by the U.S. Geological Survey, the study concludes that there is a 40 percent chance of a major earthquake in the Coos Bay, Ore., region during the next 50 years. And that earthquake could approach the intensity of the Tohoku quake that devastated Japan in March of 2011. The publication of the peer-reviewed analysis may do more than raise awareness of earthquake hazards and risks, experts say. The actuarial table and history of earthquake strength and frequency may eventually lead to an update in the state’s building codes. Geologists and civil engineers have broadly determined that the Pacific Northwest region is not well prepared for such a colossal earthquake. The tsunami produced may reach heights of approximately 30 meters (100 ft).

A comprehensive 13-year analysis of the Cascadia Subduction Zone confirms that the region has had numerous earthquakes over the past 10,000 years, and suggests that the southern Oregon coast may be most vulnerable based on recurrence frequency. Tectonic processes active in the Cascadia subduction zone region include accretion, subduction, deep earthquakes, and active volcanism that has included such notable eruptions as Mount Mazama (Crater Lake) about 7,500 years ago, Mount Meager about 2,350 years ago and Mount St. Helens in 1980. Turbidite systems along the continental margin of Cascadia Basin from Vancouver Island, Canada, to Cape Mendocino, California, United States, have been investigated with swath bathymetry; newly collected and archive piston, gravity, kasten, and box cores; and accelerator mass spectrometry radiocarbon dates.

Cascadia

Cascadia earthquake sources (USGS)

According to Chris Goldfinger, a professor in OSU's College of Earth, Ocean, and Atmospheric Sciences and lead author of the study, the southern margin of Cascadia has a much higher recurrence level for major earthquakes than the northern end and it is overdue for a rupture. However, that doesn't mean that an earthquake couldn't strike first along the northern half, from Newport, Oregon, to Vancouver Island. Major earthquakes tend to strike more frequently along the southern end - every 240 years or so - and it has been longer than that since it last happened. The probability for an earthquake on the southern part of the fault is more than double that of the northern end.

The Goldfinger-led study took four years to complete and is based on 13 years of research. At 184 pages, it is the most comprehensive overview ever written of the Cascadia Subduction Zone, a region off the Northwest coast where the Juan de Fuca tectonic plate is being subducted beneath the continent. Once thought to be a continuous fault line, Cascadia is now known to be at least partially segmented. This segmentation is reflected in the region's earthquake history, Goldfinger noted.

The Juan de Fuca Plate is pushing deep under the North American Plate. The colliding edges of these plates are locked, one plate pressed into the other. As the plates press and move, stress builds up -- until the lock breaks. The plates slip suddenly, causing a subduction zone earthquake. (Credit: ECY/Washington Coast)

Over the past 10,000 years, there have been 19 earthquakes that extended along most of the margin, stretching from southern Vancouver Island to the Oregon-California border. These would typically be of a magnitude from about 8.7 to 9.2 - really huge earthquakes. Researchers have also determined that there have been 22 additional earthquakes that involved just the southern end of the fault. The last mega-earthquake to strike the Pacific Northwest occurred on Jan. 26, 1700. Written records in Japan document how an ensuing tsunami destroyed that year's rice crop stored in warehouses.

The clock is ticking on when a major earthquake will next strike, Jay Patton, an OSU doctoral student and a co-author on the study.
Cascadia
Cascadia margin turbidite canyons, channels and 1999-2002 core locations. Major canyon/channel systems are outlined in blue. “PC” = Piston Core; “BC” = Box Core; “KC = Kasten core; “GC” = Gravity core; “TC” = Trigger core. Trigger cores omitted for clarity. Inset of Effingham Inlet shows collection site of Pacific Geoscience Centre (PGC) collected piston cores. (Click for larger image)

When a major offshore earthquake occurs, the disturbance causes mud and sand to begin streaming down the continental margins and into the undersea canyons. Coarse sediments called turbidites run out onto the abyssal plain; these sediments stand out distinctly from the fine particulate matter that accumulates on a regular basis between major tectonic events. By dating the fine particles through carbon-14 analysis and other methods, Goldfinger and colleagues can estimate with a great deal of accuracy when major earthquakes have occurred over the past 10,000 years.

