What is the expected effect of a Tsar-scale bomb at the antipodal point of the detonation site? Background
Obviously, being next to explosions is bad. The farther away one is from the explosion, the larger the hemisphere of he shock wave is, and so the more the energy from the explosion is dissipated. Typically, this rate of loss of energy density is modeled at as $1/r^2$, where $r$ is the distance from the detonation.
However, this model misses an interesting effect that occurs (at least hypothetically) for sufficiently large explosions. Supposing that the explosion in question occurred at the north pole, once the shock wave moved past the equator, the size of the circle formed by the shock wave moving along the ground decreases, and eventually converges on the south pole (i.e. the antipodal point), resulting in constructive interference that could seemingly pose more of a danger to someone who's there than someone who's much closer to the explosion.
Question
Can this phenomenon occur in practice in cases of extremely large explosions, such as the Tsar bomb? If so, then how close to the north pole would one have to be to feel the same effects that someone on the south pole feels?
Research
I know this question is somewhat silly, but I don't think it's completely absurd, either. The blast wave from the Tsar bomb circled the Earth three times, and the atmosphere can focus blast waves in such a way as to make them more deadly to somebody who's far away from the explosion. The difference in this case is that the focusing mechanism is the shape of the atmosphere, not local/regional differences in atmospheric density. Finally, I am aware of this question, but it has a different focus than mine, and the question and answers don't address the antipodal focusing principle that I'm asking about.
 A: 
Can this phenomenon occur in practice in cases of extremely large explosions, such as the Tsar bomb?

Basically yes.

Supposing that the explosion in question occurred at the north pole, once the shock wave moved past the equator, the size of the circle formed by the shock wave moving along the ground decreases, and eventually converges on the south pole (i.e. the antipodal point), resulting in constructive interference that could seemingly pose more of a danger to someone who's there than someone who's much closer to the explosion.


If so, then how close to the north pole would one have to be to feel the same effects that someone on the south pole feels?

You would not feel the same effect.
While the impulse will travel and converge it will lose energy as it travels as it has to affect the medium it travels through.
It is also not travelling on a 2-D surface (the surface of Earth), but will also radiate some energy into the interior of the Earth.  That will also disipate energy reaching the opposite pole.
So the energy reaching the opposite pole will be significantly reduced.
Also consider that if your hypothesis was correct then every nuclear detonation ever made would have refocused at the opposite side of the globe, which clearly they did not.
