$.02; OK, as often I can't take the time to double-check the details, but the Kunlun earthquake is in the Kunlun mountains. My 1980 National Geographic Atlas shows Muztag to be 7.7 km high. India is crashing into a big thick soft continent, and the effects of this collision are felt far from the actual plate boundary: or, you often can't talk about a single structure as a plate boundary in a continental collision. So, I'm reasonably sure that these Kunlun mountains are actively deforming and it is unsurprising that you would get a large EQ beneath a Mountain Range on this scale: the fault may well have been known, or it may be blind (buried).

Also, in an earthquake one block moves relative to the other, and this causes a redistribution of stresses in other areas (static stress changes). An explosion is not the same: it does create seismic waves (P wave, anyway, and probably surface waves-I don't know if shear waves are created). So, these waves may have triggering capability. But, a large underground nuclear explosion is generally smaller than M6, and I don't see any long-term triggering from that 500 km away. The Kunlun Mountains are likely more affected by larger earthquakes that have occurred over the last 100 years at similar distances (and lesser distances...) Chris