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1.
A fart which is so loud and powerful that it reverberates the entire area around the expeller.
Robin: Oh my God, everything in the house is shaking, it's an earth quake!
Chris: Don't worry about it, that was just a butt quake I let loose downstairs.

Local Newscaster: Today we mourn the loss of the twenty people that were killed on this day last year when an unsuspecting local man released a butt quake while on board a city bus.
by MerchantofVenice5 February 04, 2013
 
2.
Noun. The ripple effect you receive while having doggy style intercourse or slapping a hoe's ass.
I was banging Sheila the other night from behind and she had some epic buttquake.
by 1337x1 October 19, 2011