Do you know how does an earthquake happen? let’s know about it. Perhaps you have seen science fiction films in which great cracks suddenly appear in the ground, and the land seems to roll like the sea. Of course, if you live in Britain or Europe this does not seem very realistic, does it?
But, even though these scenes may be exaggerated to make the film more exciting there are many places around the world such as Los Angels in America or Tokyo in Japan where the ground could tremble at any time with terrible effect.
This shaking of the earth is called an earthquake you know that if you throw a stone into a pond waves spread through the water in all direction from the point where the stone fell. In a similar way if some rocks in the earth are somehow disturbed movement like a wave is set up in the ground itself.
The land can be felt to shake close to where the rocks were disturbed and depending on the force of the earthquake building may come tumbling down water pipes may be broken roads torn up and huge fires started by the breaking of gas mains or electricity cables. Last but certainly not least thousands of people may lose their lives.
Try placing a brick on a plank of wood and slowly tilt the plank from one end until the brick slips. You will find that you can tilt the plank quite a lot before the brick eventually slips and when it does it slips suddenly.
This is because both the brick and the piece of wood have a rough surface and tend to grip together. Before the brick can move this tendency ahs to be overcome by tilting the plank more and more until the brick suddenly slides.
Similarly, inside the earth, if rocks are pushed more and more they will break, or move very suddenly along existing cracks. Your brick and plank were quite small and you would feel very little when the brick slips, but in the earth, the rock masses are so big that even if they move a little an earthquake is caused.
If you were to make the surface of your brick and plank more slippery (perhaps but rubbing them with soap) you would find that the plank need not be tilted so much before the brick moved, and that it would slide more slowly.
In America, scientists are testing the effects of pumping thousands of gallons of water into the ground in the hope that rocks can be made to move more slowly, and in this way prevent earthquakes.
Apart from the harmful results of earthquakes the ways in which earthquake waves pass through the earth can tell scientists quite a lot about what the inside of our planet is like. But they have to be able to detect and measure these waves.
An instrument which measures earthquakes is called a seismometer. Simply, it consists of a masonry column bedded into the solid rock to which is attached an armload with a heavyweight called a boom.
On the end of the boom is a pen which marks a line onto a piece of paper wrapped around a drum which restates when the seismometer is in operation. Because of its weight, the boom tends to remain still during an earthquake but the rest of the instrument is shaken at this movement are recorded as a squiggly line on the paper as it rotates.
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