A typical seismogram records the arrival of the body and surface waves. The body waves consist of two waves following the same path, the p-waves and the s-waves. Because the body p-waves propagate through Earth at the highest rate, they are the first to be recorded followed by the slower s-waves. Because the body waves expend little energy moving the rocks as they pass through Earth, their amplitudes are very low. The surface waves arrive at some time following the arrival of the body waves. Because the surface Love waves are responsible for the damage caused by earthquakes, their amplitudes recorded on the seismogram are higher than those of the body waves. An important aspect of the surface waves is that they are most intense at the epi-center and decrease away from the epi-center at a regular fashion. Using the seismogram recorded at any seismic station, if the distance from the station to the epicenter were known; which can be determined by the time-travel graph, a measurement of the amplitude of the surface waves at the seismic station can be used to calculate the amplitude of the surface waves at the epi-center. This is accomplished using a nomograph which is a diagram that uses the difference in the arrival times of the body waves and the amplitude of the surface as recorded on the seismogram to calculate the amplitude of the surface waves at the epi-center which, in turn, can be converted to a value on the Richter Earthquake Scale.
richter scale, seismogram