Re: How high on the ricter scale would a 1 megaton nuclear blast produce?

The general correlation is that an earthquake of magnitude 6 on the Richter scale produces as much energy as a 1 megaton nuclear explosion (1,000,000 tons of TNT). You would not feel an explosion like this on the other side of the planet (Earth or Mars) but it would be detected by seismographs (and seismologists can generally tell the difference between a nuclear explosion and an earthquake).

There's a lot of information on the Internet about earthquakes. Start at the Seismo-Surfing the Internet page.