Avalanche Stomp is a power of the Stone Titan (all quotes are from the DDI Compendium):
Close burst 3; +21 vs Fortitude; 3d10+7 damage, and the target is grabbed by earth and stone. The stone titan does not need to use an action to sustain the grab. The earth and stone had a Fortitude and Reflex of 28 against escape attempts. Miss: Half damage.
To escape a grab:
This condition ends immediately on the creature if the grabber is subjected to an effect that prevents it from taking actions, or if the creature ends up outside the range of the grabbing power or effect.
I had a PC teleport from their currently grabbed square to a different square still in the original Burst 3 range. The question was immediately posed: did the PC escape?
Having moved to another square in the original Burst 3, my ruling at the table was: No, she did not escape.
- Avalanche Stomp is not what's doing the grabbing. As stated, it's actually earth and stone. This is enforced by the fact that the escape attempt is vs. the earth and stone's Fort and Ref.
- Avalanche Stomp has no duration and therefore the power is instantaneous (I don't have the Rules Compendium in front of me, but I'm certain there's something in there in this regard).