To determine the strength and location of earthquakes, seismograph is used which is equipped with sensors called seismometers that can detect ground motions causes by seismic waves from both near and distant earthquakes. A seismograph produces wavy lines that reflect the size of seismic waves passing beneath it.
The record of the wave, called a seismogram, is imprinted on paper film or recording tape or is stored and displayed by computes. Probably the best known guage of earthquake intensity is the local Richter magnitude scale, developed in 1935 by United States seismologist Charles F. Richter. This scale, commonly known as Richter scale, measures the ground motion caused by an earthquake. Every increase of one number in magnitude means the energy release of the quake is 32 times greater. For eg. An earthquake of magnitude 7.0 releases 32 times as much energy as an earthquake measuring 6.0 An earthquake with a magnitude of less than 2.0 is so slight that usually only a seismometer can detect it. A quake greater than 7.0 may destroy many buildings. There are about 10 times as many quakes for every decrease in Richter magnitude by one unit. Scientists locate earthquakes by measuring the time it takes body waves to arrive at seismographs in a minimum of three locations. From these wave arrival times, seismologists can calculate the distance of an earthquake from each seismograph. Once they know an earthquake’s distance from three locations, they can find the quake’s focus at the center of those three locations.
The earliest known seismograph was made in China in around AD 130. It consisted of a bronze vessel, with eight balls balanced delicately in the mouths of eight dragons placed around the vessel. A seismic wave was recorded when the vibration caused one or more of the balls to fall out of the dragons’ mouths.
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