6.S.I.3

" Crust movements The surface of earth is made up of rigid plates that are in constant motion. Plates move because molten rock rises and falls under the crust causing slowly flowing currents under the plates. Plates move at speeds ranging from 1 to 4 inches (5-10 centimeters) per year. Earthquakes usually occur where stress has been built up by plates moving in opposite directions against each other. Earthquakes cause waves (vibrations) which have: focus, the point below the surface where the quake begins epicenter, the point on the surface above the focus Severity of ground shaking is measured on the Richter scale; each unit on the scale represents a tenfold severity increase"