The only thing Arty needed footing for is the shove off the ground to flip Adrian on his side, and that would be before the tremor hit him. After that, his feet aren't even on the ground. He's laying on his side on top of Adrian's left wing, and he's latched onto him with his power, so his grip there won't be affected. Sure, it'll jostle him, but not enough to cause damage or affect his actions.