The Really Big One—An earthquake will destroy a sizable portion of the coastal Northwest. (but when?

[OKIsItJustMe]

http://www.newyorker.com/magazine/2015/07/20/the-really-big-one

[font face=Serif]Annals of Seismology | July 20, 2015 Issue

[font size=5]The Really Big One[/font]
[font size=4]An earthquake will destroy a sizable portion of the coastal Northwest. The question is when.[/font]

By Kathryn Schulz

[font size=3]…

Most people in the United States know just one fault line by name: the San Andreas, which runs nearly the length of California and is perpetually rumored to be on the verge of unleashing “the big one.” That rumor is misleading, no matter what the San Andreas ever does. Every fault line has an upper limit to its potency, determined by its length and width, and by how far it can slip. For the San Andreas, one of the most extensively studied and best understood fault lines in the world, that upper limit is roughly an 8.2—a powerful earthquake, but, because the Richter scale is logarithmic, only six per cent as strong as the 2011 event in Japan.

Just north of the San Andreas, however, lies another fault line. Known as the Cascadia subduction zone, it runs for seven hundred miles off the coast of the Pacific Northwest, beginning near Cape Mendocino, California, continuing along Oregon and Washington, and terminating around Vancouver Island, Canada. The “Cascadia” part of its name comes from the Cascade Range, a chain of volcanic mountains that follow the same course a hundred or so miles inland. The “subduction zone” part refers to a region of the planet where one tectonic plate is sliding underneath (subducting) another. Tectonic plates are those slabs of mantle and crust that, in their epochs-long drift, rearrange the earth’s continents and oceans. Most of the time, their movement is slow, harmless, and all but undetectable. Occasionally, at the borders where they meet, it is not.

…

In the Pacific Northwest, everything west of Interstate 5 covers some hundred and forty thousand square miles, including Seattle, Tacoma, Portland, Eugene, Salem (the capital city of Oregon), Olympia (the capital of Washington), and some seven million people. When the next full-margin rupture happens, that region will suffer the worst natural disaster in the history of North America. Roughly three thousand people died in San Francisco’s 1906 earthquake. Almost two thousand died in Hurricane Katrina. Almost three hundred died in Hurricane Sandy. FEMA projects that nearly thirteen thousand people will die in the Cascadia earthquake and tsunami. Another twenty-seven thousand will be injured, and the agency expects that it will need to provide shelter for a million displaced people, and food and water for another two and a half million. “This is one time that I’m hoping all the science is wrong, and it won’t happen for another thousand years,” Murphy says.

In fact, the science is robust, and one of the chief scientists behind it is Chris Goldfinger. Thanks to work done by him and his colleagues, we now know that the odds of the big Cascadia earthquake happening in the next fifty years are roughly one in three. The odds of the very big one are roughly one in ten. Even those numbers do not fully reflect the danger—or, more to the point, how unprepared the Pacific Northwest is to face it. The truly worrisome figures in this story are these: Thirty years ago, no one knew that the Cascadia subduction zone had ever produced a major earthquake. Forty-five years ago, no one even knew it existed.

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[yeoman6987]

1. Could be tomorrow or a million years from now.

Hopefully closer to a million.

[OKIsItJustMe]

2. Tomorrow is much more likely than a million years from now…

…

Thanks to that work, we now know that the Pacific Northwest has experienced forty-one subduction-zone earthquakes in the past ten thousand years. If you divide ten thousand by forty-one, you get two hundred and forty-three, which is Cascadia’s recurrence interval: the average amount of time that elapses between earthquakes. That timespan is dangerous both because it is too long—long enough for us to unwittingly build an entire civilization on top of our continent’s worst fault line—and because it is not long enough. Counting from the earthquake of 1700, we are now three hundred and fifteen years into a two-hundred-and-forty-three-year cycle.

…

[FBaggins]

4. There's a difference between a significant earthquake and "the really big one"

8.0 or higher (often called "great earthquakes&quot along that fault are closer to 500-600 years apart.

There's also a dramatic difference between the big Japanese quake and one in the low 8s... and also a difference between a low-8 that far offshore and something in the 7s... but under San Francisco.

[OKIsItJustMe]

5. I wouldn’t count on that…

http://www.crew.org/earthquake-information/history-of-earthquakes-in-cascadia

[font face=Serif][font size=5]History Of Earthquakes In Cascadia[/font]

[font size=3]…

Scientists believe the most recent subduction zone earthquake, a M9 event, occurred in January 1700. The best available evidence indicates that these earthquakes occur, on average, every 500 to 600 years. However, the years between these events have been as few as 100 to 300 years -- meaning, all Cascadia residents should prepare to experience a powerful and potentially damaging subduction zone earthquake in their lifetimes.

