When sitting on a couch with a friend you fart so hard it rumbles all the cushions on the couch. Resulting in a vibration under yours and your friends butt.
Friend 1: makes a loud rumbling fart on the couch
Friend 2: Hey what was that!
Friend 1: Sorry yo! You just experienced a california couch quake
the movements or rumbles one feels on the opposite side of the couch during a nestled flatulance.
She said "EWW, don't get up, I felt that couch quake all the way over here !"