Geologist explains some more science behind Japanese earthquake & tsunami

seismograph.jpg

Fabrizio Bensch / Reuters

I've been reading about some pretty astounding impacts of last week's 8.9 earthquake. For instance, there's the fact that the quake not only shook Japan, it moved the country. Specifically, pushed it Westward by about 8 feet. The US Geological Survey confirmed this to Space.com, as well as the fact that the quake shifted Earth's axis by 6.5 inches. And then, there's the news from a lab at NASA's Jet Propulsion Laboratory in Pasadena, Calif., where geophysicist Richard Gross has found that the quake also sped up Earth's spin, just a little a bit. Days are now 1.8 microseconds shorter, by his estimation.

How does all of this make sense? I've got a better understanding now, having read this great Scientific American guest blog post by geologist and blogger Chris Rowan. He covers all sorts of good information about the earthquake and the tsunami. From how we can tell that Japan moved, and why ... to what's going on with the changing numbers on how powerful this quake really was.

GPS stations in Japan - installed to measure the slow build-up of elastic strain in the crust between big earthquakes - show most of Eastern Honshu moving several metres to the east as a few centuries worth of that elastic strain - which pushes the crust in Japan westwards and upwards - was released over the space of a few minutes.

If you're wondering why there is some still some confusion over exactly how large this earthquake was, it's because - rather counter-intuitively - measuring the magnitude of large earthquakes is actually more difficult than it is for smaller earthquakes. To estimate earthquake magnitudes, you look at the amplitude of the seismic waves it generates: the larger the amplitude of the waves, the larger the magnitude of the earthquake that produced them. However, in very large earthquakes, this relationship starts to break down, at least for the frequencies of seismic waves that are generally used to produce the quick magnitude estimates: they 'saturate', or stop increasing in amplitude as the earthquake magnitude does. This means that the magnitude estimates for the largest earthquakes will be somewhat underestimated until seismologists look at lower frequency seismic waves, which are less susceptible to this saturation effect.

22

  1. Was there any rotation in the shift west? That could affect legal descriptions of property boundaries and other survey related information.
    As long as if just shifted as a whole unit it should be ok as the descriptions are usually relative to a known point.. at least it works that way here in the states.

  2. Well, Geocaching in Japan just got a bit more difficult. If all you have is numbers and no visual references, you are screwed. Goes for any industry/marketing effort that maps shit with GPS.

  3. It is interesting when following the link that the pre-shocks are much more clustered than the aftershocks.

  4. The link is a really good summary.

    FWIW, The Cascadia subduction zone is a pretty similar situation to this one, in that it’s a zone that occasionally kicks off really huge earthquakes, but hasn’t done so in a time that we have had instruments. (Though, we know of the date of the last big one, sometime in the 1700s based on Japanese tsunami records)

    Sometime, it’s probably going to kick off an 8+, and Seattle is going to be in trouble. From an informed guess, I’d say that Japan is far more prepared than our Zone 4 (california), and California is far more prepared than Seattle (zone3).

  5. Well, if the earth is constantly expanding like that, the same as the universe is constantly expanding, right?, that’d explain why the 130 million year old dinosaur fossils are so huge, wouldn’t it? They were once iguana sized lizards, and here we’ve built up this big boogie man theory, based on linear extrapolation of a nonlinear and constantly expanding surface-of-a-balloon universe.

    Maybe tectonic theory is all carp, and Earth has been expanding all this time, which would explain the constantly lowering ocean, otherwise over the eons we would be drowning in comet rain. Oh, did you think water magically evolved out of iron/nickel lava?

    There you go! Explanation for evolution, debunking AGW. Badda bing!

  6. Actually, didn’t Japan move eastward by about 8 feet, relative to the Pacific? The Pacific Plate is moving westward, slipping underneath Japan’s plate (generally but not universally thought to be an extreme westward arm of the North American Plate) at the Japan Trench, east of the islands. The earthquake brought Japan 8 feet closer to the Pacific Plate, i.e., 8 feet to the east.

    http://www.guardian.co.uk/world/2011/mar/11/japan-earthquake-tsunami-questions-answers

    This site says 13 feet:
    http://www.nytimes.com/2011/03/14/world/asia/14seismic.html?_r=1&hp

    The source linked to in the BB post merely says that the island moved by “about 8 feet,” without specifying a direction. Clicking through the link there, it notes that the Pacific Plate is moving westward. That would mean that Japan is, relative to the Pacific Plate, moving eastward.

    The plates meet at a subduction zone, which means that they’re at a convergent boundary, i.e., the two plates are moving in toward each other. The Pacific moving under Japan melts and then rises up to form Japan’s volcanoes (similarly, that is why there are volcanoes in the Pacific Northwest, because the Juan de Fuca Plate is subducting underneath it).
    http://en.wikipedia.org/wiki/Convergent_boundary

  7. Can someone please explain what “shifted Earth’s axis by 6.5 inches” really means? The Earth rotates about it’s axis whose angle can be expressed relative to the plane that the Earth moves in as it goes about the Sun. Saying an angle moved by 6.5 inches makes no sense. Do you mean that the r dtheta = 6.5 inches? If so, why not express the angle?

