Seismograph, instrument used to detect and record seismic waves caused by earthquakes, nuclear explosions, etc.; the record it produces is a seismogram. The simplest seismograph has a horizontal bar, pivoted at one end with a recording pen at the other. The bar, supported by a spring, bears a heavy weight. As the ground moves, the bar remains roughly stationary owing to the inertia of the weight, while the rest of the equipment moves. The pen traces the vibrations on a moving belt of paper. Seismographs are used in seismic prospecting.
See also: Richter scale.