So earthquakes generally don't and can't change the planet's orbit (the path it takes around the sun) for basically the reason you said. But an earthquake can change it's rotation (how it spins around on its axis).

Very very simply, a shift in tectonic plates, while it doesn't change the mass of the Earth can change the distribution of mass. How fast the Earth rotates, and the degree to which it "wobbles" as it does so, much like a top, is based not just on its mass, but the distribution of that mass.

Change the distribution, change the rotational axis.