When the Earth trembles, earthquakes spread energy in the form of seismic waves. A seismograph is the primary earthquake measuring instrument. The seismograph produces a digital graphic recording of the ground motion caused by the seismic waves. The digital recording is called a seismogram.
A network of worldwide seismographs detects and measures the strength and duration of the earthquake’s waves. The seismograph produces a digital graphic plotting of the ground motion of the event.