Any sudden slip on a fault can cause an earthquake. A fault is a fracture in the crust of the earth along which rocks on one side have moved relative to those on the other side. Stresses in the earth’s outer layer push the sides of the fault together, pressure builds up and the rocks slip suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake. There are 2 basic types of earthquakes. A strike-slip earthquake occurs when the rock on one side of a fault slides horizontally past the other. In a dip-slip earthquake, the fault is at an angle to the surface of the earth and the movement of the rock is up or down.