I don't know if anyone has figured this out yet, but I figured it'd be useful info to know. I got a look at some of the games source code, and the function that triggers earthquakes is a 1 in 20 chance every time you leave a shop, so keep that in mind.

Figured I would go ahead and post a picture of the function in question.

https://ibb.co/3WkPc8T

Ah, the score condition is important too! This means early game shop visits should not matter! Thank you for this.

Yes, so the game only start rolling for earthquakes once you hit 150,000 Pts.