Seismographs - Keeping Track of Earthquakes Throw a rock into a pond or lake and watch the waves rippling out in all directions from the point of impact. Just as this impact sets waves in motion on a quiet pond, so an earthquake generates seismic waves that radiate out through the Earth.
Seismic waves lose much of their energy in traveling over great distances.
When these detectors are connected to a system that produces a permanent recording, they are called seismographs. The horizontal seismograph shown here moves only in the horizontal plane. In this design, the mass hangs from a spring, which absorbs some of the motion and causes the mass to lag behind actual motion. This principle is illustrated in figure. When the pier is steady, the pen attached to the mass writes straight line.