(Via Armcontrolwonk) This map shows where the opposite (anti-podal) point on the earth is. i.e. North Pole corresponds to South Pole. One can easily check up one's anti-podal point by change the coodinate (change lattitude from N to S, then add/subtract 180 on longitude). But it is neat to view this on a globe.
ACW cited this when he read that vibration typically get 'magnified' on anti-podal points as noted by astronomers observing the moon and some planets. He thinks this may be used to measure earthquake (and nuclear tests).
The physical explanation may be quite simple, as the anti-podal point represents THE equi-distant (both on earth surface and via the mantle/core) point, such that all waves arrive IN PHASE (ignoring fluctuations in rock type/etc). This means it would be difficult to measure earthquake anti-podally since these are random events and one does not know where to place the probe in advance, but for monitoring nuclear tests with known sites this could be pretty effective.
A few interesting observations
- Very few land has land as antipodal points, mainly because there are only 30% land on earth and most of it is on the northern hemisphere. (this makes ACW's plan more difficult/costly)
- The few land-land pairs include (this site provides a great tool)- Beijing's anitpode is a few hundred km south of Buenos Aires, Xian and Santiago (Chile) are almost exact antipode pairs (perhaps the only major city pair). Jakarta/Bogota and Singapore/Quito are also close enough. New Zealand's antipode is Spain (auckland/Seville). -- play these games to test your geographic knowledge.
- The Yucatan meteorite at Chicxulub crater, which allegedly killed the dinosaurs, would probably have had created some folding under the ocean between NW Australia and Cocos Islands in the Indian Ocean. However, some people believe that the Chicxulub antipode is actually the Deccan Traps inside India because techtonic movement shifted the Indian subcontinent
The map above shows the wave-front of the DPRK shock (test). Ideally these contours should be circle, but they are distorted because of rock/water structure the p-wave passes through (changes the speed the sound wave travels).
Given these contours, it is now easy to measure future shocks (from the same location) much more accurately. The trick is to put an array of probes spaced on a chosen contour such that the signal these probes received are all "in phase". Adding these signals (time-plotted) up will result in strong signal since the noises are not in phase and tends to cancel out. This will achieve similar or even better results than a measure from the antipode.