Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little.
Beside this, what is the process that leads to an earthquake?
Earthquakes are a result of that process. Plate tectonics is the area of science that explains why the surface of the Earth changes and how those changes cause earthquakes. Scientists say the surface of the Earth is cracked like a giant eggshell. They call the pieces tectonic plates.