Earthquakes occur when tension is released from inside the crust. Plates do not always move smoothly alongside each other and sometimes get stuck. When this happens pressure builds up. When this pressure is eventually released, an earthquake tends to occur.
The effects of an earthquake can be devastating - they can destroy settlements, change landscapes, and cause many deaths.
The size of an earthquake is measured on a Richter scale 0-10 of magnitude using an instrument called a "seismograph". Seismographs are basically pens suspended over a paper-covered rotating drum. When the earth trembles the pen makes a larger squiggle on the drum, allowing the size of the shaking to be measured. Each whole number on the Richter scale represents an earthquake 30 times larger than the number below it. Earthquakes that measure less than 3.0 are not usually felt, while one of 5.0 produces the same amount of energy as the Hiroshima atomic bomb. Earthquakes measuring around 7 or 8 on the Richter scale can be devastating. The largest recorded quake happened in Chile in 1960 and measured 9.5.