  8. Geocaching is going to get even more different if everything has moved by 8 ft … as will anything that depends on GPS like finding runways or using your car’s navigation system (you bought one with all the data burned in ROM and assumed that the Geography would never change?)

  9. Beware the sign error. During the interval between quakes Japan gets scrunched up by moving west. During the earthquakes that scrunch relaxes and Japan suddenly moves east.

    Also see the IERS web page to see just how much the length of day changes and pole moves even without large quakes.
    http://hpiers.obspm.fr/
    shows that the earthquake effects are 1000 times smaller than the annual changes, immeasurably small.

  10. FWIW, translating map data by any arbitrary amount in any direction is trivial, and considering how good the motion data is, the maps will be corrected with little difficulty and they probably won’t even have to send people out to ground-truth it. Yes, you’ll be off with your older maps, but it’s not like being 8 ft off is going to be a huge problem – most GPS fixes you’ll get are not that accurate anyway!

    Anyway, I’m a geologist too, specializing in structure and tectonics (though not seismology, particularly), and I thought this summary was very nicely written. Hard to judge how easy it is for non-geologists to understand but I think anyone who’s interested in the science should find it quite enlightening.

    He linked to a video I hadn’t seen yet, available from the BBC, which shows the tsunami breaching the sea wall. Amazing footage that I think other BB readers would appreciate seeing linked to prominently.

  11. Maybe this can put to rest the lunatic tinfoilers’ speculation that HAARP is the cause of the recent quake in Christchurch and this one. The amount of energy expended by HAARP since its inception, including that expended in its construction, aggregated and released in one burst, could not have come close to something on this scale.

    (Not to mention the fact that beaming EM toward the ionosphere is a terribly ineffective way of affecting the movement of tectonic plates!)

    1. Maybe this can put to rest the lunatic tinfoilers’ speculation that HAARP is the cause of the recent quake in Christchurch and this one.

      I hate to disappoint you, but the anonymous comments don’t support your hope.

  12. Maggie- I’m confused by your part and the article quoted; did the country move 8 feet east or 8 feet west?

  13. Hmm, fresh from my encounter with not citing references when editing Wikipedia, I wrote a version of the following that had a bunch of links for references, but it seems to have been tied up in moderation limbo. So I now present to you the non-linkified version of what I intended to post earlier this morning:

    —–
    Actually, didn’t Japan move eastward by about 8 feet, relative to the Pacific? The Pacific Plate is moving westward, slipping underneath Japan’s plate (generally but not universally thought to be an extreme westward arm of the North American Plate) at the Japan Trench, east of the islands. The earthquake brought Japan 8 feet closer to the Pacific Plate, i.e., 8 feet to the east.

    (Other people say 13 feet.)

    The source linked to in the BB post merely says that the island moved by “about 8 feet,” without specifying a direction. Clicking through the link there, it notes that the Pacific Plate is moving westward. That would mean that Japan is, relative to the Pacific Plate, moving eastward.

    The plates meet at a subduction zone, which means that they’re at a convergent boundary, i.e., the two plates are moving in toward each other. The Pacific moving under Japan melts and then rises up to form Japan’s volcanoes (similarly, that is why there are volcanoes in the Pacific Northwest, because the Juan de Fuca Plate is subducting underneath it).

  14. I am not very clever. Perhaps because of this fact, I am struggling to see how the earth’s axis could have ‘moved by 6.5 inches’ by means of an earthquake.

    Surely only an ‘external'(comets or whatever) force could have this effect?

    Show me just how stupid I am, people.

  15. “Surely only an ‘external'(comets or whatever) force could have this effect?”

    No, an internal redistribution of mass can have this effect, too. The standard analogy used to show why a change in the earth’s crust could make it spin faster is to imagine an ice skater bringing her arms in closer to spin faster. (Technically what is happening here is that angular momentum is being conserved, since an equal mass needs to have more angular velocity (i.e., be rotating faster) to stay at the same angular momentum if it moves from a greater radius in to a smaller radius.)

    So now imagine that same ice skater spinning with her hands straight out, but instead of bringing them both in, she only bends her right elbow to bring one hand in toward her chest. Suddenly the distribution of mass isn’t symmetrical any more, and she’ll start to wobble to adjust to her new axis of rotation.

    What I don’t get is why Japan would be slowly inching its way to the west before springing more violently to the east with the earthquake. Wouldn’t the inching direction be the same as the violent movement of the earthquake, just slower and steadier?

  16. AS the ocean plate subducts under the Japan Island plate, it pushes it, by friction, towards the west, catching on it as it slides beneath to the west (the island, though, would rather stay in place or go east)….and then the friction lets go all at once, and the Island “snap-backs” to the east, where it “wanted” to be all along, had it not been dragged slowly to the west by the subducting ocean plate: and then subduction friction starts again catching the bottom of the overlying Island, and again inches it to the West, starting thereby to load the tension for the Island’s next “snap-back” to the East, somewhere down the line in the (hopefully far) future.

  17. feet… inches… ?

    and yeah – i had been wondering what this movement meant for GPS stuff…

Comments are closed.