Answer:
Seismic waves are tremors or vibrations in the Earth's crust that are caused by the build up or accumulation of pressure (more correctly termed stress).
This accumulation of stress causes the rocks that make up the crust to deform elastically. This is very similar to what happens when you squash or stretch a spring and causes a form of energy to be stored in the rocks of the crust - technically described as elastic potential energy.
When this stress gets too large, it exceeds the strength of the rocks in the crust and causes a brittle failure. Brittle failures are failures where fractures form through the material.
Over time the lithosphere has been repeatedly fractured in this way and so is now heavily faulted. These faults are weaker than the intact rock mass around them so tend to act as the focus point for earthquakes as they tend to slip before the surrounding rock can fail.
This sudden brittle failure causes all of the elastic potential energy to be released at one time in the form of seismic waves, just as if a spring or elastic band that was being stretched suddenly snapped.