Earthquakes are common occurrences. We often read about them in newspapers or listen over the radio. Do you know how they are caused? When an earthquake occurs, that particular part of the Earth experiences tremors. Sometimes, it is so mild that it passes unnoticed. But often, it is quite strong and creates vast openings in the Earth’s surface, because of which buildings fall down and many lives are lost. We know that the Earth’s surface is composed of various kinds of high and low rocks. Due to the internal upheavals of the Earth or the uneven pressure, malformation sets in the rocks. Due to the excessive pressure, the layers of rocks crack suddenly. After breaking, they either go up or down inside the Earth. At the places, where such changes take place in the rocks, the Earth’s crust experiences big shocks. These shocks cause vibrations, which spread through the Earth’s surface. All those areas through which these vibrations travel, are said to be affected by an earthquake. The point of origin of an earthquake is called the ‘epicentre of the earthquake’. Some places on the Earth are prone to earthquakes. Japan has the highest incidence of earthquakes. The Earth surface in Japan is uneven throughout and so the occurrence of earthquakes is very common there. San Francisco, Lisbon, China, India and Japan have all suffered great loss of lives and property, due to the occurrence of earthquakes. Scientists have developed an instrument called the ‘seismograph’ to study earthquakes. This instrument has arrangements to study the seismic waves caused by the earthquakes. Seismographs have been installed at various places in the world to record the seismic vibrations with a view to warn mankind about the dangers of earthquakes. But, it is impossible to forecast an earthquake or prevent it from happening.