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http://crew.org/sites/default/files/cascadia_subduction_scenario_2013.pdf

[font face=Serif]…

[font size=4]What Are the Odds?[/font]

[font size=3]The evidence for past earthquakes of magnitude 9.0 suggests that they recur on average every 500 years, but the actual intervals between events are far from predictable—such earthquakes have been separated by as many as 1,000 years and as few as 200. The estimates of the sizes of pre-1700 earthquakes are also uncertain. Cascadia has now been building up strain for over 300 years, so the next great earthquake could happen at any time. Reduced to simple odds, the chances that an earthquake as large as magnitude 9.0 will occur along the zone within the next 50 years are about one in ten.

While the timing cannot be forecast very precisely, great subduction zone earthquakes are inevitable—they are a fundamental consequence of plate tectonics. Whether this type of earthquake is considered alone or in combination with other earthquake sources, the odds that a large, damaging earthquake will occur in the near future are very high. The more steps our communities take now to prepare, the more resilient we will be.

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http://pubs.usgs.gov/pp/pp1661f/

[font face=Serif][font size=4]Earthquake Hazards of the Pacific Northwest Coastal and Marine Regions
Robert Kayen, Editor[/font]

[font size=5]Turbidite Event History—Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone[/font]

[font size=4]By Chris Goldfinger, C. Hans Nelson, Ann E. Morey, Joel E. Johnson, Jason R. Patton, Eugene Karabanov, Julia Gutiérrez-Pastor, Andrew T. Eriksson, Eulàlia Gràcia, Gita Dunhill, Randolph J. Enkin, Audrey Dallimore, and Tracy Vallier[/font]

[font size=3]…

Time-independent probabilities for segmented ruptures range from 7–12 percent in 50 years for full or nearly full margin ruptures to ~21 percent in 50 years for a southern-margin rupture. Time-dependent probabilities are similar for northern margin events at ~7–12 percent and 37–42 percent in 50 years for the southern margin. Failure analysis suggests that by the year 2060, Cascadia will have exceeded ~27 percent of Holocene recurrence intervals for the northern margin and 85 percent of recurrence intervals for the southern margin.

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[phantom power]

8. It's mean period is ~250 years.

I wish they'd posted the variance, but they mentioned the probability of a major fault slip is 30% in the next 100 years. I'm actually a bit surprised the probability is that low.

[PearliePoo2]

3. Yes, I live here...

I live in the San Juan Islands in Washington State. I'm on solid rock like most islanders, but Seattle and many places in Washington are on glacier fill. They will have it bad, really bad.
I felt the Nisqually quake ( 2/28 2001-10:54 am 6.8) and my dog jumped into my arms right before I felt it and actually heard it. I am about a 100 miles northwest of Seattle.

[oldandhappy]

6. Thanks. Did not know.

Was just up in the Bellingham area and was curious re the san juan islands. So beautiful. Will the islands be flooded out?

[OKIsItJustMe]

7. A tsunami is a risk in some places

http://joomla.sanjuandem.net/index.php?option=com_content&view=article&id=117&Itemid=114

… According to the latest models, most of the shorelines of the islands will be unimpacted, but in some spots the waters will reach a height of 10 feet above mean high tide, and potentially a height of 20 feet in a few high risk areas. …

[oldandhappy]

9. thanks again -- will be watching -- new things to think about for me

[Auggie]

10. So much for the thought about retiring to the Pacific Northwest

[MisterP]

11. single-story well-insulated balloon-frame homes aren't that vulnerable to even Mercalli X

even steel-frame high-rises are pretty sturdy: the real danger is brick cladding (which just gets hurled onto anyone nearby in three seconds) and soft-story buildings with weak garages as the ground level

I have no idea how log cabins fare, but I don't think they did too well

[Auggie]

12. Read the article. The story cites the devastation from the ensuing tsunami of a 9.0 quake,

a 40 to 45 ft high wall of water. So maybe the structure survives the seismic occurrence but it won't survive the water. Furthermore, the surrounding infrastructure is wiped out for months or maybe even years. No electricity, no water, no communications, etc.

[MisterP]

13. well the song goes "no more water but fire next time"

[sue4e3]

15. best to live inland then

[sue4e3]

16. best to live inland then

[happyslug]

14. Wood frame houses have a lot of give to them, most survived the 1906 San Francisco Earthquake

And thus wood frame and "log cabins" do quite well in earthquakes. Bricks, stone and block do no "Give" and thus do NOT do well in earthquakes unless the designers of the building are careful. In the San Francisco earthquake of 1906, most of the wooden housing survived the earthquake, the subsequent fires is what did them in. If you have methods of fire control, such as turning off gas pipelines that are leaking gas, and pre planning your water distribution system so can have water everywhere even in a major quake, fire is not a problem.

