It can look complicated, but is very simple. Te newton's principle (first or second ???) tell us:

F = dP/dt

Where P is the momentum and F is the force. To change the momentum of a body you must apply a force to it. If no force is appled the momentum remain unchanged and it doesn't matter what happens inside the body.

If the Earth momentum has changed some external force must have been applied and if you take into account the third Newton't principle (action and reaction), the Earth will react with a reaction force to whom applied the force. To whom will react the Earth with a reaction force? to the earthquake ????!

Stupidity also evolves!