The elastic rebound theory of an earthquake source envisages the flinging of rock masses in opposite directions on each side of the rupturing fault as the fault rupture progresses along the fault.

In the rupture, the rock masses spring back to a position where the elastic strain is less.

This movement at any point may not take place at once but rather in irregular steps.

These sudden stoppings and startings give rise to the vibrations that propagate as seismic waves.