Going back further than 10,000 years has been difficult because the sea level used to be lower and West Coast rivers emptied directly into offshore canyons. Because of that, it is difficult to distinguish between storm debris and earthquake turbidites.

Cascadia

SeaMarc 1A sidescan mosaic of the Daisy bank Failt Zone on the upper slope off central Oregon. Sinistral motion and a left bend at center have opend a small pull-apart basin. drag folding with a sinistal motion sense visible at right.

The turbidite data matches up almost perfectly with the tsunami record that goes back about 3,500 years. Tsunamis don't always leave a signature, but those that do through coastal subsidence or marsh deposits coincide quite well with the earthquake history.

Unlike Japan, which has frequent earthquakes and thus is more culturally prepared for them, the Pacific Northwest have not had a mega-quake since European settlement. And since there is no culture of earthquakes, there is no culture of preparedness.

When it hits 

Violent shaking will last at least two minutes. The ocean floor will drop, most likely creating a tsunami that will strike long stretches of the coast. The central Olympic peninsula will suddenly rise, while some coastal areas will sink below sea level.

If an earthquake occurs at the coast

  • Drop, cover and hold. Get under a sturdy object and hold on. Watch for falling objects.
  • As soon as the shaking is over, move to high ground or inland. Do not wait for an official warning.
  • Stay away from the coast. Waves may continue to arrive for hours.
  • Listen to your local radio station for an official "All Clear" notice before returning to the coastal area.
  • Be alert for aftershocks.

 

Protect yourself and your family 

  • Develop a family disaster plan. Everyone needs to know what to do on their own to protect themselves from an earthquake.
  • Be familiar with local Emergency Management earthquake and tsunami plans. Know where to go to survive a tsunami.
  • Be prepared to survive on your own for a minimum of three days.
  • Prepare a disaster supply kit for your home, automobile and work.
  • Take a first aid course and learn survival skills. Knowledge is your greatest defense against potential disaster.

Report PDF (27 MB)

Comments

Cindy 3 years ago

I would like to see how Sacramento, CA.
will be affected.

S Jason Metcalf (@Cindy) 2 years ago

Looking at moving to Sacramento from Cape Cod. Looks like there is a huge potential for a big quake, tsunami affecting Sacramento. No report says what might happen to Sacramento.

tom mallard 4 years ago

I've done a lot of research on Japan's plate-boundary release & tsunamis and see that there are two critical places that can replicate the unusual tsunami that hit the NE coast.

It was caused by a sediment layer above the boundary with splay faults so when the release happened it accelerated those as a separate mass from the main edge to jump what was a 2m wave height 20km at sea to 7m/23ft, this doubling approaching shore.

The Nitinat Fan and Astoria Fan could also do that and throw a really large volume tsunami into the Straits of Juan de Fuca or the Columbia River, consider 15m amplified entering the Straits, which it fills for a solid hour to get to Admiralty Inlet and any subsequent tsunamis will just add height on a flood tide, it's all downhill to South Sound.

This puts some 15km³ to divide up against Whidbey Island if 5km³ enters Admiralty that's a static 2.8m/5ft with triple that possible worst-case steep-wave 15ft of surge, total wipeout to the entire area's economy.

On the coast any flat land needs escape towers for beach areas spaced so people can get to safety on a large draw-down preceding the first wave, they serve as lookouts for tourists and Japan reacted by building these first in towns now rebuilding.

On hillsides you can get a 40m/130ft run up into a small bay the worst case from the 7m areas in Japan. Their seawalls were high enough for a normal tsunami.

Consider with such a release odds go up for one on this locked plate, a basic visualization of that fact to consider this video; 9:03; https://www.youtube.com/watch?v=Ar0c1vOLcfs

My estimate is a total wipe-out to Puget Sound with a hypocenter release off Vancouver Island and a possibility of a sediment layer able to replicate the freak tsunami in Japan by separating from the strata to cause a much steeper wave.

