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An earthquake is caused by a sudden slip on a fault(fault is defined as fracture in the earth's crust in which the rock on one side of the fracture has measurable movement in relation to the rock on the other side). Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the earth's crust and cause the shaking that we feel during an earthquake.