The idea is that every few real time weeks an earthquake would happen in a random supergrid. This earthquake would have the following effects:
Beforehand, several rumblings would give a warning about the upcoming earthquake
High Chance that existing mines would collapse, and have to be reprospected and rebuilt
Small chance that existing mines would collapse and vanish completely (Unless you took the chance to bail out when the warning rumbles occured, you are fucked.)
Small chance of new mining spots appearing randomly
Also, unrelated, a temporary reduction of LP gain when smoking cured hemp pots. Just because.