When underground rock breaks along a fault, the sudden release of energy causes the seismic waves that cause the ground to shake. When two pieces of rock or two plates are rubbing against each other, they begin to stick slightly, meaning although they are pushing against each other they do not move. Through time the rocks break causing an earthquake.
Both during and afterwards the plates and rock continue to move, this occurs until they are restricted again by one another and therefore become stuck once again.
The focus is known as the spot underground where the rock breaks, the place above the focus on top of the ground is called the epicenter.
The most severe damage is found closest to the epicenter and then dispursed from there outwards, earthquakes can alter in time depending on teh amount of energy that build up before the release, as well as the size of the breaking rocks or plates underground.
BBC (2014) GCSE Bitesize- Earthquakes and Global Warming. [online] London: BBC. Available from http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway_pre_2011/energy_home/6_stable_earth1.shtml