Weather weapon.
That’s what a lot of people on voat are suggesting. I always thought that controlling hurricanes sounds ridiculous, but creating earthquakes by doing something where the fault line is via explosives or low frequency sounds is very plausible. Heck, a thug mobile across the street can make your car vibrate horribly with its woofers. Imagine if you actually spent a billion dollars for the purpose of deliberately making the most powerful low frequency possible.