Kesennuma got 7 tsunamis, in a tidal basin like Puget Sound including Hood Canal the water has no where to go on a flood tide. This covers it in 17-minutes: https://www.youtube.com/watch?v=6xOFNgpLzVE

Larz 5 years ago

Get Prepared...

Lynn 5 years ago

hi, I live in Keizer, OR and I have many stories. One of them being the coastline will fall into the sea. I would like to know what to expect and will the water from Tsunamis flood inland rivers and streams causing flooding? Thankyou

Fargo (@Lynn) 5 years ago

what to expect? How 'bout having your living room full of aquatic life...

Larz 5 years ago

Are your kids, relatives prepared? PRAY, it is...

Larz 5 years ago

Nepal 8.1 very large quake. Swarms in Oregon, a quake is coming. Quakes jump faults and both together could activate Volcanoes. The Pacific Ring of Fire has that name for a reason. GLTA

Larz 5 years ago

Cascadia has gone Silent. That in itself should be a wake up call. Now no one knows for sure when the fault will break but certainty is just around the corner. Japan was bad, this will be much worse. Preparedness is a must for you and loved ones. Store water and can goods. A first aid kit, and blankets etc. My senses are heightened. Be safe know your surroundings. Hopefully only a fraction of fault breaks. Stop complaining about retrofits and seek knowledge. If Cascadia does not break in your lifetime at least prepare your kids-family cause every day the eventful day will come!

Larz 5 years ago

Watchers are Angels according to Biblical text. They are active and reach us through our ears. Suggestions to steer us in one direction or another depending on whether they are fallen angels or of the righteous kind.

Larz 5 years ago

1/2 way through a 32 year earth Volcanism. Danger approaches, there is a 0-16 year window making urgency to be prepared even more prevalent. 100 ft waves will be much higher at shore and remember Tsunami's will rap around. 9.2 is only an estimate of severity could be higher or slightly lower. It's coming folks. This is not a scare tactic. If you get prepared and it does not happen in your lifetime...your family will be prepared cause it surely will come! How difficult is it to know you can save lives? One of the hidden dangers are all the Volcanoes in this 600 mile + zone. Plus several faults that just might be triggered. I will stop here because the rest is just too much to comprehend. Preparedness is your only chance to survive. Prayer is also a good thing no matter your religious or non-belief. Be safe and don't act like research is the enemy here. Knowledge is your friend!

Larz 7 years ago

Movement is underfoot, severe consequence will push inevitable forward. Use inner prayer, a life savior to calm beast. Minutes pass yet rumble peaks. Not one not two but many seen rampant. Nature spent due time relevant exposure. Faith runs deep to quell fears saught norm. Trust self not neighbor, not anyone vacant. 2013 revs head towards inevitability becomb.

facepalm 7 years ago

@MsAnnThrope How very educational and completely unrelated to the Cascadia Subduction Zone. Thank you for you pointless political rant, now if you please shut up.

MsAnnThrope 8 years ago

Sorry for the cynicism, however, I find the timing, and therefore, the accuracy of the findings suspect. It is a well known fact that the level of corruption in Portland specifically, and Oregon State in general, is out of control. Developer/Gov't collusion in the raping of the taxpayer is just barely covered up and the hubris of this cabal is evident in the funding of "studies" published by "academic experts". For example - there is a well funded push to levy a $500million tax on the homeowners of Portland to fund "upgrades" in certain schools. It was defeated once and has resurfaced and is a ballot measure in the upcoming Nov voting. The supporters learned from their defeat and changed their tactics from jobs creation, which has been shown to be utterly false, to a chicken little "sky is falling" scare tactic. And this "study" is the primary tool of choice. I'm sorry, but corrupt politics and the promise of a fat paycheck has usurped the ethics of State funded inquiry. This comes from a City/State that has spent $150million+ on a bridge without lifting a shovel. We all know there will be earthquakes, and preparedness is prudent, however, to use, and therefore diminish the value of objective scientific study - to line the pockets of billionaire developers is wrong - especially in the 21st century where internet education is proving to be a viable alternative to brick and mortar. Ms Ann Thrope, Portland OR

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