Earthquake is special skill used by only Stumps to disable their enemies.
Utilizing a weapon with the Sure Footed ability will prevent you from being knocked down by this attack.
It is actually possible to avoid the attack. A stump will bend his knees before jumping, at this point a pirate can jump and avoid the attack. If a pirates does this when the stump is in the air he will stop short and fall to the ground.
If a stump makes "Earthquake" when a pirate is trying to swim away (in Outcast Isle and he is actually swimming, but close enough to the stump to receive damage) and the pirate loses all health in the attack, he will sink as if on a ship, the music played when a ship sinks will be played, and the player won't even get knocked out. The pirate will wake up in jail with a timed shield around him/her.