<A) The disaster timed test.

Everything is ready to go for a test. They wait until an earthquake occurs within a thousand miles. Boom. Within hours one of the stronger "aftershocks" is really an underground test.

B) The intelligence leak test.

Russia, China or now Pakistan and India plan a big test. The date and time of the test is divulged through leaks, bribes and espionage. Boom. They time their test to coincide with the "known" test.



Earthquake and bomb detectors can find location as well as magnitude.

As I understand it, two simultaneous shocks at different locations on Earth would give a completely different seismic signature as a single shock of the same combined magnitude at a single location. Assuming you are reading from multiple locations, of course.

For the same reason, they could only get away with scenario "A" if they performed the test at a location where an aftershock to the earthquake would be believable. In other words, it would only work if there was an earthquake near their test site.