There are tectonic plates in the earth's crust. When these tectonic plates come in contact with each other i.e. they rub each other or the plates are upside down, creates some vibrations called seismic waves that radiate up through the crust to the earth's surface thus causing the ground to shake, thus earthquakes are caused.
1 5 1
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.