The correct answer is - waves.
When there's an earthquake, it means that there's a big shock deep inside the ground where a large amount of energy is released, This energy, since it is inside of the Earth, is transferred through the crust in the shape of waves, or rather, seismic waves. The bigger the stress inside the Earth, the more intense the seismic waves, and vice versa, the lesser the stress, the lesser the intensity of the waves. There's special scales that are measuring the frequencies of this waves, and some of the most popular in usage are the Richter scale, and the Mercalli scale.