The magnitude is estimated from the amplitude of the seismic waves that are recorded by sensitive instruments called seismographs. The magnitude relates to the energy released at the focus. Earthquakes with magnitude of 2.0 or less are usually called micro-earthquakes and are normally too small to be felt. Earthquakes with a magnitude of about 5 or greater are strong enough to be recorded by seismographs all over the world and earthquakes of 6 or greater can do damage. Great earthquakes have magnitude of 8 or higher - on average, one such event occurs somewhere in the world each year.
Hot material pushes up along the ridge and then pushes the plates on either side further away.
The Modified Mercalli Scale measures the intensity of the effects of an earthquake. The crust of the earth is made up of about a dozen large plates and several smaller ones that are in constant motion. The movements are very slow - only a few centimetres each year. When the plates rub against one another, strain builds up, especially at the edges. If the strain builds quickly, the earth's crust may suddenly shift by several metres causing an earthquake.