Difference between Earthquake Seismographs
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/