Wood's ability to "give" and to "bend" under compression and then slip back to its original position was the reason most houses in San Francisco were made of wood in 1906 and why they survived the earthquake (they were lost in the subsequent fire, but that is a different matter then the earthquake).

http://www.nytimes.com/1999/01/31/weekinreview/the-world-one-defense-against-quakes-build-homes-of-wood.html

http://oregonstate.edu/instruct/oer/earthquake/07%20chapter%206_color.html

http://cenews.com/article/9036/wood-frame-construction-advantageous-in-areas-prone-to-seismic-activity

http://www.tennesseeheritageloghomes.com/Myths_and_Truths.html

Report on a log house after an earthquake in New Zealand (only minor repairs were needed):
http://www.naturalloghomes.co.nz/pictures/earthquake/MtPleasantLoghomeReport.pdf

http://www.naturalloghomes.co.nz/earthquakeRESISTANCE.html

Video of a Log Home in an earthquake test:

Thus Log Homes can do well in an earthquake, as good as a wood frame or modern steel frame building. The wood is both light weight and strong but in addition can "flex" and retain its structure and strength. That combination is what made wood the choice of people in California before the 1906 Earthquake.

Please note, most "Brick" homes today are wood framed but brick faced. The brick facing may fall off do to the earthquake but the rest of the house will continue to stand for the brick is just to make the house look good, it adds no structure the house.

Now, I live in an area where we have brick FRAMED houses, mostly built pre Civil War, through some as late as the 1920s (These tends to be warehouses and other large buddings not homes). The bricks are NOT one layer but three to four layers, all interconnected. One way to tell is to look at the bricks, if all you see in the long sides of each brick, it is a wood frame building with a brick veneer. On the other hand, if you see one long brick, then a short side of a brick, then a long side of a brick, then a short side, you have a brick framed home. Rare even by the 1900s, but many still exists in older inner cities.

Now, steel frame buildings held up well in earthquakes, mostly do to steel's ability to bend and give AND the structural integrity of solid connections, either welds or nuts and bolts. Now you will read reports about older steel frame buildings have been known to fail in earthquakes, mostly due to the steel frame NOT being attached to other parts of the frame at their base except by whatever footer used in building the structure. Today, in most construction steel ends run right above such footers to keep the steel structure intact, much like wooded 2x4s and 2x6s are used in wood construction.

Nut and bolt construction is more expensive then welding but permits more flex of the resulting connection between two pieces of steel. During WWII the US made its famous "Liberty Ships" and increased production of them by mostly welding them, previous ships had all been bolted. In cold seas the welds were found to make the ship to rigid so that the welds would fail and the ship would sink. The solution was to bolt a long piece of metal on both sides so if the weld failed, the bolts would give enough for the ship to survive. Today ships are built with mostly welds but certain parts are still bolted to give the ship some flex when it is needed.

You see this is auto manufacturing, welding is faster and cheaper, but every so often a car is designed so be so rigid that parts of it has to be bolted to give it enough flex. I had a sister who had just bought a then new 1990 Mercury Sable, that was wrecked when it was three months old. Rather then scrapping it the insurance company had it "repaired". Before the accident it drove nicely in dry or wet weather, but afterward it never drove right in wet weather. We did not figure out why, till we had to replace a transmission five to six years later. To replace the transmission you had to unbolt the front. The problem was due to the "Repairs" done when it was new, the front had been welded not bolted. The Sable had a super tempered frame, to give it a very rigid frame to improve its fuel economy. The reason the front end was designed to be bolted was NOT only to repair transmissions but to give the front end the flex it needed to operate in wet weather. When the repair shop (and it was a Dealer) had welded the front end, they made the front end to rigid to be driven in wet weather. After the accident my sister hated driving that car, but was a good enough driver to know how to drive that death trap in bad weather. We used it for about 10 years (my family has a habit of buying new cars, then keeping them 10-15 years) and then scraped it. We insisted on the person who purchased it that it be scraped and told him why. The car passed annual inspections with ease, on dry pavement it was a good car, wet whether was a different bar game. I bring this up for it shows the difference between welding and bolts and why BOTH are used to this day.

Thus Steel and Iron Frame buildings (most were large buildings) like wood frame homes, survived the 1903 Earthquake quite well do to each material's ability to flex and bend and return. Masonry buildings can not do that and you have to be careful how to build them in earthquake prone areas.

Please note, I am only addressing the issue of log homes and earthquakes. In any subsequent Tsunami, hopefully you are outside the range of the destruction by the Tsunami (Which varies depending on your location in height above sea level AND distance from shore AND any inlet to the sea).