How can we tell the difference between a seismograph of a local earthquake and that of a distant earthquake?
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An earthquake generates waves that move through the earth in different ways. Two of the principle waves are the compressional P wave and the shear S wave. P waves move faster than S waves, so by timing the difference in arrival time of the two waveforms we can estimate the distance to the epicentre.
https://www.bgs.ac.uk/discovering-geology/earth-hazards/earthquakes/how-are-earthquakes-detected/
