Ball stays still, ground moves

quick question, i think i know the answer:

If my ball is in some heavy rough, if i ground my club and the ground under the ball moves down from, say the grass compressing, is it a foul?

in other words, the ball doesn't move on its own, but the ground underneath it does, and the ball stays in the same relative spot to its location in on the ground.   is this still a penalty?  and thus should i be more careful about grounding my club in rough like this?

thanks!

Yes the ball will be deemed to have moved.

That is exactly what happened to Corey Pavin this year.  I don't remember the tournament, but he was assessed appropriate penalty for having the ball move and not replacing the ball to its original position.

18/1 Ball Moves Vertically Downwards

Q. A ball lying in long grass slips vertically downwards. Or a ball is accidentally stepped on and pressed down, say a quarter of an inch, in the grass or into the ground. In each case, has the ball moved?

A. Yes, unless the ball returns to its original position. The direction of movement is immaterial.

18/3 Ball in Fork of Tree Moves in Relation to Ground But Not in Relation to Fork

Q. A ball rests in the fork of a branch of a tree. The player climbs the tree to play his next stroke. The branch bends under his weight. Although the ball has moved relative to the ground, it has not moved relative to the fork. Is the ball deemed to have moved?

A. The ball is deemed not to have moved since it did not move in relation to the fork of the tree in which it was lodged.