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Mechanics of possible Pacific NW earthquake

  1. Jul 16, 2015 #1


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    The New Yorker magazine of 20 July 2015 had an interesting article about the "Cascadia" subduction zone along the coast of N.Calif, Oregon, Washington, Br.Columbia
    I looked up some background. Wikipedia seemed quite informative.
    A seismology outreach site called IRIS also had this little animation.
    The animation shows how subduction (one plate sliding under another) can store and then release energy, and the mechanics of how abrupt changes in altitude of coastal seafloor can occur.

    Rock is actually compressible and elastic.
    Here is some more background by IRIS
    I'll quote a brief excerpt of this longer article:
    ...As frictional stress builds along the fault boundaries, it is accompanied by an increase in strain in the adjacent rocks. When the frictional stress exceeds a critical value, a sudden failure occurs along the fault plane that can result in a violent displacement of the Earth’s crust. When this happens, the ensuing earthquake releases elastic strain energy and seismic waves are radiated. The process of strain, stress, and failure is referred to as the elastic-rebound theory. (See page 6 for more on this topic.)

    Earthquakes generated in this setting are called Great Subduction Zone earthquakes. They are the largest earthquakes in the world and can exceed magnitude 9.0. The devastating Sumatra-Andaman earthquake of December 26, 2004 (star on Figure 2) had a magnitude of 9.3.
    The earthquake occurred on the interface between the India and Burma tectonic plates where the India plate is subducting beneath the overriding Burma plate. Earthquake size is proportional to fault area which was was about 1200 km long and as much as 200 km wide (See Figure 3 for a comparison of the area of the Sumatra-Andaman earthquake with the size of California.)
    ...The uplift caused by the elastic rebound of the overlying plate is what caused the deadly tsunami that killed over 225, 000 people. For more detail on Sumatra:
    In 1960 and 1964, destructive magnitude-9 earthquakes occurred in Chile and Alaska respectively.
    A similar configuration of plates can be found along the Cascadia Subduction Zone (Figure 4). This is a very long sloping fault that separates the Juan de Fuca and North America plates and stretches from mid- Vancouver Island to Northern California. The contact between the two plates, the
    area of the subduction zone fault, could
    also produce a magnitude 9.0 earthquake, if rupture occurred over its whole area. It last ruptured on January 26, 1700 (for details see Orphan Tsunami.)
    Last edited by a moderator: Jul 16, 2015
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  3. Jul 16, 2015 #2


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    I thought the Wikipedia article was as least as well written and illustrated as this IRIS material, so I'll include a link.

    BTW the Cascadia zone (CSZ) is not to be confused with the San Andreas Fault. The better-known California earthquakes are part of a different system.
    It seems that the last major CSZ quake was in 1700 and there was no written historical record. (Only First Nation oral tradition). However an estimated 10 hours after the 1700 quake there was an unexplained tsunami in Japan. This was recorded and helped to date the 1700 quake. The wave had traveled some 5000 miles. It's interesting how seismologists have put together different kinds of evidence (tree rings, ocean sediment cores) to construct a long-term record of major CSZ quakes now going back some thousands of years. The importance of the CSZ has only recently been discovered.
  4. Jul 16, 2015 #3


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  5. Jul 16, 2015 #4
    I'm a geologist but not very clued in when it comes to seismology.

    However, I would be leary of living in the lower mainland of British Columbia or on Vancouver Island. I've had opportunities of relocate to Victoria and...nah...the chance of a 'big one' has influenced my decision. Perhaps not the only variable but certainly something I've thought about.

    As for all of the research, references, etc. A big asterisk can go beside every thing...'educated speculation'. Get three guys from the GSC together and you'll get three opinions on risk. The bottom line is we really don't know.

    As a geologist I do know that the public has no concept of just how massive and destructive geological forces can be. What we've experienced in civilized history are pinpricks barely registering on a geological scale. The odds are slim (odds based on speculation) but Seattle or Vancouver or Portland could literally cease to exist tomorrow...or next year or in a hundred years. One day a couple million people are there...the next, bang, gone. This may happen with some large city in the world in the next couple decades...after that reality check, living in a high risk area will have the similar anxiety of living on a nuclear military site during the Cold War. Nature is very powerful.
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