Seismograms record the energy emitted by seismic or earthquake waves that radiate from the focus (point at which earthquake begins). Seismograms are created by machines called seismographs. Seismograms are used by seismologists (scientists who study earthquakes) to mathematically describe the magnitude of an earthquake.

Earthquakes generate many different types of seismic waves. Two major types are P (push/pull, compressional, or primary) and S (shear or secondary). Seismograms will record P-wave arrivals before S-wave arrivals. Surface waves are also generated that travel on the surface of the crust.