What is the back and forth movement cuased by an earthquake?

It result from the moving of rock deep under the ground. This movement is caused by the rubbing of one plate into another. The plates have jagged edges, and so when they rub against each other, they get stuck. Since plates constantly move, the plates that are stuck create great tension, and when they are finally free, a huge amount of energy is released. Shock waves move from focus in all directions, and it causes the earth to vibrate.