Geoff FordenCaging the Dragon: Antipodal Seismometry

1: of or relating to the antipodes ; specifically : situated at the opposite side of the earth or moon (an antipodal meridian, an antipodal continent)
2: diametrically opposite (an antipodal point on a sphere)
3: entirely opposed (a system antipodal to democracy)

This post represents a little light summer reading. I’ve been working my way through “Caging the Dragon,” the book on containing underground nuclear explosions that Jeffrey recommended in a recent post, and found it contains a wonderful set of tantalizing tidbits of information. One of them, and I think I will be writing on others in the near future as well, is a casually dropped comment that if more energy of an underground explosion was coupled to the Earth, its possible a nuclear detonation could set off volcanoes at the point directly opposite. The spherical shape of the Earth actually focus all—except for that small fraction lost as the waves pass through the mantel to friction etc.—the radiated energy onto a the antipodal spot.

This is not simply science fiction! Astronomers have found evidence on both the Moon and Mercury that the seismic waves generated by a meteor impact on one side has created mountains and valleys at the antipodal point.

Fortunately, nuclear bombs don’t couple to the Earth very well. (As a rule of thumb, also from Caging the Dragon, only about 1 part in 10,000 of the bomb’s energy is radiated away as seismic waves.) But perhaps, for those hard to quantify nuclear explosions, setting up a seismic station at the antipodal point to a suspected test site might give a better measurement of the yield. Such a station might receive an enhanced seismic signal because of the focusing produced by the spherical nature of the Earth. Unfortunately, there are very few points on dry ground that actually have an antipodal point also on dry ground. Of course, seismic stations could be placed on the ocean floor.

Would it be worthwhile placing underwater seismic stations in certain areas of the South Pacific or South Atlantic? Probably not. Consider, for instance, the 2006 North Korean test, for which there apparently was no strategic warning and came in with a very low strength (about ½ kiloton). <a href= It was detected by CTBTO stations all over the world. There is a primary CTBTO seismic station in Russia fairly close to the DPRK test location; much closer than the one in Japan. CTBTO has demonstrated considerable competence for the detection of future nuclear explosions using normal seismic monitoring.


  1. Carey Sublette

    All those who are interested may download “Caging the Dragon” from my website:

  2. sun bin (History)

    1) there may be an explanation in physics, the wave reach anti-podel (from all longitude) “IN PHASE”.

    2) for the ‘astronomer study’, i wonder if they looked at the famous yucatan meteor site (dinosaur extinction), which is west of australia in your globe/map

  3. kme

    South Pacific? That map shows the Korean peninsula as being antipodal to an area just off the Atlantic coast of South America.

  4. jasline (History)

    Based on your map, maybe a land monitoring station in Argentina, Uruguay or Paraguay would be a better bet, if underwater stations would be too hard to construct. Assuming they would agree to it. Perhaps persuading the UK to put some in on the Falklands, although they’re a bit far south to do much good.
    Interesting concept, although it sounds like the DPRK would have to have a much larger test to make it worthwhile.

  5. Geoff Forden (History)

    Sun Bin, of course physics explains it. That wasn’t the point. The point is that it might be more sensitive if you could place them very close to the antipodal point; closer than you might be able to place a more standard seismic station. Thus, its real importance would be for those tests that are less powerful than even the 1/2 kiloton 2006 test.

  6. Josh (History)

    Fascinating. Utterly fascinating.

    It’s a bit hard to make out, but the antipodal point of Russia’s test site at Novaya Zemlya would appear to be on — or immediately adjacent to — unclaimed islands just off the Antarctic shore.

  7. Oliver Morton (History)

    There used to be a fair amount of interest in this among people in the mass extinction debates, I think because it allowed them to reconcile the asteroid impacts people and the igneous provinces (volcano) people: If big impacts lead to antipodal volcanoes then you got two kill mechanisms for the price of one. But I think that’s all gone a bit off the boil recently, perhaps because the chicxulub antipodes are pretty featureless, IIRC.

    That said, the planetary evidence is interesting — not just mercury but also, Mars, where the best preserved very large impact (Hellas) is antipodal to what I believe is still reckoned to be the largest volcanic structure in the solar system (by volume) Alba Patera.

  8. John Blankenbaker

    To amplify Oliver’s point — the antipodal point of the K-T impactor has been identified as the Deccan Traps in India. Plate tectonics have moved things around since the end of the Cretaceous, so the current map isn’t what one should look at. In particular, India has been cruising north at a fair clip, which is why the Himalayas are so big.

  9. Geoff Forden (History)

    Hi Josh, Thanks! I think that Novaya Zemlya’s antipodal point is on dry land (at least parts of Novaya Zemlya). But it also looks like it might be under an ice sheath:

    By the way, if you look at the GoogleEarth land mass outlines for those Antarctic islands, you really see a large displacement from the photos. Why is that? Which is correct? The land mass outlines or the photos?

  10. Hairs (History)

    Before we get too carried away it’s worth remembering that Mercury, Mars and the Moon have solid cores; the lack of damping is the reason that “moonquakes” last for hours. In contrast, the earth’s liquid core causes considerable damping of seismic activity; so earthquakes typically last for only 30 seconds, and even the longest mega-thrust earthquakes only last a few minutes. Thus measuring at the antipodes means that the shear waves (not transmitted through the liquid core) will only reach the detector via surface transmission – with an awful lot of reflections and damping. Consequently a lot of potentially interesting information could be missed.

    Additionally, while it’s nice to rely on phase effects to give a bigger signal at the antipodes, doing so creates the risk that an underground test would actually be hidden because of one signal path creating enough delay for destructive interference. Also, all the extra dispersion over longer paths would surely blur some of the interesting features (e.g. sharp rise time) that distinguish a nuclear test from an earthquake.

    I suspect that some very well qualified seismologists and geologists in Russia and America would have studied this concept back in the cold war, and the fact that no nation (to my limited knowledge) went on to actually target the antipodes for extra monitoring suggests that they didn’t (don’t?) think it would give any significant advantage over the existing methods.

  11. sun bin (History)

    well, the significance of the “phase” theory (if true) will mean that the size of the anti-pode is fairly small (ie less than quarter wavelength), which restricts where you can place your probe.

  12. Colin (History)

    This reminds me of the atmospheric pressure wave generated by the 1883 eruption of Krakatoa. That wave not only got focused at the antipodes, it bounced, making multiple passes of barometric observation stations. Obviously the physics of seismic wave propagation is different, but antipodal focusing isn’t sci fi…

  13. sun bin (History)

    for air, the medium is much more uniform than rock+water with very different depth/density. so that is not s surprising observation

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