How do earthquakes start?
Earthquakes are a sudden release of energy in the Earth's crust.
Earthquakes most commonly occur at the boundaries of tectonic plates where stresses accumulate in the earth's crust over time, causing energy to be stored in the form of elastic strain (like that in a compressed spring).
Ultimately this stress will exceed the shear strength of the crust in the fault or boundary zone causing a sudden brittle failure or rupture.
This in turn causes movement and a sudden release of the stored elastic strain energy in the form of seismic waves (as well as heat) causing an earthquake.