How The Earthquakes Are Recorded On A Seismograph?

Sometimes there is big news hitting the headlines of national dailies about an earthquake that happened is some part of the world, possibly in South America or Japan. The news is about cities being destroyed, hundreds of people getting killed. Yet one in the house does not feel much. If the earth “shook” why did not one feel it.

Maybe, one did not feel it. But not far away, in a big city or university, Scientists probably made a complete and exact record of the said earthquake. They registered the feel on their instruments.

Such an instrument is called “a seismograph”. The study of earthquakes we name as “seismology”, why an individual did not feel but on contrary the scientists were able to record it and create history.

An earthquake is a trembling of the earth’s surface. The word used is surface, meaning the crust of the earth. An earthquake is generally caused by a fault line in the rocks of the earth’s crust – a break along which one rock mass rubbed on another with great force and friction. This energy of rubbing is changed to vibrations in the rocks. The vibration may travel thousands of kilometres.

Thus earthquake vibrations are a kind of wave motion which travels at different speed through the rocky crust of earth. Because the vibrations have a long distance to go and, are travelling through rock, by the time they reach your town you cannot even notice them. But the seismograph can. This is how it works.

Imagine a slab or block of concrete. Suppose a chart is sticking out of this slab, that is fixed to it. It is parallel to the ground, like a sheet of paper. Above it, a rod sticks out from which is hung a weight. At the bottom of the weight is a pen which touches this chart. Now comes an earthquake wave. The concrete block moves and the chart with it. But the weight, which is suspended, does not move. The pen makes the markings on the chart as it moves. The earth-quake is recorded. The instruments of course are arranged very delicately, so the slightest motion will be registered and recorded.