The records obtained from seismographs , called seismograms, provide useful information about the nature of seismic waves. Seismograms reveal that two main group of seismic waves are generated by the slippage of a rock mass. One group is called surface waves because their motion is restricted to near Earth' s surface. Others travel through Earth' s interior and are called body waves. Body waves are divided into two types: primary, or P , waves and secondary , or S , waves