I can't find any reliable sources, so treat this as my own speculation. I heard that there were programs for a while in two big countries with acronyms starting with US that aimed into exploration of causing earthquakes/tsunamis that would hit across the word by using multiple underground nukes and constructive interference.

There was an American program towards the end of the second world war aiming at causing tsunamis to weaken Japan: https://en.wikipedia.org/wiki/Tsunami_bomb

I would suspect this was more for propaganda effect -- local populations would understand better a threat if it was related to something devastating they actually had experience off.

Certainly reasonable to believe. Those countries were exploring literally any and all avenues to undermine the other.